A multiple source heat pump for heating in cold climates has been investigated. The heat pump had four modes of operation, two solar source modes, an ambient air source mode, and a storage source mode. Refrigerant 12 cooled solar collectors served as the evaporator of the heat pump. The solar energy collected was used to heat space air or a water storage if heating was not required. During periods of low solar radiation and at night time, ambient air was circulated through the collectors which then served as the evaporator to extract . the heat from the ambient air.

A tempering heat exchanger was used in series with the storage evaporator to enable the heat pump to extract heat, under stable operating conditions, when the storage water was heated to a high temperature.

The solar source mode operated with minimum collector efficiencies of 0.5 at ambient temperatures as low as -23 C (-10 F). The air source mode operated with a minimum output of 268 watts, per square meter of net frontal collector area (85 Btuh/ft² ) at an ambient temperature of -18 C (0. F). The water storage was heated to 52 C (125 F) and cooled to 5 C (42 F).

It was noted that at low refrigerant flowrates the time averaged temperature of the refrigerant at the location controlled by the expansion valve was higher than the saturation temperature, however, the condition of the refrigerant at this location was actually wet. This possible condition should be taken into account when calculating, from experimental data, the energy absorbed by collectors.