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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.

  4. #4
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    It works

    I hooked up the X to the C common, on the Nest, and it all seems to work fine.
    THanks so much!!

  5. #5
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    I think I'm in a similar predicament and although I've received some guidance, I'm still a little fuzzy. When I called NEST for support, the wire that used to be connected to my Honeywell's "X" connection is no longer connected. I get heat and AC but I think if push came to shove and the temps got low enough, I won't be able to use my emergency heat on my heat pump.

    I've been looking but I am finding it difficult to find exactly what or how I need to wire this Nest correctly. Firstly, I just bought this house and this is my first heat pump ever. There is no alternative heat just electric. I have attached the front and back of what my Honeywell Thermostat looked like and how it was wired.
    I had it connected all summer for AC ( how i thought it should be) and it worked flawlessly however when heating season showed up, it didn't work out too well.

    My concern is when I called Nest for support, they told me I did not need the wire that was connected to X on my Honeywell, connected to the Nest. When I trace this up to the furnace it is connected to what appears to be a small transformer. I think, but not certain that it is for emergency heat. I've attached pics of my old setup and how Nest said to connect it.

    My concern....I do have heat now but I've been told that if the temps drop below 38, I should move the thermostat to EM Heat which is on my old Honeywell but I cannot find this on the Nest. Please if you have any advise, I would welcome it.

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  6. #6

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    Todd -
    If you don't have gas/oil/propane then most likely the "emergency heat" is an electric coil and can be configured as auxiliary heat which will make your life easier. Move the white wire to W2. Then read
    Also, most likely the X is a common (and you found it goes to a transformer) so put the brown wire in C.

    Lastly - Should verify equipment. Check model number of outside unit to make sure it really is a heat pump. If using B then normally a Rheem or Ruud unit.
    Last edited by Stuff; 12-08-2019 at 09:35 PM.

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