Quick answer
The basic unit of heat energy used to measure HVAC capacity.
A BTU is the amount of heat needed to raise one pound of water by one degree Fahrenheit. HVAC capacity is rated in BTUs per hour — how much heat a system can add or remove. Furnace output and air-conditioner/heat-pump tonnage both trace back to BTUs.
Why BTU matters
Fundamental HVAC terms help you understand capacity, comfort, and whether a quote is based on your actual home instead of a rule of thumb. They are the language behind repair, replacement, and sizing decisions.
When you'll see this term
You will usually hear this term during diagnostics, system sizing, replacement estimates, and explanations of heating or cooling capacity.
Questions to ask your technician
- How does this apply to my home's heating or cooling load?
- Was this number measured, calculated, or estimated?
- What changes if my insulation, windows, or ductwork are improved?
Go deeper
What size HVAC system do I need?