Boilers
What size boiler do I need for my house?
3 min read · Reviewed for accuracy before publishing
For a combi, size is driven mostly by hot-water demand: a one-bathroom home typically suits 24–30kW; larger homes with a big bath or two bathrooms, 30–35kW; beyond that, a combi may be the wrong type entirely (see system boilers). For heating alone, most UK homes need far less than people think — oversized boilers cycle on and off inefficiently. Proper sizing counts your radiators, bathrooms and mains flow rate; it isn't read off the house price.
Why bigger isn't better
An oversized boiler short-cycles: fires hard, overshoots, shuts down, repeats. That wastes gas and wears components. Right-sized, it runs longer and gentler — cheaper and quieter. The '35kW to be safe' instinct costs money twice.
What we actually measure
Radiator count and sizes, number of bathrooms and how they're used, incoming mains flow and pressure, and the pipework. Ten minutes of measuring beats any online calculator — and it's part of the free survey, not an extra.
When the answer isn't a combi
If your mains flow is weak or your household runs simultaneous showers, the honest answer is a system boiler and cylinder, not a bigger combi. We'll say so, even though it's a different quote.
“The 35kW-to-be-safe instinct costs money twice — once on the boiler, then every month it short-cycles. We measure; we do not guess.”
Size it properly — free survey →
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.