The formula
output = max(volume × W/m³ × factor, flow × rise × 4.186 ÷ 60) ÷ efficiency
Where it comes from
A boiler covers two demands that do not add up, because they do not coincide: you size for the larger of the two. And the larger one is not always the one people expect. Heating ten litres a minute from ten to forty degrees is almost 21 kW, more than the heating load of a well-insulated ninety-square-metre home. In flats the shower sizes the boiler; in large or poorly insulated houses, the heating does.
How to work it out by hand
- Multiply floor area by ceiling height for the volume
- Multiply by the watts per cubic metre and the climate factor
- For hot water: flow times temperature rise times 4.186, divided by 60
- Take the larger of the two and divide by the efficiency
What is worth knowing
Mains water comes in colder in winter — five degrees in the north against fifteen in the south — and that difference alone changes the hot water demand by nearly 40 %, in exactly the months when the heating is on as well. So it is worth sizing on the winter inlet temperature rather than the yearly average. Oversizing is not free either: a boiler far larger than the real demand cycles on and off constantly, which burns more fuel and wears it out sooner.