Why Does My Outside Air Conditioner Unit Frost up?

Broken Evaporator Fan

• The compressor cools the air around it and lowers the humidity, but without the evaporator fan, this cold air doesn't move. Humidity drawn out of the air gathers on the compressor as well. The evaporator fan moves the air and helps moisture evaporate, but when this fan stops working, the water stays on the cold compressor and freezes. Frost will continue to build up until it covers the compressor and spreads to other parts of the unit.

Dirty Filter

• Even if the air handler and evaporator fans run at full capacity, low airflow lets temperatures around the compressor coils drop to below freezing. One common cause of reduced airflow is a dirty and clogged air filter, says Coolray Cooling and Heating. A blocked filter in any part of the heating and cooling system could cause frost buildup. The filters closest to the compressor coils and those located at air intake points cause the most strain and lead to the fastest ice development, so check these often.

Low Refrigerant

• The compressor coils are full of refrigerant that works to cool the metal surface of the coils and therefore the air around them. A leak in the pressurized refrigerant system of your air conditioning unit leads to low pressure in the coils, according to G&S Mechanical Services. Low pressure makes the coils cooler than they are designed to be. When the surface of the coils drops below 32 degrees F, any moisture on them freezes.

Outdoor Temperatures

• If your outdoor air conditioning unit is running when temperatures are below 60 degrees F, the unit is likely to develop ice or frost buildup, says Horizon Services. Outdoor air conditioning units aren't designed to run in the fall and winter. Indoor floor units may run if other heating is available because the air entering it is warm enough to drop in temperature without going below the freezing point. Outdoor units drawing in cool air drop the temperature by 30 degrees or more even on low settings, leading to frosty conditions where moisture condenses.