HVAC crew - rapid cycling of a heat pump

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  • mitchntx

    Sarcasm Sensei
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    Couple years ago we had a high efficiency hvac system installed with a new programmable thermostat.
    It will quickly pay for itself.

    Like closkwork, the filter is changed.
    1800sq/ft house
    Unit was installed after a heat/load study.
    I feel confident that the unit is properly sized and the duct work/return air is adequate.


    Last month we had a cold snap (teens) with snow/ice.
    Went outside and heard the condenser unit rapid cycling ... on 15 seconds, off 30, on 15, off 30 and so on.

    Went inside and turned on emergency heat (heating strips) and the unit outside stopped.

    After the cold, I returned the control to normal heat.

    Hit the teens again last night (but no moisture and low humidity) and sure enough, the outside unit was again rapid cycling.

    The first time I attributed the cycling to defrosting logic for the coils. But there was no build up at all last night and humidity was in the low 20%s.

    Is this normal operation?

    I fully understand how a heat pump works and how it becomes extremely innefficient heating when outside temps drop below 40 or so.

    But the rapid cycling has me wondering exactly what's going on.

    Also, why wouldn't the unit automaticallu engage E-heat when the outside unit is in this mode?

    I walked quickly through the thermostat set up and it appears to be properly configured.


    My best guess is a temp related defrost mode.

    But why wouldn't that mode trigger the thermostat to go to E-heat and auto-stop the rapid cycling?

    I do think the strips energize, but the cycling continues.

    With a programmable T-stat, it's basically set it and forget it.
    Having to remember to manually select e-heat when temps drop kind of defeats the purpose of set it and forget it.

    It seems to me it has to be with the way the T-stat is programmed and I'm just not seeing it.

    Of course, it's Texas and I only get a couple opportunities a year to "test" new T-stat settings ...


    I welcome any input.
     

    mitchntx

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    American Standard
    4A6B4042E1 or 4A6H6048E

    In looking a the bid, vendor offered two options and I can't recall which one was installed.

    I think it was the 4 ton 4A6H
     
    Last edited:

    DMC

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    If I had the wiring schematic for that model I would have a better guess, but it sounds like a pressure switch is kicking in and out. Could be a refrigerant leak, or incorrect charge. Either way get the installer out asap and use the emergency heat till then. The rapid cycling of the compressor will kill it quick.
     

    DMC

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    Whoops, just reread part of your post. If the compressor is still cycling pull the disconnect to shut it down before the compressor dies.
     

    mitchntx

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    Clarification:

    It only rapid cycles when in normal heat (not emergency heat) AND the outside temps drop below ~40.

    Works just fine otherwise.
     

    Vaquero

    Moving stuff to the gas prices thread.....
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    Call the installer. Strip heat shouldn't be needed until around 20 degrees.
    Strip heat should run in 5kW stages if needed.
     

    mitchntx

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    Installer scheduled for a visit tomorrow.
     
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