What is a back boiler?

The short answer

A back boiler is a gas boiler that sits inside a chimney breast, hidden behind a gas fire in the living room. The fire and the boiler share the fireplace opening and use the chimney as their flue. The boiler heats your radiators and, in most cases, a hot water cylinder in an airing cupboard upstairs, fed by cold water tanks in the loft.

They were built to be simple and long-lived, and many are still running. But they are no longer made, they burn more gas than a modern condensing boiler to do the same job, and when one fails the fix is often a full back boiler replacement rather than a repair.

How to tell if you have one

You can usually tell without taking anything apart. Look for these signs together rather than any one on its own:

• A gas fire in a fireplace, often in the front room, with a fire surround or hearth around it.
• No boiler on a wall anywhere else in the house: not in the kitchen, a cupboard or the loft.
• Heating controls or a programmer near the fire, or boiler controls on the fire itself.
• A hot water cylinder in an airing cupboard and water tanks in the loft.
• A pilot light you can see or have had to relight, on the fire or the boiler.

If you’re still not sure, take a photo of the fireplace and the airing cupboard and send them with your quote request. Don’t take the fire apart or remove panels to look behind it. The boiler’s casing and gas connections are for a Gas Safe engineer only.

Why so many older homes have one

Across Croydon, back boilers are common in 1930s semis and post-war houses, in areas such as Shirley, Addiscombe, Selsdon, Coulsdon and Purley. These homes were built with fireplaces and chimneys in the main rooms. When central heating was added, a boiler that slotted into the existing fireplace was an easy option: the chimney already gave it a flue, and nothing had to be fixed to a kitchen wall.

The result was a compact setup that kept the living room fire and added radiators and stored hot water. It made a lot of sense at the time. Many of those boilers have now been running for decades.

Why replace a back boiler

A working back boiler doesn’t have to come out tomorrow. But there are good reasons most owners plan the change rather than wait for it to fail:

• Parts. Back boilers are no longer manufactured, so spares for many models are scarce or discontinued. A failed part can mean the boiler can’t be fixed.
• Efficiency. Back boilers are not condensing boilers. A modern condensing boiler recovers heat from the flue gases that an old back boiler sends up the chimney.
• Timing. A breakdown in winter means a conversion done in a hurry. Planned, you can choose where the new boiler goes and what type it is.
• Space. Removing the back boiler frees the fireplace, and if you move to a combi, the cylinder and loft tanks can go too.
• Controls. A new boiler comes with time and temperature controls that meet the Boiler Plus rules, so you can heat the house when you need it.

What replacing one involves

There’s no like-for-like swap, because new back boilers aren’t made. Replacing one usually means fitting a new boiler somewhere else, most often in the kitchen, a utility room or an upstairs cupboard, with its own flue through an outside wall or the roof. Flue position is regulated near windows, doors, boundaries and eaves, so the survey checks where it can go.

1. 1Survey: we look at the fireplace, the cylinder and tanks, the gas supply and where the new boiler and flue could go.
2. 2Choose the boiler: a combi if you want to lose the cylinder and tanks, or a system or regular boiler if your household needs stored hot water.
3. 3Removal: the back boiler and the gas fire in front of it come out, and the gas is capped safely.
4. 4New pipework: the heating and hot water pipes are re-routed to the new boiler’s position, and the gas pipe is checked and upgraded if it’s too small.
5. 5Clean and commission: the system is cleaned and treated with inhibitor to BS 7593, the boiler is commissioned and the Benchmark checklist completed.
6. 6Paperwork: the work is notified through the Gas Safe Register and you receive a Building Regulations compliance certificate.

It’s a bigger job than a straight boiler swap, and the fireplace opening is left to be made good, boarded or fitted with a new fire. See boiler conversions for what the conversion to a combi looks like in practice.

Asbestos: check before anything comes out

It’s worth knowing about before the work is booked rather than finding out in the middle of it. Mention the age of the fire and surround on your quote, if you know it, and we’ll talk through what checks are needed.

Which boiler should replace it?

For many homes with one bathroom, a combi is the natural choice. It heats water on demand from the mains, so the cylinder and loft tanks can go. If you have 2 or more bathrooms in use at once, or the mains flow is low, a system boiler with a cylinder may suit you better. The survey confirms which works for your home and what size it should be.

You can start with a fixed quote for back boiler replacement. Tell us where the fire is, add photos of the fireplace and the airing cupboard, and say where you’d like the new boiler if you have a preference.