A back boiler is a device that is fitted to a residential heating stove or open fireplace to enable it to provide both room heat and domestic hot water or central heating. The device is water-filled and typically used with a gravity feed circuit to the hot water cylinder, with a vent or overflow to prevent excess pressure build-up. It can also be connected to a series of radiators to provide central heating but usually requires an electrical pump to circulate the hot water, although some systems can use the gravity circuit to include radiators.

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.

Back boilers can improve the efficiency of a stove by acting as a heat-sink and can also act as a method of extracting additional heat from the flue system which would otherwise have been lost. Manufacturers of stoves with back boilers quote efficiency figures of up to 80% on oil-fed models.

However, back boilers require that the hot water or central heating system be able to disperse all of the heat captured from the fire, otherwise boiling or overheating of the water can occur. Back boilers are strictly regulated in Ireland and the UK following explosions relating to incorrectly serviced or installed back boilers.

Replacing an old back boiler with a new boiler at over 90% efficient can reduce fuel bills by up to £305, according to the Energy Saving Trust. When replacing a back boiler, there are several extra factors that need to be considered, which should be discussed with a Gas Safe registered installer.