General Glossary of Terms

relating to

Modelling and Simulation for Environmental Engineering


Related glossaries:



absolute ventilation efficiency

A quantity which expresses the ability of a ventilation system to reduce a pollution concentration relative to the feasible theoretical maximum performance.


The process of one substance entering into the inner structure of another. Likewise, the process of turning the energy incident into inner energy of the absorbing media.

absorption capacity / absorptivity

Absorptivity of a surface is the fraction of radiant energy incident which is absorbed. It is numerically equal to the emmisivity for any given surface.

absorbing media

The media in which the process of absorption and reflection of energy incident is completed.

AC pressurization technique

This technique allows building airtightness to be examined at small (<4 Pa) pressure differentials with minimal interference from climatic forces. The air flow through the building envelope can be evaluated by using a piston assembly to vary the effective volume of the structure. Measuring the amplitude of the pressure response inside the building and phase relationship between this pressure and the velocity of the piston, enables the air flow through the building to be determined.

ACH (air changes per hour)

Unit used for quantifying air flow.

acceptable air quality

Air in which there are no known contaminants at harmful concentrations as determined by specialist authorities and with which a substantial majority (80 % or more) of the people exposed do not express dissatisfaction. (ASHRAE 62-1989).

access time

The time interval between the request for imformation and the instant that this information is available. This is often the time taken for one complete access of a peripheral by the central processing unit.

accumulated temperature difference

The concept of accumulated temperature difference or degree days allows the need for heating in a building to be assessed.

accuracy of an input

The maximum expected deviation of the indicated value from the true value.

accuracy of an output

The maximum expected deviation of the actual value from the desired value.

Adventitious opening (Also known as unintentional opening & fortuitous leakage)

An opening within the building envelope which, in terms of ventilation, is unintentional, for example, cracks around doors and windows.

acoustic technique

A method of detecting cracks in a building where leakage may occur. A steady source of high pitched sound is placed within the building and a microphone is used outside the building as a detector. Leaks correspond to an increase in volume of the sound transmitted. This technique provides qualitative information only.


A means provided to execute the output from the controller, (i.e., the control action), through operating a final control element. This control action is determined such as to reduce the error between the measured value and desired value of the controlled variable.


changes in an organism's structure or behavior that help it adjust to its surroundings. An increase or decrease in sensitivity to a given stimulus that results from exposure to that stimulus.

adaptive control

The primary objective of adaptive, or 'self-learning', or 'heuristic' control is to reduce uncertainties concerning knowledge of the environment and system dynamics in an on-line or real time fashion and to alter controller parameters (gain,integral such as to cause system operation to continuously seek better performance. Thus more accurate control is achieved over a wide range of external conditions since control parameters are automatically adjusted as conditions vary. The need for manual re-tuning to adjust for seasonal variations is eliminated.

adaptive models for thermal comfort

See thermal comfort models.

admittance method

The CIBSE method of determining the peak inside environmental temperature.


A number or code that defines the position of the information in the memory of a computer. It is also used to control the destination of data sent on a bus.


The identification of a physical or virtual distinct entity in a network. On the Internet, this network address is called a URL (Uniform Resource Locator). For instance:


The adhesion of a thin film of liquid or gases to the surface of a solid substance.


An acronym for air handling unit; a component of an HVAC system that includes the fan(s), filters, and coils to condition the air.


a mixture of gases constituting a compressed fluid tied to the planet by gravitational attraction. Air on Earth is 79.0% nitrogen, 20.9% oxygen, and less than 0.1% a mixture of carbondioxide, argon, helium, and hundreds of other gases originating from natural and man-made sources. Air in the context of building simulation is often used to infer dry bulb temperature.

air barrier

(See air curtain)

air change

A quantity of fresh air equal to the volume of the room or building being ventilated. (See air change rate and ACH)

air change efficiency

A measure of how quickly the air in the room is replaced. It represents, the ratio between the nominal time constant, and the air change time, for the room. (See also air change time; coefficient of air change performance; local air change index; nominal time constant; specific flow)

air change rate

The volumetric rate at which air enters (or leaves) a building or zone expressed in units of building or zone volume. (See specific flow)

air change time

This is the time for all of the air in a room to be changed, and is equal to twice the room mean age. (See air change efficiency; coefficient of air change performance; local air change index; nominal time constant; room mean age; specific flow)

air channel

(See flow path)

air-conditioning (Also known as environmental control)

Treatment or control of the air in buildings so as to render it more comfortable or healthful for human beings or more suitable for manufacturing process. Alternatively, air conditioning refers to the control of temperature, moisture content, cleanliness, air quality, and air circulation as required by occupants, a process or a product in the space; Air conditioning is always associated with the cooling and dehumidification process of air and is always therefore identified with refrigeration equipment.; Control over relative humidity by the addition of moisture constitutes full air conditioning. However, the more often used partial or comfort air conditioning (cooling and drying) which uses refrigeration equipment only is still referred to as air conditioning; In the context of HVAC systems, ventilation should not be confused with air conditioning as refrigeration equipment is not necessarily provided with ventilation equipment. (See cooling; dehumidification; heating; humidification)

air cooled chillers / air to water chillers

Chillers which consist of condensers and compressors which are usually located externally and are air cooled. Chilled water is provided from these chillers. (see water to water chillers)

air curtain

A stream of high velocity, temperature controlled air which is directed downward across an opening. It is designed to exclude exterior draughts, and pollutants blown in from outside. It also prevents the transfer of heat across the boundary, and permits the air-conditioning of a space with an open entrance.

air distribution

The delivery of outdoor or conditioned air to various spaces in a building, usually by mechanical means.

air exchange rate

Used in two ways: 1 ) the number of times that the outdoor air replaces the volume of air in a building per unit time, typically expressed as air changes per hour; 2) the number of times that the ventilation system replaces the air within a room or area within the building.(See air change rate or ACH)

air exfiltration

The uncontrolled outward leakage of indoor air through cracks, interstices, and other unintentional openings of a building, caused by the pressure effects of the wind and/or stack effect.

air flow

The mass/volume of air moved between two points. (See laminar flow; turbulent flow)

air flow rate

The mass/volume of air moved per unit of time through a space opening or duct. (SI units: mass flow rate - Kg/s ; volume flow rate - m3/s)

air infiltration

The uncontrolled inward leakage of outdoor air through cracks, interstices, and other unintentional openings of a building, caused by the pressure effects of the wind and/or the stack effect.

air inlet

A deliberate opening in a room or a duct wall for the provision of outdoor or conditioned air into the room.

air leakage

The leakage of air in or out of a building or space usually driven by artificially induced pressures. (See pressurization test)

air leakage characteristic

An expression that describes the air leakage rate of a building or component. This may be:

(See flow equation; equivalent leakage area)

air leakage rate

The rate of air leakage in or out of a building or space.

air outlet

A deliberate opening in a building envelope or a duct through which air is expelled to the outside.

air pollutant

any unwanted substance in air.

air pressure

The force per unit area that air exerts on any surface in contact with it.(SI Units, Pascal (Pa), 1 Pa is equivalent to 1 N/m2)

air quality

(See acceptable air quality)


A general descriptive term for the leakage characteristics of a building. The smaller the air leakage rate at a given pressure difference across a building envelope, the greater the airtightness. (See air leakage characteristic)

airtightness standard

A standard value of building or component air leakage corresponding to a reference pressure difference across the building envelope or component. Standard values may be expressed in terms of air change rate, flow rate per unit area of opening, flow rate per unit length of crack, or equivalent leakage area.

air to air cooling pump

As air to air heat pump but for cooling only

air to air heat pump

A packaged piece of equipment that contains one fan to be connected to ductwork to cool or heat the indoor atmosphere, and one fan to reject (or reclaim) heat to the external atmosphere. Thes units may be positioned externally or they may be manufactured to be positioned internally (less usual).

air to water chillers

See air cooled chillers

air to water heat pump

As air to air heat pump but provided with a reversing valve and hot water as well as chilled water.

air speed

The speed of the air relative to its surroundings.

air velocity

The rate and direction of air movement.(Important when considering cooling effects and comfort criteria).

air vent

A purpose provided air inlet or outlet.

air vapour barrier

A moisture impervious layer applied to the surfaces enclosing a space to limit moisture migration.


A calculation method that produces a control output by operating on an error signal or a time-series of error signals.


a substance that induces allergic reaction.

ambient air

the air surrounding an object.

ambient temperature

The temperature of the air within a room or zone. (See environmental temperature)

analogue energy model

The analogy that exists between electrical flow and heat flow may be used to construct electrical analogue devices for the study of complex heat flow phenomena. The technique, although having had its use as a research tool, has little application in a design context.

Analog-to-Digital Converter (ADC or A/D)

A hardware device used to convert an analog signal into discrete voltage or current values proportional to the analog input.

analog signal

A continuously changing variable.

angle factor/local angle factor

Relation between the radiation from the surface of one body to the surface of another and the full radiation

anonymous ftp

A publicly available Internet file site. Users must sign on as anonymous and enter their email address to connect to an anonymous ftp site.


An acronym for the American National Standard Institute


A separate room or suite of rooms in a building occupied by one party. Many of these apartments form apartment blocks, or blocks of flats.

aperiodic damping

A system of damping so large that after having being subject to a single constant or instantaneous disturbance, the system tends to a state of equilibrium, without oscillating around it.

applications software

In a BEMS, programs that provide functions such as DDC algorithms, energy management, and lighting control; cf. Operating Software.


A program that locates files that are freely available on anonymous ftp sites across the Internet. To use Archie, telnet to one of these sites and login as archie. Type help to obtain full instructions.


the product, created in the process of applying art and science in the designing buildings.


The storage of data in a retrievable data set.

arithmetic summation of uncertainties

The sum of the moduli of uncertainties. The most pessimistic method of combining uncertainties. If three uncertainties are estimated at ±3% each then the arithmetic sum is ±9%.

artificial intelligence

The process of enabling computers to mimic human learning and decision-making.


An acronym for American Standard Code for Information Interchange. A standard for digital representation of letters, numbers, and control codes; understood by most computers.


An acronym for the American Society of Heating, Refrigerating and Air Conditioning Engineers (Select link to web site)

assembler language

Programming language which allows the programmer to define labels and fixed values to then use these labels with a mnemomic instruction to produce a machine code computer program.

asynchronous communication

Data communication in which the timing of information transmission is not related to any sytem frequency, e.g. 50 Hz.


An open space in the middle or at the edge of a building, usually enclosed, but still allowing the penetration of light.


A low storey or structure above the main part of a dwelling. Alternatively known as a loft or roofspace.


The proportion of the switched-on time during which the equipment is available for work; cf. maintainability and reliability.

automatic control system (also automatic regulating system)

A system that reacts to a change in the variable it controls by adjusting other variables to restore the system to the desired balance.

Auliciems model

Auliciems model for thermal comfort. An adaptive model developed by Auliciems fits sensation data based on field investigations of thermal comfort in Australia spanning several climates.
See thermal comfort models.

average control

If the value of the control variable depends on the location of the sensor, it may be necessary to apply corrector action in proportion to the average deviation measured in several locations.

axial or vaneaxial fan

A fan in which the airflow, at all times, from entry to exit is predominately parallel to the axis of rotation.



The Central network infrastructure of the Internet is often referred to as the backbone and its allows data to travel from one network to another.


The reversed flow of polluted air (or flue gases) in a chimney, flue or other air outlet, back into the room or building.

background leakage

Unidentified openings or gaps in a building envelope through which infiltration can take place.

bag sampling method

May refer to:

balanced fan pressurization

A measurement technique using two or more blower doors to evaluate the leakage of individual internal partitions and external walls of multizone buildings. The technique involves using the fans to induce a zero pressure difference across certain building components, thus eliminating their leakage from the measurement.

balanced supply/extract ventilation system

A ventilation system in which fans both supply and extract air from an enclosed space at equal rates.


A Term used to describe how much data you can send through a connection to the Net. The transmission capacity of a given medium, in terms of how much data the medium can transmit in a given amount of time. The greater the bandwidth, the faster the rate of data transmission. Information carrying capacity of a communication channel.

`bang-bang control'

see On-Off control.


Acronym for Building Automation System. (See BEMS.)

base case model

(Standard) computer model of a particular building. The base case model can be used to assess the relative performance of a certain (new) feature of the building by changing the model parameters associated with that feature. Comparison of the results for the base case model with those for the changed model will reveal the relative performance of the feature.
In case of an existing building the "as built" situation is often used as the base case.


(See cellar)

baud rate

The number of BITS of information transmited per second in a serial dat transmission system.

BEMS (Also 'BAS', 'BMS', and 'CEMCS')

Acronym for Building Energy Management System. A computerised system which operates to monitor and control the energy usage in a building.Energy management, HVAC, safety, security, operations schedules, commissioning, and maintenance functions can all be provided under the overall supervision of a central stationcomputer and operator.

Biot number

Dimensionless number, equal to Bi =(alo)/la , where a is the heat transfer coefficient from the surface to the environment (or from the environment to the surface), lois specific dimension and la is the thermal conductivity coefficient of the body.

BIT or binary digit

A single item of digital data. A BIT can only take the value 1 or 0 (usually representing true or false).


An acronym for BInary digiT. It is the basic unit of information in the computer world. A bit is a digit in binary form and carries one of two values, 0 or 1.

binary switching

A facility which can be provided by a micorprocessor based step controller. Each physical step controls a load which is twice the size of the one preceed- ing it, i.e. the loads are spli in the ratios 1:2:3:4 etc. The order in which the physical steps are switched is such that a four step controller can control in 10 stages, a six step in 63 stages, and so on. The most common application is in the control of electric heater batteries.

blower door

A device that fits into a doorway of a building, containing a powerful fan, for supplying or extracting a measured rate of air flow. It is normally used for testing air leakage by pressurization or depressurization. (See AC Pressurization; DC Pressurization; Balanced Fan Pressurization)

body odour

The odour originating from sweat and secretions from the skin, foul breath, and gases from the digestive tract. Odour emission is dependent upon diet, activity, and personal hygiene. (See olf, decipol)

boiler compensation

An operation which changes the operating temperature of a boiler usually according to the outside air temperature.

boiler optimisation

An energy management function which acts to balance boiler operation to loads and control combustion air. See also oxygen trim.

boost period

The period immediately prior to the occupancy period during which plant is operated at its full rate capacity. See also Optimum Start Controller.

boundary condition

These are the temperature, flux and other environmental conditions that pertain on either side of a surface. According to the particular surface, they may be obtained from the climate data file, from the calculated values in an adjacent zone, or from user-specified values.


Software that allows users to access and navigate the World Wide Web. Some Web browsers, such as Mosaic and Netscape, are graphical. Lynx is a text-based browser.


The unit B.T.U./hour is a unit of heat flow still widely used in North America, Canada and parts of Asia, whereas Europe uses the 'watt'. The origin of the term is the amount of heat absorbed by one ton of ice when melting from solid to liquid state at 32oF and assuming a latent heat of ice of 144 B.T.U.s/lb. The heat absorbed is found to be 288,000 B.T.U.s over 24 hours, or 12,000 B.T.U.s/hour (in reality the latent heat of ice is slightly less than 144 B.T.U.s/lb.) (See Refridgeration (`ton'of))

building envelope

The total area of the boundary surfaces of a building through which heat, light, air and moisture are transferred between the internal spaces and the outside environment.

building related illness

Diagnosable illnesses whose symptoms can be identified and whose cause can be directly attributed to airborne building pollutants (e.g., Legionnaire's disease, hypersensitivity, pneumonitis).


A single storey building, which may be detached or semi-detached whose primary purpose is for living accommodation.


A cable that is connected to a number of different devices, sensors, controllers, outstations, etc., that acts a means of data exchange. There are two main types, serial and parallel. In building services it is most common to use serial types where data flows on just two cables.


A single computer character, generally eight bits. For example, the letter "G" in binary code is 01000111.


cascade control

A control system in which one controller provides the setpoint for one or more other controllers.


Technique for making airtight joints by applying a sealing material. A form of weatherstripping. (See weatherstripping)


An acronym for constant air volume (see constant air volume).

cavity barrier

A form of vapour barrier where a moisture impervious layer is introduced inside the "cavity" of a cavity wall, usually polythene sheeting. (See vapour barrier; cavity wall)

cavity wall

A wall built of two leaves, separated usually by a continuous gap. The two leaves are connected by ties at intervals. The inner layer may be double for floor bearing.


Compact Disc-Read Only Memory: A record like storage medium that uses digital and optical laser technology to store about 600Mb of text, pictures, and sound on a single disk. With newer versions (CD-ROMXA, CDTV, CD-i) animations and moving pictures can be retrieved from the discs.

ceiling plenum

Space below the flooring and above the suspended ceiling that accommodates the mechanical and electrical equipment and that is used as part of the air distribution system.


A storey in a building whose floor line is below ground level at any entrance or exit, the ceiling of which is not more than 5ft above ground level whose primary function can be accommodation or storage.


Acronym for Comprehensive Energy Management and Control System. (See BEMS.)

central station

This is the heart of a BEMS, and also the main communication channel for the operator. Here is contained the software and the main storage of data relating to the plant and buildings controlled.

centralised intelligence

Description of a system where algorithm processing is only possible at the central station. The outstations are dormant when not in contact with the central station.

centralised system

A BEMS in which all executive control takes place at the central station.

centrifugal fan

A fan in which the air is turned from parallel to the axis of rotation on entry to a direction tangential to the arc described by the tips of the rotating blades or vanes.


An acronym for the Commission of European Community


Acronym for Chlorofluorocarbon, which is a family of refrigerants that if released into the atmosphere are associated with the destruction of the Earth's ozone layer. A typical example is Freon (tradename) or R12


Acronym for The Chartered Institution of Building Services Engineers. (select link to website)


prevalent and predictable meteorological conditions of a geographical area; determined air temperature, solar radiation, humidity, wind parameters, clouding and precipitation data for the surrounding geographic region.

closed-loop control

A monitoring control system. This type of control system possess monitoring feedback, the deviation signal formed as a result of this feedback being used to control the action of a final control element in such a way as to tend to reduce the deviation to zero; cf. open-loop.

coefficient of air change performance

This is the ratio between the nominal time constant and the room mean age. The coefficient of air change performance equals twice the air change efficiency. This term is equivalent to the ventilation effectiveness definition of ASHRAE standard 62-1989, "Ventilation for Acceptable Indoor Air Quality". (See also air change time; local air change index; nominal time constant; specific flow)

collector chamber

Sealed box or other enclosure used to isolate a building component when conducting pressurization tests.


Burning or rapid oxidation accompanied by a release of energy.


measure of human acceptability of the physical environment

comfort in buildings

perceived acceptability of various internal environmental parameters such as air temperature, lighting, noise, etc.

comfort conditioning

comfort conditioning as the name implies is to provide a comfortable environment for the majority of occupants. Humans are reasonably comfortable with a range of between 20% and 55% relative humidity at normal comfort temperatures. It is therefore common when specifying, to limit the humidity in summer and not specify a limit in winter. A typical specification would state an internal condition of 22 oC / 50% relative humidity being maintained at 30o C / 20 oC wet bulb external conditions in summer. In winter, 21oC internal temperature at -3 oC saturated outside air temperature. (see saturation point)

comfort zone

The range of indoor conditions considered acceptable by a certain proportion (e.g. usually more than 80%) of the people working or living in the space.

commercial building

A building whose primary purpose is to provide space for commercial activity rather than domestic. This includes offices, storage, plant, farm, public and some factory classifications.


The Start-up phase of a building that includes testing and adjusting HVAC, electrical, plumbing, and other systems to assure proper functioning and adherence to design criteria. Commissioning also includes the instruction of building representatives in the use of the building systems.

communications module

Controls the transmission between other controllers and between controllers and a central computer based on an established bus protocol.

comparing element (also `error detector')

A control element which compares the measured value and the desired value then sends an error signal (measured value minus desired value) to the part of the controller which determines control action. The control action implemented acts to reduce the magnitude of the error signal.

compensation control

A process of automatically adjusting the control point of a given controller to compensate for changes in a second measured variable (e.g., outdoor air temperature). For example, the hot deck control point is normally reset upward as the outdoor air temperature decreases. Compensation control isone form of open-loop control. See also Boiler compensation and Weather compensation.

component leakage

The leakage of air through the building envelope which is directly attributable to flow through cracks around specific doors, windows or other components.


The precipitation of liquid from its vapour phase resulting from the lowering of temperature at constant pressure: especially the deposition of water from moist, warm air onto a relatively cold surface or between two surfaces such as within a cavity wall.

condensing unit

As heat pump except cools only and does not therefore incorporate a reversing valve(see heat pump).


As condensing unit except does not incorporate compressor, which is then incorporated in the indoor unit(see condensing unit).

conditioned air

Air that has been heated, cooled, humidified, or dehumidified to maintain an interior space within the "comfort zone." (Sometimes referred to as "tempered" air.)

computer program

A schedule or plan that specifies actions which may or may not be taken, expressed in the form of a set of instructions suitable for execution by a computer.


Heat conduction involves the transfer of heat from one molecule to an adjacent one as an inelastic impact in the case of fluids, as oscillations in solid nonconductors of electricity, and as motions of electrons in conducting solids such as metals. Conduction is the only mechanism of heat transfer through an opaque solid. Some heat may be transfered through transparent solids such as glass, quartz and certain plastics, by radiation. In fluids the conduction process is supplemented by convection and if the fluid is transparent, by radiation.


Conductivity is the measure of conduction within a material. The conductivities of materials vary widely, being greatest for metals, less for nonmetals, still less for liquids and least for gases. Any material which has a low conductivity may be considered an insulator.

conservation of energy (Also known as energy conservation)

The principle that in any system energy cannot be created or destroyed, although it can be changed from one form to another, e.g. from potential to kinetic to heat, etc. (See energy conservation)

constant air volume system

An air handling system that provides a constant air flow while varying the temperature to meet heating and cooling needs.

constant concentration

A Tracer gas method for measuring ventilation rates, whereby an automated system injects tracer gas at a rate required to maintain the concentration of tracer gas within a room or zone at a fixed, pre-determined level. The ventilation rate is proportional to the rate at which the tracer gas must be injected. (See constant flow/emission; decay tracer gas method)

constant flow/emission

A Tracer gas method for measuring ventilation rates whereby tracer gas is continually emitted at a uniform rate. The equilibrium concentration of tracer gas in air is then measured. (See constant concentration; decay tracer gas method)


An unwanted airborne constituent that may reduce the acceptability of the air (quality) and may be detrimental to the health of building occupants.


any physical, chemical, biological, or radioactive substance that can adversely affect air, water or soil.


The part of a computer used for communication between the human operator and the comnputer.

continuous action

The action of an element, regulator, or automatic control system whose output is a continuous function of it's input signal.


The control is the ability of the system to respond to the changing requirements imposed upon it by the fluctuation of outside conditions.


Degree of difficulty in controlling the controlled variable. This depends on the the delays which occur between a change in conditions at one point and it's manifestation at another point. This delay can be of two kinds: distance-velocity and transport lag, which have different effects on the system. Together they determine the dynamic characteristics of the plant.

controlled medium

The medium in which the controlled variable exists. In a space temperature control system, the controlled variable is the space temperature and the controlled medium is the the air within the space; cf. Controlled agent.

controlled variable

The quantity or condition that is measured and controlled.

controlled sequence

Equipment operating order established upon a correlated set of environment data conditions.


A piece of equipment which combines the functions of at least the setpoint input element, comparing element, and the amplifying and signal processing element for a automatic control system. Its purpose is to receive input from a sensor and then derive the proper correction output which is then sent to the actuator.

control action

The action generated by the controller and fed to the correcting unit, i.e., the relationship between the input signal and the output signal of a control element; cf. Control mode.

control agent

The medium in which the manipulated variable exists. In a steam heating system, the control agent is the steam, and the manipulated variable is the flow of steam; cf. Controlled medium.

control element

A general term for a constituent part of a control system.

control function

Generally, this term is used for the operations carried out by an automatic control system.

control law

This defines the control algorithm which represents the logic or "control action" of a controller. For example, the control law implemented may be PID control.

control loop

Generally, this term means any control network consisting of the control elements required for automatic control.

control mode

The OVERALL type of control exercised over the process; cf. Control action.

control parameter

A variable used in the control algorithm e.g. setpoint, proportional band.

control point

The actual value of the controlled variable.

control point adjustment

The procedure of changing the operating point of a local loop controller from a remote location.

control range

The change between the initial and the potential value of the controlled condition.


The essential process in the case of convection is the flow of a fluid over a solid surface, accompanied by a transfer of heat between the surface and the fluid. The movement of the fluid may be due to changes in its density caused by changes in its temperature, by natural convection; or it can be created by mechanical means, by forced convection. (See free convection; forced convection; conduction; radiation; heat transfer)

conventional controllers

All controllers with the exception of microprocessor-based controllers. Thus the term `conventional controllers' describes the following analogue controllers:- pneumatic controllers, hydraulic controllers, fluidic controllers, electrical controllers, and electronic (solid-state) controllers.


The transfer of energy from a body of solid, liquid or gas by the existence of a temperature gradient from that body to its surroundings which are at a lower temperature, and may also be solid, liquid or gas. This process is the opposite of heating.

corrective action

Control action that results in a change of the manipulated variable. Initiated when the controlled variable deviates from the setpoint.


Acronym for Central Processing Unit. The unit that coordinates and controls the actions of all the other units. It includes circuits controlling the interpretation and executions of instructions.

crack / crackage

Small gaps around doors, windows and other parts of a building envelope through which ventilation air may pass.

crack length

The total length of the narrow gaps found around doors and windows etc, through which ventilation air may pass. (See component leakage)


A shallow space in a building, usually under the floor, which provides access to pipes, wires and other equipment.

cross contamination (of air or masses)

The contamination of one stream of air by pollutants in another, due to air movement between the two streams (or masses).

cross ventilation

Air enters on one side of a room and leaves on a different side of the same room. Airflow between the entry and exit provides ventilation. Also used for flow between rooms, where the inlet is in one room and the outlet is in another.


Acronym for Cathode Ray Tube. A display device (screen) used in computer equipment.


Acronym for Carrier Sense, Multiple Access with Collision Detection. It is a method used to control access to shared transmission medium, such as a coaxial cable bus to which a number of stations are connected.

cumulative uncertainty

Cumulative uncertainty will arise in successive stages of the calibration chain of a measuring instrument. this is usually compounded into a single value supplied by the calibration laboratory.


One complete execution of a repeatable process. In basic heating operation, a cycle comprises one on period and one off period in a two-position control system.


A periodic change in the controlled variable from one value to another. Uncontrolled cycling is called hunting.

critical damping

The limiting degree of damping such that any decrease in the amount of damping would result in a change from aperiodic damping to under-damping.



A final control element which acts to regulate the flow of air through a duct.


A representation of facts, concepts, or instructions in a formalised manner suitable for communication, interpretation, or processing by humans or by automatic means.


A structured collection of data. This is usually arranged in a series of files with the access to the database controlled by a computer program called the 'database managementsystem'.

Data Gathering Panel (DGP)

See Outstation.

data highway

Communication link between the various components of a BEMS through which digital data is transmitted.

data point

See Point.

day economization

A control scheme which permits heating or cooling plant to be turned off completely if the anticipated fall or rise in internal temperature does not exceed pre-selected limits within a period, usually 1 hour.

daylight factor distribution

This is included to indicate artificial lighting needs.

DC pressurization

Building airtightness levels can be measured by using a fan, temporarily installed in the building envelope (a blower door) to pressurize the building. Air flow through the fan creates an internal, uniform, static pressure within the building. The aim of this type of measurement is to relate the pressure differential across the envelope to the air flow rate required to produce it. Generally the higher the flow rate required to produce a given pressure difference, the less airtight the building. (See blower door; internal fan pressurization; external fan pressurization)


an acronym for Direct Digital Control. See Direct Digital Control


A range of the controlled variable in which no corrective action is taken by the controlled system and no energy is used. See also Zero energy band.

dead time

The time interval between a change in a signal and the initiation of a perceptible response to that change.

dead band

The change of input for which there is no discernible change in output. A dead band may be imposed on an output device e.g. an actuator to avoid continuous movement due to small changes in the input signal.

decay method (tracer gas)

A tracer gas method for measuring the ventilation rate whereby a quantity of tracer gas is released and the decrease in concentration is measured as a function of time. (See constant concentration; constant flow/emission)


In HVAC terminology, the air discharge of the hot or cold coil in a duct serving a conditioned space.

decentralised intelligence

A system where data processing is carried out at outstations as well as at the central station.


The decipol attempts to quantify the concentration of odour as perceived by humans. The decipol represents the perception of odour measured by the "pol" unit. To obtain a usable unit it has been suggested that one tenth of the pol unit is used, the "decipol". The perceived air pollution is defined as that concentration of human bioeffluents that would cause the same dissatisfaction as the actual air pollution concentration. (See olf)

defrost circuits(s)

As heat pumps extract heat from the external atmosphere even at very low temperatures itis inevitable the external coil freezes with ice. The coil therefore has to be warmed periodically in order to remove the ice, this is achieved by running the refrigeration circuit in reverse for a brief period of time. This cycle is referred to as the defrost cycle and therefore unlike a condensing unit (i.e. cooling only unit)heat pump units form water externally. Therefore, consideration has to be given to the removal of the water formed, by the provision of an external tray or other device.

degree day(s)

The concept of degree days or accumulated temperature difference allows the requirement for heating in a building to be assessed.


The process of reducing the moisture content of the air; serves to increase the cooling power of the air and can contribute to occupant comfort. (See air-conditioning; humidification; cooling; heating)


The term used to describe the maximum rate of use of electrical energy over a specific period of time.

demand control (also 'Demand Limiting', 'Load Limiting', 'Load Control'

An energy management technique used to monitor a facilty's energy use in order to limit the peak demand by automatically shutting down selected equipment, on a priority basis,for short periods of time. Demand limits are pre-programmed into the demand control softwarefor this purpose. Demand control is most often applied to electrical usage, and sometimessteam plant. Unlike most other BEMS techniques, monetary benefits are not a direct result of energy savings since electricity usage is often merely postponed, not eliminated. Thebenefits are reduced demand charges to the customer and alleviated peak demand for utilities.

Demand-Controlled Ventilation (DCV)

A ventilation strategy where the airflow rate is governed by a chosen pollutant concentration level. This level is measured by air quality sensors located within the room or zone. When the pollutant concentration level rises above a preset level, the sensors activate the ventilation system. As the occupants leave the room the pollutant concentration levels are reduced and ventilation is also reduced. Common pollutants are usually occupant dependent, such as, carbon dioxide, humidity or temperature.

demand limiting

See Demand control


In a BEMS, a device used to separate two or more signals previously combined by a compatible multiplexer for transmission over a single circuit.

dimensionless number

Dimensionless grouping of important coefficients for the current process.


A measurement technique used to evaluate the airtightness of a building or component. The air inside the room or building is extracted by the use of a fan, creating a lower pressure inside, than outside the room or building. (See blower door; DC pressurization; pressurization)

derivative action

The action of a controller in which the output signal is proportional to the direction and the rate of change of deviation of the input signal. This means that the derivative action term `looks' to the future, by examining the rate of change of the error. It is the controller's ` accelerator' and `brake', and is used in addition to proportional action, and possibly integral action, to improve on a controller's response to sudden or very large load changes. It cannot be used by itself since it does not respond to a constant error.

derivative action time (DAT)

In a controller having proportional + integral action, the time interval in which thepart of the signal due to proportional action increases by an amount equal to the part of the output signal due to derivative action, when the derivative action is changing at a constant rate.

derivative Kick

Electronic `noise' can cause sudden changes in the sensor signal input to the controller resulting in sudden error changes in the controller output. This is especially true in the derivative part of a P+I+D controller. where this sudden rate of change causes the derivative action term to change dramatically. A change in set-point can also produce derivative kick and, to a lesser extent, proportional kick.

deviation signal

The difference between the setpoint and the measured value.

diffusers and grilles

Components of the ventilation system that distribute and diffuse air to promote air circulation in the occupied space. Diffusers supply air and grilles return air.

dillution ventilation

dilution of contaminated air with uncontaminated air in a general area, room, or building for the purpose of health hazard or nuisance control.


A list of files or other directories on a computer at an Internet site.


See effective temperature.

discharge coefficient

A dimensionless coefficient relating the mean flow rate through an opening to an area and the corresponding pressure difference across the opening.

Digital-to-Analog Converter (DAC or D/A)

A hardware device used to convert a digital signal into a voltage or current proportional to the digital input.

digital signal

A discrete-time signal with quantized amplitude. If the discrete-time signal can assume a continuous range of values, then it is called a 'sampled-data signal'.

Direct Digital Control (DDC)

A control loop in which a microprocessor-based controller directly controls equipment based on sensor inputs and setpoint parameters, i.e. the plant is under the direct control of software (either in an outstation or central station) and not through the intermediary of some non-programmable controller. The programmed control sequence determines the output to the equipment.

direct expansion equipment

'Direct expansion,' 'DX,' 'refrigeration' or 'split' units are all generic terms used to identify the same equipment. It is accepted that the terms refer to two or more units, one usually positioned externally and one or more usually positioned internally. The units are connected together by site installed refrigeration pipework which is charged with a refrigerant. The external unit may take one of three forms:

  1. The heat pump - which consists of a fan, compressor, coil and reversing valve, and rejects unwanted heat to atmosphere during the cooling cycle and extracts heat from the atmosphere during the heating cycle.

  2. The condensing unit - which is as described above but does not have a reversing valve and therefore cools only.

  3. The condenser which consists of a fan and coil (as the compressor is contained in the indoor unit); the condenser is used less often than (1) and (2).

The indoor units consist of fan coil units or air handling units which may be located in the atmosphere being air conditioned or remotely in a plantroom. Some manufacturers produce 'external' units that may be located internally and in the case of these units ductwork is usually connected to atmosphere to reject heat or extract heat. DX systems are in direct contrast to hydraulic systems or chilled water systems. With these systems cooling is achieved by circulating chilled water with a hydraulic pump

discontinuous control

The controller produces a maximum or minimum output signal at a upper and lower pre-set limits of the measured variable, in order to maintain this measured variable between the limits (differential). Although these may be an optimum desired value, this is never maintained. The measured value is always increasing or decreasing. There are basically two types of discontinuous control:- (1) Step control; (2) Float control. Discontinuous control is an inexpensive and relatively simple form of control. However, when the desired value has to be constantly maintained, some form of continuous control must be used.

discrimination control

A control mode in which sensor signals from a number of sensors are fed into the controller which then decides on which sensor value to use when comparing a sensed value with the desired value. For example, in some multi-zone air-heating systems, the sensed temperature `selected' is the one indicating the zone with the maximum heating requirements. In this way the heat energy input to the system supply air is kept to a minimum.

displacement flow

The displacement of internal room air by incoming outdoor or conditioned air without appreciable mixing of the two masses. Very precise temperature and control conditions are required. (See piston flow).

displacement flow ventilation

With displacement ventilation, air is introduced into the air conditioned space at low level and at low velocity. Displacement air distribution has gained in popularity, mainly due to comfort and cleanliness considerations.

distance/velocity lag

The dead time between an alteration in the value of a signal and its manifestation unchanged at a later part of the system, arising solely from the finite speed of propagation of the signal. For example, if a flow detector is located at a distance of 10 metres from a mixing valve and the position of the valve is suddenly changed, then if the water velocity is 0.5 m/s, it will take 20 seconds for the new temperature front to arrive at the detector; thus the distance/velocity lag equals 20 seconds. As long as the distance/velocity lag has not elapsed, the controller is neither in a position to counteract the effect of a disturbance nor to correct that of any action it may have initiated.

distributed intelligence

See decentralised systems.

Distributed Processing Unit (DPU)

See outstation


An airstream with a significant downward directional component of velocity. Often occurs adjacent to cold surfaces. It may be generated artificially by air curtains, air doors etc.


An acronym for the USA Department of Energy


The part of the Internet address that specifies your computer's location in the world. The address is written as a series of names separated by full stops. The most common top level domains: .edu education (US) .net network resource .com commercial (US) .gov public bodies .mil military


To download is to transfer a file from another computer to the user's computer. To upload is to send a file to another computer.


The time to locate a fault and then repair it.


Excessive air movement in an occupied enclosure causing discomfort.


The action of filling the gaps around doors and windows, in order to prevent outside cold air leaking into the building, causing draughts. (See weatherstripping; caulking).


A sustained deviation between the control point and the setpoint in a two-position control system caused by a change in the heating or cooling load.

dry bulb temperature

The temperature indicated by a dry temperature sensing element (such as the bulb of mercury in a glass thermometer) shielded from the effects of radiation. (See wet bulb temperature)

duplex transmission

Simultaneous independent transfer of data in two directions; cf. Half-duplex.

Duration Adjust Type signal (DAT signal)

This scheme is used to modulate an intermediate device by sending it a train of on/off signals where the on to off ratio varies, according to proportional duty cycle. A common example is DAT control of a thyrsitor which in turn varies the amount of energy supplied to an electric heating element; cf. Position Adjust Type Signal.

duty cycling

A control method which alternates or cycles the sequence of plant.


economiser control

An energy management function whichaims to minimise energy consumption by the use of 'free-cooling'. Internal heat generation in a building may require the HVAC to provide cooling, even though the air temperatures are lower than the thermostat setpoint. Under this condition, it is possible to introduce outdoor air into the building to provideall or part of the cooling normally accomplished by refrigeration equipment. To use this'free-cooling', the economiser measures the dry-bulb temperature of the return air and the outdoor air, and selects an appropriate amount of the cooler air for the building conditioning by adjusting outdoors, return, and exhaust dampers.

effective dead time

In order to emphasize the essential difference between transfer lag and exponential lag, the term `effective dead time' is used to to denote the time interval between the change of a signal to an element or system and the build-up of the response to a specific proportion, say until 5% of the final change has taken place.

effective temperature (ET, ET*, ET*-DISC)

A temperature index that accounts for radiative and latent heat transfers. ET* represents the new effective temperature which evolves with time rather than being steady-state.
TSENS epresents the model's prediction of a vote on the seven point thermal sensation scale, while DISC denotes a verbal scale of thermal discomfort.
Also see SET*.

electrical analogue

The analogy that exists between electrical flow and heat flow may be used to construct electrical analogue devices for the study of complex heat flow phenomena. The technique, although having had its use as a research tool, has little application in a design context.

electrical control

A control system that operates on line or low voltage and uses a mechanical means, such as a temperature-sensitive bimetal, to perform control functions, such as actuating a switch or positioning a potentiometer. The controller signal usually operates or positions an electric actuator, or may switch an electrical load either directly or through a relay.

electronic control

A control circuit that operates on low voltage and uses solid-state components to amplify input signals and perform control functions, such as operating a relay or providing an output signal to position an actuator. The controller usually furnishes fixed control routines based on the logic of the solid-state components.


Allows users to send and receive messages to each other over the Internet.


any pollution discharge from a source.


The ratio of the emissive power of the surface to that to a perfect black surface. The physical nature of the surface has a marked effect on the emission of heat by radiation.


Smileys [ these things :-) ] and other character art used to express feelings in email communication.


Real time simulation. An emulator consists of a real time simulation of the building and plant together with a hardware interface that is used to connect the simulator to a BEMS. The outputs from the control system are read by the hardware interface and used as the boundary values for the simulation of the building and plant. The simulated outputs of the sensors are transmitted through the hardware interface to the control system, which then responds by producing a new set of outputs.

energy balance

The arithmetic balancing of energy inputs versus outputs of an object or processing equipment; it is positive if energy is released, and negative if energy is absorbed.

energy conservation

The deliberate design of a building or process to reduce its energy usage, or to increase its energy efficiency. (See conservation of energy)

energy consumption

This represents running costs which can be broken down to indicate the principal causal factors. Issues such as: - larger windows; - heat gains from lights; are inter-related and affect energy consumption. For example, the electrical power savings which result from from enhanced daylight utilisation can significantly outweigh the higher heating energy consumption.

energy efficiency

The efficient use of energy with minimum waste.

energy recovery ventilation system

a device or combination of devices applied to provide the outdoor air for ventilation in which energy is transferred between the intake and exhaust airstreams.

cnthalpy control

An energy management function which is similar to economiser control, only more sophisticated. In enthalpy control, the TOTAL heat content (sensible + latent) of the building return air and outside air is measured, and the enthalpy controller adjusts the dampers to select the air with the least total heat content for cooling.

environmental chamber

(See testing chamber)

environmental control

(See air-conditioning)

environmental factors

Conditions other than indoor air contaminants that cause stress, comfort, and/or health problems (e.g., humidity extremes, drafts, lack of air circulation, noise, and over-crowding).

environmental temperature

The temperature of the air outside a room or zone. (See ambient temperature)


An acronym for the United States Environmental Protection Agency.

equal-percentage valve

This type of valve is designed to produce equal percentage change in flow for equal increase change in valve lift.

Equalised Run Time (ERT)

A facility which can be provided by a microprocessor based step controller. Rather than load in one directionj and unload in the other (last on - first off), ERT loads and unloads to equalise the time in which each step is made, or is in the on position by adopting a first on - first off schedule. In this way, the run times for each plant item in multiple compresor and boiler installations are equalised; cf. Step control and Intelligent step control

Equivalent Leakage Area (ELA)

The equivalent amount of orifice area that would pass the same quantity of air as would pass collectively through a building envelope or component at a specified reference pressure difference.

error detector

See comparing element.

error signal

The difference between the measured value and the setpoint, or desired value.

estimating Summer-time temperature - the CIBSE method

The CIBSE method of determining the peak inside environmental temperature.


See effective temperature


See effective temperature


An acronym for Environmental tobacco smoke.

expert system

A formal definition is: 'The embodiment in a computer of a knowledge-based component from an expert that offers intelligent advice or takes an intelligent decision and, in addition, is able to justify its own reasoning'. This means that an expert system is a computer program that learns, deduces, diagnoses, and advises. The style adopted to attain these characteristics is rule-based programming. The rules are based on logic and standard computer statements. Expert systems can be useful for setting up control strategies in a BEMS.

event initiated programs

Computer programs initiated on the occurrence of an input/output operation or an alarm condition.

exhaust air

Air removed from a space and not reused therein.

exhaust ventilation

Mechanical removal of air from a portion of a building (e.g., piece of equipment, room, or general area).


air leakage outward through cracks and intersices and through ceilings, floors, and walls of a space or building.

extract air

Air that is removed from a building or space. A proportion is often used for recirculation and added to incoming air. Alternatively it is all exhausted to the outdoors, sometimes via an air to air heat exchanger or a heat pump.

extract ventilation

A mechanical ventilation system in which air is extracted from a space or spaces, so creating an internal negative pressure. Supply air is drawn through adventitious or intentional openings. Such a system allows heat to be recovered, using an exhaust air heat pump.

external fan pressurization

A blower door is fitted to a building, induced air flow through the fan creates an artificial, uniform static pressure within the building. Internal and external pressure taps are made and a manometer is used to measure the air flow required to produce a given pressure difference. The higher the flow rate required to produce a given pressure difference, the less airtight the building. (See blower door; DC fan pressurization; internal fan pressurization)


fabric protection control

A control function, the aim of which is to prevent condensation occurring.

fabric leakage

(See background leakage)


A mechanical device employing rotating aerofoil blades or vanes to continuously move air from one place to another. (See axial or vaneaxial fan; centrifugal fan)

fan coil units and air handling units

These may be manufactured for the direct expansion market or for the hydraulic market. They generally consist of a filter, fan and a cooling and/or heating coil. The difference between a fan coil unit and an air handling unit has become increasingly blurred. A fan coilunit generally is a small unit quite often located in the conditioned environment, they may also be located above a false ceiling and connected to a very small amount of ductwork. Air handling units are generally always intended to be connected to distribution ductwork, and the fan is sized to facilitate this. Large fan coil units have fans capable of providing up to 150 Pascals static pressure (~0.6 inches water gauge).


Frequently Asked Questions. Files on the Net which store the answers to common questions. If you are stuck, check the FAQs first, before you ask you own question. The following ftp site holds every FAQ on the Net. Ftp to: Go to the sub-directory pub/usenet/news.answers

feedback control

The simplest way to automate the control of a process is through feedback control. Sensors, or measuring devices, are installed to measure the actual values of the controlled variables. These actual values are the transmitted to feedback control hardware which makes a comparison between the setpoint or the desired value of the controlled variable and the measured value of these same variables. Based upon this `error signal' between the measured and the desired value of the controlled variable, the feedback controller calculates signals that reflect the required value of the manipulated variable. These signals are then transmitted automatically to the final control elements which act to alter the manipulated variable in such a manner as to reduce the error signal. Feedback control acts to eliminate errors. This is in contrast to feed-forward control which operates to stop the error occurring in the first instance.

feed-forward control

A formal definition of this control mode is:- `The transmission of a supplementary signal along a secondary path, parallel to the main forward path, from an earlier to a later stage'. This means that it works to eliminate errors occurring in the first place by forecasting the likely disturbance. However, if not ALL disturbances are forecast correctly then poor control will result. Feedforward control, while conceptually more appealing, signifi- cantly escalates the technical and engineering requirements of the overall design. The very sophisticated calculations must reflect an awareness and understanding of the EXACT effects that the disturbances will have on the controlled variable. With such understanding, the feed-forward controllers are able then to compensate for the disturbances. Feed-forward control is reserved for only a very few of the control loops within a plant. Thus, pure feedforward control is rarely encountered and the more common situation is for mainly feedback control with some feedforward control loops included. See Open-loop control and Compensation Control.

Field Interface Device (FID)

In a BEMS, this serves as a point of consolidation for sensors and controllers.

field processing unit

See Outstation.


Hardware or software designed to restrict access to certain areas on the Internet.

final control element

A device such as a valve or damper that acts to change the value of the manipulated variable. It is positioned by an actuator.


Software that allows the user to enter the address of an Internet site to find information about that system's users or a particular user. Some finger addresses return other topic- specific information.


Programmed microprocessor-based controllers that cannot be re-programmed or altered.


To send a harsh, critical email message to another user, usually someone who has violated the rules of netiquette.


(See apartment)

floating action

This term refers to a controlled device which can stop at any point in it's stroke and can be reversed without completing its stroke. The controller must have a `dead-spot' or neutral zone in which it sends no signal but allows the device to `float' in a partly open position. For good operation, this system requires a rapid response in the controlled variable, otherwise it will stop at an intermediate position.

flow coefficient (C)

Parameter used in conjunction with the "flow exponent" in a flow equation. (See flow exponent; flow equation)

flow equation

Equation describing the airflow rate through a building (or component) in response to the pressure difference across the building (or component). This equation takes the form of Q = C delta P^n; Where C is the flow Coefficient, delta P is the change in pressure over the component or envelope, and n is the flow exponent. Q represents the resulting volume flow rate expressed in m3/h. (See flow coefficient, flow exponent)

flow exponent (n)

Parameter which characterises the type of flow through a building (or component) and is used in conjunction with 'flow coefficient' in a 'flow equation'. (When n=1 flow is laminar, and when n=0.5 flow is assumed turbulent). For most openings, n takes a value between these two extremes.

flow hood

A device that measures airflow quantity

flow network

A network of zones or cells of differing pressure connected by a series of flow paths.

A passage/duct for smoke and fumes from a boiler/fire etc.

flue gas

The air exiting from a chimney after combustion and venting from the burner.

fluidic controller

A controller that exploits the DYNAMIC properties of a fluid (e.g., the `Coanda Effect'), as distinct from hydraulic controllers and pneumatic controllers, which utilise the STATIC properties of a fluid.

forced convection

Heat transmission by mechanically induced movement of a fluid.

forcing function

Externally applied time-dependent function, e.g., a step change in the setpoint or disturbance.

fortuitous leakage (Also known as adventitious leakage)

(See air Infiltration)

forward path

The path that connects the reference value to the controlled variable.

Fourier number

Dimensionless number, equal to Fo=(at)/lo2 , where a is the thermal diffusivity, lo is specific dimension and t is time.

free convection

Heat transmission by movement of a fluid caused by density differences. (See convection;forced convection)

free-float control

A building/plant system in which there is no active control strategy.

fresh air

(See outdoor air)

fuzzy logic

Fuzzy logic is basically a multivalued logic that allows intermediate values to be defined between conventional evaluations like yes/no, true/false, black/white, etc. Notions like `rather warm' or `pretty cold' can be formulated mathematically and processed by computers. In this way an attempt is made to apply a more human-like way of thinking in the programming of computers, controllers, etc.


The general expression of friction of fluids in pipes is known as the Fanning formula: P=f*(S/a)*D*(u2/2g), where:
P is the pressure required to overcome friction
f is the coefficient of friction or "friction/skin factor"


File Transfer Protocol. An application program that uses TCP/IP protocol to allow you to move files from a distant computer to a local computer using a network like the Internet.

function sequence systems

Many digital control systems require control events to occur in a sequence for which an output state produces a change in the input state. That is, where each event is not so much time dependent as dependent on the completion of the previous event. This process of change producing change continues until some overall objective has been met is termed 'function sequence system control'; cf. Time-sequence control.


A group of organisms that lack chlorophyll, including molds, mildews, yeasts, mushrooms. They receive their nutrition from decomposing organic matter. Some cause disease in humans. These fungi often live in damp conditions within buildings and can produce polluting microtoxins which are harmful to humans.



This is defined as 100%/PB. See Proportional band.

gain margin

The factor by which the gain must be increased in order to produce instability.

gap leakage

(See component leakage)

gas chromatography

A process by which gases can be separated from one another. This technique is used to separate tracer gases from each other and from the constituents of air, thus allowing individual quantitative analysis to be performed.

gas sorption devices

Devices used to reduce levels of airborne gaseous compounds by passing the air through materials that extract the gases. The performance of solid sorbents is dependent on the airflow rate, concentration of the pollutants, presence of other gases or vapors, and other factors.

glare index

A glare index illustrates the probability of the occupants being satisfied with a particular view direction. In modelling and simulation this allows the environmental engineer to predict visual discomfort and thereby ensure that it is avoided.

global points

Allows designated points to share their data with other bus connected devices.


A menu-based system for browsing Internet information.


Graphical user interface. Software designed to allow the user to execute commands by pointing and clicking on icons or text. It's pronounced "Gooey".



A computer user who illegally visits networked computers to look around or cause harm.

half-duplex transmission

Transfer of data in two directions but by alternate, one way at a time, independent transmission; cf. Duplex transmission.


A term used to describe how computers or peripherals can communicate without conflict.A handshake can be hardwired, where status is indicated by on/off electrical levels or software based where control characteristics are transferred along with the information to be exchanged.


An acronym for Halogenated Chlorofluorocarbon, which is a family of refrigerants that if released into the atmosphere are destructive to the Earths's ozone layer

head pressure controllers

See low ambient controllers


The transfer of energy to a space or to the air by the existence of a temperature gradient between the source and the space or air. This process may take different forms,ie, conduction, convection or radiation. (See conduction; convection; heat transfer; radiation)

heat exchanger (air-to-air)

A device designed to transfer heat from two physically separated fluid streams. In buildings, it as generally used to transfer heat from exhaust warm air to incoming cooler outdoor air.

heat balance

A statement of the heat input to, and heat loss from, an appliance, plant or structure, intended to account for all sources of heat and equivalent energy.

heat flux

The amount of heat passing through any surface per unit time.

heat flux per unit area / specific heat flow

Heat flux related to the unit square surface.

heat pump (air-to-air)

A device operating on a refrigeration cycle in which both evaporator and condenser are refrigerant/air heat exchangers. As a heating season heat recovery device, the evaporator transfers heat from the exhaust warm air to the refrigerant and the condenser transfers heat from the refrigerant to the incoming air. Arrangements are often made to allow the refrigerant flow to be reversed making the condenser the evaporator and vise versa - thus energy may be recovered in the cooling season.

heat pump

The outdoor unit associated with direct expansion equipment incorporating a compressor, coil, reversing valve and fan and provides heating and cooling.

heat recovery

(See heat exchanger (air-to-air); heat recovery effectiveness)

heat recovery effectiveness

Often referred to as heat recovery efficiency. The proportion of heat recovered from otherwise waste heat passing through a heat recovery system. Normally expressed as a percentage.

A device operating on a refrigeration cycle in which the evaporator is a water/refrigerant heat exchanger and the condenser, a refrigerant/air heat exchanger. The circuit normally includes an arrangement which allows the refrigerant flow to be reversed thus allowing heat to be transferred in either direction. In one system, a number of small air/water heat pumps installed in various zones around a building are used to transfer heat into or from a common water circuit. Thus heat unwanted in one zone may be transferred to another where it is needed.

heating load

Diversified total heating loads, by fuel, represent critical plant sizes and hence capital costs. The breakdown of the total, by zone, highlights areas of concern. A comparison of the as-built and reference IPVs for the building can be made.

heat transfer

Heat, a form of kinetic energy, is transferred from one body to another (gas, liquid or solid or combinations thereof) by the following means or combinations thereof:

Heat can be transferred only if a temperature difference exists, and then only in the direction of decreasing temperature. (See heating; hensible heat transfer; latent heat transfer)


An acronym for high efficiency particulate arrestance (filters).


An acronym for Hydrofluorocarbon, which is a family of refrigerants that if released into the atmosphere cause destruction of the Earth's ozone layer

heuristic controller

See adaptive control

homeostatic control

In a BEMS, providing toward a predetermined state of equilibrium between adjacent but interdependent elements of a system. This is obtained by use of digital metering devicescombined with a microprocessor-based controller.

home page

The first page a user sees when visiting a World Wide Web site.


Hypertext Markup Language. The programming language of the World Wide Web, HTML software turns a document into a hyperlinked World Wide Web page.


Hypertext Transfer Protocol: The protocol used to provide hypertext links between pages. It is the standard way of transferring HTML documents between Web servers and browsers.


The process of transferring a mass of water to the atmospheric air. (See latent heat transfer)


A device used in control systems for switching plant to maintain a relative humidity at some setpoint. The output signal is usually sent via a relay device to the final control element.


The measure of moisture in the atmosphere. In building simulation it is often refered to as the relative humidity within a space.

Humphreys model

Humphreys model for thermal comfort. Humphreys equation is a fit to considerable data for climate-controlled and non-climate controlled buildings.
See thermal comfort models.


Prolonged self-sustained oscillation of undesirable amplitude.


An acronym for Heating, Ventilating, and Air-Conditioning.

hydrocarbons (HC)

Chemical compounds made up entirely of carbon and hydrogen. When combusted they release carbon dioxide, nitrogen oxide, sulphur dioxide and water vapour. The carbon dioxide emissions released to the atmosphere are the primary cause of the "greenhouse effect" leading to global warming of the Earth's atmosphere.


A highlighted word or graphic in a document that, when clicked upon, takes the user to a related piece of information on the Internet.


For sensors, a measure of the diferences in indicated steady state value for identical conditions when approached from higher and lower states. For actuators, a measure of the differences in output for a system responding to the identical steady state input when approached from different directions. Hysteresis may have a physical cause e.g. wear in actuator gearboxes or a control effct e.g. deadband or failure. It can be confused with failure to appraoch steady state in a reasonable time.



An acronym for Indoor Air Quality

I/O List

Input/Output list. A list of variables specifying the storage elements into which data is to be written or from which data is to be read.

`ideal' control

Control that involves no time lags and with all control elements behaving linearly.

impedance - electrical

The electrical impedance defines the relation between current and voltage in a circuit. In a simple case, the impedance may be purely resistive such that the current and voltage are directly related by a constant of proportionality (the "resistance"). When the circuit incorporates reacive components (inductance, capacitance), the impedance is a complex quantity.

imperfect mixing

The combination of two or more substances such that the parts of one are unevenly distributed among the parts of another.

incremental control

A form of modulating control where the control device is sent an increase - hold - decrease signal from two binary outputs working as a pair. with this type of control it is possible to provide a form of proportional+integral (PI) control without position feedback from the actuator of the controlled device.

indoor air

The air that people breathe inside a built environment.

indoor air pollution

Pollution occurring indoors from any source, indoors or outdoors (see sources of indoor pollution).

indoor climate

The synthesis of day-to-day values of physical variables in a building e.g. temperature, humidity, air movement and air quality, etc, which affect the health and/or comfort of the occupants.

indoor environment

(See indoor climate)

indirect control

This means automatic operation of a control device located in the energy flow stream via an intermediary system.

industrial building

A building in which the main purpose is to provide space for manufacturing and assembly processes. These are characterised by high levels of activity both mechanical and human, and often by the generation of internal pollution and heat.

industrial conditioning

Industrial conditioning refers to a process which requires a controlled atmosphere. A typical specification would provide for an internal environment of 21 oC +or- 0.5 oC and 50% relative humidity +or- 2.5% at all external conditions. Industrial conditioning has clearly defined limits as opposed to comfort conditioning which is based on statistical surveys of occupants feelings. (see comfort conditioning)


Infiltration is the movement of air from the outside (ambient) to the inside through cracks in the building envelop. (See air infiltration)

infiltration heat loss/or gains

Heat lost from a building which is directly attributable to the effects of the cooler outside air leaking into a building and of warm indoor air leaking out.

infiltration rate

The rate at which outside air infiltrates a room or building. Equivalent to the fresh air change rate, usually expressed in air changes per hour (ach) or litres per second (l/s). (See air change rate, ACH)

infra-red gas analyser

An instrument used to determine tracer gas concentrations by determining the transmission of infra-red radiation at a specific absorption frequency through a fixed path length.

inherent regulation (also `self-regulation')

Many simple processes are characterised by the fact that a new steady- state is reached automatically, i.e., without interference by manual or automatic control.


The magnitude of solar energy which is incident on a particular building element (W/m2). Both the direct and diffuse components must be considered.

institutional building

A building with mixed occupational activities where special requirements arising from those activities may be needed; such buildings include hospitals, prisons etc.

insulation / economic thickness of insulation

thickness, giving the largest energy savings for the lowest investment costs

insulation materials

materials with low heat conductivity

integral action

The action of a control element whose output signal changes at a rate which is proportional to its input signal size. The integral term may be considered as the `memory' of the controller, looking at past errors. It acts to remove offset. It can be used in conjunction with derivative control action (I+D) or, more commonly, with proportional action (P+I) or proportional and derivative action (P+I+D).

integral action time (IAT)

In a control system having P+I control action, the time interval in which the part of the output signal due to integral action increases by an amount equal to the part of the output signal due to the proportional action, when the deviation is unchanging.

integral Gain

Defined as the ratio of proportional gain to integral action time.

integral wind-up

It is possible for a final control element to be in a `fully open' position but the controlled variable to be some way from the setpoint. For example, such an occurrence often occurs during the morning heat-up period in a heating system. The error will persist for some time and the integral action term will become enormously large. This error is called the `integral wind-up error'.

intelligent step control

This is a microprocessor based step controller or sequencer, which can be programmed or selected for different operational modes such as binary switching, and equalised run time. A second advantage which this type of step controller has over the motorised version is its ability to provide easily adjustable time delays.

intentional opening

(See purpose provided opening)

intentional ventilation

Ventilation provided through the use of purpose provided openings, such as through windows or airbricks.


The maximum difference in indicated value between two sensors chosen at random, connected to the same interface and exposed to identical conditions.

intermittent heating

It can be demonstrated both theoretically and by actual tests, that a great deal of heat can be conserved by cutting off the heat supply in any type of building during periods of non-occupancy.

internal fan pressurization

The building's own mechanical ventilation system can be used to provide the required pressure differential from within. The supply fans are operated while all return and exhaust fans are turned off, and all return dampers are closed, so that air can only leave through the doors, windows and other leakage sites. (See blower door; DC fan pressurization; external fan pressurization)

internal pressure

The pressure inside a building envelope or space. Usually expressed with respect to outside or atmospheric pressure.

internal pressure distribution

The pattern of static pressure variation at various points inside a building due to variations in air density and air flow into and out of the building.


The global network of networks that connects more than three million computers (called hosts). The Internet is the virtual space in which users send and receive email, login to remote computers (telnet), browse databases of information (gopher, World Wide Web, WAIS), and send and receive programs (ftp) contained on these computers.

Internet account

Purchased through an Internet service provider, the account assigns a password and email address to an individual or group.

Internet server

A computer that stores data that can be accessed via the Internet.

Internet site

A computer connected to the Internet containing information that can be accessed using an Internet navigation tool such as ftp, telnet, gopher, or a Web browser.

interzonal air flow

The process of air exchange between internal zones of a building.

isothermal surface

Surface with even temperature in all its points.

IP address

Every computer on the Internet has a unique numerical IP address assigned to it, such as 123.456.78.9.

IPV - integrated performance view

An IPV allows the building-design/environmental engineer to analyse the building performance from a variety of aspects such as: - peak demands; - typical patterns of energy use; - levels of pollutant (eg carbon dioxide) corresponding to the energy consumption.


An acronym for Inches of water column, still commonly used in the USA. An old measure of air pressure used in HVAC systems.




A word or words which can be searched for in documents or menus.



A delay in the effect of a changed condition at one point in the system, or some other condition to which it is related. Also, the delay in response to the sensing element of a control loop due to the time required for the sensing element to sense a change in the sensed variable.

laminar flow

Flow in which fluid moves smoothly. In this flow form cross stream momentum transfer takes place by viscous action alone and mixing between flow strata does not occur. (See transition flow; turbulent flow)


Local Area Network: A private transmission that interconnects computers within a building or among buildings for the purpose of sharing voice, data, facsimile, and/or video.

latent heat transfer

Heat added or removed during a change of state of a substance ie solid, to a liquid to a gas or vice versa, the temperature remaining constant. (See sensible heat transfer)

large opening

Hole or gap in a building envelope which is generally purpose made, for example, a door window or vent.

leakage area

The actual open area of a hole or gap.

leakage component

(See component leakage)

leakage distribution

leakage path

A route by which air enters or leaves a building or flows through a component.

limit control

A control system in which, for reasons of comfort or efficiency, the controlled variable is prevented from exceeding or dropping below a certain value.

linear control system

A control system in which the transfer function between the controlled condition and the command signal is independent of the amplitude of the command signal. The characteristics of a linear system may be described by a linear differential equation of the first degree, which remains true over the range being considered.

load control

See demand control.

load limiting

See demand control.

load shedding

See demand control.

local feedback

See Cascade control.

local air change index

An index that characterises conditions at a particular point within the room and may be largely due to the position of the measurement point. (Equation 40 Sutcliffe) (See air change efficiency; air change time; coefficient of air change performance; nominal time constant; specific flow)

local mean age of air

The average time it takes for air to travel from the inlet to any point P in a room or enclosure. The mean age of the air at the point P can be found from the centroid of the frequency curve, by taking moments about the vertical axis. (See Sutcliffe (1990).


(See attic)


A chronological record of a series of events.. Each event is identified by time, day,value, and physical reference.

logging capacity

The number of data elements that can be stored by an outstation when undertaking a log.


To sign on to a computer system.

long term stability

The maintenance of repeatability over a long period of time e.g. one year.

low ambient controllers / head pressure controllers

When equipment is required to provide cooling in cold weather the external unit will over condense i.e. will reject too much heat. To overcome this problem it is necessary to fit a low ambient controller also known as a head pressure controller. This device slows down the condenser fan or uses other methods to prevent over condensing. Some equipment has this fitted as standard others do not.

low energy house

A house designed or retro-fitted with energy savings in mind, to provide overall lower energy consumption.


mailing lists (or Listserv)

There are more than 4,000 topic-oriented, email-based message bases that can be read and posted to. Users subscribe to the lists they want to read and receive messages via email. Mailing lists are operated using listserv software. Thus, many users call mailing lists 'listservs'. There are two types of lists: moderated and unmoderated. Moderated lists are screened by a human before messages are posted to subscribers. Messages to unmoderated lists are automatically forwarded to subscribers. Menu A list of information that leads to documents or other menus.


A measure of the speed with which loss of performance is detected, the fault located, repairs carried out and completed, and a check made that the equipment is functioning normally again; cf. availability and reliability.

majority voting

A form of discriminatory control. Sensed values are input to the controller, which establishes, for each input signal, a corresponding `dummmy' output signal. The `resultant' controller output signal is determined from the majority of the dummy outputs. For example, if there are 3 input sensors to an on-off controller, and 2 result in an ON status and 1 results in an OFF status, the resultant controller output signal will indicate an ON status.

makeup air

Outdoor air supplied to replace exhausted air, brought in through a ventilation system without previous circulation in the system.

master station

A station that can select and transmit a message to a sleve station.

master-submaster (also master-slave control)

See Cascade control.

mean active repair time (MART)

This applies to repairable items, and means the average time an item may be expected to be out of service for maintenace and repair, given that the require tools/parts are to hand.

mean radiant temperature

Mean radiant temperature is not equal - but may be calculated from - the reading of a globe thermometer.

mean time between failures (MTBF)

This applies to repairable items, and means the that if an item fails, say, five times over a period of use totalling 1000 hours, the mean time between failures would be 1000/5, or 20 hours. The MTBF is a measure of the likelihood that equipment will break down in a given period. MTBF = MTTF + MTTR

mean time to failure (MTTF)

This applies to non-repairable items, and means the average time an item may be expected to function before failure.

mean time to repair (MTTR)

This applies to repairable items, and means the average time an item may be expected to be out of service for maintenance and repair, including any order/delivery times.

means and swings method

The CIBSE method of determining the peak inside environmental temperature.

measured value

The physical quantity that is measured by the transducer, i.e., the input to the transducer.

mechanical ventilation system

A ventilation system in which air is extracted from a space so creating an internal negative pressure. Supply air is drawn through adventitious or purpose provided openings.

mechanical extract ventilation system

A ventilation system in which air is extracted from a space so creating an internal negative pressure. Supply air is drawn through adventitious or purpose provided openings.


Biochemical reactions by which energy is made available for an organism to use. Includes all chemical transformations that occur in an organism from the time a nutrient substance enters until it has been used and the waste products eliminated.


a particular part of the large environment that is in some way whole by itself. Used to describe a subset of the global environment such as the indoor environment.

microprocessor-based control

A control circuit that operates on low voltage and uses a microprocessor to perform logic and control functions, such as operating a relay or providing an output signal to position an actuator. Electronic devices are primarily used as sensors. The controller often furnishes flexible DDC and BEMS control functions.

mimic display

A method of representing an HVAC system in the form of a graphic display. It usually shows the current values or status of plant.

minimum phase systems

See Non-minimum phase systems.

minimum ventilation requirement

The minimum quantity of outdoor or conditioned air entering a building, which is needed to maintain acceptable indoor air quality. (See ccceptable air quality)

mixing fan

Small fan used to aid the mixing of room air and tracer gas before and/or during ventilation rate measurements.

mixing flow ventilation

mixing flow describes a method of air distribution from an air conditioning or ventilation system. It is the most widely used method of supplying air into an atmosphere being air conditioned. This form of air distribution commonly uses ceiling diffusers or wall grilles at high level. Most air conditioning and air handling units are manufactured for the mixed flow air distribution market.


An acronym for MOdulator/DEModulator. This is a hardware device used in a BEMS to change digital information to and from an analog form to allow transmission over communication links.


Acronym for MOulate DEModulate. An electronic device that attaches to a computer and links that computer to the online world via a telephone line. Modems are available for any computer, can be internal or external, and come in several speeds, known as the baud rate. The higher the baud rate, the faster the modem. The most popular modem speed have been 9,600 and 14,400 baud, but 28,800 baud modems are now considered the standard. Most Internet service providers allow you to dial into their systems at rates up to 33,600 baud and beyond.


A control action that adjusts by minute increments and decrements.


The process of collecting, analysing, and reporting data.

multiple tracer gas technique

A Measurement method using two or more tracer gases. This method is often used to evaluate interzonal airflows.


A type of field panel used in BEMS to minimize data transmission costs by using time-sharing transmission techniques.


A building or part of a building that comprises a number of zones or cells.


natural ventilation

The movement of outdoor air into a space through intentionally provided openings, such as windows and doors, or through non powered ventilators or by infiltration.

negative pressure

A condition that exists when less air is supplied to a space than is exhausted from the space, so the air pressure within that space is less than that in surrounding areas.


The rules of conduct for Internet users. Violating netiquette could result in flaming or removal from a mailing list. Some service providers will even cancel a user's Internet account, denying him or her access to the Net, if the violation is severe enough.


A group of computers that are connected in some fashion. Most school networks are known as LANs, or Local Area Networks, because they are networks linking computers in one small area. The Internet could be referred to as a WAN, or a Wide Area Network, because it connects computers in more than one local area.

network technique

Theoretical method for estimating the magnitude of air infiltration, ventilation and interzonal air movement, using a model which considers a building to comprise a number of enclosed spaces, each at its own internal pressure and linked by flow paths.

neutral pressure level

Level at which the air pressure difference, derived from the stack effect between inside and outside a building is zero.

night purge

An energy management function (rarely used in the U.K.) in which 100% outdoor air is introduced to the building prior to starting up the air-conditioning equipment, thereby reducing the cooling load.

night set-back control

An energy management function which acts to reduce the occupancy temperature by a few degrees, with the heating ticking over to maintain it. It is not usually as effective compared to the modern practice of intermittent heating, where the system is switched off overnight.

normalised leakage area

Equivalent leakage area expressed per unit area of building envelope. (See equivalent leakage area)

nominal time constant

The inverse of specific flow. Under piston conditions the nominal time constant is the time it will take to exchange all of the air in a room or zone with fresh air. (See also air change efficiency; air change time; coefficient of air change performance; local air change index; specific flow)

non-minimum phase systems

In terms of feedback control theory, minimum phase systems are stable systems having zeroes in the left-half side of the complex s-plane, i.e. all denominator terms contribute phase lag whilst numerator terms contribute phase lead. Systems with zeroes in the right half of the s-plane (non-minimum phase systems) are inherently unstable (due to additional phase lag).



Obstacles such as other buildings and trees can prevent direct insolation depending upon the time of day. Obstruction therefore introduces time-varying shading and insolation which must be taken into consideration when calculating the building thermal and lighting performance.


The time during which people are in a building (generally expressed in hours per day)

occupant behaviour

The pattern of activity of occupants in a building, including the number of occupants, their distribution, activities and time spent within the building, and how they interact with the buildings facilities, such as ventilation systems, window opening etc.

occupied zone

The region within an occupied space between 75 and 1800 mm above the floor and more than 600mm from the walls or fixed air-conditioning equipment.


A sustained deviation between the control point and the setpoint of a proportional control system under stable operating conditions.


The olf unit attempts to quantify odorous pollution sources. One olf is the emission rate of odours (bioeffluents) from a standard person. Any other odour source is then expressed by the equivalent source strength, defined by the number of standard persons (olfs) required to cause the same dissatisfaction as the actual pollution source. The olf is a relative unit expressing the pollution source by a comparable known reference source. (See decipol)

on-off control

A special case of two-step control in which one of the output signal values is zero.


When you are logged onto a computer through your modem, you are said to be online. When you are using your computer but are not connected to a computer through your modem, you're said to be working offline.

open-loop control

A control system in which no monitoring feedback is used. An open-loop system assumes a fixed relationship between a controlled condition and an external condition. It does not take into account changing space conditions from internal heat gains, infiltration/exfiltration, solar gain, or other changing variables in the building. Open-loop control alone does not provide close control and may result in underheating or overheating. For this reason, open-loop systems are not common in residential and commercial buildings. See also Feed-forward fontrol and Compensation control.

operating software

In a BEMS, the main operating software and program that schedules and controls the execution of all other programs; cf. Application software.

optimum start/stop controller

This controller alters the time that the HVAC equipment starts/stops depending on the weather conditions. It works by using an external sensor and, occasionally, an internal sensor, to bring in the heating/cooling plant at the latest possible time to get the building/zone(s) to the required temperature by the start of occupancy.

outdoor air

Air taken from the external surroundings and therefore not previously circulated through the system.

outstation (Also field processing unit, data gathering panel)

In a BEMS system, this is the unit (with inputs from the sensors and outputs to the actuators) which controls the plant. It is often situated in the plant room.

overlay system (Also, BEMS reset system)

A BEMS which is overlaid onto conventional analogue control systems providing reset signals to change temperature, humidity settings, etc. This contrasts with Integrated DDC. where the BEMS provides direct digital control of temperature, humidity and pressure, eliminating the need for discrete analogue controller.


An induced pressure above ambient atmospheric pressure or other given reference pressure.


Aperiodic damping in which the degree of damping is greater than that required for for critical damping.


A term used for proportional controllers when the load changes sufficiently large to cause the control point to move above or below the limits of the proportional band.

oxygen trim

An energy management function in which an attempt is made to reduce heat losses from a boiler's exhaust gases, i.e., the aim is to try and approach the stochiometric ('ideal') air/fuel ratio.



An acronym for pascal; An SI unit of pressure measurement equal to 1 newton per square meter.


See performance assessment method

panel building or panel-type building

A building composed of pre-fabricated concrete facade and other building elements. Usually the panels have a low level of thermal insulation, and often the panels are poorly jointed which leads to relatively high levels of infiltration. Consequently many panel buildings suffer from high levels of energy consumption, and often poor comfort conditions inside.

parallel data transmission

A transmission method where the data (in the form of 'words', or groups of BITS) are transmitted simultaneously.


Fine liquid or solid particles such as dust, smoke, mist, fumes, and fog found in air and emissions.

particulate matter

A state of matter in which solid or liquid substances exist in the form of aggregated molecules or particles. Airborne particulate matter is typically in the size range of 0.01 to 100 micrometers.

particulate tracer

Solid particles of aerosol or bubbles used as a tracer for measuring the rate of air movement. These particles usually have diameters of 2 to 3 microns, and can be detected by using either a) a fluorescent light scattering detector; b) a photomultiplier (P-M) detector, or c)a phosphorescence with a P-M detector.

passive adsorption

A process by which a gas or vapour is condensed (out of the air) and held on the surface of a piece of solid material by natural forces only.

passive sampling

A method of sampling tracer gas in a building by the process of passive adsorption.

passive smoker

A non smoker who shares the same room, building, or space as a smoker, and thus is exposed to the products of tobacco combustion.


See predicted percent dissatisfied due to draft.


Awareness of the effects of stimuli.

perfect mixing

(See uniform mixing)

performance assessment method (PAM)

A performance assessment method defines best practice simulation procedures in order to assess the performance of a building. (See also integrated performance view or IPV )

phase-cut signal

This is where a sine wave is cut off part way through the cycle to produce a continuously varying output signal from a controller. The signal is then used to directly control a magnetic actuator.

PID controller (also P+I+D controller)

A three-term controller having proportional action, integral action, and derivative action. This algorithm enhances the P+I control algorithm by adding a component that is proportional to the rate of deviation (derivative action) of the deviation of the controlled variable. This compensates for system dynamics and allows for faster control response.

piston flow

Also known as plug flow, and displacement flow, and is regarded as the most efficient form of ventilation. The ventilation air acts as a piston, which pushes the "old" air in the room in front of it without actually mixing. Therefore all of the air that reaches an arbitrary point from a small packet of fresh air at the inlet does so at the same time; this time is by definition, the local mean age of air at this point. (See local mean age of air)

plant component

Device within a plant network such as a boiler or pump.

plant connection

Topology of plant network.


Air compartment connected to a duct or ducts.

plenum chamber

A chamber, at higher/lower pressure than surrounding air, that receives air before/after delivery to a conditioned space or combustion system.

plug flow

A flow regime where the flow is predominately in one direction and contaminants are swept along with the flow (See piston flow).


A visible or measurable discharge of a contaminant body from a given point of origin. Can be a visible body of pollution such as smoke coming from a stack or a measured amount such as the heat in water coming from a power plant boiler.


See predicted mean vote.


(See contaminant)

pollutant concentration
pollutant migration

The movement of indoor air pollutants throughout the building between rooms or zones.

The concentration within a given portion of air of harmful or unpleasant contaminants such as noxious gases or dust particles. Concentrations are often expressed as time weighted values over 24 hours, a working day or a working week.


An unwanted by-product of human activity. the presence of matter or energy whose nature,location, or quantity produces undesired environmental effects.

pollution source

Any object, usually within the building, which produces a substance which will contaminate the internal environment, for example, human bioeffluents caused by man, or pollutants outgassed from carpets or furniture.

pneumatic controller

A controller which uses compressed air as the control medium. All the required control terms are readily available, e.g., step control, PID, etc.


A physical source or destination for data in the form of analogue or digital signals.

point descriptor

The information giving the characteristics of a monitoring point.


A method of interrogating a computer to determine whether each device is ready to perform its specific task and/or communicate with the computer.

position adjust type signal

A controller output using two binary signals, working as a pair. Normally, they drive a reversing actuator by sending either an increase, hold, or decrease comand. When used with DDC the controller may calculate the percentage opening of the controlled device based on the actuator running time to go from fully closed to fully open

positive pressure

A condition that exists when more air is supplied to a space than is exhausted, so the air pressure within that space is greater than that in surrounding areas.

potential value

The limiting value of the controlled condition that tends to be attained following a particular adjustment of the corrector unit, all other factors which may effect the value of the controlled condition being maintained constant.

power factor

A term used to quantify the amount of reactive power being generated/absorbed at a particular point in an electrical circuit.


See predicted percentage dissatisfied


An acronym for parts per million

preconditioning period

One or more days of simulation before the requested simulation period. These are required to remove any effects that may result from the assumed initial conditions of the simulation. Calculated values during the preconditioning period (or start-up period) are not saved.

predicted mean vote (PMV)

Comfort rating derived from the work of Fanger. PMV is derived from the physics of heat transfer combined with an empirical fit to sensation. PMV* represents a new temperature index that incorporates skin wettedness into the PMV equation using SET* or ET* to characterize the environment.

predicted percentage dissatisfied (PPD)

Comfort rating derived from the work of Fanger. PPD is the predicted percent of dissatisfied people at each PMV.

predicted percent dissatisfied due to draft

PD is a fit to data of persons expressing thermal discomfort due to drafts. The inputs to PD are air temperature, air velocity , and turbulence intensity.

predictive control

A control system which attempts to predict the effects of a disturbance on the future output. Predictive control forms the basis of many self-tuning control methods; cf. adaptive control.

preheat period

See boost period

pressure attenuation technique

A method of estimating the leakage of a building by releasing air inside the building causing instant pressurization, the pressure returning to normal as the air leaks out. The rate of reduction of the pressure is proportional to the leakage.

pressure coefficient

A dimensionless coefficient relating the velocity pressure on the outer surface of the building to the velocity pressure derived from the mean wind velocity at a reference point.

pressure differential

The difference in pressure across a building envelope or component whether caused by natural or artificial means.

pressure distribution

(See internal pressure distribution; surface pressure distribution)

pressure, total

In flowing air, the sum of the static pressure and the velocity pressure.

pressure, velocity

In flowing air, the pressure due to the velocity and density of the air.


A method of testing air leakage of a building or component by installing a fan in the building envelope, for example through a door or window, and creating a static pressure excess inside the building. The airflow rate through the fan and the pressure difference across the envelope are measured from which the air leakage is assessed. (See AC pressurization; DC pressurization)


A rank assigned to a task that determines its precedence in receiving system resources.

problem configuration

This is the core description of a problem which contains references to the various geometry, scheduling, mass flow network, plant and thermophysical property files which constitute the description of a problem.


A general term that describes a change in a measurable variable (e.g., the mixing of return and outdoor air streams in a mixed-air control loop and heat transfer between cold water and hot air in a cooling coil). Usually considered separately from the sensing element, control element, and controller.

process control characteristic

This gives information on how the controlled variable is affected by small changes in the intermediate position of the final control element.

process reaction rate (PRR)

The rate at which the plant responds to a disturbance. Usually plotted with the control variable on y-axis and time on the x-axis.


The efficiency with which a person performing a specific function does a job, or the output of a worker under specific environments and conditions.

programmable point

A control or monotoring point for which the user may program an associated control scheme.

programmed start/stop

An energy management function which operates to selectively shutdown electrically-operated equipment. This is accomplished on a predetermined time-schedule, usually paralleling occupancy schedules.

proportional action

In this type of control action, the output of the controller is proportional to the error. If the error is large, the signal output to the actuator is large, and if the error gets smaller, the output signal gets smaller proportionally. The relationship between the two is determined by a constant called the proportional gain. The error band within which the output is between 0% and 100% is called the proportional band. The higher the gain, the higher the proportional band. The main problem with proportional control is offset. This can be reduced by the addition of integral action, although proportional action alone is used successfully in many situations where the degree of offset is tolerable. A sluggish response to sudden, or very large, load changes may be improved by incorporating derivative action.

proportional band (PB)

That range of values of deviation corresponding to the full operating range of output signal of the controlling unit resulting from proportional action only. The PB can be expressed as a percentage of the range if the controlled condition which the measuring unit of the controller is `designed' to measure. With HVAC control systems, the proportional band is expressed in absolute units (for example: K); with industrial control, the proportional band is expressed in percentages (of the input value).

proportional controller

A controller with proportional action only.

proportional control factor

The broken loop amplification at an infinite period with any integral action removed from the controller.

proportional gain

The inverse of the proportional band.

proportional kick

See Derivative kick.

proportional-speed controller

A variation of floating control is proportional-speed control. In this type of controller, the farther the control point moves beyond the deadband, the faster the actuator moves to correct the deviation.

pull down period

The time taken for a measured temperature to reduce from some given higher value to a required set-point.

pulsed input

The representation of a value by a series of abrupt and relatively short cyclic changes in a signal.

purpose provided opening

An opening in the building envelope for the specific purpose of supplying or extracting ventilation air, ie, air bricks, vents, extractor fans, intake and exhaust for HVAC systems, etc.

purpose provided ventilation

Ventilation provided to a space as the result of specific action to ensure its introduction. Such ventilation may be provided by natural means through purpose provided openings of the required size and position, or by mechanical means.

public building

A building which is open to the public, such as museums, clubs, public houses, exhibition halls etc.

pseudo point

See soft point.


The PS equation predicts the air velocity that will be chosen by a person exposed to a certain air temperature when the person has control of the air velocity source. See thermal comfort models.



radiant heat transfer

Radiant heat transfer occurs when there is a large difference between the temperatures of two surfaces that are exposed to each other, but are not touching (see heat transfer).


The transmission of heat through space by the propagation of infra red energy; the passage of heat from one object to another without necessarily warming the space between. Radiation does not need a transport medium and so it can take place in vacuum (or in space; i.e. solar radiation). Heat and light are forms of electromagnetic radiation; other forms are micro-waves, X-rays, radio broadcast waves. The different forms of electromagnetic radiation differ in wave length and frequency. (See conduction; convection; heat transfer)

random uncertainty or random error

The likely difference between a single measurement and the mean of the distribution of such measurements.


A colorless, odorless gas that occurs naturally in almost all soil and rock. Radon migrates through the soil and groundwater and can enter buildings through cracks or other openings in the foundation. Radon can also enter well water. Exposure to radon can cause lung cancer.

rate action

See Derivative action.

ratio control

A controller which operates to maintain a pre-determined ratio between two quantities (e.g., two air-flows).


Ratio of maximum controllable flow to minimum controllable flow. It follows that there is a minimum quantity which a valve, for instance, can reliably control. Rangeabilities of the order of 30:1 or better, are considered reasonable; cf. Valve turn-down ratio.

real time

A situation in which a computer monitors, evaluates, reaches decisions, and effects control with the response time of the fastest phenomenon.

real and reactive power

The real power is measured in kilowatt (kW) and represents the portion of electrical power which is available for useful work.

The reactive power results from the presence of inductive (or capacitive) components and, while it does not represent useful work, it does augment the total current flow. Units: Volt-Ampere-Reactive (VAR).

recirculated air

Extracted air which is re-supplied to a space. Recirculated air is normally blended with outside air and is reconditioned. It can then be used for ventilation, heating, cooling, humidification, or dehumidification.

reductive sealing method

A method of determining the leakage of specific building components by pressurizing the building and recording the leakage changes as the components are successively sealed.


A situation that occurs when the air being exhausted from a building is immediately brought back into the system through the air intake and other openings in the building envelope

reference value

See setpoint.

reflectance / reflectivity

The fraction of radiant energy incident upon it which is reflected

refridgeration (`Ton' of)

One ton of refrigeration is the term used to refer to 12,000 B.T.U.s/hour (British Thermal Units/Hour) of cooling effect. (see BTU)

relative humidity

A ratio between the actual moisture content of the air compared with the moisture content of the air required for saturation at the same temperature, i.e., at 100% relative humidity (also known as saturation point).

relative humidity (comfort)

Indoor relative humidity in the range of 30 to 70%

relative ventilation efficiency

A quantity describing how the ventilation ability of a system varies between different parts of a room.

regulating system

See automatic control system.


The characteristic of an item expressed by the probability that it will perform a required function under stated conditions for a stated period of time; cf. availability and maintainability.


For sensors, a measure of the differences in the indicated steady state value when exposed to identical conditions on different occassions. For systems, a measure of the differences in output for a system responding to the identical steady state input on different occasions. In repeatability testing the component approaches the steady state condition from the the "same direction" e.g. from cold to hot for a temperature sensor, thus repeatability excludes hysteresis.


Similar to repeatability but inclusive of hysteresis.

reset action

See Integral action.

reset rate

Defined as the inverse of the Integral action time.

residential building

A building whose main purpose is to provide living space for the occupants. Activities within them are limited to those of a domestic nature. Such buildings includes single-family; multi-family, communal, institutional and intermittent use building classifications.


The minimum change in a variable that can be observed. In digital systems this is related to the number of bits used by the analogue to digtal converter (ADC) in the controller. For an 8 bit system the resolution is 1 in 28 i.e. 0.4% of range. 8 bit resolution is now considered inadequate for BEMS inputs and most systems use 12 bit (or better) ADC although for technical reasons, the resolution may be quoted as 11 bit i.e. 0.05% of range.

respirable particles

Particles that penetrate into and are deposited in the nonciliated portion of the lung. Particles greater than 10 micrometers aerodynamic diameter are not respirable.

response time

The time interval, with regard to a step input signal, between the input and the first coincidence of the output signal with the final steady-state value of the output signal.


The action of improving a buildings performance by increasing various aspects for its design. For example improving the energy efficiency of existing buildings, by enhancing its thermal performance and by systematically sealing infiltration flow paths. (See weatherization)

return air

Air removed from a space and then recirculated or exhausted (see recirculated air).

Reynolds number

Dimensionless number, equal to Re=rwolo/ m, where r is the density, m is the dynamic viscosity, wo is the specific fluid velocity and lo is specific dimension.


An acronym for relative humdity (see relative humidity)


(See attic)

room mean age

The average value of the local mean ages of air flow for all points in a room. The room mean age cannot therefore be measured easily. It is necessary to express the room mean age in terms of a measurable quantity, such as a tracer gas concentration in the exhaust duct. This requires a mass balance equation to be constructed for the room. (See air change rate; local mean age)

root mean square of uncertainty

The square root of the sum of the squares of the uncertainties. If the uncertainties are estimated at ±3% each then the root sum of squares is ±5.2% (compare with the artihmetic sum of the 9%).

run time

Accumulates equipment on or off time and tranmits totals periodically to the central station. On-off cycle counting can also be accumulated as a maintenace indicator. Alarm annunciation ocurs if run time or cycle time count limits are exceeded.


sampled-data signal

A digital signal, the amplitude of which can assume a continuous range of values.


An acronym for Sick Building Syndrome (See "Sick Building Syndrome).

sensible heat transfer

The heat absorbed or evolved by a substance during a change of temperature that is not accompanied by a change of state. (See latent heat transfer)

sensing element

A device or component that measures the value of a variable.

self-acting controller

A controller where no external connections are necessary. Many work by means of a capillary tube. Most self-acting controllers are of the proportional control type.

self-acting controller

A controller where no external connections are necessary. Many work by means of a capillary tube. Most self-acting controllers are of the proportional control type.

self-adapting optimiser

Optimisers are based on the linear approximation of the the relationship between the preheat time and the setpoint temperature. However, as the cooling of the building changes with the weather, so does the preheat curve. Thus, a new 'straight-line relationship' is required, otherwise the optimiser will be very inaccurate. Flexible movement of the line is achieved on microprocessor-based controllers by employing 'self-adaptive' techniques to tune theline to the response of the building and heating system.


See Inherent regulation.

self-tuning controllers

A type of controller which has the capability to determine, dynamically, its control parameters such as: gain, integral action time, and derivative action time.


The minimum change in input value that can be reliably detected. For a sensor, the minimum change in the sensed variable that can be reliably detected. This may be depend both on resolution, external influences and "noise".

serial data transmision

A transmission method where data characters are transmitted in sequence one at a time over a single path; cf. Parallel data transmission.

settling time

The time required following the initiation of a specified stimulus (step change, ramp or sinusoid) to a system for the output to enter and remain within a narrow band centred on its steady state value e.g. within ±% of the steady state value.

sequential control (Sequencer)

Refer to Step controller.


See standard effective temperature


The value on the controller scale at which the controller is set (e.g., the desired room temperature set on a thermostat). The desired control point.


sulfur hexafluoride; a physiologically inert gas used as a tracer in building investigations.

shaft-type buildings

A building with large vertical connecting openings.

shelter belt

A natural or planned barrier of trees or shrubs used to reduce wind velocity, giving shelter.


The degree of protection from wind offered to a building by upstream obstacles. These may be windbreaks, shelter belts, or other buildings.

shielding coefficient

The ratio of average total exterior wind pressure to the stagnation pressure at ceiling height.


In the context of air or mass flow, a situation that occurs when the supply air flows to exhaust registers before entering the breathing zone. To avoid short-circuiting, the supply air must be delivered at a temperature and velocity that results in mixing throughout the space.

Sick Building Syndrome (SBS)

A collective term sometimes used to describe situations in which building occupants experience acute health and/or comfort effects (such as headaches, eye/skin irritation, shortness of breath and nausea) that appear to be linked to time spent in a particular building, but where no specific illness or cause can be identified. The complaints may be localized in a particular room or zone, or may be spread throughout the building.

single tracer gas technique

A method for determining the air change rate within a room or zone using only one tracer gas.

single zone

A building or part of a building comprising of one zone of uniform pressure.


This includes the latitude, longitude, average ground reflectance as well as an index to the exposure of a specific location. Site information forms part of the problem configuration. Climatic patterns are taken from one or several climate databases which are representative of particular regions.

smart sensor

A sensor in a BEMS which has an in-built ADC, a communication section, some memory to store look-up tables, and also the ability to directly control the final control element.


The airborne solid and liquid particles and gases that evolve when material undergoes pyrolysis or combustion.

smoke visualisation

A method of detecting leaks in the building fabric, by pressurizing the building and using smoke to trace the paths followed by the escaping air.

soft point

A point that can be referenced as if it were a monitoring or control point in a BEMS although it has no associated physical location. It may have a set value or be the result of a given algorithm.

soil gases

Gases that enter a building from the surrounding ground (e.g.,radon,volatile organics(VOC's) and pesticides).

solar radiation

An important item to the cooling load, which causes an increased heat flow through walls and roofs by their absorption of radiant heat; and it penetrates through windows on the sunny sides of the building unless they are externally shaded. There are several modifying factors: the latitude, the season of the year (both of which govern the solar elevation), the orientation and angle of the intercepting surface and the condition of the atmosphere.

sources of indoor air pollution

Indoor air pollutants can originate within the building or be drawn in from outdoors. Common sources include people, room furnishings such as carpeting, photocopiers, art supplies, etc. (see indoor air pollution).


The quoted range of operation from minimum to maximum input or output e.g. 10 degrees celcius to 60 degrees celcius. Accuracy, hysteresis, etc, may be quoted as "% of span". Equivalent to full scale deflection (FSD).

specific flow (a)

Defined as the total volumetric supply airflow rate per unit volume of the room. Specific flow is expressed by the equation; a=Q/V , where Q is the ventilation air flow rate (m3/s) and V is the total volume of the room (m3). Specific flow is often called the air change rate (NOTE: Specific flow is usually expressed by the letter "n", but "a" is used here as not to confuse with the Flow exponent "n".) (See also air change rate; air change time; coefficient of air change performance; local air change index; nominal time constant)

specific heat

The heat necessary to raise the temperature of a unit weight kg of a substance through 1 Deg K. Units of measure are: [J/kg.K]

specific leakage area

Leakage area, expressed per unit floor or wall area.

speed of response

A measure of how quickly a system responds to a change in input.

split-range control

A control system in which a definite sequence of events takes place in order that a certain mani- pulated variable may have first preference as a means for the process control.


Two types of instability occur in control systems:- `Monotonic' instability, and `Oscillating instability'. In monotonic instability, the controlled variable begins to increase as a function of time. In oscillating instability, the value of the controlled variable begins to oscillate with growing amplitude as a function of time. The first type of instability is usually due to a failure of the control system to function, whereas the second is usually caused by a mismatch of the control system to the process. The control system begins to amplify an error at some critical frequency instead of driving it to zero. Note that ins- tability is as much a part of microprocessor-based controller control systems as in conventional control systems. This is in part because the strategy of control is the same in either case and is the strategy which can create instability.


A single chimney/flue or a cluster of chimneys/flues. That part of a flue above roof level.

stack effect

Pressure-driven airflow produced by convection as heated air rises, creating a positive pressure area at the top of a building and a negative pressure area at the bottom of a building. The stack effect can overpower the mechanical system and disrupt ventilation and circulation in a building. (See pressure level, neutral).

stack pressure

(See stack effect)

stagnation pressure

The pressure of air if it were brought to rest.

standard effective temperature (SET*)

A temperature index that accounts for radiative and latent heat transfers.
See also effective temperature.


A description used when, for normal operation, the item of equipment does not depend upon a host or other processor.

state-space analysis

The state-space approach is used by control engineers for studying the internal `states' as well as input/output relationships. The state-space is a vector space of dimension equal to the order of the system. The transfer function is usually transformed into a state-space form by the `direct-programming' method.

static pressure

The condition that exists when an equal amount of air is supplied to and exhausted from a space. At static pressure, equilibrium has been reached.

steady-state error

The difference between the measured value and the desired value as the time tends towards infinity. There are three types of steady-state errors:- (1) Constant steady-state, or `Proportional Error'; (2) Constantly varying, or `Velocity Error'; and (3) Constantly accelerating, or `Accelerating Error'.

step control

A control method in which a multiple-switch assembly sequentially switches equipment (e.g., electric heaters, multiple chillers) as the controller input varies through the proportional band. Step controllers may be actuator driven, electronic, or directly activated by the sensed medium (e.g., pressure, temperature).

step input

An instananeous change between two steady sate input values. Thsi is commonly used to determine the response characteristic of a system.

storey-type buildings

A building comprising of floors seperated by impermeable layers.

stratified air

The formation of layers of different densities, in a body of fluid that is not mixed well. The variation in densities may be due to difference in temperatures. The term "Thermal Stratification" is often ascribed for this condition.


See outstation.

Summer-time temperature

See estimating Summer-time temperature - the CIBSE method

supervisory control

The predecessor of DDC. In supervisory control, it is the central computer (and not the outstation's microprocessor) which determines the analogue controller's setpoint. (Note that the controller in 'Supervisory control" is still of the analogue type, as opposedto DDC in which the controller is microprocessor based). The main computer coordinates and balances the operation of all the outstations in order to achieve optimum efficiency and comfort levels, and also to supervise energy management functions. Typical supervisory control functions include programmed start/stop, economiser cycle, and load control.

supply air

Air delivered to a conditioned space and used for the purpose of ventilation, heating, cooling humidification or dehumidification.

supply ventilation

A system in which air is supplied to a space, so creating an internal positive pressure. Air leaves the building through adventitious or purpose provided openings. (See adventitious opening; fortuitous leakage purpose provided opening)

surface pressure distribution

The pattern of positive (or negative) pressure relative to the static pressure of the prevailing free wind, at various points on the external surface of a building, caused by the flow of the wind onto or around the building.

system-level controller

In a BEMS, a microprocessor-based controller that controls centrally-located HVAC equipment such as VAV supply units. The controllers typically have a library of control programs, and many control more than one mechanical system from a single controller; cf. Zone-level controller.

systematic uncertainty or systematic error

A constant uncertainty or error in one direction, due to an experimental factor e.g. calibration uncertainties in reference instrumentation



Allows users to access computers and their data at thousands of places around the world, most often at libraries, universities, and government agencies.


A property of an object which determines the direction of heat flow. When the object is placed in thermal contact with another object, heat flows from the higher temperature object to the lower temperature one. It is measured either by an empirical temperature scale based on some convenient property of a material or instrument, such as the Celcius scale, or by a scale of absolute temperature, such as the Kelvin scale.

temporal insolation pattern

See "insolation". The magnitude of the incident solar energy is a time-varying quantity and often requires sophisticated point projection or hidden line/surface techniques for its estimation; i.e. the insolation varies in magnitude and position w.r.t. time, and constitutes a temporal insolation pattern.

terrain roughness

The character of the terrain over which wind passes upstream of a building, causing the wind velocity to be modified. It is common practice to characterise terrain according to roughness and express the variation in terms of roughness constants.

testing chamber

Also known as environmental chamber. A specially designed room, to enable experimenters to vary all of the environmental parameters. Thus, given conditions can be tested. It is used mainly for the reconstruction of environmental conditions that represent extreme cases, and to simulate conditions which are difficult to measure in the

thermal comfort

Thermal comfort is generally defined as that condition of mind which expresses satisfaction with the thermal environment (e.g. in ISO 1984). The thermal comfort condition is a subjective feeling of satisfaction, building designers attempt to satisfy as many of the occupants as possible (usually 80 % or more). Dissatisfaction may be caused by the body as a whole being too warm or cold, or by unwanted heating or cooling of a particular part of the body (local discomfort).
See also thermal comfort models.

thermal comfort models

Theoretical models:

Empirical models:

Adaptive models:

thermal stratification

(See stratified air)

thermal transmittance ("U"-Value)

The heat flow transmitted through a unit area of a given structure, divided by the difference between the effective ambient temperature on either side of the structure, under steady state conditions. Expressed as a "U"-value.


The process of converting the heat emitted from an object into visible pictures. It is used to indicate the temperature distribution over part of a building envelope and is useful for locating infiltration flow paths.

thermal zone

A thermal zone is a region in a building where we may assume the temperature to be more or less the same. Sometimes a thermal zone is only part of a room, (in case there are important temperature differences within the room), sometimes it represents the entire room, and sometimes a thermal zone may represent several rooms (in case these rooms have more or less the same temperature).
The part of a building (or other space) represented by a zone may vary according to the aims of the particular simulation study, and it requires experience to determine how a particular problem might best be abstracted.


A device used in control systems for switching plant to maintain a relative humidity at some setpoint.The sensor and controller elements are combined in the one element. The output signal is usually sent via a relay device to the final control element.

three phase

A system of voltages (or currents) used for large-scale electrical power generation and transmission/distribution.

Three-term controller

A controller with proportional + integral + derivative action (PID).

Threshold Limit Value (TLV)

The limit of an environmental conditions to which any person may be exposed repeatedly without adverse effect. Typically, the air concentration of chemical substances to which healthy workers can be exposed for an 8-hour working day during a 40-hour working week without suffering an adverse effect.

throttling range

In a proportional controller, the control point range through which the controlled variable must pass to move the final control element through the full operating range. Expressed in values of the controlled variable (e.g., Kelvins, per cent relative humidity). Also called the proportional band. In a proportional room thermostat, the temperature change required to drive the manipulated variable from full off to full on.

time and event programs

Initiates a predetermined series of control actions based on time of day, elapsed time,alarm condition, or a point status change.

time-sequence control

In digital control, systems, the state of the next output is determined by the lapse of time from the start of the previous state. Such a process is usually encountered as part of an overall sequence involving functional dependence. (See Function sequence systems.) It is a characteristic of this type of control sequence that it depends only on the passage of a fixed amount of time and not on the state of any variable in the system.

time constant

The time required for a dynamic component such as a sensor or a control system, to reach 63.2% of the total response to step change in its input. The 95% response time is then approximately equal to 3 times the time constant. Typically used to assess the the responsiveness of the component.

time proportional action

A control action in which on/off devices are switched on for a certain time proportional to the error signal. The time proportional controller keeps the plant on for a defined fraction of a period of time. The period of time is set by the operator when the system is configured.

time schedule

Record of the desired changes of a variable or operational status with respect to time.


An acronym for Threshold Limit Values.

tracer gas

A detectable, non toxic, non reactable gas, such as sulfur hexafluoride(SF6), used to identify suspected pollutant pathways and to determine ventilation rates. Tracer gases may be detected qualitatively by their odor or quantitatively by air monitoring equipment.

tracer gas analyser

Any instrument used to evaluate the concentration of tracer gas in a sample of air over time.

tracer gas technique

A method employing tracer gases to determine air infiltration and ventilation rates. (See constant concentration, constant emission; decay tracer gas method)


A device which converts the quantity being measured into an optical, pneumatic, mechanical, hydraulic, electrical, or electronic signal. Transduction is the energy conversion process that takes place, i.e., from one form of energy to another, e.g., pneumatic to electronic.

transfer air

The movement of indoor air from one space to another.

transfer index method

A method of measuring ventilation rates, by determining the transfer index between two points. The time integral of tracer gas concentrations is determined at one point, following the liberation of a fixed volume of tracer to another. Several sample points are usually employed. The reciprocal of the transfer index has dimensions of ventilation rate.

transfer function

Is a `prescription' which will give the output response for any specific history of input.

transient response

The time variation of an output signal when an input signal or disturbance of specified nature is applied.

transient peak value

The maximum value of the output signal in controller response to a step function input signal.

transition flow

The unstable region of flow that occurs when there is a change from a laminar to a turbulent flow regime. (See laminar flow; turbulent flow)


The process in which the radiation passes through a body or medium.

transport lag

Occurs when energy is transferred through a resistor to or from a capacity.


In electronic control systems, a relay switch used to bring in other equipment, e.g., a pump.


TS is an equation that predicts thermal sensation vote using a linear function of air temperature and partial vapour pressure. See thermal comfort models.


See effective temperature.


An acronym for Total volatile Organic Compounds.


Thermostatic Radiator Valve. This is a control valve which has the sensor, controller, and actuator functions combined in one device. Wax expands with an increase of space temperature to throttle the water flow rate through the radiator.

turbid /turbidity

An unclear condition or haziness in air caused by particles, or cloudy condition in water caused by suspended silt or organic matter.

turbulent flow

Motion of fluids in which local velocities and pressures fluctuate irregularly. (See laminar flow; transition flow)

two-step controller

A controller whose output signal changes from one predetermined value to another predeter- mined value when the deviation changes sign.

two-step controller with overlap

A controller where the output signal has one predetermined value when it's input signal exceeds a certain threshold value, and another when its input signal is less than a second threshold value. The difference between the two thresholds is the `differential' or `overlap'.

two-term controller

A controller with either proportional + integral (PI) action, or proportional + derivative (PD) action.



A degree of damping sufficiently small that after the system has been subjected to a single disturbance, one or more cycles or oscillations are executed by the system.

unitary controls

Some components, such as chillers, boilers, and air-handling units, come packaged with their own controls or control systems, but which are capable of interfacing with a BEMS.

unintentional opening

(See adventitious opening; fortuitous opening)

uniform mixing

The combining of two or more substances such that the parts of one are wholly distributed throughout the parts of another.

unity feedback

This is where the feedback through the sensor is accurate, the sensor has little thermal mass and lags are short.


Unix is a computer operating system. An operating system is the program that controls all the other parts of a computer system - both the hardware and the software. Most importantly, it allows you to make use of the facilities provided by the system. Every computer has an operating system. Please note that there exists a separate glossary for Unix terms

"U"- Value

(See thermal transmittance)


valve authority

Rate of pressure across a fully opened valve to the pressure drop across the remainder of the circuit.

valve regulation

Ratio of maximum controlled flow to minimum controlled flow.

valve turndown ratio

Ratio of maximum normal (usable) flow to minimum controllable flow; cf. Rangeability.

valve positioner

A control valve accessory which transmits a loading pressure to an actuator to position the valve exactly as required by the controller. It helps overcome hysteresis and lags due to friction.


Air handling system that conditions the air to a constant temperature and varies the outside airflow to ensure thermal comfort. Ventilation Air-Defined as the total air, which is a combination of the air brought into the system from the outdoors and the air that is being recirculated within the building. Sometimes, however, used in reference only to the air brought into the system from the outdoors.


An acronym for a Variable Air Volume system (See Variable Air Volume System).


A substance in gas form, particularly one near equilibrium with its condensed phase, which does not obey the ideal gas laws; in general, any gas below its critical temperature.

vapour/vapor barrier

A moisture impervious layer applied to the surfaces enclosing a space or to the surface of thermal insulation to limit moisture migration through the surface.

Variable Air Volume (VAV)

A ventilation system that controls the dry bulb temperature within a space by varying the volume of supply air, rather than the supply air temperature.

velocity profile (for a room)

The relationship between the height above a surface and the mean velocity of a fluid (air) at that point.


The process of supplying or removing air, by natural or mechanical means to and from a space. Ventilation refers to air movement between zones.

ventilation air

That portion of supply air that is outdoor air plus any recirculated air that has been treated for the purpose of maintaining acceptable indoor air quality.

ventilation effectiveness

An expression describing the ability of a mechanical (or natural) ventilation system to remove pollution originating in a space, either of a steady state or transient nature.

ventilation efficiency

A measure of how quickly a contaminant is removed from the room. (See absolute ventilation efficiency; relative ventilation efficiency)

ventilation heat loss/or gains

The heat lost or gained by virtue of warm and/or humid air flowing into or leaking from a space.

ventilation rate

The rate at which outside air is intentionally supplied to a building or zone. Sometimes ventilation is used to describe the total mechanical air change in a room or building. This rate may then frequently comprise a considerable proportion of recirculated rather than outdoor air. Hence when apparently very large ventilation rates are quoted, it is important to establish the proportion of flow representing outside supply air. The remainder will be recirculated air.

ventilation strategy

A plan by which ventilation air is purposefully provided to a space. When such a strategy is employed, it is normal to take action to minimise background leakage.

ventilation system

(See mechanical ventilation system; mechanical extract ventilation system; mechanical supply ventilation system; balanced supply/extract ventilation system)


An acronym for Variable Frequency Drive - Electronic speed control for motors.


A computer-generated environment.


Friction or resistance to the flow of a liquid.

Viscous Flow

(See laminar flow)

vitiated air

Spoiled, impure or polluted air.

visual discomfort

The most common cause for visual discomfort is glare, which is the discomfort or impairment of vision caused by an excessive range of brightness in the visual field. Glare can be caused by lamps, windows and painted surfaces appearing too bright in comparison with the general background. Glare can be further described as disability and discomfort glare.

virtual point

See soft point.

visual environment

The internal visual environment is composed of various items such as:


An acronym for Volatile Organic Compounds (See Volatile Organic Compounds).

volumetric specific heat

The heat necessary to raise the temperature of a unit volume of a substance by 1 K


1. Able to evaporate readily. 2. Able to go to gas phase from a liquid or solid phase.


Compounds that evaporate from the many housekeeping, maintenance, and building products made with organic chemicals. These compounds are released from products that are being used and that are in storage. In sufficient quantities, VOCs can cause eye, nose, and throat irritations, headaches, dizziness, visual disorders, memory impairment; some are known to cause cancer in animals; some are suspected of causing, or are known to cause, cancer in humans. At present, not much is known about what health effects occur at the levels of VOCs typically found in public and commercial buildings.


Airflow with rotary, rather than translatory motion. It occurs in the wakes of buildings etc. and also in the presence of strong updraughts. A standing eddy or stationary vortex may be formed in the lee of a building arising from the airflow around it.



Wide Area Information Servers These servers allow users to conduct full-text keyword searches in documents, databases, and libraries connected to the Internet. Pronounced "Ways"

water to water chillers

(see Air cooled chillers or air to water chillers) The same as except water is used to reject heat instead of air, the circuit rejecting the water is known as condenser water and is usually provided from a cooling tower.

water to water heat pump

(see air to water heat pump) The same as except unit provides hot water as well as chilled water.


An acronym for water column, the old measure of air pressure used in HVAC systems, see Inches of Water Column(IWC)


(See weatherstripping)


a) The fitting of a strip of flexible material to seal a joint between a movable component and its seating of the application of mastic sealants to seal infiltration openings. The strip is attached to one edge and excludes air by pressing tightly against the other. b) Fixing a piece of material to stop a draught passing through the joints of a closed component (such as a door or window). (See Caulking)

weather compensation

A control algorithm which varies the the temperature of the heating and cooling medium with respect to the outside temperature. Can incorporate internal temperature reset which senses the space condition and applies a correction to the heatingmedium temperature if the desired space temperature is not achieved. See also Boiler compensation.


(See World Wide Web)

wet bulb temperature

Air temperature indicated by a sensing element kept wet (usually by a wick), the indicated temperature thus being related to the rate of evaporation from the wetted bulb. This Wet Bulb Temperature is used by psychrometers to measure the relative humidity. (See dry bulb temperature)

wild coil

A coil where no control is exercised over either the temperature or the the flow-rate of cooling or heating.

wind barrier

(See windbreak)


A barrier designed to obstruct wind flow and intended for protection against excessive wind pressure. (See shelterbelt)

wind pressure (on a facade)

The difference between the local pressure on the exterior of a building induced by the action of the wind and static outdoor pressure far away from any building or shield.

World Wide Web

A revolutionary Internet browsing system that allows point-and-click navigation of the Internet. The WWW is a spiderweb-like interconnection of millions of pieces of information located on computers around the world. Web documents use hypertext, which incorporates text and graphical links to other documents and files on Internet-connected computers.


(See World Wide Web)




zero energy band

An energy conservation technique that allows the temperatures to float between selected settings, thereby preventing the consumption of heating or cooling energy while the temperature is in this range.

zone-level controller

A microprocessor-based controller that controls distributed or unitary HVAC equipment such as VAV terminal units, fan coil units, and heat-pumps. These controllers typically have standard control sequences, relatively few connected input/outputdevices, and are dedicated to specific applications.


The practice of dividing a building into sections for heating and cooling control so that one controller is sufficient to determine the heating and cooling requirements for the section.