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Thermostat “C” wire?

footsox1 | Posted in Energy, Heating & Insulation on August 18, 2005 11:25am

We are trying to install a new thermostat which requires a “Common wire” or “C” wire.  Our air conditioner was installed new, 2 years ago.  It is a Bryant brand.  The installers connected it to a 4-wire thermostat (the cheap thermostat which came with the unit).  We are now trying to intall a new thermostat which requires hooking up the “C” wire.

There are more wires are in the wall than just the 4 wires which are currently hooked up (7 or 8 total).  When it was installed, the “C” wire was not connected into the air handler.  I am trying to figure out which wire in the air handler would be the “C” wire, or “Common wire.”  The wires are color coded.  Does anyone know which color wire designates the “C” wire?

Thanks for any help on this.  I appreciate it.

Sue

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  1. User avater
    NickNukeEm | Aug 19, 2005 12:38am | #1

    You may or may not want to do the following.  If you're leary of electrical devices, or have never read a schematic, continue on to the next post.

    Turn the power off to the compressor by opening the breaker at your main panel (there should also be a local service disconnect.)  Go out to the compressor.  Remove the panel that protects the electrical components.  Somewhere inside, you should find a service guide packet, which should include an electrical schematic.  It might appear to be some sort of hyroglyfics, but it should show you which wires go where.  Take a decent digital picture, post it, and maybe someone can decipher it.

    I am used to the Common wire being one of the two legs from the 24v transformer, with the other leg (typically a red wire) being the lead that is fed through switches/relays and such to operate the controls.

    But that is a Trane convention, and they don't have a C terminal in any of the three main components (outdoor compressor, indoor air handler, and T-stat.)

    Good luck.

    Edit:  Some manufacturers include service packets in the air handlers as well.  If yours is readily accessible, it might be easier to check that first.

    "I am the master of my fate, I am the captain of my soul."  Invictus, by Henley.



    Edited 8/18/2005 6:43 pm ET by NickNukeEm

  2. Tim | Aug 19, 2005 10:22pm | #2

    Thermostat wires are typically coded as follows:

    white (W) is the "call for heat" signal wire

    yellow (Y) is the "call for cool" signal wire

    green (G) is the "run the fan" signal wire

    red (R) is the 24V power supply

    the others, blue, brown, black, orange can be used as necessary, black being the most common "common".

    1. footsox1 | Aug 19, 2005 10:59pm | #3

      Thanks for the replies. I think we will try the red wire and hope we don't blow up the house. With this fixer-upper house, it probably wouldn't make much difference.... Thanks!Sue

      1. prout35 | Aug 20, 2005 02:52am | #4

        i changed my thermostats this passed winter to digital ones. it was great the furnace went right back on and every thing was fine. this spring. the first hot day i turned on the ac and nothing. i thought the worst seeing that the system is 20 years old,and called the ac guy. he first went to the thermostats, but i told him they were fine I replaced them and they are working. he told me that there are two wires that can go either way.for the heat, but not for the ac. he switched them and it went on. i felt like a fool, and still dont ounderstand what he did.

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