Trying to attach a c-wire to old rheem air handler for a cooling system only. System is so old I didn't see what I thought I'd see when I got up into the attic. In going over the pictures I think I might have figured it out but could use confirmation.

There's a 4 conductor thermostat wire going from the thermostat up to the air handler. Only 3 wires were used I was hoping to attach the blue wire as the C-wire.

Red Rc on thermostat White (or maybe yellow) Y on Thermostat Green G on Thermostat.

There's also a second 2 conductor thermostat looking cable (brown jacket) with a white and red wires.

On top of the air handler is a switch (regular light switch) I guess as a quick disconnect. enter image description here

This is the wiring diagram I found inside the air handler wiring box.enter image description here

There's a block that's obviously the line voltage terminal that I didn't take a picture of.

I think this is the MIB? (Motor, indoor blower)enter image description here

And this RM? (Relay, blower motor) with the transformer on the right.enter image description here

I think I can connect the C-wire to the yellow wire that comes off the left of the transformer or the blue wire it attaches to on the top right of the relay. Easiest is probably to cut the yellow wire and splice the 2 new ends of the yellow wire together with the blue wire from the thermostat cable. Does that sound right?

And out of curiousity what could the other 2 wire cable be going to? It's not to another thermostat.

  • There's an outdoor unit that goes with this air handler, right? Jun 25, 2017 at 0:19
  • @ThreePhaseEel yes the compressor unit. Can you confirm which wire I need to tap into it's such a pain getting into my attic I'm hoping I get it right the first time. well second trip :-D Jun 25, 2017 at 0:21

2 Answers 2


This is easier than it looks even

Simply nut the blue wire in the four-wire cable to the junction of the white wire from the two-wire cable and the brown? wire going into the air handler. We know it's your C wire because that two-wire cable has only one place it can head: to your outdoor unit and its contactor. Since the red wire in that cable is the Y wire that switches the contactor, the white wire, and by extension the wire connected to it, must be C wires otherwise the contactor would never work.

  • Going to try it now. That's much easier gladg I asked. Jun 25, 2017 at 0:41
  • I thought I replied on my phone earlier. It worked. Had the thermostat already connected and when I came back upstairs after turning on the breakers the thermostat was on. A lot easier than what I was planning to do with splicing it in the yellow wire. Thank you so much! Jun 25, 2017 at 1:46
  • Your research is right and I confirm that adding a C wire to the thermostat and simply grounding it to Rheem (or Ruud) metal chassis works perfectly well. My Nest thermostat now recharges fine, thank you!
    – HO guy
    May 28, 2020 at 11:33

After much research I can tell you that the C wire is nothing more than Ground. The red wire is the power. Some electrical idiot somewhere decided to call the 'Ground' connection 'Common'. So for old Rheem units that are missing the C wire, just run an extra wire to Ground (anywhere on the Rheem metal chassis is ground) and you now have a C wire connection. Twist it into an exiting bundle of ground wires if they exist. You can verify with a volt meter from Ground to Red wire that you have the 24 volts. Your welcome.

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