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This article is about Boilers used for Central heating and, usually, hot water. Such boilers usually use Natural Gas, Oil or LPG as fuels.

Many people seem to be confused by the words Condensing, Combi, System and Heating which are applied to types of boilers.

Types of Boiler


Condensing - also known as High Efficiency - boilers gain extra heat from the fuel they burn by cooling their exhaust (flue) gases so that the steam present in hot flue gas turns to water giving up its Latent Heat. This typically adds about 10% to the boiler's efficiency.


Combination boilers produce Domestic Hot Water directly (rather than the boiler heating a cylinder of water as in a conventional system). Since they are usually intended to provide most of the components of a central heating and DHW system in one package they usually contain the components of a sealed system, and often a timer as well.


System boilers contain the components (pressure vessel, pressure relief discharge valve, pressure gauge and sometimes filling arrangement) to make a Sealed System (see Ed Sirett's Sealed system FAQ). Combi boilers also usually contain these components, so a System boiler can be regarded as a Combi boiler without the DHW-producing components (and some boilers are exactly so: there are spaces where the DHW components of the Combi version of the boiler would fit). System boilers also tend to include the circulation pump within the boiler.


A Heating-only (or regular) boiler just heats the primary water circulating through the radiators and hot water cylinder etc. It does not have the components for a sealed system and may not even contain a pump.


A boiler may be:

  • condensing and a combi
  • condensing and non-combi: i.e. system or heating
  • non-condensing and combi
  • non-condensing and non-combi: i.e. system or heating

Current Building Regulations require that new and replacement boilers must generally be Condensing types. The choice of combi, system or heating is not stipulated.

See Also