Part J of the Building Regulations requires combustion appliances to have an adequate air supply, safe discharge of combustion products, carbon monoxide detection and protection from fire and burns, and Approved Document J (published for use in England) sets out how to show compliance for a new or replacement gas boiler.

The four requirements that matter to a boiler

Approved Document J, published on GOV.UK for use in England, sets out requirements J1 to J4 for combustion appliances. J1 says appliances must be installed so that there is an adequate supply of air for combustion, to prevent overheating and for the efficient working of any flue. J2 requires adequate provision for discharging the products of combustion to the outside air. J3 requires that where a fixed combustion appliance is provided, appropriate provision is made to detect and warn of the release of carbon monoxide. J4 requires installation that reduces to a reasonable level the risk of burns or of the building catching fire. Requirements J1 and J2 apply to fixed combustion appliances, and J3 applies only to fixed appliances in dwellings.

GOV.UK states that the amended provisions and guidance on carbon monoxide alarms came into effect on 1 October 2022. The document also stresses that following its guidance gives a presumption of compliance, but that presumption can be overturned and other ways of meeting the requirements are permitted.

Air supply

In England, the guidance says combustion appliances need ventilation for combustion and for the proper operation of flues, and that room-sealed appliances, or those with a directly connected external air supply, minimise ventilation energy losses and cold draughts. A room containing an open-flued appliance may need permanently open air vents, depending on the appliance type and rating; infiltration through the building fabric can be enough in some cases. Compartments that enclose open-flued appliances need vents large enough to admit all the air the appliance requires, and cooling air vents at high and low level may be needed where the appliance requires it.

Adjustable vents installed to meet the separate ventilation requirement in Part F cannot substitute for the vents needed for Part J unless they are fixed permanently open. If several appliances share a room, the air supply usually has to be increased.

Flues and how they are checked

In England, Approved Document J says flues should be checked at completion to show they are free from obstruction, satisfactorily gas-tight and made of suitable materials and sizes. Where an appliance is installed, tests should cover the fluepipes and the joints between fluepipes and appliance outlets. A spillage test to check for compliance with J2 should be carried out with the appliance firing, as part of commissioning to check compliance with Part L and, where relevant, the Gas Safety (Installation and Use) Regulations.

For gas appliances the guidance adds that flues should be sized following the manufacturer’s instructions for CE-marked appliances, that connecting fluepipes should match the appliance flue outlet in size, and that outlets should be positioned to allow the dispersal of combustion products and, for balanced flues, the intake of air.

Competence and notification

The document says gas installations must also comply with the Gas Safety (Installation and Use) Regulations, which require gas work to be done by a competent person and gas businesses to be members of a class approved by the Health and Safety Executive, which it says means registration with Gas Safe Register. Because of this, the Building Regulations allow that work need not be notified to a building control body if it solely comprises installing a gas appliance and is done by a member of that approved class.

Carbon monoxide alarms for new or replacement gas appliances

Under the 2022 amendments, where a new or replacement fixed gas burning appliance, excluding gas appliances used solely for cooking, is installed in a dwelling, a carbon monoxide alarm should be provided in the room where the appliance is located. Alarms should comply with BS EN 50291-1:2018 and be battery powered with a battery designed to last the alarm’s working life, with a warning when the alarm is due to expire; mains-powered Type A alarms with fixed wiring and a sensor failure warning may be used instead.

The alarm should be in the same room as the appliance, on the ceiling at least 300mm from any wall or, if on a wall, as high as possible above doors and windows but not within 150mm of the ceiling, and between 1m and 3m horizontally from the appliance. The guidance adds that an alarm should not be regarded as a substitute for correct installation and regular servicing. Similar provisions apply to solid fuel and oil-burning appliances.

FAQs

Does this apply in Scotland, Wales or Northern Ireland?

The GOV.UK publication is the version for use in England. Scotland and Northern Ireland have their own building standards, and readers elsewhere should check the rules that apply in their nation.

Do I need a carbon monoxide alarm if I only replace my boiler?

Approved Document J says an alarm should be provided in the room of a new or replacement fixed gas appliance in a dwelling, other than a gas appliance used solely for cooking.

Does an alarm replace a boiler service?

No. The guidance states that an alarm should not be regarded as a substitute for correct installation and regular servicing.

Bottom line

For a gas boiler in England, Part J means adequate air, a sound and tested flue and, since 1 October 2022, a carbon monoxide alarm in the same room for a new or replacement appliance. A Gas Safe registered installer is expected to handle competence and notification. This is general information, not advice on any specific installation; confirm the details with your installer and local building control body.

Sources

This guide draws on the following primary sources, current as of 20 September 2026:

• Ministry of Housing, Communities & Local Government via GOV.UK, “Combustion appliances and fuel storage systems: Approved Document J”
• Ministry of Housing, Communities & Local Government via GOV.UK, “Approved Document J: Combustion appliances and fuel storage systems” (PDF, 2022 amendments)

Related guides

Best Boiler Deals is an independent guide. We may earn a fee from some links; this never affects what we write. This article is general information, not regulated advice on any specific installation — always confirm the details with a Gas Safe registered engineer.