Approved Document J (Combustion Appliances): What Homeowners Need to Know

Part J covers:

• Air supply — combustion appliances need a supply of air. In very well-insulated or airtight homes, a dedicated air supply duct may be needed.
• Discharge of combustion products — flues and chimneys must be sized, designed and maintained to safely carry combustion gases (including CO) to outside.
• Protection of the building — hearths, walls and floors near appliances must be non-combustible and properly spaced.
• Provision of information — a carbon monoxide alarm must be fitted, and a notice plate should be fixed in the building showing key appliance details.
• Fuel storage — oil tanks and solid fuel stores must be sited safely, with appropriate fire separation and containment.

Installing a wood-burning stove is one of the most common Part J projects for homeowners. Key requirements include:

• Hearth: a non-combustible hearth at least 840 mm × 840 mm (for freestanding stoves) and at least 125 mm thick under the appliance.
• Flue liner: existing chimneys usually need a stainless steel flexible liner installed inside the chimney breast. The liner must be sized to match the stove’s flue outlet.
• Air supply: stoves with an output above 5 kW may need a dedicated combustion air duct (especially in newer, airtight homes).
• Carbon monoxide alarm: a CO alarm must be fitted in the room containing the appliance.
• Competent Person Scheme: HETAS-registered installers can self-certify the installation without building control notification.

Gas appliances (boilers, gas fires) are governed by the Gas Safety Regulations as well as Part J. All gas work must be carried out by a Gas Safe registered engineer. A Gas Safe engineer can self-certify the installation, so building control notification is not required separately for like-for-like boiler replacements.

For oil-fired boilers, an OFTEC-registered engineer can self-certify. Key Part J requirements for oil boilers include correct flue termination height, oil tank siting (at least 1.8 m from the building and boundary if unprotected, or closer with a fire-rated wall), and an oil interceptor if the tank is near a watercourse.

Flues must be designed to ensure adequate draught and to prevent combustion gases re-entering the building. Minimum flue terminal heights above a flat roof or pitch are specified in Part J. For a solid fuel appliance:

• The flue terminal must be at least 600 mm above the highest point of the roof covering within 600 mm of the flue, or at least 1 m above the intersection with the roof slope for a pitched roof.

For gas appliances, the terminal must be at least 300 mm from a window, air vent or door that can be opened. Always check the appliance manufacturer’s instructions alongside Part J, as manufacturer requirements are often more demanding.

Since October 2022, carbon monoxide alarms are required in any room of an existing dwelling that has a fixed combustion appliance (except gas cookers). For new installations, the alarm must be fitted as part of the work. CO alarms must:

• Meet British Standard BS EN 50291-1
• Be positioned on the ceiling at least 300 mm from a wall, or on a wall between 1 m and 3 m from the appliance
• Be tested when the work is completed