Adventitious air versus purpose-provided ventilation

An open-flued appliance burns air taken from its room, so the room has to replace that air as fast as it is used. It arrives in two ways.

Adventitious air

Incidental air leaking in through gaps in the building fabric. It is uncontrolled. Modern, well-sealed homes have much less of it, and draught-proofing or alterations reduce it further, so it cannot always be relied on.

Purpose-provided ventilation

A fixed, permanent opening provided specifically to supply combustion air, so the appliance does not depend on chance leakage.

• Use a non-adjustable opening: no built-in shutter, timer or closable cover. A fixed vent can still be obstructed later by furniture, decorating or a separate cover, so check that it remains clear.
• An openable window does not count, because it can be shut. Nor does a promise to leave a door ajar, or a CO alarm.
• The free area needed rises with heat input: burn more gas, need more air. It also depends on the type of space: a room, a compartment, or air drawn through another room. Room size and supply pressure do not set it.
• Two open-flued appliances in one room are sized for both running together.

In Approved Document J (England), deducting the first 7 kW of net input applies only to older dwellings with air permeability above 5 m³/(h·m²). A 6 kW appliance is not automatically vent-free.

Before you rely on the 7 kW allowance

Approved Document J asks for 5 cm² of vent per kW of net input for an open-flued appliance in a room. Check every condition before you deduct the first 7 kW.

• Building: homes built after 2008 are likely to have air-tightness test evidence, and an older home that has been thoroughly draught-proofed may be below the 5 m³/(h·m²) threshold.
• Room: a single room with solid walls, a solid floor and modern glazing can be tight even in a leaky house. If in doubt, treat it as airtight and fit the full vent.
• Arrangement: the deduction only comes off a room's vent to outside. A compartment's high and low vents are sized on the full net input, because all of the appliance's air passes through them.
• Appliance: a decorative fuel-effect fire has its own rule, normally at least 100 cm² in a fireplace with a throat, and flueless appliances follow a separate table.

An 11 kW net open-flued boiler in a room of a leaky older house needs (11 − 7) × 5 = 20 cm². In an airtight house the same boiler needs 11 × 5 = 55 cm².

What changes the assessment

A replacement window, a blocked air brick, a new extractor fan or a higher-output boiler can change the air supply. Assess the complete air supply against the appliance now fitted. A previously working appliance is no evidence that an altered room is still safe.

Flueless appliances

A flueless appliance, such as a gas cooker, releases its products straight into the room, so the room itself has to clear them. The open-flued vent sizes and the 7 kW allowance do not apply.

• The room needs an openable window, door or similar opening direct to outside; in a kitchen, mechanical extract can do that job instead.
• For a cooker, any permanent vent depends on the room's volume, not the cooker's input: under 5 m³ needs 100 cm², 5 to 10 m³ needs 50 cm² unless a door opens straight to outside, and over 10 m³ needs none.
• A permanent vent for a flueless appliance must open directly to outside, never into another room.

Room-sealed appliances

They take combustion air from outside, so they need no room vent for combustion. A cupboard around one may still need cooling vents and clearances: the boiler's instructions decide. A disused chimney is not automatically a flue route for one either: the manufacturer must permit a suitable flue system in it, and its condition must meet the requirements.

Cupboards around a room-sealed boiler

Cooling is a separate check from combustion air. The case still gives off some heat, so the instructions set minimum clearances around the boiler. Some boilers lose so little heat that their instructions allow a cupboard with no vents, as long as the clearances for an unventilated compartment are kept. Where the instructions call for cooling air, for example because the cupboard is tighter than those clearances, fit high and low vents: Approved Document J gives 5 cm² per kW of net input at each level direct to outside, or 10 cm² per kW at each level into a room.