BOILER SIZE
BOILER SIZE
BTU/hr = ft² × climate_factor
ft²
RESULT
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FILL IN ABOVE
Rule of thumb based on climate zone. Cast-iron boilers commonly 82% AFUE; modulating condensing boilers 90-95%. Radiant systems sometimes specced 10-15% lower than radiator systems.
About this calculator
This boiler size calculator returns the heating output needed for a residential hydronic system, whether the distribution is radiators, baseboard, or radiant floor. The rule of thumb is the same as forced-air furnace sizing — 25-60 BTU/ft² depending on climate zone — but boilers are quoted in input BTU at the gas burner and the typical efficiency profile (AFUE) ranges from 82% non-condensing to 95% modulating-condensing. For radiant floor systems specifically, design heat loss is typically lower than radiator systems because the entire floor is the emitter, but the boiler sizing follows the same envelope load.