HVAC crew - rapid cycling of a heat pump

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  • mitchntx

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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.
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    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
     
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    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

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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.
     

    shortround

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    You most likely have a low charge of refrigerant in the system. Mine behaved exactly the same way the last cold spell. HVAC tech found a leak. Fix was quick, and we had heat soon thereafter. If I had to do it all over, I would have put a propane heater in my home.
     

    scap99

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    Cypress
    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

    The first digit is refrigerant 4 is 410a
    (2 was for 22)

    042 is 3.5 tons
    048 is 4 tons

    Could be there's a pressure switch banging on and off, and/or there's also a reversing valve that could be hung up.
    Was it making any usable heat at all?
     

    scap99

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    No idea. Either its cycling in defrost mode or a bad charge. Put some guages on it . If the charge is good I'd replace the defrost control board.

    Do not do this!
    Unless you have the right tools.
    It's a 410 system, and if you put 22 gauges on it, you'll over pressure them.
     

    itchin

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    You cannot install a r22 system now a days. Only replace the condenser on am already existing system. He said he just had the whole thing replaced. Hopefully a technician would know to use r410 guages on a r410 system. R410 runs at higher pressure. That's why you can use 410 indoor equipment with r22 by changing the orifice or a r22 txv. But not the other way around.
     
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    scap99

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    You cannot install a r22 system now a days. Only replace the condenser on am already existing system. He said he just had the whole thing replaced. Hopefully a technician would know to use r410 guages on a r410 system. R410 runs at higher pressure. That's why you can use 410 equipment with r22 by changing the orifice or a r22 txv. But not the other way around.

    You said "put some gauges on it" like your were telling OP to do it himself.
    I was just trying to prevent injury because many of us with gauges, that aren't working out in the field, don't have 410 compatible gauges/hoses...
     

    itchin

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    I got ya man. I am not a hvac tech and would not recommend anyone messing with hvac equipment unless qualified. There are many nuances. I wouldn't mess with my equipment even though I know a little about hvac. I find it interesting. Especially heat pumps. It seems impossible to grab heat out of the outside air when its cold like today or worse.
     
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