With the increasing popularity of wifi thermostats I know this question has been asked many times, however I'll ask it again fr my specific situation.

How do I add a "C" wire to my existing air handler terminal and thermostat.

Here is a picture of the terminal (Magic Aire Dux-8 air handler) enter image description here

Starting from the left, the C terminal has two wires a large White/Yellow and a green, neither of these wires come out of the wall where the thermostat is mounted. (The wire bundle that comes out wall has 6 wires - Red, Blue, White, Yellow, Green and Black. The Black is not in use at the terminal end or the thermostat end.) Next to the C terminal is the R terminal and has both the red and blue wires attached, part of the 6 wire bundle. The green wire is attached to the G terminal, also part of the bundle. Y has a yellow wire, part of the 6 wire group, and a red wire not a part of the wire group. W has a white wire, part of the bundle, and a large White/Blue wire not a part of the bundle. W1 is not in use.

Now for the thermostat image enter image description here

This picture is self explanatory with two exceptions: - the black wire mentioned earlier, part of the 6 wire bundle, is present but not in the picture. - the blue wire is connected to Rc

So, now the questions are: 1.) how to I connect a wifi thermostat that requiers a C wire? 2.) can i simply use the unused black wire as a C? 3.) does the blue need to be connected the Rc on the wifi thermostat?

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    I think most of us are worn out now on answering C-wire questions. I would suggest doing a search. There are only about 3 million C-wire questions/answers here over the past couple years. – Jeff Pritchard Dec 18 '15 at 12:27
  • Can you take another photo the same as the first, just zoomed out a bit? It looks like there's two or three cables involved, but it's hard to tell exactly what's going on with the wiring. – Tester101 Dec 18 '15 at 13:55
  • Jeff, I fully understand you position on answering the C wire question. I did look on the form for an answer before I posted my question and none of the responses seemed to fit my situation. – Kallen74 Dec 21 '15 at 18:58
  • Tester101, there are a few other cables involved but it doesnt have anything to with the thermostat in question. Thanks for the reply. – Kallen74 Dec 21 '15 at 18:59
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    @Jeff-Pritchard Maybe it's time for thermostatcwire.stackexchange.com ;) – Stavr00 Nov 28 '17 at 14:43

In your case, the red and blue wires are connected to the same terminal on the air handler, and that's just not necessary. Connect the blue wire to the C terminal on the air handler along with the yellow/white and green.

On the new thermostat, the blue will then be your C wire, and there will normally be a jumper or small wire that connects the Rc and R terminals, so the red wire only needs to go to one of the R terminals. The two different "R" terminals are used if the air handler/heater and outside AC unit happen to use different power sources. In my experience this is very rare to the point that thermostat manufactures pre-install a jumper between them.

  • Thank you very much for the answer JPhi1618. I'll try it and let you know how it works out. – Kallen74 Dec 21 '15 at 18:55
  • @Kallen74 be sure power is off to the unit when messing with the wires, or even the thermostat. The thermostat wires are only 24v, and not really a danger, but momentarily touching the wrong wires could be bad for the A/C or air handler. – JPhi1618 Dec 21 '15 at 18:59

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