Abstract: Disclosed are apparatus and methods for controlling the defrosting of the outdoor heat exchange coil of a heat pump. In general, the disclosed apparatus and methods use the differencee between the outdoor heat exchanger and the outdoor ambient temperature to determine when to initiate a defrosting operation. Only a single sensor is employed, this single sensor being positioned at least proximate the outdoor heat exchanger such that the sensor responds to ambient temperature when the heat pump is first started and the heat exchanger is free of frost, and such that the sensnor becomes surrounded with frost as frost builds up on the heat exchanger during operation. At the beginning of a run the sensor is employed to read the ambient temperature, which is memorized. As frost builds, the sensor begins to reflect the temperature of the heat exchanger as the sensor becomes increasingly insulated by the frost from the surrounding air.

Abstract: A defrost system for refrigerating apparatus having a refrigerating mode and a defrosting mode including an evaporator and an electric resistance heater in heat transfer relationship with the evaporator. There is provided a timer to actuate a normally open defrost initiation switch to energize the heater periodically and initiate a defrosting mode. A defrost terminating thermostat switch is in the heater circuit and will open at a predetermined elevated temperature to deenergize the heater and close at a lower temperature. A voltage sensor across the normally open defrost initiation switch senses periodically and provides a signal when voltage in the heater circuit is interrupted. The multiple voltage sensor signals are stored in a memory bank over a predetermined period of time and when the voltage is interrupted for the predetermined period of time, an alarm is actuated to indicate failure of the defrost system.