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Why window Air Conditioners Ice up

One of the most frustrating problems with a window air conditioner is if it starts to ice up. For the owner of the air conditioner is can be perplexing.  man with fan in heat wasveWhen 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. 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,  freezing onto the cooling coil where it will appear as ice.  

It must be understood that the primary job of an air conditioner is to de-humidify, not refrigerate, the room air. Removing moisture from the room air gives us a feeling of comfort. This is at the central principle in the operation of the air conditioner. To do this though the temperature of the cooling coil must always be higher than the room’s dew point. If allowed to drop too low the cooling coil will begin to refrigerate and produce the icing-up affect so many people encounter.

With this aforementioned airflow problem in mind the following are a few problems that can result in your air conditioner icing up:   

1...   Dirty filter.
To avoid this problem replace or clean your filter every couple of weeks of the cooling season. If a smoker do it every week. To clean filter remove from air conditioner, wet thoroughly, and lay in bottom of a sink. Sprinkle detergent (laundry detergent works well) onto filter surface. Allow to sit for a few minutes. Next, add warm water to sink until filter is completely covered. 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. Every two or three years is typical. Every year would be best, but 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 that might trap or hold air borne particles. Dirt particles on the coil will lower heat transfer. Eventually excess dirt results in the cooling coil becoming partially blocked which reduces airflow.

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 regularly. Since the condenser is on the outside of the home it becomes exposed to a lot of dirt, pollen, and smog. To clean it the air conditioner must be completely disassembled. If not cleaned 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. If it is unable to pump the refrigerant properly the cooing coil may not get cold enough to shut off the cold control. It can hover just above the cut off point. When this happens the cooling coil will begin to refrigerate. Ice on the cooling coil will result. If the compressor is at fault the cost of replacing it is prohibitive. 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. If the outdoor temperature is too low the air conditioner pressure-temperatures can be affected. When the outside temperature falls so will the the cooling coil temperature. So much that the coil will refrigerate the room air resulting in the cooling coil producing ice. This problem is prevalent in the fall. Therefore, if it's a hot day followed by a cold night suspect this as the cause of icing. To alleviate the problem run the air conditioner in the 'fan only' position at night while leaving the re-circulating vent open. This will circulate the room air without cooling it, while bringing in a small amount of cool outdoor air during the night.

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 results in 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.

  By Donald Grummett 

Copyright © 2006 Donald Grummett. All rights reserved

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