Glossary
BTU
Also known as British thermal unit
A unit of heat energy equal to the amount needed to raise one pound of water 1°F—the standard measure for heating and cooling capacity.
BTU stands for British thermal unit and is the baseline measurement for heat output in HVAC and water heating equipment. One BTU is the energy required to raise the temperature of one pound of water by one degree Fahrenheit at standard conditions. In the field, BTU ratings tell you the heating or cooling capacity of a furnace, boiler, air conditioner, or water heater—a 50,000 BTU boiler outputs that much heat energy per hour under full load.
When sizing equipment, BTU/h (per hour) is what matters. A residential water heater might be 40,000 BTU/h, while a commercial boiler could run 500,000 BTU/h or higher. Load calculations determine what BTU capacity a system needs to meet demand; undersizing leaves customers cold or with slow recovery, oversizing wastes fuel and money. BTU is also used to rate burners, radiant heating loops, and heat pump output, so you'll see it everywhere on spec sheets and nameplate ratings.