How UK boiler sizing really works
For a combi boiler, the kW rating on the box mostly describes hot water performance, not heating. Even a large house rarely needs more than 18 kW for radiators — but a decent shower needs 24 kW+ of instant water heating, and two simultaneous showers push you into the 30–42 kW class. That is why this calculator sizes combis by bathrooms first.
For system and regular boilers the hot water lives in a cylinder, so the boiler only needs to cover the radiator load — roughly 1.5–2 kW per average radiator including pipe losses, adjusted for insulation.
| Home | Combi | System/Regular |
|---|---|---|
| 1–2 bed flat, 1 bath, up to 10 rads | 24–27 kW | 12–15 kW |
| 3 bed semi, 1 bath, 10–15 rads | 27–30 kW | 15–18 kW |
| 4 bed, 2 baths, 15–20 rads | 30–35 kW | 18–24 kW |
| 5+ bed, 3 baths, 20+ rads | 35–42 kW (or system) | 24–30 kW |
Bigger combi ≠ better heating
A 35 kW combi in a small flat does not heat the flat faster — it just modulates down (or short-cycles if it can't modulate low enough, wasting gas). Choose the smallest combi whose hot water flow rate (litres/min at 35°C rise) matches your shower habits. If two showers must run at once reliably, a system boiler with a cylinder usually beats a giant combi.
Frequently asked questions
What size boiler do I need for a 3-bed house?
A typical UK 3-bed semi with one bathroom and 10–15 radiators needs a 27–30 kW combi, or a 15–18 kW system/regular boiler with a hot water cylinder.
Is a 30kW boiler too big for my house?
For heating alone, probably — most homes need under 18 kW. But combi ratings are about instant hot water: 30 kW delivers a strong single shower. Oversizing beyond your hot water needs adds cost without benefit.
How many radiators can a 24kW boiler run?
Comfortably around 10–12 average radiators (roughly 15 kW of heating load), with capacity left for hot water in a combi. With a cylinder (system boiler), 24 kW runs 15+ radiators.
Combi or system boiler for 2 bathrooms?
Two bathrooms used simultaneously is the classic tipping point: either a 32–42 kW combi (works if mains flow is strong) or, more robustly, a system boiler with a 170–210 litre cylinder.