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Thread: Nest with Heat Pump wiring question

  1. #1
    Junior Member

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    Nest with Heat Pump wiring question

    My wife bought me a Nest 2.0 thermostat for Christmas and I'm trying to use with with a Heat Pump with a gas furnace for aux heat (Trane XB80). I'm currently having two problems

    1) After wiring I'm getting error N25 - No power to AUX/W2 wire detected. This leads to problem #2...
    2) The heat pump seems to work fine, but when the temperate drops to the point the heat pump shuts off, the aux heat never comes on.



    I'm not sure if this is a wiring issue or setup issue. Emergency heat is wired to * and it works fine. I've included the before (old thermostat) and after (nest 2.0 thermostat) wiring for reference.

    Previous Thermostat
    Click image for larger version. 

Name:	before_thermo.JPG 
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ID:	251


    Nest 2.0
    Click image for larger version. 

Name:	after_thermo.JPG 
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ID:	250


    Any help would be greatly appreciated.

  2. #2
    HVAC Thermostat Specialist

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    The component that is shutting down the heat pump when temperature outside is lower is most likely also opening the wiring circuit from W2 to the furnace when it is warmer outside. When the furnace circuit is open the Nest thermostat is unable to detect the wire. Do you have a electrical meter like this one for testing

  3. #3
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    That makes sense for why I'm getting the error code.

    Any idea why Aux Heat isn't coming on?

  4. #4
    HVAC Thermostat Specialist

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    Quote Originally Posted by mhoggatt View Post
    Any idea why Aux Heat isn't coming on?
    Not 100% sure but I think that because the Nest did not detect the wire it will not energize the terminal.

  5. #5
    HVAC Thermostat Specialist

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    Do you know if you have a dual fuel kit? If so, the NEST is supposed to be designed to take place of it. IMO, the NEST is not a good thermostat choice for your system.

  6. #6
    Junior Member

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    I'm fairly sure I do. The old thermostat was very simple with no intelligence to switch heating systems. It does seem odd that the Nest is trying to do this when there is another system doing it as well. I have the Nest set to 'dual-fuel' mode. Does it make sense to set it to another mode and let the dual-fuel kit do it's thing?

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