The purpose of the defrost cycle on a heat pump is to remove the accumulation of frost built up on the outdoor coil during the heating cycle. To accomplish this task, the reversing valve directs the flow of hot refrigerant vapor to the outdoor coil. This hot vapor causes the frost to melt.
When the system goes into defrost, the outdoor fan is stopped to permit the outdoor coil to heat up much faster. The indoor fan continues to run and cold air will blow from register. This is necessary because the indoor coil is now absorbing heat from the indoor air to aid in the defrosting process.
The unit should not stay in the defrost cycle any longer than is required, usually, 1 to 10 minutes, because energy is used without heat being discharge into the building. Thus, the overall cost of heating the building is increased.
When the heat pump is operating in the heating mode, refrigerant is evaporating in the outdoor coil. If the temperature of the coil falls below 32 degrees, frost will begin to form on the coil. Eventually, the frost can build up enough on the coil to restrict the air passing through the coil, causing reduced efficiency. The frost will also act as in insulator on the finned surface and reduce heat transfer, thus reducing coil efficiency even further. When the coil efficiency is reduced enough to appreciably affect system capacity, the frost must be removed.
Frost is removed by causing the reversing valve to switch positions, putting the system in the cooling cycle and stopping the outdoor fan. This directs the hot discharge refrigerant vapor to the outdoor coil to melt the frost.
There are several methods of automatic defrost initiation and termination. The most popular are (1) air-pressure differential across the outdoor coil, 2) outdoor coil temperature, (3) time, (4) time and temperature, and (5) solid-state defrost control.