British thermal unit
Energyexact by definition
The British thermal unit used here is the International Table BTU, exactly 1055.05585262 joules. That value is exact by definition, following from the International Table calorie of exactly 4.1868 J. The informal description is the heat needed to raise one pound of water by one degree Fahrenheit, which like the calorie is only approximately true because water's specific heat varies with temperature.
Watch out: At least five BTUs are in circulation. The International Table BTU is 1055.05585262 J, the thermochemical BTU about 1054.35 J, the 39 °F BTU about 1059.67 J, the 59 °F BTU about 1054.80 J, and the mean BTU about 1055.87 J. They span about half a percent, which matters in fuel contracts and rarely elsewhere. Note also the therm, which is 100 000 BTU and is the unit natural gas is often billed in.
| 1 BTU | 1,055.0559 J |
The unit arose in nineteenth century British steam engineering and, despite the name, is now used mainly in the United States and Canada. British practice moved to the joule and kilowatt.
About the british thermal unit
The BTU is the customary counterpart of the calorie, built on the pound and the Fahrenheit degree instead of the gram and the Celsius degree. Its rough definition, the heat to raise one pound of water by one degree Fahrenheit, has the same defect: water's specific heat is not constant, so the answer depends on which degree you pick. That is why the unit fragmented into a family of definitions labelled by reference temperature, and why standards bodies eventually pinned the International Table version to an exact number of joules and stopped talking about water.
In North American HVAC the BTU is inescapable. Furnace outputs, air conditioner capacities, water heater ratings and gas appliance inputs are all in BTU per hour, usually spoken as "BTUs" with the per-hour dropped, which turns an energy into a power by convention alone. A 60 000 BTU/h furnace is a 17.6 kW furnace. A "3 ton" air conditioner is 36 000 BTU/h, which is 10.55 kW of cooling. Natural gas is sold by the therm, 100 000 BTU, or in Canada by the cubic metre with an energy content around 37.5 MJ.
The conversion worth memorising is 1 BTU/h ≈ 0.293 W, or equivalently 3412 BTU/h per kilowatt. That last number turns any electric heater rating straight into a heating capacity, since resistance heating is 100 percent efficient at converting electricity to heat.