Worked high- and low-vent sizes for an open-flued boiler in a cupboard, vented into a room or direct to outside, and the checks before relying on them.
An open-flued boiler in a cupboard takes its combustion air from the cupboard, and the cupboard also has to shed the heat the boiler gives off. So it needs two openings: a low one where cooler air enters and a high one where warmed air leaves. Check each opening on its own. Adding the two areas together proves nothing, and they are not always equal.
The rates depend on where the openings lead. For an open-flued appliance in a compartment, Approved Document J gives, per kW of net input:
- Openings direct to outside: 5 cm² high and 10 cm² low.
- Openings into an adjoining room: double, 10 cm² high and 20 cm² low. That room then needs its own vent to outside of 5 cm² per kW.
Example 1: 24 kW net boiler, compartment vented into a room
- Low opening: 24 kW × 20 cm²/kW = 480 cm².
- High opening: 24 kW × 10 cm²/kW = 240 cm².
- Room vent to outside: 24 kW × 5 cm²/kW = 120 cm². Without it, the air path ends inside the building.
Example 2: same boiler, compartment vented direct to outside
- Low opening: 24 kW × 10 cm²/kW = 240 cm².
- High opening: 24 kW × 5 cm²/kW = 120 cm².
- In square millimetres, multiply by 100: 240 cm² = 24,000 mm².
Before you rely on the numbers
- Confirm the figure applies to this appliance type and this arrangement. A correct value from the wrong table can still leave the appliance short of air.
- A room-sealed boiler in a cupboard takes its combustion air from outside. Whether the cupboard needs cooling vents depends on the boiler's instructions and the clearances; some need none.
- Do not deduct a room's 7 kW allowance from compartment openings.
- Choose ventilators by their declared equivalent area, not the frame size. Keep them permanently open: a user-operated cover does not meet a permanent requirement.