Heat Pump Calculator

Heat pump heating capacity decreases as outdoor temperature drops because there is less heat available in cold outdoor air for the refrigerant to absorb, and the outdoor coil has to defrost. AHRI rates heat pumps at two outdoor temperatures: 47°F, the standard rating point, and 17°F, a low-temperature point that includes the effect of defrost. A standard heat pump typically delivers roughly 55–70% of its 47°F capacity at 17°F, while cold-climate models hold much more, so using a unit's own two ratings is far more accurate than a generic derate. Between and beyond those points capacity changes roughly linearly with outdoor temperature. Below a certain outdoor temperature, known as the balance point, the heat pump alone cannot meet the building heat loss, and supplemental heating (electric strip heat or gas backup) is required. The balance point depends on the building load, the unit's capacity at each outdoor temperature, and the local design temperature. Altitude does not change the capacity here; thinner air carries less heat per CFM, so the same capacity produces a larger temperature rise.