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Gas appliances need enough air to burn their fuel safely and to clear the products of combustion. Whether an air vent is needed, and how big it should be, depends on the appliance type, its heat input, and whether it sits in a room or an enclosed compartment. The sizing is set out in BS 5440-2.
Reviewed by Jordan Valentine-Dunn, Gas Safe registered engineer · Portsmouth Gas Heating · Last reviewed July 2026
Gas appliances need a supply of air for two reasons: to burn their fuel safely, and to help clear the products of combustion. Sometimes the air that leaks naturally into a room is enough; sometimes an appliance needs extra, purpose-provided ventilation in the form of an air vent.
The detailed sizing of any vent is set out in the relevant standard, BS 5440-2, and a Gas Safe registered engineer works out what a given appliance needs. It is not a guess or a rule of thumb: it follows the standard for that appliance and location.
A key safety point for householders and landlords: never block, cover or seal an existing air vent, because it may be there to keep an appliance burning safely. Only a Gas Safe registered engineer can assess and set up ventilation correctly. This is general information, not safety advice for a specific installation.
Last reviewed July 2026. This guide is general information, not legal or safety advice, gas safety work must be carried out by an appropriately Gas Safe registered engineer. Rules can change, so check the linked official sources for the current position.