thermostat-c-wirewiring
My old thermostat LUXPRO TX 9600tsa uses 4 wires:
This thing is not working right, so I purchased a Honeywell RTH9585WF WiFi thermostat. The issue is I have is no C wire and on this new one there is no RH block so I hooked the R red to red but do I need to loop it to RC to get power?
I was also told I could relocate the G to C and go that route. just not sure what to do? I am getting no power to it…
There should be no problem connecting the C wire from your thermostat to the C terminal in the furnace. Take a look at this diagram, which is a rough approximation of your system.
Notice the cable going to the condensing unit has a red wire connected to the Y terminal in the furnace, and a white wire attached to the C terminal. When the thermostat calls for COOL, it does so by energizing the Y terminal (connecting R to Y). This provides power to the coil of the contactor, which allows it to close the contacts and turn the condensing unit on. The other side of the contactor coil is connected to the C terminal, as to provide a complete circuit.
You'll notice in the first image that two relays in the furnace, the thermostat, and the contactor coil all connect to the same leg of the transformer. This is because the thermostat basically works like a switch, connecting the R wire to the C wire through control circuitry. Anything that needs 24VAC power, will be connected to both legs of the transformer.
After looking closely at the images you've supplied, this is what it appears is going on in the last photo.
From what I know about HVAC systems, I would guess a broader view of the system would look something like this.
COOL call from the thermostat.C).When the thermostat calls for COOL, the BLUE wire is energized. This energizes the WHITE wire leading to the contactor, which pulls the contactor closed. The GREEN wire connects to the BROWN wire, which leads back to the transformer, thus completing the circuit.
Therefore, if you want to use the YELLOW wire as C. You should include it in the twist-on wire connector with the GREEN and BROWN wire.
You'll just want to make sure the transformer is sized properly to supply the additional current draw of the thermostat.
If it were me, I'd rewire it so that the YELLOW wire was the COOL call, and the BLUE wire was C. As that's a more common color coding.
Best Answer
The RTH9585WF should have a Rh/Rc jumper from the factory, if it is missing then you must add one. If you use the green wire as a "c" wire you need to change it on the thermostat and in the air conditioner. You will lose the "fan only" option at the stat.