Why window Air Conditioners Ice up

Why window Air Conditioners Ice up by Donald Grummett
One of the most frustrating problems with a window air conditioner is if it starts to ice up. For the owner it can be perplexing.
When this problem occurs most people immediately think it is the fault of the refrigerant gas. More often the cause is the result of other difficulties.
It is most often the result of poor airflow.
Any time the airflow through an air conditioner is restricted the cooling system becomes affected. If taken to extremes the critical pressure-temperature balance of the cooling coil can be changed. If they drop too low the cooling coils surface temperature can drop below the dew point temperature of the room. When this happens the cooling coil will begin to operate as a refrigerator rather than an air conditioner. Rather than simply cooling the air it will collect and hold moisture. This moisture will then freeze onto the cooling coil where it will appear as ice.
With this information in mind the following are a few problems that can result in your air conditioner icing up:
1. Dirty filter.
To avoid this replace or clean your filter every couple of weeks of the cooling season. If a smoker, clean every week.
To clean filter remove from air conditioner, wet thoroughly, and lay in bottom of a sink. Sprinkle cleaner (laundry detergent works well) onto filter surface. Allow to sit for a few minutes. Then add enough hot water to to cover filter. Soak for 15 minutes. Remove from water and rinse. Allow to air dry.
2. Dirty or blocked cooling coil.
An air conditioner requires regular maintenance. Usually every two or three years. Every year would be best, but this can be costly unless you do it yourself. During cleaning the cooling coil should be degreased and washed to remove accumulated dirt and debris.
Degreasing is important to remove any coatings on the coil. If not done greasy residue can trap and hold air borne particles. They will build up on the coil and affect heat transfer. If left too long this can result in the cooling coil becoming partially blocked. This will produce a lower airflow, which can lead to icing.
3. Dirty or blocked condenser coil.
The condenser coil is the one at the rear of the air conditioner. Its job is to dissipate the heat that is being removed from the room. Just like the cooling coil it too must be cleaned every few years.
Since the condenser is outdoors it becomes exposed to a lot of dirt, pollen, and smog. Since airflow direction is from inside to outside it is the inside surface of the condenser that becomes dirty. Therefore to clean this part the air conditioner must be completely disassembled. If not cleaned regularly an airflow blockage here can even burn out the compressor. Before this happens though the lowering of airflow will affect the overall operation. This can result in the compressor efficiency dropping, the internal pressure-temperature relationships being affected, and the resultant production of ice on the cooling coil.
4. Inefficient compressor.
As describe above an inefficient compressor can cause icing up. When the compressor is inefficient the cooing coil may not get cold enough to shut off the cold control. It can hover just above the cut off point. This causes the cooling coil to refrigerate, rather than dehumidify. Ice on the cooling coil will result.
If the compressor itself is at fault the air conditioner will usually be replaced. But note that many icing problems are misdiagnosed as bad compressors when they were actually one of the other faults discussed in this article.
5. Not enough refrigerant. Too much refrigerant.
Both scenarios can result in an icing condition. If your air conditioner was repaired recently suspect too much refrigerant. Mixed with an airflow problem this can be difficult to diagnose.
If not repaired recently then suspect airflow problems before considering a refrigerant imbalance.
6. Outdoor temperature too low.
Icing can occur if the outdoor temperature falls below 60 Degrees Fahrenheit. When the outdoor temperature is too low the air conditioner pressure-temperatures can be affected, causing the cooling coil temperature to fall so low that the coil will refrigerate the room air. This results in ice production.
This problem is more prevalent in the fall. If it is hot during the day, but cold at night, suspect this problem as the cause of icing up.
If this problem is suspected try running the air conditioner in the 'fan only' position. Leave the re-circulating vent open. This will circulate the room air without cooling it, while bringing in a small amount of outside air during the night.
Alternately, turn of air conditioner at night and restart next morning as day becomes warmer.
7. Oversized air conditioner.
If the air conditioner is too large for the room size icing up can result. If oversized the air conditioner can short-cycle. This condition is usually seen as the air conditioner starting and stopping every few minutes. Even though it runs almost constantly the air conditioner will give poor cooling. Use a sizing chart to determine what is the proper air conditioner size for that particular room.
8. Cold control not shutting off.
If the cold control does not shut off the cooling coil surface temperature will drop below the room's dew point and begin to refrigerate. This will allow ice to build up onto the cooling coil.
9. Cold control bulb broken or loose.
As in the previous paragraph this will result in the cold control not shutting off and ice to build up on the cooling coil. If this is suspected remove the front grille and inspect the bulb. If broken replace cold control. If bent, kinked, or not securely fastened suspect a problem here.