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Rules of thumb only. A proper heat-loss calculation by a Gas Safe engineer is the right way to size a boiler, especially for heat pumps or poorly insulated homes; oversizing wastes money and causes short-cycling.
What The Boiler Size Calculator Works Out
- Recommended size (combi): combis are sized by hot water demand, not heating, because heating a shower on demand is the hardest job they do. The usual bands are 24 to 27kW for one bathroom, 28 to 34kW for two and 35kW plus for three or more. A combi can only really serve one shower at a time whatever its size.
- Recommended size (system or heat-only): sized by heating demand at a rule of thumb of 1.5 to 2kW per radiator, with a 12kW floor. A typical 10-radiator three-bed home comes out at 15 to 20kW.
- Heating demand estimate: the radiator-count figure shown for both boiler types, so you can see how far below a combi's output the heating load usually sits. Modern condensing boilers modulate down, but a grossly oversized boiler still short-cycles, wearing itself out and wasting gas.
- kW per radiator (advanced): edit the low and high estimates. Use nearer 1kW for small singles in a well-insulated flat and 2.5kW or more for big doubles in a draughty Victorian house.
- Heat loss from floor area (advanced): floor area multiplied by a heat-loss rate of 50 W/m2 (well insulated), 70 W/m2 (average) or 100 W/m2 (poor). A 90 m2 average semi is about 6.3kW, which is why many homes are heated by boilers far larger than they need for heating alone.
- Combi output for your hot water (advanced): litres per minute x temperature rise x 4.18 kJ divided by 60. Heating 10 litres a minute by 35 degrees (cold winter mains to a hot tap) needs about 24kW; a 12 l/min power shower needs 29kW.
- Hot water from the recommended size (advanced): the same sum in reverse, giving the flow rate each end of the band can deliver. Manufacturers quote flow at a 35 degree rise; in winter, when mains water is colder, real flow is lower.
- Sizing check (advanced): flags when your flow target exceeds the band, or when the floor-area heat loss disagrees with the radiator count. Before buying, insist on a room-by-room heat loss calculation from a Gas Safe registered installer, essential if a heat pump is on the table.
Source and further reading: Energy Saving Trust.