Boiler horsepower

Boiler horsepower is a rating of heat output, not shaft power, and it is 13 times larger than mechanical horsepower — 9.81 kW against 0.746 kW. The definition comes from the 1876 Centennial Exhibition in Philadelphia, where a committee decided that the boiler serving a good steam engine of one horsepower had to evaporate about 34.5 pounds of water per hour "from and at 212 °F", meaning feedwater already at boiling point. Multiply 34.5 lb/h by the 970.3 BTU/lb latent heat and you get the defining 33 475 BTU/h.

The unit is still printed on North American firetube boiler nameplates, where it is a convenient proxy for physical size — roughly 5 square feet of heating surface per boiler horsepower. Actual fuel input is higher than the rating by the reciprocal of efficiency, and feedwater below 212 °F needs extra sensible heat, so a 100 bhp boiler at 80 % efficiency burns closer to 4.2 million BTU/h of gas.