# What size boiler do I need for my house?

*David · Founder, Datum · Datum · updated 2026-10-19 · Boilers*

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.
>
> — David, Founder, Datum

## Related questions

### Is a 24kW boiler enough for a 3-bed house?

For heating, we'd expect so — EnergyPages puts 24–30kW as suiting most 3-bed homes. For hot water it depends on bathrooms and usage: one bathroom, likely; two busy bathrooms, look at 30kW+ or a system boiler. The survey settles it.

### Does an online boiler calculator work?

As a rough first pass. It can't see your mains flow rate or pipework, which are exactly what changes the answer in older homes.

### Can you fit a bigger boiler on the same pipes?

Often, but gas-supply pipe sizing must be checked — bigger boilers can need a larger gas run, which affects the quote. Another reason the survey is free and the price is then fixed.

## Sources & verification

- [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)](https://www.worcester-bosch.co.uk/support/literature/download/OPOLife)
- [Worcester Bosch — Greenstar 4000 combi datasheet (25 and 30kW; verified 8 Jul 2026)](https://www.worcester-bosch.co.uk/files/Gstar_4000_Combi_One_Pager_CONSUMER.pdf)
- [Baxi — 800 Combi 2 (24 and 30kW; verified 8 Jul 2026)](https://www.baxi.co.uk/professional/products/boilers/combi-boilers/baxi-800-combi-2)
- [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)](https://energypages.co.uk/boilers/costs)

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