Boiler size for a 100 m² home, by insulation
A 100 m² home is a typical three-bedroom semi-detached house. The heating load is the heat the home loses on a cold day. A system or regular boiler is sized on the heating with 10% for reheating the cylinder. The yearly gas cost assumes an 88% efficient boiler and 6.3p per kWh of gas, and the heat pump size is the next common model above the load.
| Insulation | Heating load | System boiler | Gas for heating a year | Heat pump size |
|---|---|---|---|---|
| Poor (solid walls, single glazing) | 10.0 kW | 12 kW | £1,289 | 12 kW |
| Average (cavity walls, double glazing) | 7.0 kW | 12 kW | £859 | 7 kW |
| Good (well insulated) | 5.0 kW | 12 kW | £573 | 5 kW |
| Excellent (new build) | 3.0 kW | 12 kW | £358 | 5 kW |
Combi boiler size by hot water use
A combi heats water as it flows, so its size comes from the hot water you draw at once, heated by 30°C. The load is the larger of the hot water and the 7.0 kW heating load.
| Hot water at once | Load | Combi size |
|---|---|---|
| One shower, 8 litres a minute | 16.7 kW | 24 kW |
| One shower, 10 litres a minute | 20.9 kW | 24 kW |
| One shower, 12 litres a minute | 25.1 kW | 28 kW |
| Two showers at once, 10 litres a minute | 41.8 kW | 42 kW |
| Two showers at once, 12 litres a minute | 50.2 kW | ask an engineer |
What sets the size
For a 100 m² home the heating load is smaller than the power a combi needs for one shower (20.9 kW), even with poor insulation, so the hot water sets the size of a combi. A modern boiler turns down (modulates) to run the radiators at a fraction of its rating.
These are rule-of-thumb figures, not a heat loss survey. Ask a Gas Safe registered engineer for a room-by-room heat loss calculation and quote, and size each room with the radiator size calculator. The boiler size guide explains combi and system boilers.
Want to change anything else? The calculator above is the full Boiler Size Calculator (UK): What kW Boiler Do I Need?: edit any number and the results update instantly.
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Frequently asked questions
What size boiler do I need for a 100 m² house?
A 100 m² home with average insulation loses about 7.0 kW on a cold day. A combi boiler supplying one 10 litre a minute shower should be 24 kW, because the hot water needs more power than the heating. A system or regular boiler with a cylinder can be 12 kW.
Is a 24 kW combi big enough for a 100 m² house?
Yes for one shower at a time: a 100 m² home needs 7.0 kW of heating and one 10 litre a minute shower needs 20.9 kW. Two showers at once need 41.8 kW, which is a 42 kW combi, or a system boiler with a cylinder.
What size heat pump for a 100 m² house?
About 7 kW for average insulation, from a heat loss of 7.0 kW. Over a year the heating uses about 12,000 kWh of heat, costing about £859 with gas or £1,040 with a heat pump at a SCOP of 3, at 6.3p gas and 26p electricity.
Related sizes and guides
Related sizes
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.