BTU (British Thermal Unit)
Also known as: British thermal unit
Standard unit of heat energy. One BTU is the energy required to raise one pound of water by one degree Fahrenheit. HVAC equipment is rated in BTU/hour.
A BTU is a unit of heat energy. In HVAC contexts, equipment capacity is usually expressed in BTU/hour — the rate of heat delivery (heating) or removal (cooling) the equipment can produce.
For reference: 1 ton of cooling = 12,000 BTU/hour. A typical residential central AC system is 2-5 tons (24,000-60,000 BTU/hour). A small mini-split might be 9,000-12,000 BTU/hour for a single room. A whole-house furnace is typically 60,000-120,000 BTU/hour. Customer-facing conversation often uses 'tons' for AC and 'BTU/hour' for furnaces, but the underlying unit is the same. Selecting the right BTU/hour capacity for a building requires Manual J load calculation; under- or over-sizing both produce poor performance and shortened equipment life.
Related terms
SEER (Seasonal Energy Efficiency Ratio)
Cooling efficiency rating for air conditioners and heat pumps. Higher SEER = more efficient. Federal minimums in 2026: 14-15 SEER for residential central AC.
Manual J load calculation
ACCA-standard methodology for calculating the heating and cooling load of a building. The required first step before sizing an HVAC system properly.