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Thread: Honeywell T841A to Nest Gen 3 questions

  1. #1
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    Honeywell T841A to Nest Gen 3 questions

    Hi,

    I have a general wiring question.

    Original Wiring to the Honeywell (model T841a):

    Orange Wire -> O
    Brown Wire -> B
    Blue Wire -> X
    Black -> E

    The rest are connected correctly, but i'm not a 100% on the correct terminal connections to the nest for the ones referenced above.



    I have:
    O (orange) going to Ob,
    E (black) going to *,
    X (blue) going to C,
    Brown currently not connected.

    Please advise correct connections

    My Air Conditioning is running, but I feel like it's not as responsive as it should be (not shutting off once it's hit the designated temperature).

    I have read a lot, but I cannot find a definitive answer. I've read that B is supposed to be the common wire, but that sometimes O can actually be the common wire and it's throwing me off. Also, in regards to B i've read that the common wire provide "direct power to the thermostat". What's the difference between direct power to the thermostat and the R wire that provides power in general.

    Greatly appreciate the assistance.

  2. #2
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    Here are a couple of pics of the original thermostat.

    Click image for larger version. 

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ID:	1197Click image for larger version. 

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  3. #3
    Administrator

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    Orange Wire -> O > Nest O/B terminal
    Brown Wire -> B > Not used. For the Honeywell T841A the B terminal is used to energize the revising valve in heating.
    Blue Wire -> X > Nest C terminal
    Black -> E > Nest * if the heat is emergency heat or W1 if the heat is aux heat


    Quote Originally Posted by grandlarsony View Post
    I've read that B is supposed to be the common wire, but that sometimes O can actually be the common wire and it's throwing me off.
    B is only the common wire on some Trane A/C systems. O is never the common wire






    Quote Originally Posted by grandlarsony View Post
    Also, in regards to B i've read that the common wire provide "direct power to the thermostat". What's the difference between direct power to the thermostat and the R wire that provides power in general.
    Read this article for more information on this

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