Your Heat pumps Icing up will be natural during the winter. It is considered normal if the entire coil becomes lightly covered in ice but only in certain weather conditions. It is not normal if whole unit becomes completely covered in Ice for a long period of time. This includes the top of the unit as well as the inner coil. Turn off your heat pump to stop ice from building up. You need to have this checked out as it may cause severe damage to your unit. The Defrost Cycle Heat pumps are designed to pull cold air from a home and not freeze up.