System boilers are heat only boilers but with the components that are normally found in the cylinder cupboard pre-fitted inside the boiler whilst still compact, this makes the boiler larger than a standard heat only boiler. There are two types of system boiler, vented and sealed system, a vented system boiler would still require an expansion tank in the loft, a sealed system boiler has a built in expansion tank together with other safety controls. Both require a hot water cylinder, however if sealed system boiler is combined with an unvented cylinder there is no need for any cold water storage tanks, this is an ideal arrangement for loft conversions in larger homes.

As system boilers components are built-in, they are pre-wired, pre-plumbed and pre-tested for a neater quicker and more reliable installation.

Most heat only boilers are available as a system version, in Natural Gas or LPG, some good models are the Ideal Icos system HE, the Worcester Greenstar i System series or the Glow worm Ultracom 30SXI System Boiler.