Found in over two thirds of homes in the UK, a Combi (Combination) Boiler is an instant domestic water heater and a central heating boiler combined. Water is heated directly via a thermal store when a tap is turned on, saving not only on cost, but also on the space required for a hot water storage tank. Providing instant access to hot water, this type of boiler is particularly suited to smaller households with one bathroom and shower / en suite.

A System Boiler which is also referred to as a 'sealed system boiler', provides central heating and direct hot water via a storage cylinder, which is usually housed in an airing cupboard. Unlike a regular boiler heating system, there is no requirement to have water tanks situated in the loft. System boilers are generally easier to install, as the heating and hot water system components form an integral part of the boiler itself.