What Is BTU?

A BTU (British Thermal Unit) is the energy needed to raise one pound of water by one degree Fahrenheit. HVAC equipment is rated in BTU per hour — a rate, not a quantity — describing how fast a system adds or removes heat. Gas appliances are also rated in BTU/h for their fuel input.

Also called: British Thermal Unit · BTU/h

Why it matters to you

BTU/h is the common unit that lets you compare unlike equipment: a furnace, a heat pump and a gas grill connection are all rated the same way. On a gas piping quote it matters directly, because total connected BTU/h load is what determines the pipe size feeding your appliances.

What to do about it

Note whether a figure is input or output BTU/h on fuel-burning equipment — an 80,000 BTU/h input furnace at 80% AFUE delivers about 64,000 BTU/h of heat. For gas piping, give your installer every appliance's rating so the line is sized for the whole load.