Boiler Sizing Assumptions and Calculations

Energy Systems

The existing energy systems taken into account to estimate the heat load are the following:
-7 electric boilers
-1 oil-fired boiler in the old building
- 1 restaurant LPG combi boiler
-Workshop LPG radiant heaters
-2 GEC AHU heaters (sport hall)
-2 oil boilers (for Nith-Cree)

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Average Winter Heating Load and Hot Water Demand

The non-heating electric demand was considered to be constant throughout the year and equal to the electric day consumption in September (little or no heating, college fully occupied). The electric heaters do indeed only store energy at night when the price of electricity is low.
The average energy consumption in winter (December, January, February, March) was estimated on the basis of the winter energy bill. The overall figure obtained was then broken down into:
-hot water
-heating

The hot water demand was estimated on the following basis (figures taken from the Ashrae and occupancy level specified by Pearl Edgar):
-75 boarders (49.7 l/(day student)), (19 l/hr max)
-225 day students (6.8 l/(day student), (3.8 l/hr max)
-110 members of staff (3.8 l/(day staff)) (1.5 l/hr max)
-75 breakfasts, 100 lunches, 80 dinners every day (9.1 l/meal) (5.7 l/hr max)
Tap and shower hot water was taken at 40°C, meal-related hot water was taken at 60°C and refill water was taken at 15°C.

Once the hot water demand calculated, it was deducted from the overall energy consumption and heating figures were obtained thus leading to an assessment of the average winter heating load. A peak hot water demand was also derived from the maximum hourly hot water consumption.

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Extreme Winter Conditions

To assess the extreme weather heating load, the global UA value of the buildings was calculated via the degree day sum for December, January, February and March and the heating load obtained previously for this period. The degree days were obtained from the vesma.com site and correspond to the 20 year average for the British North-western region (which encompasses the Dumfries) with a 15.5 base temperature.
The outside extreme temperature was assumed to be -7°C (temperature under which outside temperatures do not drop, on average, more than one day a year in Glasgow and Edinburgh according to the CIBSE guide A). The extreme heating load was then calculated as if no casual gains were available (ie with a 21°C inside temperature).
The main results have been gathered in the following tables:

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