Part J · England · Free to use
Combustion air vent calculator
Permanent ventilation for open-flued stoves, boilers and fires by fuel, output and airtightness.
Approved Document J (2022)
Your inputs
Results update as you type.
Part J · Result
1,650 mm² permanent vent
550 mm² per kW above 5 kW.
Calculated result- Rated output: Calculated result8 kW
- Minimum free area: Calculated resultappliance in the room, direct to outside1,650 mm²
- Room-sealed appliances need no room vent. Appliances in a compartment need the larger vents in Diagrams 32 and 40.
- Pre-2008 homes are unlikely to be below 5 m³/(h·m²).
Clauses this relies on
Source edition and scope
Approved Document J (2022).
Gas calculations cover open-flued appliances in a room with direct ventilation to outside, excluding decorative fuel-effect and flueless appliances. The first 7 kW net input can be ignored only in older dwellings with air permeability above 5 m³/(h·m²), as stated in Diagram 32 note 5.
This is a preliminary check of the conditions shown. Confirm the edition, project circumstances and any additional requirements with your architect or building control.
A design aid, not legal advice. Checked against the Building Regulations 2010 for England; confirm with building control.