Work out boiler sizing instantly with clear inputs, formula shown and shareable results.
Boiler duty is the space heating load plus a domestic hot water allowance, uplifted by a small margin for recovery after setback. Dividing that output by seasonal efficiency gives the gross input rating used to check gas or oil supply capacity and flue sizing.
Boiler output
Output kW = (space load + DHW allowance) x (1 + margin)
Gross input
Input kW = output kW / efficiency
Preliminary estimate only. Boiler selection, flue routing and gas supply must be verified by a qualified heating engineer against local regulations.
Output kW = (space load + DHW allowance) x (1 + margin). Boiler duty is the space heating load plus a domestic hot water allowance, uplifted by a small margin for recovery after setback.
For a combination boiler the hot water duty usually dominates and is not added to space heating, because the two are not served simultaneously. For a stored-cylinder system the cylinder reheat allowance is added as shown.
This calculator takes 4 inputs: Space heating load, Hot water allowance, System margin, Seasonal efficiency. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.