Compressor heat pump system with maximum and minimum evaporator .DELTA.T control

In an air source heat pump system of the type including a first heat exchanger forming an indoor coil, a second heat exchanger forming an outdoor coil and positioned in an ambient air flow, a compressor, and conduit means carrying a refrigerant and connecting said compressor between said coils and in a closed series loop, an expansion means provided within the conduit means adjacent to the inlet end of the outdoor coil to permit the outdoor coil to act as an evaporator when the system is in the heating mode, and wherein means are provided for loading and unloading the compressor to vary the capacity of the compressor and the system compression ratio, the improvement comprising:

• (a) temperature sensing means positioned adjacent the outdoor coil and within the ambient air flow passing over the outdoor coil;
• (b) means for sensing the evaporating temperature of the refrigerant available to the compressor from the outdoor coil;
• and
• (c) control means operatively connected to said sensing means and responsive to a predetermined temperature decrease of evaporating temperature below ambient temperature for at least preventing further loading of the compressor, thereby tending to prevent the outdoor coil surface temperature from dropping below the dew point and virtually eliminating frosting of said outdoor coil.