Open-Flued Ventilation
Air-vent free area for an open-flued appliance (revision rule).
Free area required65 cm²
Method: BS 5440-2 (air-vent sizing)
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Open-Flued Ventilation Calculator
Estimate the purpose-provided air vent an open-flued appliance needs, using the standard revision rule.
How it works
An open-flued appliance draws its combustion air from the room, so the room needs a purpose-provided air vent once the appliance is large enough. The revision rule: provide 5 cm² of free area for every kW of total rated heat input above 7 kW (an appliance of 7 kW or less generally needs none in an average room).
Free area (cm²) = (total rated input kW − 7) × 5. "Free area" is the actual open area of the vent, which is less than its overall size once the grille is accounted for — always check the vent’s stated free area.
This is a revision approximation. Room-sealed appliances need no purpose vent; compartments, multiple appliances and high-rise situations follow the full BS 5440-2 procedure.
Worked example
A 20 kW open-flued boiler: (20 − 7) × 5 = 65 cm² of free area required.
Ventilation questions
How much ventilation does a 20 kW open-flued boiler need?
By the revision rule, (20 − 7) × 5 = 65 cm² of purpose-provided free area. The full BS 5440-2 procedure and appliance instructions decide the live answer, especially for compartments or multiple appliances.
Do room-sealed boilers need an air vent?
A room-sealed appliance takes its combustion air from outside through the flue arrangement, so it does not need a purpose-provided combustion vent in the room. Compartment cooling and manufacturer instructions can still apply.
Does an openable window count as ventilation?
No. Combustion air needs permanent, non-closable provision. A window can be shut, so it cannot be relied on as the air supply for an open-flued appliance.
Method: BS 5440-2 (air-vent sizing)
Training & revision aids — live installations follow the full standard, the manufacturer’s instructions and calibrated instruments.