BTU
British Thermal Unit — the standard unit of heating and cooling capacity in North American HVAC.
Definition
A BTU (British Thermal Unit) is the amount of energy required to raise one pound of water by one degree Fahrenheit. In HVAC, equipment capacity is rated in BTU per hour (BTU/h, often shortened to just 'BTU' on spec sheets). A 12,000 BTU air conditioner removes 12,000 BTU per hour from a room. The metric equivalent is the kilowatt (kW); 1 ton of cooling = 12,000 BTU/h = 3.5 kW. For sizing residential HVAC, the rule of thumb is roughly 20 BTU per square foot of conditioned space, but proper sizing uses Manual J load calculations that account for insulation, window area, climate, and occupancy.
Where this term appears
BTU comes up in product spec sheets, service reports, and troubleshooting documentation across most major appliance brands. If you encountered it in a manual or a service description and want context for how it fits into the larger picture, the related articles in the sidebar drill into the practical details. The glossary as a whole is intended as a quick lookup reference; for a deep dive on any specific topic, jump to the related buying guide or troubleshooting article.