Open-flued ventilation: choosing the rule before calculating

An open-flued appliance takes combustion air from its room. Establish the required air supply from the appliance instructions and applicable guidance before calculating an opening. The appliance type, net input, room or compartment, air route and building air permeability matter.

The 7 kW allowance is conditional

Approved Document J for England gives a room-vent method for specified flued gas appliances, excluding decorative fuel effect fires and flueless appliances. Its allowance for the first 7 kW net is linked to older dwellings with air permeability above 5 m³/(h·m²). Do not apply that allowance to every home, appliance or compartment opening.

Worked comparison

For an 18 kW net appliance, suppose the applicable room-vent method requires 5 cm² per kW and the 7 kW allowance has been established as applicable:

(18 − 7) × 5 = 55 cm².

Without that allowance, the same 5 cm²/kW calculation is 18 × 5 = 90 cm². The arithmetic is easy; selecting the permitted assumptions is the important decision. These are design exercises, not permission to reduce an existing vent without assessment.

Check the installed route

Use the ventilator manufacturer's declared equivalent area where available. The outside frame dimensions do not establish capacity. Check all parts of an indirect route, the permanence of openings and the risk of blockage. An openable window or a CO alarm does not replace required permanent ventilation.

Do not add fine flyscreen to a gas-appliance combustion-air vent simply because its nominal area looks large enough. Suitability and resistance must be checked as well as area. A vent is an air supply, not an acceptable escape route for spilled flue products.