Back Boiler Replacement: What Is Involved?

If your heating comes from a boiler hidden behind a gas fire in the living room, you have a back boiler. They are still found in older homes across the TW area, usually where gas central heating was added decades ago. They were built to last, but most are now well past their best, and replacing one is a bigger job than a straight boiler swap.

This guide explains what a back boiler is, why people replace them and what the work involves, so you know what to expect before anyone starts lifting floorboards.

What is a back boiler?

A back boiler is a gas boiler that sits inside the fireplace opening, behind a gas fire. The fire and boiler usually share the chimney as their flue. Most back boiler systems are “open vented”: the boiler heats a hot water cylinder in an airing cupboard, and a cold water storage tank and a small feed and expansion tank sit in the loft.

You may have one if:

• there is a gas fire in a chimney breast and no other boiler anywhere
• the boiler controls are behind a panel on or under the fire
• you have a hot water cylinder and tanks in the loft

Not sure what system you have? Our guide to combi, system and conventional boilers will help you work it out.

Why are back boilers replaced?

It is not illegal to keep using a back boiler that is working safely and has been serviced. People replace them because:

• They are inefficient. Back boilers are non-condensing. A new gas boiler must be a condensing model with an efficiency of at least 92%.
• Parts are getting harder to find. When a key part fails, a repair may not be possible or worthwhile.
• They rely on the chimney. An open flue depends on a clear, sound chimney. Problems with the flue can let combustion fumes, including carbon monoxide, into the room.
• They take up space. Removing the fire, boiler and often the loft tanks and cylinder can free up a fireplace, a cupboard and loft space.

If a Gas Safe registered engineer finds the appliance or flue unsafe, they may classify it as “At Risk” or “Immediately Dangerous” and, with your permission, turn it off. That is often the point people start planning a replacement.

What is involved in replacing a back boiler?

Every home is different, but a back boiler replacement usually follows these steps.

1. Survey the system and choose the new boiler

The engineer looks at your hot water use, the gas supply, the radiators and where a new boiler and its flue can go. Common locations are the kitchen, a utility room, an airing cupboard or the loft.

2. Check for asbestos before anything is disturbed

In older installations, materials around the fire, boiler or flue can contain asbestos. If anything is suspect, it should be sampled before removal starts (see below).

3. Isolate and remove the old fire and back boiler

The gas is isolated and the old supply capped safely. The fire, back boiler and any old flue liner are removed, and the heating pipework is drained.

4. Fit the new boiler and flue

A modern condensing boiler usually has a room-sealed flue that goes straight out through an external wall or the roof. It also needs a condensate pipe to a suitable drain.

5. Upgrade the gas pipe if needed

New boilers, combis especially, can need a larger gas supply pipe than the old back boiler did, so the run from the meter may need upgrading.

6. Decide what happens to the tanks and cylinder

You can keep a cylinder with a regular or system boiler, or switch to a combi and remove the cylinder and loft tanks. The right choice depends on how much hot water you use and your mains pressure.

7. Clean the system and add inhibitor

Old open vented systems often hold years of sludge. The system is flushed and treated with inhibitor so the new boiler is not damaged by it.

8. Commission the boiler and register it

The engineer tests the installation, sets up the controls, notifies the job under Building Regulations through Gas Safe Register and registers the manufacturer’s warranty.

9. Make good the fireplace

The chimney is capped with ventilation and the fireplace is closed off with an air vent, or opened up for decorative use. Many people have a builder or decorator finish this part.

What about asbestos?

The Health and Safety Executive says asbestos can be found in any home built or refurbished before 2000. With older boilers and fires, it can turn up in asbestos insulating board around the boiler, in rope seals and gaskets, and in asbestos cement flue pipes.

Asbestos that is left alone and in good condition is low risk. The danger comes when it is broken, drilled or cut during removal. So:

• do not take apart an old fire or back boiler yourself
• ask your engineer to check for suspect materials before the job starts
• if anything is suspect, have it sampled and tested, and removed by a suitably trained contractor, licensed where the material requires it

Who can replace a back boiler?

By law, any work on a gas appliance must be done by a Gas Safe registered engineer. Ask to see their Gas Safe ID card and check that it covers the work they are doing. Removing the old system and fitting the new boiler is gas work. Making good the fireplace and decorating can be done by a builder.

How long does it take?

A back boiler replacement takes longer than a like-for-like boiler swap, because of the new boiler position, new flue, extra pipework and system cleaning. Your engineer will give you a timescale with the quote, so you can plan around having no heating or hot water for part of the job.

Ready to replace your back boiler?

If you have an old back boiler that keeps breaking down, or one an engineer has turned off, we’ll put you in touch with a local Gas Safe registered engineer who will quote for the boiler you choose plus the installation. For a no-obligation quote, call 07360 268948 or WhatsApp photos of your current boiler, the fire and any tanks or cylinder, or visit our boiler replacement page.