Heat pump control/defrost circuit

Claim 1

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 mode selection switch means for placing said air conditioning unit in said heating or cooling cycle; a thermostat means having a first stage switching means operable for energizing said compressor in said heating cycle when the temperature of said enclosure is a predetermined comfort level, and a second stage switching means operable for de-energizing said compressor when the temperature of said enclosure drops to a second lower predetermined level during the operation of said compressor in said heating cycle; a heating means arranged in the path of air through said indoor heat exchanger being energized by said second stage switching means when the temperature of said enclosure drops to the second lower preselected temperature; a frost control means including a sensing element being exposed to the surface temperature of said outdoor heat exchanger, switch means under control of said sensing element movable between a first position for maintaining operation of said compressor in the heating cycle when said heat exchanger surface temperature sensed by said portions are above a preselected frost accumulation level and being movable to a second position for de-energizing said compressor when either of said sensing elements sense a preselected frost accumulation level to place said unit in a defrost mode and including circuit means operable when said frost control switch means is in the second position for energizing said heating means through said mode selection switch and said thermostat first stage switching means thereby energizing said heater at the start of said defrost mode to provide uninterrupted heating when said compressor is de-energized by said frost control means; and a second control including a sensing element having one portion being exposed to said ambient outdoor temperature means under control of said sensing element for maintaining operation of said outdoor fan independent of said first control when the ambient sensed by said sensing element is above a frost accumulation temperature and to complete a circuit to said fan through said first control if said sensing element of said second control senses a preselected frosting temperature to de-energize said outdoor fan when said compressor is de-energized.