Heat Pump Short Cycling to Hold Temp.

Discussion in 'HVAC Heating & Cooling' started by Phillbo, Nov 29, 2010.

  1. Phillbo

    Phillbo New Member

    Messages:
    77
    Location:
    Arizona
    Hi all, I have a roof mounted heat pump that when in the heat mode it appears to cycle normally until it reaches the set temp on the t-stat, then it really short cycles to hold the temp.

    Once it reached the set temp it will cycle on for about 6 minutes and then off for only 3-4 minutes before cycling on again. If i increase the t-stat a couple degrees warmer it appears to cycle as it should until it reaches the new desired temp. I suspect it's t-stat related but plan to call my AC guy out to take a look. Just curious if there is anything obvious that would be causing this ? It does not let it drop a single degree before cycling on... shouldn't there be a degree or 2 fluctuation?

    T-stat is a Honeywell IV.

    One other item, it appears to defrost more than it should - 2-3 times during the morning after I turn it on to warm the house up.


    Any insight? This is my first winter in this house.
    Last edited: Nov 29, 2010
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,824
    Location:
    New England
    SOme thermostats have the ability to program them for the type of heat you are using...ensure if yours does that it is set to indicate it is controlling a heatpump. Some can also enable you to set the hysterisis of the heat control (i.e., to let it swing more). If the house is cooling off enough in 3-5 minutes to trigger the thermostat, you need to find the air leaks and maybe beef up the insulation, though, unless it is sitting in a drafty area. Placement of the thermostat can sometimes be critical.
  3. Phillbo

    Phillbo New Member

    Messages:
    77
    Location:
    Arizona
    It does not cool off in the 3-5 minutes between cycles.. I can decrease the temp setting a couple degrees and it takes a long time ( almost a couple hours ) to drop enough for the unit to cycle on again. The house is older but has great insulation, teh temp does not even need to drop (according to the t-stat reading ) for the unit to cycle back on.

    I find myself having to manage the t-stat.. turn it up to 2* higher than I want and when it passes the desired temp, set it for 1* below what I want... otherwise it short cycles and does not increase the temp of the house at all.
  4. Thatguy

    Thatguy Homeowner

    Messages:
    1,460
    Location:
    MD
    It almost has to be the hysteresis adjustment.
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,824
    Location:
    New England
    The electronics can detect much smaller temperature differences than a whole degree, so depending on how it is set, it is trying to maintain as close to the set point as possible. If you tell it it is a heat pump (and not say the strip heaters), it should be less aggressive in trying to maintain exactly the setpoint.
  6. Phillbo

    Phillbo New Member

    Messages:
    77
    Location:
    Arizona
    OK. I walked up to it and said " you are a heat pump" and it ignored me :)


    How do i set it to "heat pump" ? Do I need to take the face off like I was changing the batteries?
    Last edited: Nov 29, 2010
  7. Dana

    Dana In the trades

    Messages:
    2,790
    Location:
    01609
    Which EXACT model is the thermostat? Is it from the Honeywell Chronotherm- IV series? If yes, it already knows it's a heat pump, and has a programmable hysteresis. Some variants had a "learning" function to limit overshoots & short cycling as well. Dig up the manual (it might be online, might not.)
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