De-Icing AC Evaporator Coils
An AC unit uses a set of evaporator coils to cool the air in your house. As a fan pushes, the hot air from inside your house over the coils, the refrigerant inside the coils vaporizes. During the refrigerant's phase change from a liquid to a gas, it absorbs heat from its surroundings. However, if there is not enough air flowing over the coils, they can absorbs so much heat, that the water vapor in the air flowing over the coils freezes.