Heat load (also called heating load or heat loss) is the rate at which heat escapes from a conditioned space through the building envelope — walls, windows, roof, floor, and air infiltration. It is measured in BTU per hour and represents how much heating capacity the HVAC system must provide to maintain the desired indoor temperature on the coldest expected day.
Heat load is calculated using ACCA Manual J, which accounts for: the difference between indoor and outdoor design temperatures, R-value of all insulation, U-value of all windows and doors, surface area of each building component, and air infiltration rate.
Understanding heat load is essential for proper HVAC sizing. An oversized heating system short-cycles (turns on and off frequently), wastes fuel, and fails to run long enough to properly mix air. An undersized system cannot maintain comfort on the coldest days. Heat load calculations should be performed by every HVAC contractor before recommending equipment.