# Combi or system boiler — which is right for my home?

*David · Founder, Datum · Datum · Boilers*

For most homes with one bathroom, a combi wins: instant hot water, no cylinder, no tank in the loft, lower install cost. For households running two or more showers at once, a system boiler with a hot-water cylinder wins — a combi shares its output between taps, so the second shower goes lukewarm. The right answer is about how your household actually uses hot water, not which boiler is 'best'.

## When a combi is right

One bathroom, gas main of decent pressure, no space for a cylinder, and you value instant hot water with the lowest install cost. A like-for-like combi swap is also the cheapest, fastest job — usually one day.

## When a system boiler is right

Two-plus bathrooms in regular use, a big family, or weak incoming mains pressure. The cylinder stores hot water so everyone showers at full flow — and it pairs naturally with future upgrades like solar or a heat pump.

## The conversion question

Converting from one type to the other costs more than a like-for-like swap (pipework, cylinder in or out). Worth it when your household has genuinely changed; not worth it for the sake of it. We'll tell you which side of that line you're on.

> Nobody should sell you a boiler type — they should ask how many showers run at once in your house. That one question settles most of it.
>
> — David, Founder, Datum

## Related questions

### Is a combi boiler cheaper to run?

Slightly, in most homes — you heat only the water you use, with no cylinder standing losses. Modern cylinders are well insulated though, so usage pattern matters more than boiler type.

### Can a combi run two showers?

It shares its hot-water output, so two simultaneous showers both run reduced. Larger combis help; they don't fully solve it. Regular two-shower households should look at a system boiler.

### What does a conversion cost?

Typically £3,200–£4,300 installed depending on pipework and cylinder work — versus £2,310–£3,010 for a like-for-like swap. Fixed price confirmed at a free survey.

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