Got a wifi thermostat. Behind the old thermostat was a blue "C" wire. The other side of the blue wire in the equipment room isn't plugged into anything. I need to get the 24v common hooked up to this blue but don't know where it goes. Any help is greatly appreciated.



2 Answers 2


The wiring diagram you posted is quite clear on what it wants you to do; namely, it wants the C wire connected to the furnace chassis ground. To do that, simply drill/tap a 10-32 hole in a suitable spot on the furnace chassis if there isn't one present already, screw a premade ground pigtail into it, and nut the C wire to that pigtail.

  • I could simply unscrew the Chassis Ground (there is a screw that already has a blue wire from the condensing unit going to it) and put this blue wire there and screw back in. I don't think I would need to drill a new hole. I'm not following how this would provide constant power to a smart wifi thermostat..... if I am hooking it up to a ground, how it is getting power? Feb 15, 2017 at 23:43
  • @user3330299 -- the thermostat already has the 24VAC side in the form of the R wire, you just need to close the circuit with a C wire to provide a return path for that juice besides through the furnace board :) Feb 15, 2017 at 23:53
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    While technically this will work, it's not exactly the correct way to do it. By connecting the C wire to the chassis, you're effectively using the chassis as a current carrying conductor. I guess it's only possibly ~1A @ 24V, but still. It's like telling somebody to connect the neutral to the ground on a receptacle, and you wouldn't do that, would you?
    – Tester101
    Feb 16, 2017 at 13:45
  • @user3330299 If you know anything about AC wiring, you can think of the R wire as the ungrounded (hot) conductor, and the C as the grounded (neutral) conductor. In the furnace, C (neutral) is bonded to ground (the frame). So connecting the neutral from the thermostat to ground, gives it a path back to the source (albeit and inappropriate path, in my opinion).
    – Tester101
    Feb 16, 2017 at 13:49
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    @ThreePhaseEel that's likely because the manufacturer doesn't want installers messing around with the factory installed wiring. It's an accepted practice in the HVAC industry, but in my opinion it's a bad practice. Telling the OP to connect the thermostat C to the same chassis screw as the condensing units C (using properly sized ring terminals), would have been a bit of a better answer. Telling the OP to willy-nilly ground the wire to the chassis, is not so great (while technically acceptable). I mean what are we, the auto industry?
    – Tester101
    Feb 17, 2017 at 11:57
  • If there's a C terminal on your board (can't tell from picture), use that.

  • If you have an air conditioner, it will have two wires that connect back to the thermostat (usually via the furnace): so long as one is going to R, the other will be C and you can splice into the same spot.

  • Lastly, you can splice into a wire from the transformer. You'll need a multi-meter to figure out which one it is (measure 24V between the C side of the transformer and the R terminal).

    • It does look like common is grounded to the chassis, so you can also use that to identify the side of the transformer -- find the side that has a wire connected to the chassis.

    It'll be one of the bottom two terminals:

    enter image description here


You should use the multi-meter to test -- no matter which method you use -- before you actually splice into anything.

With the furnace powered on but not actually running (fan off, heat off), you should measure 24V AC between the R and the place you're hooking into.

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