Combi, system or regular: which boiler do you actually need?

There are three types of gas boiler in British homes, and the right one depends less on preference than on how many people are trying to shower at once.

Combi

A combi heats water on demand. No cylinder, no tanks, no airing cupboard taken up. It suits flats and smaller houses with one bathroom, and it is by far the most common replacement.

The catch: run two showers at once and both will disappoint you. Combis also depend on decent mains water pressure. If your cold tap dribbles, a combi will not fix it.

System

A system boiler heats a hot water cylinder. That means stored hot water, so several taps and showers can run at once without a fight. Most of the components are built into the boiler, so it is tidier than a regular setup.

The catch: it takes up space for the cylinder, and when the cylinder is empty you wait for it to reheat.

Regular, also called conventional or heat-only

The traditional setup: boiler, hot water cylinder, and a cold water tank in the loft. Usually found in older houses with older pipework.

It is rarely the choice for a new install unless the existing system is regular and the pipework or water pressure makes a change expensive. If an installer suggests keeping a regular system, ask them to explain why, and the answer should be about pressure or pipework, not convenience.

A rough rule

One bathroom, decent mains pressure: combi. Two or more bathrooms, or a big family who all shower at 7am: system. An older house where changing the whole layout is expensive: talk properly to your installer before committing.