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Thread: HVAC freeze up

  1. #1
    David Edens's Avatar
    David Edens Guest

    Default HVAC freeze up

    Recently an electric single service HVAC system was checked in heating mode only (winter) and the unit dropped in output from 110 degrees at the register to 95 degrees after an hour. After going back outside, the unit's coils were freezing up. The HVAC tech later came and said nothing was wrong with this 12 year old system. Am I missing something as I think I should have called this (and I did)?!

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  2. #2
    Join Date
    Mar 2007
    Location
    Santa Rosa, CA
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    2,286

    Default Re: HVAC freeze up

    David,

    I can't imagine a resistance heater freezing up. Are you referring to a heat pump?

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  3. #3
    Join Date
    Mar 2007
    Location
    Dallas, Texas
    Posts
    4,112

    Default Re: HVAC freeze up

    It is normal for heat pump exterior coils to frost up in the during cold weather and also normal for the output to drop when the coils frost over.

    What is not normal is for the frost to remain long enough to totally block the air flow over the exterior coil.
    A heavy frost coat that still allows some air flow might be OK depending on the unit.
    Different manufacturers use different methods to initiate and terminate defrost.
    Solid ice is not OK, it is a sign the defrost cycle is not working properly.

    If you were getting 110 degree out of the supply grills, I'll be the backup heat was also running, unless the indoor air temp was over 90 degrees.

    I always advance the temp setting on the stat just enough to make the unit come on and watch the temperature split and the outdoor unit, it should be running and have between 15-25 degree split on a moderate day with the house close to 70 degrees.
    After I'm satisfied that it is working satisfactorily in the heat pump mode, I advance the stat setting to bring on the backup heat and watch for a substantial jump in the temp split.
    Once I'm satisfied with both heat pump and the backup, I switch it to emergency to make sure the heat still works and the outdoor unit shuts down.
    Take into account the frost level on the outdoor unit before getting too excited about any temp. split, it is all relative.

    Jim Luttrall
    www.MrInspector.net
    Dallas, Texas

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