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Why Air Conditioners Freeze And What You Can Do About It

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If you discover your air conditioner isn't cooling as well as it should, you should check to see if the system is frozen. A frozen air conditioner will prevent your system from functioning properly, and it may cause mechanical damage if allowed to operate in its frozen state. Fortunately, freezing within the system is not usually a serious problem and can be corrected by homeowners or by simple maintenance conducted by a professional. Below are some common causes of freezing and what you can do in response:

What causes air conditioners to freeze?

While it may sound desirable to have an air conditioner that produces ice, the laws of physics provide a clue that it is a sign of a system malfunction. Air conditioners operate by continuously circulating a compressed gas, Freon, throughout a closed system. As Freon circulates, it draws heat from the air inside your home through the evaporator coil, and it releases this trapped heat to the outside at the condenser coil. For Freon to properly absorb heat from inside your home, it must enter the evaporator coil in a highly compressed state to maintain the temperature of the coil above freezing. Otherwise, if the pressure is too low, the premature expansion of gases inside the coil will lower the temperature of the unit and allow ice to form. When ice forms, it actually serves as an insulator to prevent heat from leaving your home's interior and entering the Freon. Often this low pressure is caused by low Freon amounts.

Low Freon is not the only cause of a frozen air conditioner. Dirty or missing air filters also can lead to freezing within the evaporator. Whenever airflow is restricted to the coils due to dirt that clogs the filter or blankets the coil itself, heat transfer from the air is inhibited. This lack of heat transfer permits the evaporator coil to lower in temperature, eventually passing below the freezing point. At that point, water condensing on the coil will freeze and form a layer of ice.

What can be done to prevent air conditioner freezing?

Since the causes of air conditioner freezing are due, ultimately, to a lack of heat transfer from interior air to the coil, you should address the possible specific problems that may exist. Here is what you should do:

Replace the air conditioner filter

Part of the normal maintenance routine for any air conditioning system is to change the system air filters. There are a variety of air filters available on the market, including those that use electrostatic action, paper pleats and fiberglass strands, but they all need replacing or cleaning at some point.

If your system is freezing, turn it completely off, and allow it to defrost for a couple of hours. Next, check the filter for debris accumulation and replace it if it appears dirty or if you don't remember the last time it was replaced. Mark the date you replaced the filter on a label near or attached to your thermostat to serve as a reminder for regular future replacement.

Clean the evaporator coil

If the filter has been replaced or is clean, but the system still freezes, the next place to check for trouble is the evaporator coil itself. The evaporator coil is located adjacent to the air handler that blows cooled air into the ducts. Check closets, the basement or attic if you aren't sure where to find the evaporator coil.

Once you locate the evaporator coil, remove the sheet metal access panel to gain entry. After the panel has been removed, you will notice the evaporator coil is shaped like a tent, with the coil fins located on both sides. If these coils are dirty, then this debris will need to be removed. While you can use a brush or vacuum to clean the coils, this poses some risk due to the delicate nature of the fins; the fins will restrict airflow into the evaporator if bent. Instead, the best option is to spray the coils with a no-rinse coil cleaner available from hardware stores or air conditioning parts suppliers. Be sure to cover the coils completely with the foaming spray, and allow it to run off and evaporate. The dirt will be pulled out of the fins and drop into the bottom of the evaporator where moisture collects. Your evaporator coil should be inspected annually to determine if it needs cleaning, but regular filter replacement should maintain its clean condition for a considerable period of time.

Contact a professional for Freon charging

Should your air conditioner still freeze after filter replacement and coil cleaning, the next option is to contact a qualified HVAC professional for Freon charging. Never attempt to recharge your own system, as you can introduce moisture or cause other serious problems much worse than system freezing.

For more information or help with repairs, contact a local HVAC company like Cape Fear Air Conditioning & Heating Co., Inc.