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.)
Best Answer
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.