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Why is My Air Conditioner Freezing Up?

It may sound strange that in the heat of summer your air conditioner can actually develop frost but it is possible – and it isn’t good. Understanding the causes, however, can help you prevent your A/C from freezing up.

What Causes an Air Conditioner to Freeze Up?

You may not realize it, but a key component of your air conditioning system is actually in the ductwork inside your home. Your furnace’s fan blows air past the super-cooled indoor air conditioning coils, which remove heat from the air, cooling your home. However, without enough air being blown over these coils, not enough heat is absorbed, and the coils can actually freeze. When this happens, your air conditioning system must shut down until the coils thaw out. In the mean-time, your home’s air isn’t being cooled anymore. Pay special attention to the items below to prevent this from happening in your house this Summer:

  • Dirty Air Filter(s)
    This is the leading cause of freezing coils, and also the easiest to fix yourself. Dirty air filters block the free flow of air through your system, which increases the likelihood of frozen coils. Change (or clean) your air filter(s) at the start of the Summer, and check the status every month or so to keep the air flowing smooth.
  • Insufficient Airflow / Fan Speed
    The correct balance of airflow and air pressure is necessary for the proper operation of your air conditioner. A damaged furnace blower fan may not be running at the right speed. Blocked air ducts, closed or blocked vents or closed dampers can also reduce the flow of air through your system. The resulting lack of airflow can cause condensation to build up on the coils, which then freeze. Turn your fan to “On” at the thermostat to keep a constant flow of air over the coils, even when your A/C isn’t running. It costs less than leaving on a light bulb and can make a big difference. Make sure you keep vents and dampers open to maximize airflow through the entire system. And consider having your ducts cleaned at the beginning of the Summer.
  • Dirty Coils
    Dirt on the coils can prevent them from functioning properly. During normal operation, condensation develops and drips off the coils. Dirt prevents the proper heat transfer between the coils and the air. The coils get too cold and freeze. Having a professional A/C tuneup at the beginning of the summer (or even in the Spring) is a great way to make sure your coils are clean and ready for cooling season.
  • Low Refrigerant Levels
    The air conditioner cools the air in your home by drawing the hot air over evaporator coils filled with refrigerant. The coils absorb the heat and transfer it to the outside unit, where it is blown into the air by the large fan. A refrigerant leak can result in an insufficient amount of refrigerant to remove the heat from the air. Condensation builds up on the coils and eventually freezes. Again, a professional A/C tuneup will detect and fix any refrigerant issues in your system.
  • Thermostat Issues
    A poorly functioning thermostat can cause the air conditioner to run all night when it doesn’t need to. In addition to wasting energy and money, this can also cause the air conditioner to freeze. Make sure your A/C is shutting off when your home reaches the desired temperature, and if not, call a professional to diagnose what’s going on.
  • Closed Supply Registers
    People often close the supply registers in unoccupied rooms to save money and energy, but this can actually make your air conditioner less efficient. Having too many closed registers can even cause the air conditioner to freeze. Keep them open to maximize comfort throughout your entire home.