Boiler Size Calculator

How the boiler size is worked out

Heating load

The heating side is the whole-house heat loss on the design day, in kilowatts. A house that loses 7,370 W needs a boiler that can deliver 7.4 kW to its radiators. Adding a large margin on top used to be common; a modern condensing boiler turns itself down to match the load, but one that is much too big spends mild days switching on and off.

Estimating the heat loss from the house

If you do not have a heat loss figure, the estimate panel builds a notional house from its floor area, storeys, built form and age, and runs it through the same room-by-room method as the heat loss calculator. Its assumptions are a square plan, 2.4 m ceilings (2.7 m for a house built before 1930, unless you enter the height), RdSAP's default window area for the age of house, two external doors, and the air changes and room temperatures of a typical room mix. Unless you choose one, the loft takes RdSAP's default insulation for the age. If yours has been topped up the estimate reads high, and if it has less it reads low. We checked it against full room-by-room calculations of three test houses (a 1930s semi, a mid-terrace and a Victorian terrace), and it came out between 4% under and 6% over. It gives you a starting figure. Measure the rooms for one you can rely on.

Hot water on a combi

4.186 kJ heats a litre of water by one degree. Multiply by the litres a minute, by how many degrees the water has to rise, and divide by 60 seconds. A combi has no store, so it has to do this while the tap runs. The formula agrees with the spec sheets. Worcester Bosch's specifier guide for the 30 kW Greenstar 4000 (read September 2026) gives 12.3 l/min at a 35 °C rise, and 12.3 × 4.186 × 35 ÷ 60 = 30.0 kW. Vaillant's and Ideal's figures agree to within 1%.

Reheating a cylinder

A system or regular boiler heats a cylinder instead, and its hot water load depends on how quickly you want the cylinder back to temperature. The flow at the tap does not come into it. The Health and Safety Executive's legionella guidance says hot water should be stored at 60 °C at least (hse.gov.uk, page updated 26 September 2024).

Sizing a boiler for a 1930s semi

The 1930s semi in Leeds from the heat loss calculator's test set loses 7,370 W. With a combi and one mixer shower at 11 litres a minute, from 8 °C January mains to 41 °C at the head:

• Heating: 7,370 W = 7.4 kW
• Hot water: 11 × 4.186 × (41 - 8) ÷ 60 = 25.3 kW
• The hot water load is the larger, at 3.4 times the heating load. Nearest common combi: 26 kW. On a spec sheet, look for at least 10.4 l/min at a 35 °C rise.

The same house with a system boiler and a 150 litre cylinder reheated in an hour: 150 × 4.186 × (60 - 8) ÷ 3,600 = 9.1 kW for the cylinder. With hot water priority, where the heating pauses while the cylinder reheats, the boiler needs the larger of the two: 9.1 kW. If the controls run both at once, the two add up to 16.4 kW, or 11.9 kW with a two-hour reheat.

Two showers at once from a combi would take 50.7 kW. The biggest combis are badged at 50 kW, but Worcester Bosch's product page gives its 45 and 50 kW models the same flow, 16 litres a minute at a 40 °C rise, which is about 45 kW. So a house that runs two showers together needs a cylinder, and the calculator treats anything over 45 kW of hot water as beyond a combi.

In a large or draughty house the heating can be the bigger load. The badge size does not tell you the heating output: the combis we checked from Worcester Bosch, Vaillant and Ideal (read October 2026) give between 19.8 and 33.8 kW to the radiators. So when the heating is the bigger load and is above 19.8 kW, the calculator gives the heating output to look for instead of naming a size, and above 33.8 kW it points to a system boiler. If you are weighing a heat pump instead, the heat pump size calculator uses the same heat loss figure, and the Boiler Upgrade Scheme checker shows the grant.