A BTU (British Thermal Unit) is the energy required to raise one pound of water by one degree Fahrenheit. In HVAC, capacity is measured in BTU per hour (BTU/hr).
The conversion you'll see most: 12,000 BTU/hr = 1 ton of cooling. So a 3-ton AC = 36,000 BTU/hr.
Heating capacity is also rated in BTU/hr. A 60,000 BTU/hr gas furnace at 90% AFUE delivers about 54,000 BTU/hr of usable heat. A 3-ton heat pump in heating mode might deliver 30,000-45,000 BTU/hr depending on outdoor temperature.
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