A "3-bed house" covers an enormous range of properties — from a compact 1970s semi with one bathroom to a large Victorian terrace with a bathroom and separate shower room. Sizing by bedroom count alone is inaccurate. Here's how proper sizing works.
The key factors
- Number of radiators: More radiators need more boiler output for the heating circuit
- Hot water demand: For a combi, the kW rating determines hot water flow rate. One bathroom = 24–28 kW. Two bathrooms with simultaneous use = 32–40 kW or system boiler
- Insulation: Well-insulated home needs less output than a draughty one
- Property age: Modern homes lose heat much more slowly than pre-1970s homes
Typical 3-bed semi (1970s–1990s, one bathroom)
Usually appropriate: 24–30 kW combi. Our calculation approach: count the radiators (typically 8–10 in a 3-bed semi), multiply by approximately 1.5 kW each, add domestic hot water demand. 10 radiators × 1.5 kW = 15 kW heating + 9–10 kW hot water = 24–28 kW total.
Don't oversize
An oversized boiler short-cycles — it heats the system quickly, turns off before warming up properly, then fires again. This wastes gas and causes premature wear. Accurate sizing is as important as brand selection.
Our sizing process
We do a proper heat loss calculation at every survey — not just an estimate from the bedroom count. Book a free survey.