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  1. #1
    Join Date
    Jan 2008
    Posts
    7

    Trane/Honeywell very short cycling issue

    My house has 2 - 2 ton Trane heat pumps with Honeywell thermostats... they are about 10 years old and work very well. About 8 months ago one of the units started short cycling... very short on-offs, just of the fan unit the outside unit does not turn on when it does this. I had a HVAC contractor look at the unit and he seem to think that everything was fine. I have started paying more attention to it and it seems to only happen in the morning and evening, and it happens in both heating and cooling modes. The therm will be set to 70 and it displays 70 then the fan unit will switch on-off in very short cycles <1 sec. Sometimes 2 or 3 times sometimes more then it will stop... then some time later when the therm goes down to 69 the outside unit turns on then the fan and it heats or cools fine... the therm reaches 71 and the unit shuts off. I pointed this out to the tech and he explained that this is sometimes a normal operation... I don't believe this. The other unit does not do this! I think that this is an issue with the therm's hysteresis... can someone help me out?

    Thanks,
    Kevin

  2. #2
    Join Date
    Dec 2002
    Location
    Rochester, MN
    Posts
    5,304
    It could be the t=stat, what is the model # of the t-stat?

  3. #3
    Join Date
    Jan 2008
    Posts
    7
    It is a Honeywell Chronotherm III.

    Kevin

  4. #4
    Join Date
    Jan 2008
    Posts
    7
    The Model number is T8601C... does that help?

  5. #5
    Join Date
    Dec 2002
    Location
    Rochester, MN
    Posts
    5,304
    Do you know what the screws settings are set at for system type?

  6. #6
    Join Date
    Jan 2008
    Posts
    7
    Hi, sorry I didn't see the response until now... the settings are 1A - out, 1B - out, 3A - in.

    But here is what I've observed... I switched the 2 Thermostats that I have (both the same) expecting the "bad" one to cause the other Fan unit to short cycle... but it didn't. And the "good" thermostat did not fix the short cycling of the Fan unit that was doing it. So, I think it's not the thermostat, but I could be wrong... thoughts?

    Question: does the Fan unit have a relay that could be going bad?

    Here are more observations: the short cycling, remember it's less than 1 sec, only happens when the unit has been running for a while... early morning or evening when the program is raising the temp. One time when it was doing this I pulled the thermostat off it's base and it continued to cycle the fan on & off... that seemed strange? Which made me think that it could be the relay that turns the Fan unit on... maybe it's spring or contacts are old/dirty and it just arches sometimes... thus causing the fan to spin momentarily then it's off again... what do you think??

    Kevin

  7. #7
    Join Date
    Jun 2007
    Location
    Pigeon Forge, TN
    Posts
    295
    wow that is weird!

    If it would do this dance when the tech is checking it, he could put his meter on it and see what's happening, otherwise it could be hard to find. Sounds like a loose connection somewhere. The fan relay could be the culprit but this is not typical behavior of a bad relay. My best guess would be a loose screw at the t-stat, that's taking a stab from cyberspace.

  8. #8
    Join Date
    Jan 2008
    Posts
    7
    I checked all the connections early on and all were tight with no loose wires. I had a technician come out and he couldn't explain the behavior and claimed it was normal on older (10 year units)... I didn't buy that then and I don't buy it now.

    Any ideas on how to test or determine what the problem might be??

    Thanks,
    Kevin

  9. #9
    Join Date
    Jan 2008
    Posts
    7
    Anyone??

  10. #10
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,915
    Probably a bad blower off delay timer on the fan relay in the indoor unit.
    What you describe is one of the typical failure modes for that control.
    If more government is the answer, then it's a really stupid question.

  11. #11
    Join Date
    Jan 2008
    Location
    Las Vegas, NV
    Posts
    81
    is your unit gas heating? could be the limit is going bad or has lint build up. Falsifying the true temp. alowwing fan to come on and cool, then shutting back off

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