Why back boilers are being replaced
A back boiler is a boiler that sits in the fireplace, behind a gas fire, and shares its flue with the fire. They were fitted in large numbers in UK homes from the 1960s to the 1980s. Most still in use are well beyond the 10 to 15 years a modern boiler is designed for, are far less efficient than a condensing boiler, and are hard to get parts for.
If you have one and it still works, you do not have to rip it out tomorrow. But if it is breaking down, or the gas fire in front of it has been condemned, replacement is the sensible move, and it is a bigger job than a normal boiler swap.
Why it costs more than a standard swap
A modern boiler cannot go back into the fireplace, so the job almost always means moving the boiler somewhere else and changing the hot water setup too.
| Extra work | What it involves |
|---|---|
| Removing the boiler and fire | Disconnecting gas, water and flue, then lifting out a heavy cast unit |
| New boiler position | Usually a kitchen wall or airing cupboard, with a new flue through an outside wall |
| Rerouting pipework | Gas, heating and water pipes run from the fireplace to the new position |
| Hot water | Often a combi conversion, removing the cylinder and loft tanks |
| Capping the chimney | Fitting a cap and ventilation so the stack stays dry |
| Making good | Closing the fireplace opening or preparing it for a new fire |
Because most back boiler jobs are also conversions, the typical conversion price of £3,000 to £4,200 fitted (checked October 2026) is a starting point. Fireplace work, a longer gas run or a new flue route add to that. Ask for each item to be listed so you can see where the money goes.
Choosing the replacement boiler
Most back boilers run a conventional system, with a cylinder in the airing cupboard and tanks in the loft. You have three options:
- Combi. The most common choice for one bathroom homes with good mains pressure. Frees up the cylinder and loft space.
- System boiler. Keeps stored hot water at mains pressure, good for two bathrooms. See system boiler replacement.
- Regular boiler. Keeps the tanks and cylinder, least pipework change. See conventional boiler replacement.
Back boilers are often far bigger in output than the house needs, so do not let anyone size the new boiler by copying the old one. Our guide what size boiler do I need? explains why.
Asbestos and safety
Some older back boiler units, fire surrounds and flue seals contain asbestos. Your installer should look before starting work and recommend a test if anything looks suspect. Do not drill, break or sand any part of an old fire or boiler yourself.
If you ever smell gas near the fire or boiler, turn off the gas at the meter if you can, open windows and call the National Gas Emergency Service on 0800 111 999. Do not use light switches or naked flames.
What happens on the day
The first day is usually removal and first fix: taking out the fire and boiler, running new pipes and making the flue opening. The second day is fitting and commissioning: mounting the new boiler, connecting controls, filling and testing the system, and capping the chimney. If tanks and a cylinder are coming out, that is fitted around the same two days.
The installer finishes by commissioning the boiler, testing the gas pipework for tightness and registering the work for Building Regulations through Gas Safe.
Before you sign
Ask each installer where the new boiler will go and where the flue will exit, who makes good the fireplace and to what standard, and whether the asbestos check is included. Check their Gas Safe ID and confirm it on the Gas Safe Register. City Boiler connects you with independent installers who are Gas Safe registered, and we check before they quote.
For a broader view of what all this costs, read our boiler replacement cost guide.
Common questions
How much does back boiler removal and replacement cost?
Expect it to cost more than a straight swap. Most back boiler jobs are also conversions, which typically cost £3,000 to £4,200 fitted (checked October 2026), and removing the fire, capping the flue and making good the fireplace come on top. Ask for these items to be priced separately.
Can I keep my gas fire when the back boiler is removed?
Usually the old fire comes out with the boiler, because they share a flue and a casing. You can fit a new gas fire or an electric fire afterwards, but that is a separate job and needs its own Gas Safe check for a gas fire.
Do back boilers contain asbestos?
Some older back boiler units and fire surrounds can. A careful installer looks before starting and will recommend a test if anything looks suspect. Never break into old panels or seals yourself.
How long does it take to replace a back boiler?
Usually two days, sometimes three. The extra time goes on removing the unit from the fireplace, rerouting pipes to the new boiler position and capping the chimney.