A biomass system needs a chimney to release the gases and particulates emitted. It ss important to be sure that the chimney is correctly sized, providing enough draft for the circulating gas. A negative pressure on the boiler side may cause smoke to be redirected to the boiler room or even the storage room, cause the combustion process to be inefficient, etc. The balance between the inlet and outlet pressure depends on several factors like kind of boiler, building structure (which will define pipework routing), emissions removal system and the possibility (or requirement) to add fans to create a forced draft, etc.

This variety of possible answers to a single question lead our group to decide that creating a general tool to size the chimney would end up with something too detailed and overcomplicated. For this reason, a sizing sheet was not created.

It is important though to be aware that when selecting the boiler room, a chimney will need to be installed and some limits need to be respected, as shown below.

Where:

A	2.3m horizontally clear of the weather skin.
B	1m provided A is satisfied or 0.6 m where above the ridge.
C	1m above the top of any flat roof and 1m above any openable rooflight, dormer or ventilator, etc. within 2.3 m measured horizontally.
D/E	Where D is no more then 2.3m, E must be at least 0.6m.

Notes:

1 - Horizontal dimensions are to the surface surrounding the flue

2 - Vertical dimensions are to the top of the chimney terminal

Source: The British Flue and Chimney Manufacturers’ Association. A guide to choosing and using flues and chimneys for domestic solid fuel and wood burning appliances.

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