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    Furnace Wiring - Adding a common "C" wire

    Asked Sep 19, 2008, 04:17 PM 19 Answers
    My new thermostat has uses a W,R,Y,G and now a "C". My wiring has only W,R,Y, and G and these are needed.
    How do I, or can I add the needed "C" wire? Is it just adding a wire from the transformer? :confused:

    Last edited by Bocaheat; Sep 19, 2008 at 04:26 PM.
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    KISS's Avatar
    KISS Posts: 12,510, Reputation: 838
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    #2

    Sep 19, 2008, 05:06 PM
    (C) depends on the stat. Battery powered ones make (C) an optional connection.

    (C) goes to the other terminal of the secondary that (R) does not go to. You should find a few low voltage wires on that terminal (gas valve, fan, and outside AC unit all tied together)
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    letmetellu's Avatar
    letmetellu Posts: 3,151, Reputation: 317
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    #3

    Sep 19, 2008, 06:17 PM
    Quote Originally Posted by Bocaheat
    My new thermostat has uses a W,R,Y,G and now a "C". My wiring has only W,R,Y, and G and these are needed.
    How do I, or can I add the needed "C" wire? Is it just adding a wire from the transformer? :confused:
    Are you saying that you need a fifth wire that runs from the furnace to the thermostat? If so all you have to do is to just add one 18 gage wire to the four that you already have or you can buy a different wire that has six or more wires and replace the four wire cable that you have now.
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    Bocaheat's Avatar
    Bocaheat Posts: 9, Reputation: 1
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    #4

    Sep 20, 2008, 10:05 AM
    Ok. Added the wire but now the Fan will not turn off. I used the "B" terminal for the common. Any ideas what is wrong?
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    KISS's Avatar
    KISS Posts: 12,510, Reputation: 838
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    #5

    Sep 20, 2008, 01:33 PM
    Common may not be marked. There are two wires that go to the outside unit. One is connected to "Y". The other wire will be common.
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    letmetellu's Avatar
    letmetellu Posts: 3,151, Reputation: 317
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    #6

    Sep 20, 2008, 06:05 PM
    Quote Originally Posted by Bocaheat View Post
    Ok. Added the wire but now the Fan will not turn off. I used the "B" terminal for the common. Any ideas what is wrong?
    Which fan will not turn off? The indoor fan or the outside fan. Let me know and I can tell you what to do.
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    Bocaheat's Avatar
    Bocaheat Posts: 9, Reputation: 1
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    #7

    Sep 30, 2008, 06:59 AM
    Quote Originally Posted by letmetellu View Post
    Which fan will not turn off? the indoor fan or the outside fan. Let me know and I can tell you what to do.
    Inside fan keeps running. Note, the other side of the Blue wire goes to the same common terminal inside the furnace.
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    KISS's Avatar
    KISS Posts: 12,510, Reputation: 838
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    #8

    Sep 30, 2008, 08:18 AM

    Remove the "G" wire from the thermostat. Does the fan turn off?
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    KISS's Avatar
    KISS Posts: 12,510, Reputation: 838
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    #9

    Sep 30, 2008, 08:26 AM
    Let's make this simple. G,R,Y,G and C

    Let's leave C out of this temporarily.

    When R connects to G, the fan comes ON.

    When R connects to W, the furnace turns on: The furnace controls the fan (On and Off)

    When R connects to Y, the outside AC unit comes on.

    Thermostat oddities:

    In heat, the fan is controlled by the furnace, but Turning the fan to the ON rather than the auto position sometimes doesn't work in heat mode. OFF will not turn off the fan - the furnace must do it.

    In cool the thermostat controls both the W and G terminals. In order to get cool, both Y and G are connected to R.

    When a common is used, there will be 24 VAC between R and C at all times.

    Bold items were corrected thanks to EPM.
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    Bocaheat's Avatar
    Bocaheat Posts: 9, Reputation: 1
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    #10

    Sep 30, 2008, 01:12 PM
    Quote Originally Posted by KeepItSimpleStupid View Post
    Let's make this simple. G,R,Y,G and C

    Let's leave C out of this temporarily.

    When R connects to G, the fan comes ON.

    When R connects to Y, the furnace turns on: The furnace controls the fan (On and Off)

    When R connects to W, the outside AC unit comes on.

    Thermosat oddities:

    In heat, the fan is controlled by the furnace, but Turning the fan to the ON rather than the auto position sometimes doesn't work in heat mode. OFF will not turn off the fan - the furnace must do it.

    In cool the thermostat controlls both the W and G terminals. In order to get cool, both Y and G are connected to R.

    When a common is used, there will be 24 VAC between R and C at all times.
    I will try this weekend... I'm on the road. Will let you know how I make out. Appreciate the help. If I can not get it figuredd out, I will include some pictures. Thanks
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    EPMiller's Avatar
    EPMiller Posts: 624, Reputation: 37
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    #11

    Sep 30, 2008, 04:53 PM
    Quote Originally Posted by KeepItSimpleStupid View Post
    Let's make this simple. G,R,Y,G and C

    <snip>
    When R connects to Y, the furnace turns on: The furnace controls the fan (On and Off)

    When R connects to W, the outside AC unit comes on.
    <snip>
    Uhh... KISS, I think you got it backwards. Unless they do it differently in your part of the world.

    R to Y calls the compressor, which usually is cooling. If it is a heat pump then the O or B terminal (the type of reversing valve determines which must be used) decides whether it will heat or cool.

    R to W calls heat. If it is a heat pump W often is the second stage (W2) or backup heat.

    Some systems control the fan from the control board and some from the thermostat. You program your tstat accordingly if it allows you to. Otherwise R to G will call the fan (you had that correct), and the temp limiters will overide the fan call and keep it running until the system is below the limit switch threshold.
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    Bocaheat's Avatar
    Bocaheat Posts: 9, Reputation: 1
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    #12

    Oct 3, 2008, 02:19 PM
    Ok, here is what I have found. Please see attachment. When I connect the Yellow wire to "C" the fan inside turns on and stays on. (Thermostat - Furnace) RH-R, G-G, W-W1, Y-Blue wire, and finally C-B.
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    Bocaheat's Avatar
    Bocaheat Posts: 9, Reputation: 1
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    #13

    Oct 3, 2008, 02:24 PM
    Sorry, not correct! The "O" and "B" terminal on the Furance are reversed.Name:  Furnace.GIF
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    KISS's Avatar
    KISS Posts: 12,510, Reputation: 838
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    #14

    Oct 3, 2008, 02:33 PM

    Since the new tstat has Rh, there musct also be an Rc. Rc and Rh MUST be jumpered together.
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    Bocaheat's Avatar
    Bocaheat Posts: 9, Reputation: 1
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    #15

    Oct 4, 2008, 03:41 AM

    The instrucion had me insert a jumper for my configuration. I assume that was what it was for.
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    hvac1000's Avatar
    hvac1000 Posts: 14,540, Reputation: 434
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    #16

    Oct 4, 2008, 03:59 AM


    Unless it was missed can you post the brand and model of your new thermostat?
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    Bocaheat's Avatar
    Bocaheat Posts: 9, Reputation: 1
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    #17

    Oct 4, 2008, 10:51 AM

    Ritetemp 6080
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    hvac1000's Avatar
    hvac1000 Posts: 14,540, Reputation: 434
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    #18

    Oct 4, 2008, 10:56 AM


    If you follow this all should work. Now if the wiring color has not been followed by the original installer then you might have to track down the wiring and start over.

    The RiteTemp Support Site - 6080 Technical Support Page
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    Bocaheat's Avatar
    Bocaheat Posts: 9, Reputation: 1
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    #19

    Oct 4, 2008, 11:00 AM
    I rechecked the wiring and it seem to be working now. Thanks.
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    hvac1000's Avatar
    hvac1000 Posts: 14,540, Reputation: 434
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    #20

    Oct 4, 2008, 11:07 AM


    Glad you got it going. Keep the web site for future use if necessary.
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