What Size Boiler Do I Need? kW for a Combi or System Boiler (UK)
A 100 m² home with average insulation needs about 7 kW for heating, but a combi boiler must also heat the hot water as you use it, and one shower at 10 litres a minute needs 20.9 kW. So the right combi is about 24 kW, and 42 kW if two showers run at once. A system or regular boiler with a hot water cylinder is sized for the heating alone: 12 kW for this home. The boiler size calculator does the sums for your own home.
Heating load and boiler size by floor area
The heat the home needs on a cold day, with the system boiler size in brackets, for four levels of insulation, and the combi boiler size for one shower at a time:
| Floor area | Poor (100 W/m²) | Average (70 W/m²) | Good (50 W/m²) | Excellent (30 W/m²) | Combi (one shower) |
|---|---|---|---|---|---|
| 50 m² | 5.0 kW (12 kW) | 3.5 kW (12 kW) | 2.5 kW (12 kW) | 1.5 kW (12 kW) | 24 kW |
| 75 m² | 7.5 kW (12 kW) | 5.3 kW (12 kW) | 3.8 kW (12 kW) | 2.3 kW (12 kW) | 24 kW |
| 100 m² | 10.0 kW (12 kW) | 7.0 kW (12 kW) | 5.0 kW (12 kW) | 3.0 kW (12 kW) | 24 kW |
| 125 m² | 12.5 kW (15 kW) | 8.8 kW (12 kW) | 6.3 kW (12 kW) | 3.8 kW (12 kW) | 24 kW |
| 150 m² | 15.0 kW (18 kW) | 10.5 kW (12 kW) | 7.5 kW (12 kW) | 4.5 kW (12 kW) | 24 kW |
| 200 m² | 20.0 kW (24 kW) | 14.0 kW (18 kW) | 10.0 kW (12 kW) | 6.0 kW (12 kW) | 24 kW |
| 250 m² | 25.0 kW (30 kW) | 17.5 kW (24 kW) | 12.5 kW (15 kW) | 7.5 kW (12 kW) | 24 kW |
Hot water power for a combi
The power to heat the water by 30°C, which is from cold winter mains water to a comfortable shower:
| Outlets and flow | Hot water power | Combi size |
|---|---|---|
| 1 × 8 L/min | 16.7 kW | 24 kW |
| 1 × 10 L/min | 20.9 kW | 24 kW |
| 1 × 12 L/min | 25.1 kW | 28 kW |
| 2 × 8 L/min | 33.5 kW | 35 kW |
| 2 × 10 L/min | 41.8 kW | 42 kW |
| 2 × 12 L/min | 50.2 kW | over 42 kW: use a cylinder |
The power is the litres per minute times 2.09 kW. A normal mixer shower uses 8 to 10 litres a minute, and a power shower 12 or more, so a power shower or two outlets at once needs a bigger combi.
Combi, system or regular?
- Combi boiler. Heats the hot water on demand, with no cylinder, and suits a small or medium home with one bathroom. Its flow rate is limited, so two showers at once is hard.
- System boiler. Heats a cylinder of hot water, so several bathrooms can be used at once, and the boiler is sized for the heating.
- Regular (heat-only) boiler. Like a system boiler, but with a separate tank in the loft. It suits a house with an old system and good pressure from the cylinder.
How to avoid buying too big a boiler
- Insulate first. Better loft insulation, cavity wall insulation and draught-proofing cut the heat you need, and can let you choose a smaller boiler. See the loft insulation calculator.
- Ask for a room-by-room heat loss calculation. It is more accurate than a rule of thumb, and it sizes the radiators too (see the radiator size calculator).
- Remember that a modern condensing boiler modulates. A 30 kW combi runs at a fraction of its output for the radiators, so it is not oversized for the heating.
- Think about the future: a heat pump needs larger radiators and a lower flow temperature, so a heating engineer may suggest a different plan.
Boiler and gas work must be done by a Gas Safe registered engineer. These figures are rule-of-thumb estimates, not a design.
Frequently asked questions
What size boiler do I need for my house?
A 100 m² home with average insulation needs 7.0 kW for heating. A combi boiler also has to meet the hot water, so it would be 24 kW for one shower at a time, and 42 kW for two. A system boiler with a cylinder is sized for the heating: 12 kW here.
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 from the sums here. With two bathrooms and two showers at once it is 42 kW.
Is a 24kW combi boiler enough for a 3 bed house?
For one shower at a time, yes: it supplies about 11 litres a minute at a 30°C rise, and it turns down to 7 kW for the radiators. If two showers are used at once, a larger combi or a system boiler with a cylinder is better.
Why is the boiler so much bigger than the heat the house needs?
A combi heats the hot water as it runs. The house needs only 7.0 kW for heating, but a shower at 10 litres a minute needs 20.9 kW. A modern boiler modulates, 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 switches on and off a lot can be less efficient, so choose the size for the load.
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and check the Building Regulations and any planning rules for anything structural or close to a boundary.