Air Conditioning Icing


One of the problems with home forced air air conditioning systems is icing, which you my notice during its operation on: exterior condensing unit (inside the enclosure), refrigerate lines, or evaporator coil (AC section inside the house, usually not visible without removing of the access panel).

AC lines icing, frost covered

Condition is usually associated with inadequate or lack of cooling, and might have various reasons, but first thing you should do is turn off your cooling system.  You can turn OFF your air conditioning system by:

  • switching off your thermostat
  • adjusting thermostat setting higher that room temperature – some thermostats out of calibration might require several degrees differential
  • if none of the above works, turn OFF the house AC power disconnect, which should be located by the condensing unit (exterior), or circuit breaker / fuse marked AC located in an electrical panel

and wait for ice to melt.

In the meantime:

  • make sure, that your filter is clean (replace if contaminated)
  • check if all air supply registers and return grills are open and clear
  • check air conditioning coil exterior for lint, dust contamination – clean it carefully if contaminated

After all visible ice has melted, turn your air conditioning system back on, check if the condensing unit (exterior part of air conditioning system) is blowing warm air (just put your hand above the top of its enclosure). Make sure, that there’s air coming out of the registers and monitor parts, where icing occurred before.

If air conditioning icing starts reappearing, turn your system back off, and contact HVAC professional because additional testing equipment will be most likely necessary to perform further evaluation:

  1. evaporator coil might be clogged
  2. system low or high on refrigerant (both might cause icing)
  3. bad fan motor (inside air handler and / or condenser enclosure)
  4. inefficient compressor
  5. temperature falling below 60 degrees Fahrenheit
  6. oversized cooling equipment  might be also responsible for air conditioner icing

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  1. Bob says

    Another cause of icing is low ambient (outside) temperature. This can be cured by having a “Low Ambient” control installed.

    1. CTH Experts says

      Hi Bob, As a home inspector I didn’t have much experience with those controls and from what I’ve heard, they are mostly used in commercial applications. You’re welcome to share your experience in more detail. Thanks

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