Recommended
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Why bathrooms matter more than bedrooms
A combi heats water on demand. The kW rating mostly decides how fast it can heat water flowing through to your taps and shower. Radiators in a typical home need far less than a combi's hot-water output.
Mains flow rate is the limit
A bigger combi can't give you more hot water than your mains supply delivers. If your flow rate is low (roughly under 10–12 litres per minute), a larger boiler won't help. In that case a system boiler with a cylinder may suit you better.
Rule of thumb
| Home | Typical combi size |
|---|---|
| Flat or small house, 1 bathroom | 24 – 28 kW |
| 3–4 bed house, 1–2 bathrooms | 28 – 35 kW |
| 2+ bathrooms used at the same time | 35 kW+, or consider a system boiler |
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