British Thermal Unit (BTU)

Definition

The energy needed to raise one pound of water by one degree Fahrenheit, used in HVAC sizing. Heating and cooling loads are typically expressed in BTUs per hour. A rough rule of thumb: 20 BTU per square foot of living space, adjusted upward for sun exposure, high ceilings, poor insulation, or many windows, and downward for shaded, well-insulated rooms. A 1,500 square foot home typically requires 30,000 BTU of air conditioning and 40,000 BTU of heating, though Manual J load calculations give the engineering-accurate number for new installations.