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Back boiler guide - replacement, repair and servicing


Jo Alsop

Heating Hero

The Heating Hub

It is no longer possible to replace back boilers. We guide you on repair, maintenance and replacement.


Can I repair my back boiler? How much will a new back boiler cost?

Back boilers are a form of ‘heat only’ boiler found mostly in 1960s/1970s homes. They are located in the fireplace and use the brick chimney to flue their gases out of the property. Back boilers work on open vented systems often with a gravity fed hot water system. Most are accompanied by a gas fire. Under new condensing efficiency standards, back boilers no longer meet a minimum efficiency. At the moment it is not possible to install a new back boiler, although it is still possible to repair them if you can find someone specialist enough to do it. We help you with options for replacement and replacement costs, repair and servicing requirements.

When you can repair your back boiler

The original back boilers had a gas fire on the front, the Baxi Bermuda was by far the most popular model and many are still operating today. For a few years Baxi manufactured a replacement back boiler with an electric fire on the front. The newer models will be more readily repairable than the older models as most parts are still available.

For the older models, many of the main parts – gas valves, thermostat – are widely available and many ‘universal’ parts - such as the thermocouple - can be bought from the majority of merchants. However the availability of the heat exchanger, which forms the main body of the back boiler, will be quite patchy.

Before accepting any diagnosis for a new boiler, ring the boiler manufacturer's technical help line to see if the parts are still available. You can also check for reconditioned parts on sites like ebay providing you have the part number and in some cases the 'GC' number. Again ask the boiler manufacturer for the part number.

A good way to find out if it’s worth persevering with your old back boiler is to see if you can get boiler cover for it. If you can then it might we worth going with an insurance policy for repairs and maintenance cover. A number of mainstream boiler insurance providers still cover old and newer back boilers and many will fit reconditioned parts that you have sourced and supplied. Read our Boiler Cover Guide to see who offers back boiler insurance and prices.

Options for replacing your back boiler

If it’s time to replace your back boiler then the two most popular options are to 1) retain a separate hot water tank and fit a wall hung heat only boiler elsewhere in the property or 2) take it all away and fit a combi boiler. We have provided some guidance and cost scenarios for each below.

1) Fit a new wall hung heat only boiler

A back boiler is a form of heat only boiler, so the most cost effective installation is a new wall hung heat only boiler.

A new heat only boiler will work with your existing hot water tank and the system can be left as ‘open vented’, i.e. you keep your tanks in the loft.

However if the hot water is ‘gravity fed’ it will almost definitely have to be changed to a pumped hot water system. Of course you can opt to ‘seal’ the system and put it under pressure. Just be sure you have good incoming water pressure.

If your pipes and radiators are very old and you do not plan to replace them then perhaps leave as an open vented system. We have provided some cost scenarios below. For an accurate quote, you can book a free survey visit with a heating business near you  - go to our Installer Network and enter your postcode.

The cost to replace a back boiler with a wall hung heat only boiler

Nature of the works Cost with entry level boiler Cost with mid range boiler Cost with premium boiler Time taken
Replacing a back boiler with wall hung boiler retaining the hot water tank £2,500 £2,775 £3,000 2 days
Replacing a back boiler with wall hung boiler with a new hot water tank £3,000 £3,275 £3,500 3 days


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2) Replace your back boiler and hot water tank with a combi boiler

A popular replacement is to install a combi boiler. This will involve removing the back boiler, hot water cylinder and water tanks in the loft. NB as we move to renewable heat sources we will need our hot water cylinders once again, so be mindful of removing it.

A new combi boiler is great for freeing up space and removing all future maintenance liabilities on the system compared with leaving the cylinder and tanks in place. You will also get a very long boiler warranty and reduce maintenance costs even further.

However as is evident in the table below, it is more expensive to install and if your pipework and radiators are very old there is a risk of leaks when the system is put under pressure.

Cost to replace a back boiler with a new combi boiler

Nature of the works Cost with entry level boiler Cost with mid range boiler Cost with premium boiler Time taken
Replacing a back boiler with combi boiler £2,900 £3,200 £3,500 2.5 days

Servicing your back boiler

For the old back boilers the attending engineer is servicing two appliances – the boiler and the gas fire. The work will involve stropping apart the individual components, removing the burner and dismantling, cleaning individual components and reassembling. The whole process takes a few hours if done right.

A back boiler is open flued and draws its air from the room. If it is not serviced regularly it will start to produce carbon monoxide. If there is a problem with the chimney this it could start ‘spilling’ into the room. A carbon monoxide alarm is a definite for a back boiler.

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