THE BRITISH THERMAL UNIT (BTU)

In some applications, a completely different unit of heat is used: the British thermal unit (Btu). You’ve heard this unit mentioned in advertisements for furnaces and air conditioners. If someone talks about Btus alone in regard to the heating or cooling capacity of a furnace or air conditioner, this is an improper use of the term. They really mean to quote the rate of energy transfer in Btus per hour, not the total amount of energy transfer in Btus.
The Btu is defined as the amount of heat that will raise or lower the temperature of exactly one pound (1 lb) of pure liquid water by one degree Fahrenheit (1°F). Does something seem flawed about this definition? If you’re uneasy about it, you have a good reason. What is a pound? It depends where you are. How much water weighs 1 lb? On the Earth’s surface, it’s approximately 0.454 kg or 454 g. On Mars, however, it takes about 1.23 kg of liquid water to weigh 1 lb. In a weightless environment, such as on board a space vessel orbiting the Earth or coasting through deep space, the definition of Btu is meaningless because there is no such thing as a pound at all.
Despite these flaws, the Btu is still used once in awhile, so you should be acquainted with it. Specific heat is occasionally specified in Btus per pound per degree Fahrenheit (Btu/lb/°F). In general, this is not the same number, for any given substance, as the specific heat in cal/g/°C.