What Size Combi Boiler Do I Need?
Updated June 2026 · Estimates only — verify before quoting
With a combi boiler the output (kW) has to do two jobs: heat the radiators and deliver instant hot water. Hot water is usually the deciding factor, so combi sizing is driven mainly by how many bathrooms and outlets the home has.
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Combi Boiler Sizing Calculator
Recommended boiler output from radiators and bathrooms.
Open the Combi Boiler Sizing CalculatorRough combi sizing by home
As a starting point:
- Up to ~10 radiators, 1 bathroom: 24–27 kW
- 10–15 radiators, 2 bathrooms: 28–34 kW
- 15–20 radiators, 2–3 bathrooms: 35–42 kW
Heat demand vs. hot water
The radiators might only need 8–12 kW, but a combi has to deliver enough flow of hot water on demand — which is why combis are rated higher. If hot-water demand is high (multiple showers at once), a system boiler with a cylinder may suit better. Use the calculator to get a recommended output range, and pair it with the heat-loss calculator for the heating side.
Frequently asked questions
What kW combi boiler for a 3-bed house?
A typical 3-bed home with one bathroom and ~10–12 radiators is usually well matched by a 28–30 kW combi. Add output if you have a second bathroom or high hot-water demand.
Is a bigger combi boiler better?
Only up to a point — an oversized combi cycles inefficiently. Size it to your hot-water demand and radiator load rather than fitting the biggest available.
From estimate to winning quote
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