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Thread: x terminal on Honeywell, what is that?

  1. #1
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    x terminal on Honeywell, what is that?

    This is a duplicate post, from one in the NEST forum, because it refers to both thermostats. Moderators should feel free to delete one of them, I dont know which forum it should be in.

    I would like to install a Nest Thermostat into my apartment, and it has an old honeywell thermostat, and the current wiring is a little unusual. Nest Support was unable to help today. The issue is that one wire is hooked up currently to the X terminal on the old thermostat, but the Nest doesnt have an X terminal. Would that be the same as the C terminal? I have attached a picture of it,

    It is a Honeywell t874r1442 thermostat.

    And I am very sure our system is an electric heat pump, with emergency resistive heating coils. No gas or other types of fuels involved. And the system also does cooling.

    Any help would be appreciated!
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  2. #2
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    The easiest thing I would do is eliminate the ones you know what they do (Y = compressor, G = fan, etc.) and then check the air handler to see what the other end connects to.

    Also: X or Aux – This terminal is for back-up on a heat pump and allows for auxiliary heating from the back-up heat source usually located in the air handler. X or Aux – There is no universal color used for this terminal designation but this should be wired directly to the heating relay or the Aux terminal on a terminal strip board in the air handler or package unit if you have a heat pump package unit.

  3. #3
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    Honeywell t874r1442 with base Q674L1504. The X in those is a common. Goes to C in most new thermostats. B goes to O/B terminal and gets configured as B-activate on heat. W2 is aux which goes to either W1 or W2 on Nest.


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    Last edited by Stuff; 01-17-2019 at 04:53 PM.

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