I see this is a pretty common question, but the answer seems like it might be dependent on your thermostat and air handler, so I'm posting with my specific questions.

I'm upgrading from a White Rodgers thermostat to Honeywell RTH9580WF. The Honeywell requires a C wire. My old White Rodgers thermostat does not have a C connector. It has a wire from the air handler switchboard to the thermostats RC and is has a jumper to RH.

(air handler) - (Thermostat) 
W - W 
Y - Y 
G - G
R - RC jumper to RH

I can see on the air handler that there is a COM 24v which I believe is C (COM = Common = C, right?)

There is a second 18/4 wire hooked into the air handler (C & Y). I'm not positive where that line runs, but it looks to follow the coolant line, so maybe to the condenser outside?

We only use this system for AC. I don't know if that impacts my situation at all, but I see lots of discussion on how things relate to heat, so I wanted to point it out.

I was reading about the various wire's uses in this thread.

It says RC/RH are for Cooling and Heating calls, but common is seperate, so does that mean I cannot just move the red wire from RC in the old thermostat to the new thermostat's C? (I assume no, since it travels to the R in the air handler , and not the C).

It seems like my only recourse right now is to either run new wires, or to move one of the existing 4 to the C. I saw in other threads, and this honeywell video they moved the Green wire from G to C which takes away the manual fan option, which I think would be ok provided the fan still can be called when AC kicks on.

Am I understanding things correctly? If not, please correct me.
Does moving G to C make sense in an AC only system?
Would it make more sense to just run new wires?
Do I need to do anything with the extra 18/4 that's hooked up to the air handler as it is?

(Also, I keep referring to it as the air handler because that's how I keep reading about it. If it's called something else when it's used for AC only, let me know so I can speak to it correctly.)

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  • 1
    Where are you on this planet? Also, the word you're after for your not-a-furnace is "air handler", or AHU (air handling unit). Jun 18, 2016 at 1:42
  • Where am I on the Planet? In Utah, in USA. Why do you ask? (Is my wiring insane? Not the first time I've learned things were done 'different' in this house)
    – Fuzz Evans
    Jun 18, 2016 at 2:39
  • I asked to get a sense for your local climate :) the words "hot" and "dry" apply, no? Jun 18, 2016 at 3:14
  • Hot and dry are definitely applicable.
    – Fuzz Evans
    Jun 18, 2016 at 4:23
  • Does the unit have any heating capabilities?
    – Tester101
    Jun 18, 2016 at 14:07

3 Answers 3


Since you have an air handler and not a furnace -- I bet W does nothing whatsoever. Unhook that wire from W and connect it to COM 24V at the furnace end, then use it as your C wire for the new 'stat. And yes, the cable from Y and COM that follows the refrigerant line goes to the condenser unit outside.


Run a new cable with at least five conductors. Connect a wire between COM 24V, and the C terminal on the thermostat.

The cable that connects to Y and COM 24V, goes out to the outdoor unit.

If the indoor unit only blows air, and doesn't add heat. It's called an air handler.

  • Air handler, thank you. Seemed like 'furnace' was wrong. I'll go make some quick edits.
    – Fuzz Evans
    Jun 18, 2016 at 2:38

RC and RH on the thermostat mean Red Cool and Red Heat, this is for old General Electric HVAC equipment that had 2 transformers 1 for Heat and 1 for Cool. The Transformers 24 volt hot leg from the AC unit went to RC and the 24 volt hot leg from the air handler went to RH and there was no jumper between RC and Rh, today's modern HVAC equipment uses 1 transformer for both modes Heat and Cool thus the jumper between RC and Rh the red wire can be attached to either since they are jumpered together

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