Combi boilers heat hot water as it is required, therefore there is no need to have a hot water cylinder in an attic for example. They are ideal for properties where there is a low demand for hot water.

Heat only, or "conventional" boilers are suited to properties where there is a higher demand for water than a combi can deal with. The central heating circuit is open to the atmosphere to allow for changes in pressure.

Sealed system boilers are more suited for applications where the demand for hot water is higher than households where a combi boiler would be enough. System boilers require a cylinder to act as a hot store.