Sizing Heat Pumps
The recommended source of information for sizing heat pumps is the latest edition of the Air-Conditioning Contractors of America’s Manual H—Heat Pump Systems: Principles and Applications. It covers basic principles, equipment, installation, operation, and sys- tem design.
The ability of the heat pump to extract heat from the outdoor air decreases as the outdoor temperature drops. At the same time, the heat required to maintain the desired indoor temperature increases. If lines representing the outdoor and indoor conditions
were drawn on a graph, the line representing the dropping outdoor temperature would eventually cross the line representing the rising indoor heat requirements. The point at which the two lines on the graph cross is called the balance point. The capacity of a heat pump must be sized to match the balance point for the house or building. Most balance points will occur just below and above freezing (Fahrenheit).