A window or wall air conditioner can sometimes frost over or become covered with a fine layer of ice, even when it isn't cold. This is a common problem for those with large AC units, and there are a few reasons why the wall air conditioner can become covered with frost. Once you understand the reasons for this lack of warmth, you should be able to take care of any frost which develops on your air conditioner unit.
Why Frost Forms
The AC unit has a large metal coil in its machinery, known as the evaporator coil. This is designed to evaporate water from the surface of the unit, and is part of the heat-exchange system which helps to keep the room cool. When the AC unit is not working properly, or for one of the reasons described below, the evaporator coil does not warm up enough to cause the water to heat up. It may in fact have the opposite effect of cooling the water down enough that it becomes solid, creating a layer of frost on your AC unit.
Size of Unit
By far the biggest reason for an AC unit to develop frost is through lack of efficiency in cooling the room due to its size. The AC unit may be too large for the house, in which case it will make the room colder than it is designed for. Most AC units are only intended to make a room 72 degrees or above, so any colder than this will make the evaporator coil freeze. To prevent frosting, set your temperature at 74 degrees or higher.
The size of the AC unit can play a significant part in frosting. The air conditioner will need a certain amount of airflow, relative to its area that it needs to chill, so if you have a very large AC unit in a small room, it will make the room too cold, again resulting in the evaporator coil freezing and producing frosting. To small an AC unit can prevent enough hot air from getting to the system, meaning that the internal parts of the unit are left too-cold.
Dirt Inside the Unit
Another cause of frosting in the AC unit is dirt and dust. If you have had the AC unit for a number of years, it may have become very dirty. If there is excess dust in the machine, or the filter is blocked, hot air cannot circulate around the machine, moisture will be trapped on the tubing and the unit will drop to below 72 degrees. Avoid this problem by keeping your unit clean. Dust every month or so, and wipe out the filter.
Low on Freon
Your AC unit needs the liquid known as freon to prevent the machine from becoming too cold. If there is a drop in the Freon level, either through a leak, or not enough being added during the manufacturing process, you may find that the coils develop a fine layer of frost. Fix this by calling in a AC unit repair person, or technician, who will add some more liquid to your unit's circulation.