Why some appliances need a permanent air vent and where the rest of their combustion air comes from.
Adventitious air enters through gaps in the building fabric. Its availability changes with construction, draught-proofing and alterations. Purpose-provided ventilation uses deliberate openings designed to remain available.
For an open-flued appliance, establish whether permanent room ventilation is required and how it must be sized. Do not decide from heat input alone. In the relevant Approved Document J method for England, the allowance for the first 7 kW net depends on the older dwelling's air permeability being above 5 m³/(h·m²). Other appliances and arrangements have their own requirements.
A replacement window, blocked airbrick or new extractor can change the air-supply conditions. Assess the complete arrangement and required appliance checks. A previously working appliance is not evidence that an altered room remains safe.
Room-sealed appliances draw combustion air through their outside air route. A cupboard around one may still need cooling provision and specified clearances; check the instructions for the model and arrangement.
Keep combustion air, compartment cooling and removal of combustion products distinct. Required ventilation cannot be replaced by an alarm, an openable window or a promise to leave a door ajar.
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