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Hi guys I'm trying to figure out I'm using a dual heat and cool thermostat so i decided to get myself a smart thermostat and want to connect both thermostats into 1 but I'm having trouble location the c wire to power my thermostat. As you can tell the white thermostat and red thermostat wires aren't attached to anything.

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  • Can you post photos of the wiring diagram and wiring compartment for your indoor unit (aka air handler or furnace)? What you've posted is the inside of your outdoor unit, which isn't helpful here... – ThreePhaseEel Dec 21 '19 at 4:31
  • Ok i will post it as soon as i get in my attic during the week. – Errrtannn Dec 21 '19 at 4:48
  • This outdoor unit is a single stage. The wires used are blue and green. Typically, the outdoor unit is controlled by yellow (cooling) and black or blue (common). Unused wires should not be stripped back, and should be twisted around the wire jacket. It seems like you have an opportunity to clean up your HVAC control wiring. – ndemarco Dec 21 '19 at 5:03
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    All i have is a 2 wire red and white coming from my heating source that's all. The wire that I'm holding is the one that goes to my thermostat with the white wire not connected to anything. – Errrtannn Dec 26 '19 at 1:37
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    I had 2 separate thermostats for heating and cooling thats why i want to make them into 1 unit but there is only 2 wires coming from my heating thermostat labels as w and r that is all. The one in my cooling thermostat was 4 wires white not attached the red wire on r blue wire is on y and the green wire was g. – Errrtannn Dec 26 '19 at 2:16
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This wiring could do with a bit of cleaning up

As you pointed out that Cable 3 (the rightmost cable in the photo) at the air handler goes nowhere, we can take it out of the circuit entirely by nutting the red wire from the air handler to the red wire from Cable 1 instead of the red wire from Cable 3. (This means that the blue wire from Cable 3 is no longer nutted to the red wire from Cable 1, too.)

Once that's done, finding C isn't so hard after all

Now that we have that useless cable removed from the picture, we can find C rather readily, given that we know how the outdoor unit is wired; in particular, green and blue in the cable to the outdoor unit are connected to the contactor, with the other wires left unused. Since we know that the blue wire in the cable to the outdoor unit is connected to the blue wire in the cable from the thermostat, we can safely presume that this blue wire is the Y wire, leaving the green wire to connect to C on the air handler.

Since we have a good indication that brown is C as well from the scant documentation available for your make of air handlers, we can safely assume that the junction between the thick brown wire and the thin green wire is where you need to connect the white wire in your thermostat cable to use that white wire as C.

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  • Thank you so much it worked great. I appreciated your help with the wiring. – Errrtannn Jan 17 at 1:16

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