What is a heat load calculation?

A heating load is the rate of heat loss from a conditioned space to the outdoors at the winter design conditions. A cooling load is the rate of heat gain at the summer design conditions, divided into sensible and latent components. Heating equipment must supply heat at the heating-load rate to hold the indoor design temperature; cooling equipment must remove heat at the cooling-load rate.

Loads are rates of heat transfer; in U.S. practice they are commonly expressed in BTU per hour, and equipment capacity is stated in the same unit. Both loads are peak values at design conditions, calculated for the structure as a whole and for each room.

One procedure produces both loads. It accounts for conduction through the envelope (walls, roof, floor, glazing, doors), infiltration, and ventilation. The heating load takes no credit for solar or internal gains, since peak heat loss occurs at night. The cooling load adds solar gain through glazing and internal gains from occupants and appliances. The standard's instruction on safety factors is that calculations be aggressive: full credit for the construction as built, no added margin.

The load calculation is the first step of the HVAC design sequence. Equipment selection and duct design are derived from its results.

Last reviewed 2026-09-06. Accurate to the best of our knowledge as of that date. Rules change — that's why the date is there.