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.
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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".
Boiler size by home — at a glance
Heating output should come from a room-by-room heat-loss calculation — these bands are the sanity check.
Size, type and 2026 fitted price
| Home | Right boiler | Typical fitted price (2026) |
|---|---|---|
| 1–2 bed flat, 1 bathroom | 24–27kW combi | £1,800–£2,400 |
| 3-bed house, 1 bathroom | 27–30kW combi | £1,900–£2,800 |
| 3–4 bed, 2 bathrooms | 30–35kW combi | £2,200–£3,200 |
| 4+ bed, 2+ bathrooms | 18–24kW system + unvented cylinder | £2,700–£4,500 |
| Large home, weak mains pressure | Regular (heat-only) + tanks | £2,400–£4,000 |
| Second bathroom added later | Upsize combi or convert to system | £2,500–£4,500 |
Full price detail lives in our boiler replacement cost guide; compare deals on the best boiler deals page.
The two outputs that actually matter
A combi has two ratings: central-heating output and hot-water output. The heating side of a typical 3-bed needs only 8–12kW — almost any modern boiler covers it. The number on the box (27, 30, 35kW) is really the hot-water output, which sets how fast your taps and shower run. That is why bathrooms, not bedrooms, decide the size: more simultaneous hot water needs a bigger combi — or a cylinder.
Flow rate: the spec nobody checks
A combi can only heat the water your mains actually delivers. Measure it: run the cold kitchen tap flat out into a measuring jug for six seconds and multiply by ten. Under about 10 litres per minute, a big combi is wasted money and a cylinder system will outperform it. 12–15 l/min runs a 30–35kW combi properly. A good installer measures this before quoting — treat it as a vetting test.
How a proper sizing survey runs
- Room-by-room heat-loss calculation (window areas, walls, insulation).
- Mains flow rate and pressure measured at the kitchen tap.
- Bathroom count, shower types and usage pattern discussed.
- Boiler size and type recommended in writing — with the numbers shown.
Red flags
- “We will just match what you have” — oversized boilers short-cycle, wasting gas and wearing parts.
- A 35–42kW combi pushed onto a one-bathroom house “to be safe”.
- No flow-rate measurement before recommending a combi.
- Quotes without the make, model and output in writing. Get three itemised quotes and compare the sizing logic, not just the price.
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