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Advice · 5 min read

Combi, system or conventional?

Three types, one decision, and a lot of confident advice pulling in both directions. The right answer comes down to how many people want hot water at once, and what your incoming mains is like.

Last reviewed 7 September 2026

The three types, in plain English

  • Combi. Heats water on demand straight off the mains. No tank, no cylinder, nothing in the loft. Hot water is effectively unlimited, but it comes at one flow rate shared between whatever taps are open.
  • System. Heats a stored cylinder of hot water. The expansion vessel and pump live inside the boiler, so the loft stays clear, but the cylinder needs a home. Several outlets can run at once at full pressure.
  • Conventional, also called regular or heat-only. A boiler, a hot water cylinder, and a cold tank in the loft. The oldest arrangement and usually the one being replaced.

How to choose without guessing

Four questions get you most of the way there.

  1. 01How many bathrooms, and will two of them realistically run at once? One bathroom, or two that never overlap, points at a combi. Two or more in proper simultaneous use points at a system boiler and a cylinder.
  2. 02What's your mains pressure and flow rate like? A combi is only as good as the water coming into the house. In a property at the end of a long run with weak mains, a combi will disappoint you no matter how good the boiler is. This is measurable, and it ought to be measured.
  3. 03Where would a cylinder live? An airing cupboard is ideal. If there's honestly nowhere, that narrows the answer.
  4. 04Do you want the loft cleared? Combi and system boilers both get rid of the cold tank. For anyone converting a loft, that's often the whole decision.

The conversion question

Going from conventional to combi is the most common upgrade and it's more work than a straight swap. Tanks and cylinder out, pipework capped or rerouted, and usually some making good where the cylinder stood. It costs more on the day and it's generally worth it if the answers above point that way.

It isn't automatically right, mind. Three bathrooms and a family who all shower between seven and eight, and a combi will make your mornings worse however modern it is.

If an installer recommends a combi without asking about your bathrooms or checking your water pressure, get a second opinion.

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