The formula
output = area × height × watts per m³ × climate factor
Where it comes from
What has to be heated is air, so the sum works on volume: two rooms with the same floor area but ceilings of two and a half and three and a half metres do not want the same output. The usual reference is 40 watts per cubic metre in a home with normal insulation, and a factor for climate and aspect is applied on top. With the total watts and the output of each section of the chosen radiator — it is in the catalogue — you get the number of sections.
How to work it out by hand
- Multiply the floor area by the ceiling height: that is the volume
- Multiply by the reference watts per cubic metre
- Apply the factor for climate and aspect
- Divide by the output of each section
What is worth knowing
The watts per section quoted in a catalogue are measured at a particular water temperature, almost always 75 degrees flow. A condensing boiler or a heat pump runs considerably cooler — 45 or 50 degrees — because that is where they are efficient, and at that temperature the same radiator gives roughly half. That is why an installation designed for condensing carries noticeably larger radiators than one built for a conventional boiler, and why swapping only the boiler without touching the radiators usually disappoints. The climate factor here covers weather and aspect, but it is no substitute for a heat loss calculation when the home has a lot of glazing or very poor insulation.