Combi vs System Boiler: Which Is Right for Your Home?

How combi, system and heat-only boilers differ, and what a survey considers before recommending one.

Last updated: 2026-09-01

There is no universal best boiler, only the right boiler for the property, the mains supply, and how the household uses hot water.

The three types in brief

Combi boilers heat water on demand. They are compact and need no tanks, but hot water flow is limited by the boiler’s output and the incoming mains. System boilers pair with a hot water cylinder, which suits larger households running two outlets at once, at the cost of cylinder space. Heat-only boilers suit traditional vented systems that already work well.

What a survey weighs up

Mains pressure and flow, the number of bathrooms, simultaneous demand, space for a cylinder, flue and condensate routes, and the condition of existing pipework. Conversions, for example heat-only to combi, free up space but involve more pipework, so they are quoted only after a visit. Read how SureFlow quotes new boilers.