Boiler Horsepower to Heat Output

Boiler horsepower has nothing to do with mechanical horsepower and everything to do with 1889. The ASME committee of the day defined one boiler horsepower as the steam required by a typical steam engine of one horsepower — 34.5 pounds per hour of steam "from and at 212 °F" — and since evaporating water at atmospheric pressure absorbs 970.3 BTU/lb, that works out to 34.5 × 970.3 = 33,475 BTU/h. It is roughly 13 times a mechanical horsepower, which catches out anyone who assumes the two are related.

A 100 BHP boiler is therefore rated at 3,347,500 BTU/h output, or about 981 kW. Two cautions when you use the number. First, this is output, not fuel input: at 80% combustion efficiency that same boiler burns roughly 4,180,000 BTU/h of gas, which is what the meter and the gas bill see. Second, the definition assumes feedwater already at 212 °F; a real boiler feeding 180 °F condensate or 55 °F makeup must supply the sensible heat too, so nameplate BHP always flatters the delivered steam rate slightly.