Short answer: Replacing a back boiler means taking out the old boiler behind the gas fire, capping its gas and water pipes, making the fireplace and chimney safe, and fitting a new condensing boiler elsewhere with its own flue. The cylinder and loft tanks can stay or go, depending on the new system.
Back boilers were a popular way to heat a house for decades. The boiler sat inside the fireplace, hidden behind a gas fire, and heated the radiators and a hot water cylinder upstairs. Plenty are still working in Sheffield homes, including in post war semis and in older terraces where they replaced a coal fire. But they are old, inefficient by modern standards and parts are getting scarce. Replacing one is a bigger job than a straight boiler swap, so it helps to know what is involved.
Why replace a back boiler
- Age. Most back boilers in use now are decades old. Parts are harder to find with every year.
- Efficiency. Back boilers are non condensing. New gas boilers in England must be condensing, which recover heat a back boiler throws away up the chimney.
- Reliability. When an old back boiler fails, it may not be repairable.
- Space. Many people also want the airing cupboard or loft tanks gone.
If your back boiler is still working safely, there is no legal duty to replace it. But if it fails, a like for like replacement is not normally possible, so planning ahead avoids a rushed winter decision.
What the job involves
1. The survey
The engineer looks at the fireplace, the chimney, the gas supply, the radiators and the hot water cylinder, and talks to you about where the new boiler can go. They check the mains flow at the kitchen tap, because that decides whether a combi will work.
2. Choosing the new system
| Option | What changes |
|---|---|
| Combi boiler | Back boiler, cylinder and loft tanks all removed. Hot water on demand. Suits one bathroom and good mains flow |
| System boiler with unvented cylinder | Back boiler and loft tanks removed, new cylinder fitted. Suits two bathrooms or more |
| Regular boiler | Back boiler removed, existing cylinder and tanks may stay. Suits keeping the layout |
Our post on combi or system boilers goes through the choice.
3. Removing the back boiler
The gas fire and back boiler are disconnected and the gas supply capped safely. The heating and hot water pipes running from the fireplace are drained and either capped or rerouted to the new boiler position. The boiler is lifted out, which often means taking the fire surround off.
4. Making the fireplace safe
With the back boiler gone, the fireplace opening and chimney need sorting. Options include:
- Fitting a new gas fire, which must be installed by a Gas Safe registered engineer.
- Making the fireplace decorative only, with the chimney capped and ventilated to stop damp.
- Blocking it up, with a vent left to keep the chimney dry.
5. Fitting the new boiler
The new condensing boiler goes where a flue can run safely to the outside, often on a kitchen wall, in a loft or in a cupboard on an outside wall. It needs a condensate pipe to a drain. In Sheffield, that pipe needs to be sized and routed to avoid freezing on cold nights, especially on high ground. Our post on frozen condensate pipes explains why.
6. Cleaning the system
An old back boiler system can hold decades of sludge. A new boiler on a dirty system will not last as long as it should, so the system is usually flushed and a magnetic filter fitted. Old radiators and valves that are seized or leaking may need replacing.
7. Paperwork
A Gas Safe registered installer notifies the work, and you receive a Building Regulations compliance certificate. Keep it, as you will need it when you sell.
How long does it take?
A back boiler replacement takes longer than a straight swap, because of the pipe changes, fireplace work and system clean. A few days is typical, depending on the system and where the new boiler goes. You will be without heating for part of that time, so the warmer months are a better time to plan it than mid winter.
Back boilers in Hillsborough and similar areas
In Hillsborough and other older parts of the city, a lot of terraces had their coal fires replaced with a gas fire and back boiler in the second half of the last century. Many have since gone to combis, but some are still in place. In a terrace, the new boiler often goes in the kitchen at the back or in the cellar. A cellar boiler can work well, but needs a safe flue route and protection from any flood risk in the low lying streets near the Don and the Loxley. Our Hillsborough area page explains the flood history there.
Cost
The price depends on the new system type, where the boiler goes, how much pipework changes, what happens to the fireplace and whether radiators need replacing. We give a written quote after the survey. Our new boiler page explains how our quote works.
If you have a back boiler and want to plan a replacement, send us a photo of the fire and tell us about your hot water cylinder and bathrooms.
Quick questions
What is a back boiler?
A boiler fitted inside the fireplace, behind a gas fire, that heats the radiators and the hot water cylinder. Many were fitted from the 1960s to the 1980s.
Can I keep my gas fire?
Sometimes. The old fire and back boiler are often one unit, so a new fire may be needed if you want one. A Gas Safe engineer can advise.
Where will the new boiler go?
Usually on an outside wall where a flue can go through, such as the kitchen, or in a loft or cupboard with a flue through the roof.
Does it need building regulations?
A new gas boiler is notifiable work. A Gas Safe registered installer self certifies it and you get a compliance certificate.
Last updated 23 October 2026. Written for homes in Sheffield. Rules can change, so check the linked sources for the latest position.