BTU
Initialism of British thermal unit.
BTU: the heat energy needed to warm a pound of water one degree
A BTU, or British thermal unit, is a fixed amount of heat energy: specifically, the quantity required to raise the temperature of one pound of water by one degree Fahrenheit at standard atmospheric pressure. In modern practice, one BTU equals approximately 1,055 joules. This historical unit remains the standard measure across North American HVAC, refrigeration, and fuel industries, even though most of the world has adopted the joule.
The BTU appears in three main industrial contexts. In heating and cooling, equipment ratings are stated in BTU per hour (BTU/h), indicating how much heat the unit can add or remove. A residential furnace might deliver 60,000 BTU/h; a small window air conditioner might handle 5,000 BTU/h. In energy content, fuels and combustibles are rated by their total BTU yield: natural gas is typically 1,000 to 1,100 BTU per cubic foot, while gasoline contains roughly 115,000 BTU per gallon. Refrigerant capacity and cooling load calculations also hinge on BTU measures.
Understanding BTU ratings is essential for matching equipment to actual demand. A furnace undersized for a building will fail to reach setpoint in winter; an oversized unit will cycle too frequently and waste fuel. HVAC load calculations, performed by technicians using room dimensions, insulation values, and climate data, produce a BTU requirement that the selected equipment must meet or exceed. Similarly, refrigeration capacity must be matched to the heat load generated by the cooled space.
Where confusion arises
BTU/h and watts are often conflated. One watt equals 3.412 BTU/h, so a 3,500-watt heater produces approximately 11,940 BTU/h. Confusion between total BTU content and BTU/h rate is also common: a 40,000 BTU propane tank does not mean 40,000 BTU per hour of output; rather, it contains fuel with that total energy value, which will be consumed over time depending on the appliance's firing rate.
The term survives in North America because equipment specifications, contractor training, and building codes are entrenched in BTU language. Metric conversion to kilowatts is standard in technical literature, but on job sites and in equipment nameplates, the BTU remains the working unit. Anyone buying a furnace, speccing a chiller, or calculating fuel consumption for an industrial process will encounter BTU/h as the primary specification.