What Size Boiler Do I Need? 2026 (UK)
Most UK homes in 2026 suit a 24–30kW combi boiler, but the right size depends on how many bathrooms and radiators you have far more than on floor area. Get it right and you enjoy strong hot-water flow and efficient running; over-size it and you waste money on the boiler and lose efficiency through short-cycling. Here is a simple sizing table by bedrooms and bathrooms, what the kW figure actually means, and how a proper heat-loss calculation nails the number.
Combi boiler sizing table (2026)
| Home | Bathrooms | Typical combi output |
|---|---|---|
| Flat / 1–2 bed | 1 | 24–27kW |
| 2–3 bed house | 1 | 27–30kW |
| 3–4 bed house | 1–2 | 30–35kW |
| 4–5 bed house | 2+ | 35kW+ (or system boiler) |
Combi output is driven by hot-water demand. Homes with two or more bathrooms used at once are usually better on a system boiler with a cylinder than a very large combi.
What does kW mean on a boiler?
The kW rating is the boiler's power output. On a combi, the important figure is the domestic hot-water output — higher kW means more hot water per minute at the tap. The central-heating output is usually lower and should be matched to the home's heat loss, not the hot-water figure. That is why a big combi can be great for showers but oversized for the radiators.
Why bigger is not better
Over-sizing costs more up front and makes the boiler "short-cycle" — firing in brief bursts that hurt efficiency and increase wear. The efficient approach is to match the heating output to a proper room-by-room heat-loss calculation, then choose the hot-water output to suit your bathrooms. A good installer does this rather than fitting the biggest boiler "to be safe".
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