Ventilation, flueing and indoor air quality
What this domain tests
The exam tests whether you can provide combustion air, remove products of combustion safely, and protect indoor air quality in dwellings, commercial kitchens and plant rooms. Flue design and ventilation are inseparable: a flue that cannot draft is a carbon-monoxide hazard.
Core concepts
- Combustion air supports burning; dilution air protects the flue and removes heat; ventilation air controls room temperature. These are separate requirements and must not be double-counted or omitted.
- Open-flue appliances take air from the room and rely on a draught diverter and a vertical flue. Room-sealed appliances take air directly from outside and are preferable where indoor air quality or depressurisation is a concern.
- Terminals must be clear of openings, windows, air intakes and combustible surfaces, and must avoid recirculation and downdraught. The code gives minimum terminal clearances.
- Mechanical extract can depressurise a room and reverse an open flue. Interlocks or compensating air inlets are required where the risk exists.
- Ventilation openings must be measured as free area, not gross louvre area. Keep them clear and protected against blockage.
Worked example
A 24 kW open-flue boiler is installed in a compartment. Using the code's 10 cm2/kW free-area rule for high and low level, each opening needs 24 x 10 = 240 cm2 free area. The selected louvre has 50% free area, so the gross opening must be 240/0.50 = 480 cm2 at both high and low level. If the door has a 100 cm2 transfer grille, that is not automatically an acceptable substitute: the compartment still needs the high-level opening for heat release and flue dilution. Record the calculation and inspect the installed free area.
Traps that cost marks
- Counting a louvre's overall size instead of its free area.
- Providing only one ventilation opening, or placing both at the same level.
- Ignoring an extract fan, tumble dryer or kitchen hood that can depressurise the room.
- Siting a terminal under an openable window or beside an air intake.
- Assuming a flue is safe because the appliance lights; spillage testing and combustion analysis are separate checks.