Approved Document J explained: combustion appliances, flues and fuel storage

What the document is, and what it is not

The law is Part J of Schedule 1 to the Building Regulations 2010: seven short requirements. Approved Document J is the government’s guidance on meeting them. Following it is one way to show that work complies, and the document names other routes too, such as BS 5440 for gas flues and ventilation and BS 6798 for gas boilers.

This edition is for use in England. It was written mainly with homes in mind: space and water heating, cookers, their flues, and the oil and LPG that feed them. Wales, Scotland and Northern Ireland publish their own building regulations guidance.

The seven requirements, J1 to J7

• J1, air supply: an appliance gets enough air to burn its fuel, to keep it from overheating and to let its flue work.
• J2, discharge of products of combustion: the fumes reach the outside air.
• J3, warning of carbon monoxide: where a fixed combustion appliance is provided in a dwelling, there is a way to detect carbon monoxide and give warning.
• J4, protection of the building: appliances, fluepipes, fireplaces and chimneys are installed so they are unlikely to burn people or set the building on fire.
• J5, provision of information: when a hearth, fireplace, flue or chimney is provided or extended, a durable notice in the building says what it can safely serve.
• J6, protection of liquid fuel storage: oil tanks over 90 litres, LPG installations over 150 litres and the pipes from them are built and separated so that a fire in a neighbouring building or beyond the boundary is unlikely to ignite the fuel.
• J7, protection against pollution: an outdoor oil tank of 3,500 litres or less serving a private dwelling resists leaks and carries a notice on what to do if oil escapes. Buried tanks are outside J7.

Which section covers which appliance

Section 1 applies to every installation: air vents, extract fans, flues, concealed flues, notice plates and the checks at completion. Section 2 adds the rules for solid fuel (its heading covers appliances up to 50 kW rated output), Section 3 for gas up to 70 kW net (77.7 kW gross) rated input, and Section 4 for oil up to 45 kW rated output.

Section 5 covers heating oil tanks up to 3,500 litres and LPG storage up to 1.1 tonnes. The appendices hold a checklist for hearths, fireplaces, flues and chimneys, worked ventilation examples for a gas and an oil appliance, and methods for testing flues. Larger or unusual installations, such as incinerators, may need specialist guidance.

Gas work and building control

Gas work must also meet the Gas Safety (Installation and Use) Regulations, which require competence and, for a business, Gas Safe registration. Because of that, installing a gas appliance through a registered business needs no advance notice to building control. Under a competent person scheme, the occupier receives a certificate of compliance within 30 days of completion, and building control is told.

Approved Document J also restates the room rules from those Regulations. An appliance in a bathroom or shower room must be room-sealed. A gas fire, space heater or water heater over 14 kW gross (12.7 kW net) in a room used for sleeping must be room-sealed; at or below that input it must be room-sealed or have a device that shuts it down before combustion products build up. An instantaneous water heater in any room must be room-sealed or have such a device.

Air supply (J1): vent sizes for gas appliances

A permanently open air vent cannot be closed or adjusted, sits where it is unlikely to be blocked, and should not tempt anyone to seal it against draughts or noise. Size it by its equivalent area, the figure the maker declares, which can differ a lot from the free area measured at one face. Keep it out of a fireplace recess unless specialist advice allows it.

Diagram 32 gives the areas for gas appliances other than decorative fuel effect fires and flueless appliances, per kilowatt of net heat input.

A worked vent size, and two special cases

Divide square millimetres by 100 for square centimetres. Note 5 to Diagram 32 lets an older dwelling with an air permeability above 5.0 m³/h/m² ignore the first 7 kW (net). A 15 kW (net) open-flued boiler in a room of such a house needs (15 − 7) × 500 = 4,000 mm², or 40 cm². In a newer, tighter house it needs 15 × 500 = 7,500 mm², or 75 cm².

A decorative fuel effect fire in a fireplace recess with a throat needs a vent of at least 10,000 mm² (100 cm²). A flueless instantaneous water heater should not go in a room or space smaller than 5 m³.

Extract fans and open-flued appliances

An extract fan lowers the pressure in a house and can draw fumes back out of an open-flued appliance, even one in another room. The appliance must work safely with the fans on or off. Show it with a spillage test at the greatest depressurisation: outside doors and windows shut, fans on their highest setting, and a test with the door of the appliance’s room closed and every fan in that room running.

A kitchen with an open-flued gas appliance should have extraction of no more than 20 litres a second (72 m³/h). For oil, the limit is 40 litres a second with a pressure jet burner and 20 with a vaporising burner. With solid fuel, avoid extract ventilation in the same room.

Flues and the checks at completion (J2)

At completion, show that each flue is free of obstructions, gas-tight and built from components that suit the appliance. A spillage test with the appliance running is part of commissioning. The person doing the work is responsible for compliance, and a report of the materials used and the flue tests is the evidence; Appendix A gives a checklist.

A flue hidden in a void needs inspection hatches of at least 300 mm × 300 mm, placed so someone can see that it is continuous, its joints are made and sealed, it is supported, and it falls back to the boiler where condensate must drain. A flue should not pass through another dwelling.

Where a terminal comes out through a wall or roof, the separations are in the table to Diagram 34 for gas and Diagram 41 for oil. Our flue distances guide works through the gas table.

Carbon monoxide alarms (J3)

When a new or replacement fixed appliance is installed in a dwelling, fit a carbon monoxide alarm in the room where the appliance is. This has applied to solid fuel since the 2010 edition, and since 1 October 2022 to gas appliances, except those used only for cooking, and to oil appliances. The alarm is a backstop, not a substitute for correct installation and regular servicing.

• It meets BS EN 50291-1:2018 and runs on a battery designed to last its working life, with a warning when that life is ending. A fixed-wired mains Type A alarm, not a plug-in, with a sensor failure warning is the alternative.
• On a ceiling, it sits at least 300 mm from any wall. On a wall, it goes as high as possible, above any door or window, but not within 150 mm of the ceiling.
• It sits between 1 m and 3 m horizontally from the appliance.

Hearths and shielding (J4)

A solid fuel appliance stands on one of two kinds of hearth. Where it is not in a recess and has been tested to keep the hearth’s surface at or below 100 °C, non-combustible board or tiles at least 12 mm thick will do. Otherwise it needs a constructional hearth of solid concrete or masonry at least 125 mm thick, counting any non-combustible floor or finish on it. Combustible material may sit beneath a constructional hearth only with a 50 mm air space, or at least 250 mm below the hearth’s top.

A constructional hearth for a free-standing appliance is at least 840 mm square. In a fireplace recess, it projects at least 500 mm from the jambs and runs at least 150 mm past each side of the opening.

A gas appliance needs a hearth unless every flame or glowing part sits at least 225 mm above the floor, or its instructions say none is needed. The hearth’s top layer is non-combustible and at least 12 mm thick, and its edge is marked, for example by a change in level, so carpet stays back. For a decorative fuel effect or inset live fuel effect fire, the clear surface reaches 300 mm in front of the fire’s glowing part; for other gas appliances, 225 mm in front and 150 mm to each side, or up to a suitably heat-resistant wall.

To keep a gas appliance from heating combustible material nearby, follow the maker’s instructions for a CE-marked appliance. The alternative is a non-combustible shield with a fire-resistant surface, or an air space of at least 75 mm.

Notice plates (J5)

When a hearth, fireplace (including a flue box), flue or chimney is provided or extended, including a flue added during refurbishment, leave a notice plate in the building. Diagram 16 shows one. It records where the hearth, fireplace or flue box is, or where the flue begins; the flue’s category and the generic types of appliance it can safely serve; the type and size of the flue, or of its liner, and the manufacturer’s name; and the installation date.

The plate is robust, indelibly marked and fixed somewhere unobtrusive but obvious: next to the consumer unit, beside the chimney or hearth it describes, or next to the stopcock.

Oil tanks: fire separation and leaks (J6 and J7)

A heating oil tank needs no further fire protection when it stands at least 1,800 mm from any part of the building and at least 760 mm from a boundary (Table 10). Closer to the building, either make the walls within 1,800 mm imperforate, apart from small openings such as air bricks, with 30 minutes’ fire resistance, and the eaves within that distance fire-resisting for 300 mm past each side of the tank; or build a fire wall between them that extends at least 300 mm higher and wider than the tank. Closer than 760 mm to a boundary, put a fire wall, or a boundary wall with 30 minutes’ fire resistance from either side, between the tank and the boundary.

The tank stands on a hard base, such as concrete or paving slabs at least 42 mm thick, reaching at least 300 mm beyond it. The supply pipe resists fire and has a proprietary fire valve. A bund holds at least 110% of the largest tank in it, and every installation carries a label on what to do after a spill, with the Environment Agency’s emergency hotline number.

A tank of 3,500 litres or less serving a home needs secondary containment, an integrally bunded tank or a masonry or concrete bund, wherever the document finds a significant risk of pollution:

• a total capacity over 2,500 litres;
• within 10 m of inland freshwater or coastal water;
• where a spill could reach an open drain or a loose-fitting manhole cover;
• within 50 m of a well, borehole or spring;
• where spilt oil could run across hard ground to those waters;
• where the vent pipe outlet cannot be seen from the filling point;
• within Zone 1, the inner protection zone, of a groundwater source protection zone.

LPG tanks and cylinders

An LPG tank goes outdoors, never in a pit, separated from buildings, the boundary and fixed sources of ignition. Table 11 gives 2.5 m for a tank up to 0.25 tonne, or 0.3 m with a fire wall between, and 3 m for a tank up to 1.1 tonnes, or 1.5 m with a fire wall. A fire wall stands at least as high as the tank’s pressure relief valve, resists fire for 30 minutes (60 if it is part of the building), and normally goes on one side only, so the tank stays ventilated.

Cylinders stand upright, strapped or chained to an outside wall on a firm base, such as 50 mm of concrete. Their valves sit at least 1 m horizontally and 300 mm vertically from openings and heat sources such as flue terminals and tumble-dryer vents, and at least 2 m from untrapped drains, unsealed gullies and cellar hatches unless a wall at least 250 mm high stands between.

Where Part J runs through the revision notes

On PlumbRevise, Part J’s figures run through the CCN1 notes on ventilation, flues and spillage, the HTR1 notes on gas fire hearths, vents and flue sizes, the CENWAT notes on boiler flues and the Level 3 domestic fuels notes on oil and LPG storage. All of them are free to read.