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Upgrading to a wifi thermostat and already have extra unused wire from furnace to wall, just not sure where to connect it at the furnace.

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I do have central air installed, and assume the blue wire just needs to be added to any existing junction, just not sure which one.

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  • Yellow goes to Y at your thermostat, right? Also, mind posting a clear picture of the wiring diagrams on the yellow labels in your furnace?
    – ThreePhaseEel
    Commented Nov 29, 2019 at 19:05
  • On the wiring diagram you should be able to locate a 24v transformer, one side will have a fuse, the other side is the common. You will need to trace that to look to see if there is a screw terminal on the control board.
    – NoSparksPlease
    Commented Nov 29, 2019 at 20:03
  • @ThreePhaseEel, Thanks !, yes the yellow wire is connected to Y at the stat. Trying to add pic of diagram from install manual for furnace and pic of current stat connections.
    – Kenneth Axtell
    Commented Nov 29, 2019 at 21:38

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Connect the blue C wire to the junction of the thin white wire and the thick dark wires

If you follow the yellow Y wire from the thermostat to where it goes in your furnance, it goes to a thin red wire. Since we know this wire is what energizes the coil of your A/C's compressor contactor during cooling, we know that the cable with the red and white wires in it goes off to the A/C, and that the white wire in that cable must connect to C in order for the A/C to work. From this, we can determine that you can connect your blue C wire for your thermostat to the junction of the thin white wire with the two thick, dark wires; note that you may need to upsize the wirenut at that junction to fit everything into it.

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  • Thanks @ThreePhaseEel, new stat should be here soon so this is a big help. Part of a trial program with our energy supplier and have limited time to install/register it once I receive it to qualify for rebate. Thanks again!
    – Kenneth Axtell
    Commented Nov 30, 2019 at 0:03
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    @KennethAxtell -- we thank people around here by upvoting and/or accepting their answers :)
    – ThreePhaseEel
    Commented Nov 30, 2019 at 0:19

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