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Thread: 2nd Gen Nest running AC compressor when trying to heat

  1. #1
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    2nd Gen Nest running AC compressor when trying to heat

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    I installed a 2nd Gen Nest thermostat last week on my HVAC system. It is a forced air LP furnace with an outdoor AC (Trane XL15i) compressor (not heat pump).

    Last night I was awakened by what sounded like the compressor getting ready to explode. I thought at first it was the 2nd zone heat pump dying, but turns out it was the Nest turning on my AC compressor in conjunction with my furnace to try and heat the 1st zone. (It was 30 deg outside last night and the Nest was set to Heat only function).

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    I spent an hour with their customer service today. They had me switch wires in terminals with no success. As soon as I powered the breaker to the furnace/AC with only the Nest terminal WALL PLATE installed, my AC compressor would cycle on and off for 2 min at a time.

    I re-installed my old digital thermostat on this system and everything is working as expected again (No AC compressor when heating).

    Terminal thermostat wire connections were all very basic for this system.
    W. W1
    Y. Y1
    G. G
    R. Rh
    C. C

    1) Any idea what's going on? Why does the Nest turn on my AC compressor and furnace when trying to heat?
    2) How badly did I damage my 1 year old Trane AC unit by having the AC compressor running this weekend when I was gone?

    ( Also, just for info, I do have a Nest installed on the 2nd zone standard heat pump and seems to work just fine.

  2. #2
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    Oh, I forgot to add that the Nest service consultant said this was a 'known' issue with certain Trane models and that the Nest was incompatible with those models. Sounds ridiculous to me.

  3. #3
    HVAC Thermostat Specialist

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    WHAT?! Known issue?! What a bunch of bull. If you have a conventional system (Gas furnace and A/C), no matter who the manufacturer is, the nest should work fine. The fact that it does it with only the base plate tells me that the nest (or just the base plate) is bad.

    It's hard to say what damage if any was done to the AC without being there and listening to its operation and checking it over thoroughly.

  4. #4
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    Yup. Agreed. Can't get more conventional than Trane and Carrier, right? The fact that the compressor was cycling on by simply wiring the terminal plate on the wall leads me to believe there is a short in the plate, right? I talked to another support agent this evening on the way home and he said the terminal plate was just contacts, relays and fuses. I may call a local 'certified' nest installer and see if they have heard of or witnessed such behavior. Thanks.

  5. #5
    HVAC Thermostat Specialist

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    You may also want to check the thermostat wires where the sheathing was cut behind the thermostat. Sometime when the thermostat wire sheathing is trimmed back the thermostat wires get cut. If the cut areas touch each other it creates a short. Maybe the Y wire is shorting to another wire. If this is not the issue you most likely have a bad Nest base plate.

  6. #6
    HVAC Thermostat Specialist

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    There was a post on another forum where there was a electrical draw through C and Y with just the plate on so it can be just a bad plate. The contacts and or relays may be sticking. If it doesn't do it with the old thermostat then the problem is the NEST. A new NEST should fix your problem if you still want to use one. Personally, I think there are better thermostats out there.

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