BTU (British Thermal Unit)
The basic unit of heating and cooling energy — the heat needed to raise one pound of water by 1°F. Equipment capacity is stated in BTU/h (BTUs per hour); 12,000 BTU/h equals one ton of cooling.
The longer answer
BTU ratings appear on every piece of equipment, but the number that matters is the one from your home's load calculation — not the label on the old unit being replaced. Matching 'whatever was there' is how systems end up 30–50% oversized, which wastes money and comfort for 15 years.
For ductless systems, minimum and maximum BTU output per head also matters: a head rated for 6,000–18,000 BTU/h can modulate down to serve a small bedroom; one with a 9,000 BTU floor can't — and will short-cycle in a 100-square-foot room.
The Seattle angle
Seattle's mild design temperatures mean heat pumps are often sized smaller than intuition suggests — oversizing in BTUs causes short-cycling, the comfort killer in our climate.
Where this decision happens
Nearby in the glossary
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