Back boiler replacement: your options and what the job involves

What a back boiler is and why they are being replaced

A back boiler sits inside the fireplace opening, behind a gas fire, and shares the chimney as its flue. They were a common way to heat homes for decades, and plenty are still working in London terraces and ex-local authority houses. Most are part of an open-vented system: a hot water cylinder in an airing cupboard, fed by a cold water tank in the loft, with a feed and expansion tank alongside it.

They are replaced for a few recurring reasons. The boiler is old and a major part has failed; spare parts have become hard to find; the running costs of a non-condensing boiler are high; or a buyer's survey, insurer or landlord gas safety check has flagged the appliance. If you are not sure of the age, our guide on how old is my boiler explains how to find out.

If a back boiler or its fire shows soot, yellow or lazy flames, or scorch marks, or your carbon monoxide alarm sounds, stop using it, open windows and call the National Gas Emergency Service on 0800 111 999.

Your replacement options

Because a new boiler almost always has to go in a different place, the real choice is what kind of heating and hot water system you want after the change. A survey of the hot water demand, the mains flow and the space available decides which of these suits the property.

Choosing between them

A combi frees up the airing cupboard and loft space and gives mains-pressure hot water, which is why it is the most common choice. It depends on the mains supply, though: if the flow at the kitchen tap is poor, a combi will struggle to run a shower well. Our guide to combi or system boiler covers how to judge peak demand.

Keeping the open-vented system with a regular boiler is the least disruptive route and suits homes with low water pressure. Old systems can contain sludge that a new boiler will not tolerate, so the system usually needs cleaning either way. Our guide to the types of boilers explains the three systems in more detail.

• Measure the mains flow and pressure before choosing a combi
• Count the bathrooms and showers likely to run at the same time
• Decide where the new boiler will go and check a flue route exists there
• Check whether the old cylinder is worth keeping or near the end of its life
• Ask whether the radiators and pipework need flushing or replacing

What back boiler removal involves

The removal is the part that makes this job different from a normal boiler swap. A Gas Safe registered engineer isolates and disconnects the gas supply, drains the heating system and removes the gas fire and the back boiler unit from the fireplace. The redundant gas pipe is capped safely and tested so there is no live dead leg left behind.

If there is no boiler behind your fire, it is a simpler job; see our guide to gas fire removal.

The old flow and return pipes that ran up from the fireplace are removed or rerouted to the new boiler position. If the system is being converted to a combi or a sealed system, the loft tanks and old cylinder come out too, and the open vent is removed.

The chimney and fireplace then need finishing. The opening is usually closed with a board or new fire surround, the chimney capped with ventilation so it does not collect damp, and the hearth made good. Plastering and decorating are normally done by a builder or decorator after the heating work.

Older fires and back boilers can contain asbestos, for example in seals, rope gaskets or insulation panels. If the appliance is old, ask for it to be checked before it is broken up. Asbestos should only be handled in line with HSE guidance.

How long does it take?

Converting from a back boiler to a combi or a system boiler normally takes two to three days, similar to other conversions, because of the removal, the new pipework and the change to the hot water system. Keeping the open-vented system with a regular boiler can be quicker. Cleaning the system, replacing radiators or upgrading the gas pipe adds time. Our guide on how long does a boiler replacement take breaks down the timings.

Expect to be without heating and hot water for most of the job. Plan it outside the coldest weeks if you can, and clear the fireplace, airing cupboard and loft hatch before the engineers arrive.

What affects the cost

Back boiler replacement is quoted after a survey, because no two fireplaces and systems are the same. These are the things that move the price up or down, so you can compare quotes properly:

• The type of new boiler and whether a new cylinder is needed
• How far the new boiler position is from the old pipework and the gas meter
• Whether the gas pipe needs upsizing for a modern boiler
• Removal of loft tanks, the old cylinder and redundant pipework
• System cleaning, and any radiators or valves that need replacing
• Chimney capping, closing the fireplace and making good
• Any asbestos survey or removal

Paperwork after the job

When the new boiler is fitted, the installer notifies the work under Building Regulations and you should receive a compliance certificate, plus the commissioning checklist and warranty registration. Our boiler installation certificate guide explains what to keep. If you rent out the property, the new boiler goes on the next landlord gas safety record.

Talk to us about replacing a back boiler

Loyal Plumbers replaces boilers across London and inside the M25, and all gas work is carried out by Gas Safe registered engineers. Call 020 4652 8585 or send an enquiry with a photo of the fire and back boiler, the number of bathrooms and where you would like the new boiler to go. We will survey the system, explain the options that suit the house and set out the full scope in writing, including the removal and making good.