Which appliances need a room vent, the Approved Document J room-vent rate, and when the 7 kW allowance applies, with a worked 15 kW example.
Before you size any vent, decide which rule applies. That starts with the appliance type.
Does the room need a vent at all?
- Room-sealed: combustion air comes from outside and products go outside, so no permanent ventilation is required in the room for combustion or flue draught.
- Open-flued: combustion air comes from the room, so the room must replace it through a permanent, purpose-provided vent that the occupant cannot close.
- Flueless: products are released into the room, so the room itself needs adequate ventilation, typically including rapid-response ventilation such as an openable window.
For an open-flued appliance the free area follows the net heat input and the type of space: a room, a compartment, or air taken through another room. Two open-flued appliances in one room are sized for both running together.
The room-vent rule and its 7 kW allowance
For an open-flued gas appliance in a room, other than a decorative fuel effect fire, Approved Document J (England) gives 500 mm² (5 cm²) of permanent vent per kW of net input.
The first 7 kW may be deducted only in older, leakier dwellings with air permeability above 5 m³/(h·m²). Below that, as in a new house at 3.0 m³/(h·m²), size on the full input. A small appliance is not automatically vent-free: check the appliance, the room and the building first.
Worked example: 15 kW net open-flued boiler
- Allowance applies: (15 − 7) × 5 = 40 cm².
- Allowance does not apply: 15 × 5 = 75 cm².
- In square millimetres, multiply by 100: 40 cm² = 4,000 mm².
Subtract the allowance once. Never deduct it from a compartment's high and low openings, and never use it to reduce an existing vent without an assessment.
Check the vent you rely on
- Use the ventilator's declared equivalent area, not its outside dimensions.
- Do not add flyscreen or fine mesh to a combustion-air vent.
- Air taken through an adjoining room counts only if that room has its own vent to outside.
- An openable window, a closable cover or a CO alarm does not replace required permanent ventilation.
- A vent supplies air. It never makes spilled flue products acceptable.
Common mistakes
- Deducting 7 kW in every home, for every appliance.
- Sizing from room size or flue diameter instead of heat input.
- Assuming a vent that suited the old boiler suits a higher-output replacement.