How this calculator works
A boiler has two jobs. It heats the radiators, and it heats the hot water. The heating load is the heat the home loses on a cold day, which we estimate from the floor area and how well it is insulated: about 70 watts per square metre for an average home, 100 for a poorly insulated one and 30 for an excellent new build. A combi boiler also heats the hot water as it runs, using a power of the flow in litres a minute times the temperature rise (30°C) times 0.0697 kW. It cannot heat the radiators and run a shower at once, so it must meet the larger of the two. A system or regular boiler stores hot water in a cylinder, so it is sized for the heating, with about 10% extra to reheat the cylinder.
Worked example: a 100 m² home with average insulation needs 7.0 kW for heating. One shower at 10 litres a minute needs 20.9 kW of hot water, so a combi has to be at least 20.9 kW: a 24 kW combi. With two outlets at once it is 41.8 kW (42 kW). A system boiler for the same house is 12 kW (7.7 kW with the cylinder allowance). A 200 m² home needs 14.0 kW and a 24 kW system boiler, a poorly insulated 100 m² home needs 10.0 kW, and an excellent new build only 3.0 kW.
Heat needed by home size (average insulation, 70 W per m²)
The heating load and the combi and system boiler sizes.
| Floor area | Heat needed | Combi (1 shower) | System boiler |
|---|---|---|---|
| 50 m² | 3.5 kW | 24 kW | 12 kW |
| 75 m² | 5.3 kW | 24 kW | 12 kW |
| 100 m² | 7.0 kW | 24 kW | 12 kW |
| 125 m² | 8.8 kW | 24 kW | 12 kW |
| 150 m² | 10.5 kW | 24 kW | 12 kW |
| 200 m² | 14.0 kW | 24 kW | 18 kW |
| 250 m² | 17.5 kW | 24 kW | 24 kW |
Tips before you buy
- The heat the house needs and the hot water you use at once are two separate sums. A combi boiler has to meet the larger of the two, but a system boiler with a cylinder is sized for the heating.
- A bigger boiler is not better. An oversized boiler cycles on and off and wears faster, but modern boilers modulate down to a fraction of their output.
- Better insulation and draught-proofing are cheaper than a bigger boiler, and cut the size you need.
- A combi boiler struggles to supply two showers at once. If you have several bathrooms, a system boiler with a cylinder is a better fit.
- Boiler and gas work must be done by a Gas Safe registered engineer.
Tools & materials for this project
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Frequently asked questions
What size boiler do I need for my house?
For a 100 m² home with average insulation, the heating needs 7.0 kW. A combi must also meet the hot water, so it is a 24 kW combi for one shower at a time, or a 12 kW system boiler with a cylinder.
What size combi boiler for a 3 bedroom house?
A 3 bedroom house of about 100 m² with one bathroom usually needs a 24 to 30 kW combi: 24 kW here. With two bathrooms and two showers at once it is 42 kW.
Is a 24kW combi enough?
For one shower at a time, yes: it supplies about 11 litres a minute at a 30°C rise. For two showers at once you need a 30 to 35 kW combi, or a system boiler with a cylinder.
Why is the boiler bigger than the heat the house needs?
Because a combi must supply hot water on demand as well. The house needs only 7.0 kW for heating, but one shower needs 20.9 kW. A modern boiler modulates down, so it runs at a low output for the radiators.
Does a bigger boiler cost more to run?
Not by itself: a boiler uses gas in proportion to the heat it produces. But an oversized boiler that cycles on and off can be less efficient, so choose the size for the load.
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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.