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Boiler Size Calculator
Calculate the correct boiler size (kW output) for your property. Enter the property type, floor area, insulation quality, and hot water demand to get a precise recommendation.
How to Size a Boiler
Boiler sizing depends on the property's heat loss, which is driven by floor area, insulation quality, and property type (detached properties lose more heat than mid-terraces).
A combi boiler needs additional capacity for domestic hot water generation. Oversized boilers short-cycle and waste energy; undersized boilers can't maintain temperature in cold weather.
Modern condensing boilers modulate their output, so slight oversizing is less problematic than in the past, but correct sizing still improves efficiency and comfort.
Frequently Asked Questions
What size boiler do I need for a 3-bed semi?
A typical 3-bed semi-detached house with 80–100 m² floor area needs a 24–30 kW combi boiler, or 18–24 kW system boiler if hot water is from a cylinder.
Is it bad to oversize a boiler?
Oversizing causes short-cycling (frequent on/off), which wastes energy and increases wear. Modern modulating boilers reduce this issue, but correct sizing is still best.
What size combi boiler do I need?
For a 2-bed flat: 24 kW. For a 3-bed semi: 30 kW. For a 4-bed detached: 35–42 kW. Combi boilers need higher output for simultaneous hot water.
Boiler Size Calculator
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