Heat pump control system

In a self-contained air conditioning unit for heating and cooling an enclosure, a refrigerant circuit including an outdoor heat exchanger and indoor heat exchanger, a compressor, a reversing valve for selectively connecting said compressor to said heat exchangers whereby said outdoor heat exchanger functions as an evaporator during operation of said unit on the heating cycle and said indoor heat exchanger functions as an evaporator during operation of said unit on the cooling cycle, fan means for moving enclosure air through said indoor heat exchanger, fan means for moving outdoor ambient air through said outdoor heat exchanger, an air conditioner heating cycle control system comprising:

• a supplemental and secondary heat means arranged in air flow relationship with said indoor heat exchanger;
• a thermostat, including a first-stage switching means, operable for controlling operation of said compressor and said secondary heat means in said heating cycle for maintaining the temperature of said enclosure within a selected temperature range, and a second stage switching means operable when the temperature of said enclosure drops below said selected temperature range for energizing said supplemental heating means while said compressor is operating through said first-stage switching means;
• a first frost control means including a sensing element having one portion being exposed to the surface temperature of said outdoor heat exchanger and a second portion being exposed to the drain area temperature, switch means under control of said portions including a first switch position for maintaining operation of said compressor in the heating cycle when drain area and said outdoor heat exchanger surface temperature sensed by said portions are above a preselected frost accumulation level and a second switch position operable when either of said portions senses a preselected frost accumulation level to de-energize said compressor and for energizing said secondary heat means through said first-stage thermostat switch means so that the temperature of air passing through said indoor heat exchanger is maintained at a preselected temperature by said secondary heat means when said compressor is de-energized;
• and
• a second frost control including a sensing element having one portion being exposed to the surface temperature of a portion of said refrigerant circuit and a second portion being exposed to said ambient outdoor temperature, switch means under control of said portions for maintaining operation of said outdoor fan independent of said first control when both ambient and the line surface temperature sensed by said portions are above a frost accumulation temperature and to complete a circuit to said fan through said first control if either of the portions of said second control senses a preselected frosting temperature to de-energize said outdoor fan when said compressor is de-energized.