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A heatpump is used to cool a home in Florida. A relative noticed that the temperature was 90 degrees and that the evaporator (see photo) was sweating. She thinks she heard a piece of ice fall break off inside the evaporator. The unit was installed in 2016.

What causes a heatpump to freeze when used in cooling mode?

  • Poor airflow from a dirty airfilter?
  • Low freon pressure?

The power to the heatpump has been disconnected to ensure that any ice melts.

Is the most likely culprit the AC filter? Any diagnostic instructions are appreciated.

enter image description here

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There are many possible causes but some of the more common ones are:

  1. Clogged drain
  2. Dirty filter
  3. Excess humidity in the house
  4. Blocked air return or outlet duct

Get it thawed out and make sure that the drain is flowing properly. Check the filter, check the blower for any obstructions.

  • 3
    Yes, I've seen this happen when the filter doesn't get changed. The airflow is greatly reduced, and not enough air flows over the cooling coils, allowing them to get colder and colder. The unit eventually becomes so cold that the condensation drain starts to freeze, and eventually the drain pipe becomes blocked with ice. The obvious symptom is a pool of water on the floor and people reporting that the air conditioner is leaking. It sounds like this one is so cold that the excess water is totally freezing rather than leaking out onto the floor. – Ray Butterworth Sep 3 at 0:31
  • This particular one has a float switch, so it probably isn't a clogged drain - the AC would have shut off before any water started backing up into the actual air handler. I agree with this answer though. I had an AC unit where the coils completely froze over in Florida. I think part of the issue was simply the at the thing was way too old and had poor airflow already. – conman Sep 3 at 10:10
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Low refrigerant can also cause this, although that typically happens closer to the condenser. When the refrigerant is low, the condensed refrigerant is allowed to expand earlier, which can lead to excess condensate and/or freezing.

It's unlikely this is the case here, but heat pumps can vary in how far they are from the condenser, so it's not impossible either.

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