Boilers
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 find the names - Condensing, Combi, System and Heating - of different types of boiler confusing.
Types of Boiler
Condensing
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.
Combi
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
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).
Heating
A Heating-only (or regular) boiler just heats the primary water circulating through the radiators and hot water cylinder etc.
Permutations
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.