During the winter mode, the reversing valve reroutes the refrigerant path therefore making the outside coil function as the evaporator and indoor coil as the condenser. The heat is absorbed from the outside air by the evaporator and then brought inside and released by the condenser to heat the home’s air. The outdoor expansion valve is active while the inside valve is bypassed. This is a very efficient and cost effective method for heating the home.
There is one inherent problem, though. In order for the evaporator, now outside, to absorb heat, it must operate at temperatures lower than the outside air. This very low operating temp allows ice or frost to accumulate on the outside of the evaporator. The remedy? The defrost cycle. As the defrost cycle is initiated, the reversing valve switches the refrigerant path back to the that of the summer mode. This allows indoor heat to be transferred from the inside air stream to the outside coil, thereby melting the accumulated frost.
To speed this process up, outdoor fan operation is terminated through outthe defrost cycle. The indoor air stream, now being robbed of its heat becomes very cold which is not a desirable condition during winter. To compensate for this condition, the auxiliary heat is activated for the duration of the defrost cycle. After a few minutes, the outdoor coil is frost free and the frost cycle is terminated.