Sizing summary
What size combi boiler?
A short reference. The full sizing tool, flow-rate ladder, and oversize / undersize trade-offs are at /what-size.
| Home type | Radiators | Bathrooms | Recommended kW |
|---|---|---|---|
| 1 to 2 bed flat | Up to 8 | 1 | 24 to 28 kW |
| 3 bed semi | 9 to 12 | 1 | 28 to 30 kW |
| 3 bed detached | 10 to 14 | 2 | 30 to 35 kW |
| 4 bed detached | 12 to 16 | 2 | 35 to 38 kW |
| 5 bed detached | 16 to 20 | 3+ | 38 to 40 kW+ |
Two or more bathrooms in parallel use?
Combis cannot supply two showers comfortably below 35 kW. If your household runs multiple outlets at once, head to the combi vs system page before fixing on a kW.
For the full sizing tool, flow-rate ladder, and undersize / oversize trade-offs, head to the full sizing guide.
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