What is defrost mode and why does this happen?

When a heat pump is operating in the heating mode or heat cycle, the outdoor air is relatively cool and the outdoor coil acts as an evaporator. Under certain conditions of temperature and relative humidity, frost might form on the surface of the outdoor coil. The layer of frost will interfere with the operation of the heat pump by making the pump work harder and, therefore, inefficiently. The frost must be removed. A heat pump has a cycle called a defrost cycle, which removes the frost from the outdoor coil.

A heat pump unit will defrost regularly when frost conditions occur. The defrost cycle should be long enough to melt the ice, and short enough to be energy-efficient.

In the defrost cycle, the heat pump is automatically operated in reverse, for a moment, in the cooling cycle. At this time the electric heat strips are activated so the indoor air temperature can be maintained. Sometimes the air temperature can be a little cooler while going through defrost mode, being completely normal. This action temporarily warms up the outdoor coil and melts the frost from the coil. In this defrost cycle, the outdoor fan is prevented from turning on when the heat pump switches over, and the temperature rise of the outdoor coil is accelerated and increased.
The heat pump will operate in the defrost cycle until the outdoor coil temperature reaches around 57° F. The time it takes to melt and remove accumulated frost from an outdoor coil will vary, depending on the amount of frost and the internal timing device of the system.