Boilers
What size boiler do I need for my house?
3 min read · Last updated · Reviewed for accuracy before publishing
For a combi, size is driven mostly by hot-water demand: EnergyPages puts 24–30kW as suiting most 3-bed homes and 30–35kW a 4-bed with two bathrooms; beyond that, a combi may be the wrong type entirely (see system boilers). EnergyPages also says oversizing burns more gas per cycle and shortens boiler life. Proper sizing counts your radiators, bathrooms and mains flow rate; it isn't read off the house price.
Why bigger isn't better
EnergyPages says oversizing burns more gas per cycle and shortens boiler life. The '35kW to be safe' instinct costs money twice: once on the boiler, then on every cycle. EnergyPages also says a heat-loss calculation costs nothing if your engineer does it properly at the quote stage.
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. We write the numbers on the quote so you can see what the size was based on.
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. Worcester Bosch says combis can struggle to keep up in a busy household.
A combi's hot-water flow is capped by its size: on the Worcester Bosch Greenstar 8000 Life datasheet it rises from 12.3 litres a minute at 30kW to 20.3 at 50kW.
“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 →
Sources
- Worcester Bosch — Greenstar 8000 Life datasheet (30–50kW combis; hot-water flow 12.3 L/min at 30kW to 20.3 L/min at 50kW; verified 8 Jul 2026)
- Worcester Bosch — Greenstar 4000 combi datasheet (25 and 30kW; verified 8 Jul 2026)
- Baxi — 800 Combi 2 (24 and 30kW; verified 8 Jul 2026)
- EnergyPages — Boiler costs (magnetic filter £100–£200; power flush £300–£600 on older systems with sludge; vertical flue +£200–£500 plus possible scaffolding; upsizing an older gas supply pipe £200–£600 depending on the run; 24–30 kW suits most 3-bed homes and 30–35 kW a 4-bed with two bathrooms; oversizing burns more gas per cycle and shortens boiler life; accessed 19 Oct 2026)
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.