Glossary · Boilers and boiler types
What is a back boiler?
A back boiler is a small, compact boiler fitted at the back of an open fireplace hearth, behind a gas fire. It heats water for central heating and hot water. Back boilers were popular until the Building Regulations changed in 2005, and many old standard efficiency units are still in use.
Key points
- A back boiler sits behind a gas fire in the fireplace opening and provides central heating and hot water.
- Baxi says they were popular from 1966 until the Building Regulations changed in 2005 to require condensing boilers.
- Baxi says an old standard efficiency back boiler is less than 70% efficient, so about 30p in every £1 of gas is lost up the chimney.
- Approved Document L sets a minimum of 45% for natural gas for a combined fire and back boiler unit in an existing home.
- Servicing, repair or replacement is gas work, so it must be done by a Gas Safe registered engineer.
How a back boiler works
A back boiler is a small, compact boiler fitted at the back of an open fireplace hearth. It sits behind a gas fire and provides hot water and central heating to the property.[1]Baxi, What is a back boiler? The fire is what you see in the room. The boiler is the hidden unit behind it.
Baxi says back boilers (also called back boiler units, or BBUs) became popular in 1966 with the launch of the original Baxi Bermuda. They stayed popular until the Building Regulations changed in 2005, when it became a requirement to install high efficiency condensing boilers.[1]Baxi, What is a back boiler? Vaillant gives the same date for the condensing rule: since 1 April 2005 all new gas boilers installed in the UK have had to be condensing.[4]Vaillant, What is a condensing boiler? Many back boilers have since been replaced with wall-hung condensing boilers, but Baxi says a large number are still installed in UK homes.[1]Baxi, What is a back boiler?
Efficiency
| Measure | Figure | Source |
|---|---|---|
| Old standard efficiency back boiler | Less than 70% | Baxi |
| New boiler Baxi compares it with | Over 90% | Baxi |
| Minimum for an inset live fuel-effect combined fire and back boiler, natural gas | 45% | Approved Document L |
| Minimum for the same, LPG | 46% | Approved Document L |
Baxi explains what 70% means. For every £1 of gas burned, only 70p of usable heat is generated and the other 30p is lost up the chimney.[1]Baxi, What is a back boiler? The Approved Document L figures are a separate planning rule for a combined fire and back boiler unit in an existing home. They are not a statement of how efficient old units were.[2]Ministry of Housing, Communities and Local Government, Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments If a gas fire is part of a combined fire and back boiler unit, the standards in BS 7977-2 should be met.[2]Ministry of Housing, Communities and Local Government, Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments
Safety and who can work on it
A back boiler is a gas appliance. HSE says anyone employed to work on gas appliances in domestic premises must be a Gas Safe registered engineer and competent in that area of gas work.[3]Health and Safety Executive, Domestic gas: frequently asked questions HSE also strongly advises that all gas appliances, flues and pipework are serviced at least annually.[3]Health and Safety Executive, Domestic gas: frequently asked questions Do not open the casing or work on the fire, the boiler or the flue yourself.
HSE says every gas appliance needs a flue that works and is kept clear, because without one dangerous levels of carbon monoxide can build up.[3]Health and Safety Executive, Domestic gas: frequently asked questions HSE also strongly recommends carbon monoxide alarms, which should meet British Standard EN 50291. They are not a replacement for servicing.[3]Health and Safety Executive, Domestic gas: frequently asked questions See carbon monoxide alarm, boiler flue and gas safety check.
Example
Lucy buys a 1970s terrace in Bradford. The sitting room has a gas fire in the old fireplace, and the surveyor notes a back boiler behind it. She books a Gas Safe registered engineer to inspect it and the flue. If she later replaces it with a modern boiler, Baxi’s comparison point is a new boiler over 90% efficient, which it says can cut fuel bills.[1]Baxi, What is a back boiler? Our boiler repair and boiler installation teams cover both routes.
What people get wrong
Many people assume the gas fire is the whole appliance. In a back boiler set-up the boiler is the compact unit behind the fire. The fire can look new while the boiler behind it is an old standard efficiency unit.
Baxi describes the boiler as sitting behind a gas fire.[1]Baxi, What is a back boiler? Another mistake is to wait for it to fail. An old back boiler is a standard efficiency unit, often under 70%, and it still needs an annual service.[1]Baxi, What is a back boiler?[3]Health and Safety Executive, Domestic gas: frequently asked questions Our guide to how long boilers last helps you judge age.
The bottom line
A back boiler is a compact gas boiler fitted behind a gas fire in a fireplace. It was popular until 2005 and many remain in use, but an old one wastes about 30p of every £1 of gas according to Baxi. Only a Gas Safe registered engineer should service, repair or replace one. An annual service and a carbon monoxide alarm are sensible safeguards.
Common questions
What is a back boiler?
A small boiler fitted at the back of an open fireplace hearth, behind a gas fire. Baxi says it provides hot water and central heating to the property. Many were later replaced with wall-hung condensing boilers.
Are back boilers still legal to use?
Baxi says there are still a large number of back boilers installed in UK homes. Whether yours is safe is a question for a Gas Safe registered engineer, who can inspect it and the flue.
How efficient is an old back boiler?
Baxi says an old standard efficiency back boiler is less than 70% efficient. A new boiler over 90% efficient wastes far less gas, so the difference shows up on the fuel bill.
Who can service or replace a back boiler?
A Gas Safe registered engineer. HSE says anyone employed to work on gas appliances in domestic premises must be Gas Safe registered and competent in that area of gas work.
Sources
We cite the original source for every figure: government, regulators, standards bodies and manufacturers' own documents. Each one shows the date we last checked it.
- Baxi. What is a back boiler?. Checked 11 November 2026. ↩
- Ministry of Housing, Communities and Local Government. Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments. Checked 11 November 2026. ↩
- Health and Safety Executive. Domestic gas: frequently asked questions. Checked 11 November 2026. ↩
- Vaillant. What is a condensing boiler?. Checked 11 November 2026. ↩