Because the outdoor air is relatively cool when the heat pump is on the heating cycle, and the outdoor coil is acting as an evaporator, frost forms on the surface of the coil under certain conditions of temperature and relative humidity. Because this layer of frost on the coils interferes with the efficient operation of the heat pump, it must be removed. This is accomplished by putting the heat pump through a defrost cycle.
In the defrost cycle, the action of the heat pump is reversed at certain intervals and returned to the cooling cycle. This is done to temporarily heat the outdoor coil and melt the frost accumulation. The temperature rise of the outdoor coil is hastened because the operation of the outdoor fan stops when the system switches over to the cooling cycle.
The system will remain in the cooling cycle until the coil temperature has risen to 57°F. The time of the defrost cycle will vary, depending on how much frost has collected on the coil. During this period, the indoor motor continues to operate and blow cool air. This cold condition can be eliminated by installing an electric heating element (see Auxiliary Electric Heating Elements in this chapter). The heating element is wired in conjunction with the second stage of a two-stage thermostat and will come on automatically when the heat pump is in the defrost cycle (terminals 9 and 7 on the defrost relay.
The defrost cycle control system consists of a thermostat, timer, and relay. The defrost thermostat is located at the bottom of the outdoor coil where it can respond to temperature changes in the coil. It makes contact (closes) when the temperature of the outdoor coil drops to 32°F. This action of the thermostat causes the timer motor (located in the unit electrical box) to start. After the accumulative running periods reach either 30 minutes or 90 minutes (depending on the type of cam installed in the timer), the timer energizes the defrost relays, which reverses the reversing valve and stops the outdoor fan motor. The unit remains in the defrost cycle (cooling cycle) until the temperature of the outdoor coil reaches 57°F. At that temperature the coil is free of frost and the frost thermostat opens to stop the timer and return the unit to the heating cycle. The timer will not run again until the outdoor coil temperature drops to 32°F. The timer runs only when the thermostat contacts are closed.
A defrost timer is shipped with a 30-minute cam installed. With this cam, the unit will defrost once every 30
minutes (of accumulated running time) when the outdoor coil temperature is below 32°F. If there is little or no frost on the coil, the defrost cycle will be correspondingly short (approximately 45 seconds to 1 minute). A 90-minute cam is recommended.
Defrost system wiring diagram