Boilers
Combi or system boiler — which is right for my home?
3 min read · Reviewed for accuracy before publishing
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.”
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David founded Datum after years on the tools across heating and plumbing. He writes the boiler and cost guides from what actually happens on real installs.