Boiler size calculator: methodology
Everything is worked out in your browser; nothing you enter is sent anywhere. The calculator sizes the two jobs a boiler does separately, because they need very different amounts of power.
Heating: from your gas use
A year of gas, less what heats water, is the heat your house lost over a year of weather. Degree days measure that weather: for each day, how far its average temperature fell below 15.5°C, added up over the year. Dividing the heat by the degree days gives how much heat the house loses for each degree colder it is outside:
heat loss (kW per °C) = (gas − hot water gas) × efficiency ÷ (24 × degree days)Multiplying by how cold the coldest days get gives the heat needed then:
heating (kW) = heat loss × (15.5 − design temperature)15.5°C is used rather than room temperature because heat from people, appliances and the sun keeps a house warm down to roughly that outside temperature; the degree days are counted from the same base, so the two stay consistent.
Example. Ofgem’s typical household uses 9,500 kWh of gas a year. Less 1,900 kWh for hot water, at 85% efficiency, that’s 6,460 kWh of heat. In the South East, with 2,255 degree days, the home loses 119 W per °C; at -3°C it needs 2.2 kW.
This measures the home as it is heated now. It reads low if you heat only some rooms or keep the house cool, and high if last winter was unusually cold or you were home more than usual. It assumes your gas is mostly heating and hot water: a gas fire or cooker is counted as heating.
The weather
Degree days and design temperatures come from the MCS Heat Load Calculator, design conditions: degree days by postcode area from Met Office records; design temperatures from CIBSE Guide A (2015) Table 2.5. The design temperature is the 99.6% figure, the level only 0.4% of hours fall below. We use one postcode area and the nearest listed weather station for each electricity region:
| Region | Degree days | Design temperature |
|---|---|---|
| North West England | 2,228 (M) | -4.5°C (Manchester) |
| North East England | 2,370 (NE) | -3.7°C (Newcastle) |
| Yorkshire | 2,307 (LS) | -3.3°C (Leeds) |
| North Scotland | 2,668 (AB) | -5.4°C (Edinburgh) |
| Southern England | 2,224 (SO) | -4.8°C (Southampton) |
| South and Central Scotland | 2,494 (G) | -5.6°C (Glasgow) |
| Merseyside and North Wales | 2,228 (L) | -4.5°C (Manchester) |
| London | 2,033 (E) | -3°C (London Heathrow) |
| South East England | 2,255 (CT) | -3°C (London Heathrow) |
| East of England | 2,254 (NR) | -4.6°C (Norwich) |
| East Midlands | 2,254 (NG) | -3.9°C (Nottingham) |
| West Midlands | 2,425 (B) | -5.1°C (Birmingham) |
| South West England | 1,858 (EX) | -1.5°C (Plymouth) |
| South Wales | 1,835 (CF) | -3.1°C (Cardiff) |
Hot water: from your shower
A combi heats water as it flows through, so the power it needs depends on the flow and how much the water is warmed:
hot water (kW) = litres a minute × 4.186 × temperature rise ÷ 604.186 kJ warms a litre of water by one degree. We use a 35°C rise, the figure combi manufacturers quote their hot water flow at, which is roughly cold winter mains water warmed to a hot shower. The default shower is 12 litres a minute, Triton’s average for a mixer shower, which needs 29.3 kW. We suggest the smallest common combi size that covers it.
Hot water gas
The default 1,900 kWh is 95 litres of hot water a day at 52°C, the government SAP method’s typical household, warmed from 12°C mains at 85% efficiency. A big household uses more, so change it if you know your summer gas use: with the heating off, a summer day’s gas is nearly all hot water.
Boiler specifications we checked
- Ideal Logic Max Combi2 C30 specification: 12.4 litres a minute at a 35°C rise from 30.3 kW; heating from 6.1 kW.
- Vaillant ecoTEC Pro 24 specification: 9.6 litres a minute at a 35°C rise; heating from 5.2 kW.
- Ideal Logic Max C35: 35.3 kW and 14.5 litres a minute, the most powerful hot water in the Logic Max range.
What this isn’t
It isn’t a room-by-room heat loss calculation, which an installer should do before fitting a boiler, and it doesn’t size radiators. Gas boilers must be fitted by a Gas Safe registered engineer.