Ventilation and Air Supply Requirements for Non-Domestic Gas Installations
This domain tests whether you can provide both combustion air and cooling/ventilation air to a plant room, and prove it with openings rather than room volume.
Principles
Open-flue appliances draw combustion air from the room; room-sealed appliances take it from outside but still need ventilation for heat removal. Mechanical ventilation must be interlocked so gas cannot flow without air, and extract systems in the same room (kitchen extract, fume cupboard) must not reverse the flue.
Opening sizes
Commercial practice commonly uses permanent low-level and high-level openings sized on total input: about 4 cm²/kW at low level and 2 cm²/kW at high level for natural-draught boilers, subject to the standard's tables and any appliance-specific instructions. Openings must be measured as free area; louvres and grilles reduce free area, often by 50%, so the structural opening doubles.
LPG and special cases
LPG vapour is heavier than air, so low-level ventilation and low-level detection are essential; basements and pits need particular care. Where flammable vapours, dust or corrosive atmospheres exist, the ventilation strategy must address them separately from combustion air.
Worked example
Two 150 kW natural-draught boilers total 300 kW. Low-level free area = 300 × 4 = 1,200 cm²; high-level = 300 × 2 = 600 cm². If the chosen louvre has 50% free area, the structural openings must be 2,400 cm² and 1,200 cm². Adding a 0.5 m³/s extract fan would require an interlock and a make-up air check, otherwise the openings are no longer adequate.
Traps
Using room volume instead of permanent openings; quoting grille size rather than free area; forgetting that an extract fan competes with combustion air; placing LPG ventilation only at high level; blocking openings during refurbishment.
Exam focus
Combustion air versus cooling air, cm²/kW sizing, free-area correction, interlocked mechanical ventilation and LPG low-level requirements.