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Thread: Heat Pump Short Cycling to Hold Temp.

  1. #1
    DIY Member Phillbo's Avatar
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    Default Heat Pump Short Cycling to Hold Temp.

    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 by Phillbo; 11-29-2010 at 08:58 AM.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    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.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Member Phillbo's Avatar
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    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.

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    Homeowner Thatguy's Avatar
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    It almost has to be the hysteresis adjustment.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    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.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Member Phillbo's Avatar
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    Quote Originally Posted by jadnashua View Post
    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.
    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 by Phillbo; 11-29-2010 at 05:00 PM.

  7. #7
    In the trades Dana's Avatar
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    Quote Originally Posted by Phillbo View Post
    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?
    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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