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  1. #1
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    Thermostat keeps switching from auto (cool) to on (fan) during the night

    I have a 10 year old American Standard heat pump. During the summer, I would notice on cooler nights that the unit was running constantly. When I checked the Honeywell digital thermostat, I would find it had switched from auto to on by itself and the fan was blowing outside air at a low volume. Resetting it to cool would usually get me through the night, but a day or two later, it would do it again during the night. It never did it during the day. I called my HVAC company, and the technician diagnosed it as the thermostat. He replaced the thermostat with an updated Honeywell model and checked the entire system for proper function. About 3 weeks went by and the same thing has started happening with the new thermostat. My question is: What would cause the thermostat to switch from auto to on by itself?
    I wanted to try and get some feedback from this forum before I call the contractor back. Could the compressor's main circuit board be acting up or possibly something wrong in the air exchanger? When the system stays on cool, it runs as it should, blowing cool air. The system was not low on refrigerant when the technician checked it a few weeks ago and installed the new thermostat. Any help and suggestions will be greatly appreciated.

  2. #2
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    Poltergeist

  3. #3
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    Are you the only person in the house?
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  4. #4
    Join Date
    Dec 2007
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    Model of the thermostat?

    Do you have a ventilation air timer?
    The Food Stamp Program, administered by the U.S. Department of Agriculture, is proud to be distributing the greatest amount of free meals and stamps EVER.
    Meanwhile, the National Park Service, administered by the U.S. Department of the Interior, asks us to "Please Do Not Feed the Animals". Their stated reason for this policy "... the animals become dependent on handouts and will not learn to take care of themselves."
    from an excerpt by Paul Jacob in Sun City, AZ

  5. #5
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    Oct 2019
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    Thread Starter
    Thanks for all who responded so far. martyinlincoln, my adult son said the same thing. vstech, no my adult son and his wife and their teenage son live here also. I've questioned them and they never have touched my thermostat over the years. Anything is possible, but I'm almost 100% certain no one is touching it. The thermostat is about 8" above the top of a loveseat so I even thought of the possibility (far fetched) of one of our cats touching the buttons. pacnw,
    the thermostat is a Honeywell 5000 and no, I'm almost certain there is no ventilation timer. I had to do a search to see what one was. There are no programs set on the new thermostat. I did read of a similar case where a man's thermostat kept changing the set temperature and even did the same thing after a new one was installed. In that humorous case, it actually turned out to be the man's teenage son who initially denied touching it. I guess I could put one of those clear plastic lock boxes around is as a last resort. Seriously guys, this one is driving me up the wall. Barring human touch of the thermostat, is there any possible way it could change itself or by a signal from one of the components? I did have a cut-off switch installed a couple of years ago in the air exchanger that cuts the system off if water fills the cvc overflow pipe. One time it cut the system off, but in that case the thermostat went blank until we cleared the blockage and restarted the system. The reason I (and the technician) thought the original thermostat was at fault was because a couple of the AA batteries I had in the unit leaked and corrosion seeped into the old thermostat. I had read that if the circuitry got damaged, the thermostat could have issues. I thought the problem was solved until after nearly 3 weeks with the new one, it started doing it again.

  6. #6
    Join Date
    Dec 2007
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    Complete model number.

    What is the backup heat source?
    The Food Stamp Program, administered by the U.S. Department of Agriculture, is proud to be distributing the greatest amount of free meals and stamps EVER.
    Meanwhile, the National Park Service, administered by the U.S. Department of the Interior, asks us to "Please Do Not Feed the Animals". Their stated reason for this policy "... the animals become dependent on handouts and will not learn to take care of themselves."
    from an excerpt by Paul Jacob in Sun City, AZ

  7. #7
    Join Date
    Oct 2019
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    Thread Starter
    Quote Originally Posted by pacnw View Post
    Complete model number.

    What is the backup heat source?
    I'm not sure, but the heat pump is an American Standard 4A6H3024A1000BA.


    The air handler (which I assume you mean as the back-up heat source?) is also an American Standard model 4TEE3F31B1000BA.

    I'm not allowed to post links yet. The air handler offers a variable-speed blower with Comfort-R advanced humidity control (whatever that means).

  8. #8
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    I don't think the two thermostats are switching to "fan on". Do they say fan on?

  9. #9
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    Oct 2019
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    Thread Starter
    Quote Originally Posted by timjimbob View Post
    I don't think the two thermostats are switching to "fan on". Do they say fan on?
    Yes. It happened again last night. before going to bed at 10, I checked the thermostat. It was set to 70 degrees and was on auto (cool) and refrigerated air was coming out of the vents as it should. About 1 a.m. this morning, I woke up and noticed just cool air was coming out of the vents. I went downstairs and sure enough, it had switched from auto to on meaning it was just the fan from the air handler blowing air. One note, when the weather was real hot and the low night time temperature was 70 or above, it didn't switch over and stayed on auto (cool) the whole night. Now that we're getting cooler nighttime temperatures, it's switching from auto to on during the night. I'm wondering if it's possible the air handler could somehow be malfunctioning and switching the system from auto to on?

  10. #10
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    Jul 2010
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    That would be like saying the lightbulb makes the wall switch move. The thermostat is the switch and the fan is the bulb.

  11. #11
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    Oct 2019
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    Thread Starter
    Quote Originally Posted by martyinlincoln View Post
    That would be like saying the lightbulb makes the wall switch move. The thermostat is the switch and the fan is the bulb.
    Marty, I agree. I know very little know about HVAC , but I always thought too that the thermostat sent signals to the air handler and compressor and not the other way around. I still have a year parts warranty (of a 10 year warranty) left so I might see if American Standard has any suggestions and may be willing to send someone under their contract to examine the unit. I'm quite certain both Honeywell thermostats (old and new) could not have malfunctioned in the exact same way so I'm not even considering that right now. When I did switch the unit manually from on to auto at 1 am this morning, it stayed on cool for the rest of the night and when it got warm enough later this morning, the unit cooled as it should and in fact just cycled on as I'm writing this. As long as it stays on auto, the unit(s) work as they should.

  12. #12
    Join Date
    Dec 2007
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    I was after the thermostat model number, not just 5000, but TH5220 or whatever.

    There are 2 buttons on that stat, correct?
    Left side is fan. It has auto or on.
    Right side, since it is a heat pump, should have heat, cool,em heat, auto, off.

    To the left of the smaller size number and above the System(right side) box with be displayed the operation currently happening, if anything(heat, cool).

    Is it possible you misunderstand the settings and how to operate it. I just do not see any possible way the equipment can make this stat switch either of those button settings.

    Do you have this:
    https://www.honeywellhome.com/en/pro...ble-thermostat
    The Food Stamp Program, administered by the U.S. Department of Agriculture, is proud to be distributing the greatest amount of free meals and stamps EVER.
    Meanwhile, the National Park Service, administered by the U.S. Department of the Interior, asks us to "Please Do Not Feed the Animals". Their stated reason for this policy "... the animals become dependent on handouts and will not learn to take care of themselves."
    from an excerpt by Paul Jacob in Sun City, AZ

  13. #13
    Join Date
    Sep 2007
    Location
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    I would almost guarantee that a person is making that change. There is nothing the system could do to change the setting on the thermostat.
    Can someone please explain to me -
    Why is there never enough time to do it right the first time, but plenty of time to do it twice?


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