What is a back boiler?

Back boilers (also known as back boiler units or BBUs) became popular in 1966 with the launch of the original Baxi Bermuda. A back boiler is a small, compact boiler fitted at the back of an open fireplace hearth. It sits behind a gas fire and provides hot water and central heating to the property.

Back boilers were popular until the Building Regulations changed in 2005, when it became a requirement to install high efficiency condensing boilers. Although many have been replaced with wall-hung condensing boilers, there are still a large number of back boilers still installed in UK homes.  

An old standard efficiency back boiler is less than 70% efficient – so for every £1 of gas burned, on 70p worth of useable heat is generated and the other 30p worth is lost up the chimney. By replacing it with a new boiler at over 90% efficient, fuel bills can be reduced by up to £305, according to the Energy Saving Trust.

Replacing a back boiler

When replacing a back boiler, there are several extra factors that need to be considered, which you should discuss with your Gas Safe registered installer

• What type/output of boiler do you need?
• Where will your new boiler be sited?
• Do you need to have the gas and water pipes moved?
• Does the hot water cylinder need to be removed or upgraded?
• Do tanks in the loft need to be removed?
• What are you going to do with the fireplace? If the old back boiler is decommissioned but left in situ, a fire can’t be installed in front. If the boiler is removed, you’ll need to plaster over the hole in your wall, or install a new fireplace and mantelpiece.
• Ask your installer to flush the system; as it is likely to be an old system, it could be full of sludge and debris.
• Discuss with your installer what controls you will need for your new heating system; these will include a timer and room thermostat at least, to comply with Building Regulations thermostatic radiator valves (TRVs). You may also like to consider smart controls, such as Baxi uSense.