Back boiler: what it is and how to get it replaced
A back boiler is a boiler fitted behind a fire in the fireplace, usually heating water for a hot water cylinder and often the radiators too. Most are old, non-condensing units that waste far more gas than a modern boiler, and ECO4 can pay to replace one for households that qualify.
ECO4 is funded by energy suppliers under rules set by government and runs until 31 December 2026. Start the check on this page to see if your household qualifies.
Last reviewed: 6 October 2026.
Check if ECO4 can replace your back boiler
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What is a back boiler?
A back boiler is a boiler built into the fireplace, behind a gas fire or a solid fuel fire, inside the chimney breast. With a gas fire, the fire and the boiler often come as one combined unit. The boiler heats water that flows to a hot water cylinder, normally in an airing cupboard, and in many homes it runs the radiators as well.
Baxi, whose Bermuda model made back boilers popular from 1966, says they stayed common until 2005, when Building Regulations changed. Baxi no longer makes them, but plenty are still working in UK homes.
How a back boiler works
The fire heats a water jacket that sits behind it. Hot water from the jacket moves through pipes to the cylinder and, where they are connected, the radiators. The water often reaches the cylinder by gravity, while a pump pushes it round the radiators. The fumes go up the chimney, which works as the flue.
Back boiler stoves and solid fuel fires
Some wood burners, multi-fuel stoves and coal fires have a back boiler too. They work the same way, with the fire heating water as well as the room. Mention it when you apply, because the funding route depends on the heating you actually have.
How to tell if you have a back boiler
The boiler itself is hidden behind the fire, so you usually spot it from the clues around it.
- There is a gas fire or a solid fuel fire in the chimney breast, and you can't find a boiler anywhere else. Check the kitchen, the airing cupboard and the loft before you decide.
- You have a hot water cylinder, usually in an airing cupboard, and often a cold water tank in the loft.
- Pipes run from the fireplace to the cylinder, or under the floor towards the radiators.
Still not sure? A Gas Safe registered engineer can confirm it at a service or survey. If you apply through us, the surveyor records it anyway.
Common back boiler problems
Age
Since back boilers were common until 2005, most still in use are likely to be more than 20 years old. In many homes the back boiler is the whole heating system, so one fault can leave you without heating and hot water at once.
Efficiency
Baxi says older standard-efficiency back boilers run at under 70% efficiency, so about 30p of every pound spent on gas goes up the chimney. A new gas boiler fitted in an existing home in England should reach at least 92% under Approved Document L of the Building Regulations. That gap is the main reason to replace a back boiler that still works. What you save depends on your home, its insulation and how you heat it.
Safety checks
HSE says anyone employed to work on gas appliances in a home must be a Gas Safe registered engineer, and that gas appliances, flues and pipework should be serviced at least once a year. With a back boiler the chimney is the flue, so it needs checking too. HSE also recommends a carbon monoxide alarm, but says an alarm is not a substitute for servicing. If you rent, your landlord must arrange a gas safety check every year.
Parts and repairs
Baxi no longer makes back boilers, so a repair depends on parts for a discontinued appliance. Ask the engineer whether a repair is worth paying for before you agree to one. Under Ofgem's rules a broken inefficient heating system may still be upgraded through ECO4 if your household qualifies.
Old back boiler behind a coal or wood fire
If a back boiler behind a solid fuel fire is no longer connected to anything, don't light the fire until it has been dealt with. HSE warns that a sealed redundant unit can build up pressure when a fire is lit in front of it, and the casing can explode. HSE says it should be removed completely or left vented, and points people to HETAS or the Solid Fuel Association for advice.
Are back boilers illegal?
No. You can keep using a working back boiler. Approved Document L still sets a minimum efficiency for a combined fire and back boiler unit in an existing home in England. What changed is the standard for new gas boilers, which is why a replacement is normally a modern condensing boiler. Scotland and Wales have their own building rules.
Back boiler replacement options
The main choice is whether to keep stored hot water. That decides whether your cylinder and loft tanks stay. The descriptions below follow Baxi's guide to boiler types.
| Option | Hot water | Cylinder and tanks | Often suits |
|---|---|---|---|
| Combi boiler | Heats water from the mains as you turn on a tap | No cylinder and no loft tanks | Smaller households that use less hot water |
| System boiler | Heats a sealed hot water cylinder | Cylinder stays, loft cold water tank goes | Homes that want stored hot water without the loft tank |
| Regular or heat-only boiler | Heats a cylinder fed from a cold water tank in the loft | Cylinder and loft tanks stay | Low mains water pressure, more than one bathroom, or keeping the existing system |
| Air source heat pump | Heats a hot water cylinder using electricity | Needs a cylinder | Homes moving off gas, ideally well insulated |
Where the new boiler goes
A modern boiler doesn't go back in the fireplace. It hangs on a wall, often in the kitchen, a utility room, the airing cupboard or the loft, and has its own flue through an outside wall or the roof instead of using the chimney. A condensing boiler also needs a condensate pipe to a drain. The installer chooses the spot at the survey, based on the flue route and the pipework.
Removing the fire and the cylinder
The fire and the back boiler come out together. With a combi, the cylinder and loft tanks can come out too, which frees up the airing cupboard. With a system boiler the cylinder stays and the loft tank goes. With a regular boiler both normally stay. Either way, the heating pipes that ran to the fireplace are rerouted to the new boiler, and some old pipework may be replaced.
Making good the fireplace
Once the back boiler is out, the opening has to be finished. The usual choices are to brick it up and plaster over it, keep it as a decorative fireplace, or fit a new fire. If you want to light a fire there again, have the chimney and fireplace checked by a competent person first. Ask before work starts exactly what making good is included, such as plastering and decorating.
Our condensing boiler installation guide covers modern boilers in more detail.
What back boiler replacement involves
The exact job depends on your home, but most back boiler replacements follow the same order.
- A survey of the heating, pipework and flue options. For ECO4, the survey also covers the home's insulation and its EPC rating.
- Any insulation that ECO4 requires goes in first. The section on grants below explains when this applies.
- The gas is made safe, and the old fire and back boiler are removed.
- The new boiler, its flue, the rerouted pipework and the heating controls are fitted.
- The fireplace and chimney are made safe and finished.
- The system is tested and you are shown how to use the controls. Under ECO4, Ofgem requires the installer to balance the heating system after fitting a boiler to radiators, so each room gets the right amount of heat.
How long it takes
We don't quote a fixed time. It is a bigger job than swapping one wall-hung boiler for another, because the unit is built into the chimney breast and the pipework usually needs rerouting. Your installer confirms the timescale after the survey. All gas work must be done by a Gas Safe registered engineer.
Back boiler removal grants
The main grant for replacing a back boiler is ECO4. LA Flex is a way into ECO4 for people who don't get a qualifying benefit. Our grants overview lists the other heating and insulation help we cover.
The ECO4 route
ECO4 is funded by large energy suppliers under rules set by government, and Ofgem administers it. It covers England, Scotland and Wales, not Northern Ireland, and runs until 31 December 2026. Under Ofgem's ECO4 delivery guidance, every boiler fitted must be a condensing boiler, so a back boiler is replaced with a modern condensing boiler or, in some homes, a heat pump. Our free boiler grant guide covers ECO4 boiler replacements in general.
You will generally need:
- Someone in the home who gets a qualifying means-tested benefit: Universal Credit, Pension Credit, Housing Benefit, Income Support, income-based Jobseeker's Allowance, income-related Employment and Support Allowance, or Child Benefit within the income limits.
- Or a declaration from your council through LA Flex, explained below.
- A home rated EPC band D, E, F or G.
- To own your home. Private rented homes have tighter rules, covered below.
The full ECO4 eligibility checklist goes through each rule.
Which ECO4 measure applies
It depends on what the back boiler heats. Ofgem treats a home heated by a non-condensing boiler as having inefficient heating, which can be upgraded to an efficient boiler. That covers a back boiler that runs radiators around the home. Ofgem also says a gas back boiler with one radiator, or none, does not count as central heating, so that home can qualify for first time central heating instead. The survey confirms which applies.
Insulation may need to come first
ECO4 sets insulation rules for heating work. In a band E, F or G home, the outside cavity walls and the loft must be insulated, either already or as part of the same job, unless an exemption applies. In a band D home, at least one insulation measure usually goes in as part of the job unless the home is already fully insulated. See our loft insulation and cavity wall insulation pages.
Homeowners and tenants
For boilers, ECO4 is mainly for homeowners. Ofgem's rules say a private rented home generally can't have a boiler replaced or upgraded. The exceptions include first time central heating and renewable heating such as a heat pump, in rented homes rated E to G, and the landlord must give permission. Council and housing association tenants should ask their landlord.
LA Flex
If nobody in the home gets a qualifying benefit, your council may still declare you eligible through LA Flex. The usual grounds are a household income under £31,000, or someone in the home with a health condition made worse by living in a cold home. Councils add their own criteria, and Ofgem says taking part in LA Flex is optional for councils, so check what yours offers.
If you don't qualify
- Boiler Upgrade Scheme. In England and Wales it gives £7,500 towards an air source or ground source heat pump. It does not pay for gas boilers. See our Boiler Upgrade Scheme guide.
- Warm Homes: Local Grant. In England, for privately owned homes rated EPC band D to G, where household income is usually £36,000 or less. It pays for things like insulation, heat pumps and solar panels, not gas boilers. You apply on GOV.UK and your council arranges the work.
- Paying privately. Get written quotes from more than one installer, and check each engineer on the Gas Safe Register. The back of an engineer's Gas Safe ID card lists the types of gas work they are qualified to do.
- Scotland has Warmer Homes Scotland and Wales has the Warm Homes Nest scheme, both run alongside ECO4, so check those if you live there. ECO4 does not cover Northern Ireland.
Who runs Apply Free Boiler
Apply Free Boiler is run by Reliant Energy Solutions Ltd, an ECO4 installer. We are not the government or an energy supplier, and the funding comes from energy suppliers' ECO4 obligations. We can't confirm what you will get until we have checked your eligibility and surveyed your home, and we will tell you if ECO4 isn't the right route for you.
Back boiler questions
What is a back boiler?
A back boiler is a boiler fitted behind a fire in the fireplace. It heats water for a hot water cylinder and, in many homes, the radiators too. Most were fitted with a gas fire in front, though some sit behind coal, wood or multi-fuel fires.
Are back boilers illegal in the UK?
No. You can keep using a working back boiler. England's Building Regulations guidance, Approved Document L, still sets a minimum efficiency for a combined fire and back boiler unit in an existing home. A new gas boiler in an existing home should reach 92% efficiency, which is why a replacement is normally a modern condensing boiler.
Is there a grant to remove a back boiler?
Yes, for households that qualify. ECO4, funded by energy suppliers under rules set by government, can pay to remove a back boiler and fit a condensing boiler or a heat pump. You usually need a qualifying means-tested benefit or a council LA Flex declaration, and a home rated EPC band D to G. ECO4 runs until 31 December 2026.
Can ECO4 fit another back boiler?
Ofgem's ECO4 rules say every boiler fitted under the scheme must be a condensing boiler. In practice the replacement is a modern wall-hung condensing boiler or a heat pump.
Can I keep my fireplace after the back boiler is removed?
Usually, yes. The opening can be bricked up and plastered, kept as a decorative fireplace, or fitted with a new fire. If you want to light a fire there again, have the chimney and fireplace checked by a competent person first.
Will I lose my hot water cylinder?
It depends on the new boiler. A combi heats water on demand, so the cylinder and loft tanks can come out. A system boiler keeps the cylinder but not the loft tank. A regular boiler and most heat pumps keep a cylinder.
Can tenants get a back boiler replaced under ECO4?
Private tenants usually can't get a boiler replaced under ECO4. Ofgem's rules make exceptions for first time central heating and renewable heating, such as a heat pump, in rented homes rated EPC band E to G, and the landlord must give permission. A gas back boiler with one radiator or none can count as first time central heating.
How much does it cost to replace a back boiler?
There is no single honest figure. The price depends on the new boiler, how much pipework needs rerouting, the flue route and the work on the fireplace. If you pay privately, get written quotes from more than one installer on the Gas Safe Register. If you qualify for ECO4, the funding can cover some or all of the cost.
How long does a back boiler last?
There is no reliable single figure. It depends on the model, the fuel and how well it has been serviced. Baxi says back boilers were common until 2005, so most still in use are likely to be more than 20 years old. A Gas Safe registered engineer can tell you whether yours is worth repairing.
Replace your back boiler before ECO4 closes
ECO4 closes on 31 December 2026. Checking costs nothing and you are not committed to anything.