A worked free-area calculation for an open-flued appliance, framed so you verify the rate in the standard.
A 16 kW net open-flued boiler is in a room. For this exercise, the applicable method permits a 7 kW allowance and requires 5 cm² of permanent vent area per remaining kW.
- Chargeable input: 16 − 7 = 9 kW.
- Required area: 9 × 5 = 45 cm².
- In square millimetres: 45 × 100 = 4,500 mm².
The allowance is a supplied condition, not a universal rule. Approved Document J in England links the 7 kW allowance to older dwellings with air permeability above 5 m³/(h·m²), within its specified appliance scope. Establish the actual requirements before transferring this example to a site.
Choose a suitable ventilator using its declared equivalent area where available. A grille measuring 100 mm by 100 mm overall does not necessarily provide 10,000 mm² of effective air passage. For an assembly of components, check the restriction through the whole route; do not add the front and back openings as though they were independent vents.
An appliance compartment or a route via another room changes the checks. High and low openings may have different duties and sizes. Never divide a room-vent answer between cupboard openings without establishing the applicable compartment method.
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