During the winter months when your heat is on, there will be moisture draining from the surface of the outdoor coil and around the base when the unit is in the heating mode. This moisture will turn to frost when the outdoor temperature is below 40° F. The outdoor unit may become totally white with frost/ice during cold weather. Unless this condition persists for over 2 hours, you should not be concerned. Frost will build up rapidly during damp cold weather (28° to 38° F). When the outdoor unit starts to defrost, it will, without stopping, make a “swoosh” sound, stop the outdoor fan, and briefly deliver slightly cooler air inside. The duration of this defrost cycle may take up to 10 minutes depending on the amount of frost on the outside coil. Steam or fog from the outdoor unit is normal during the defrost cycle.
Is it normal to have frost on my outdoor unit in winter?