I moved into a home recently and found the old thermostat temperature controls to be inaccurate at turning on and turning off A/C as set so I purchased a Honeywell RTH6580WF thermostat as a replacement. The current thermostat is an old Honeywell round mercury bulb model. In attempting to replace the current thermostat I found that I need a C wire for the new thermostat to work correctly, although my old thermostat only had G, Y, W, and R wired up. I read more into this issue and found that I should be able to take the G wire and move it to the C on the new thermostat as well as at the furnace, and then place a jumper from Y to G at the furnace and that should take care of it.

The problem I run into is that the Y slot on the furnace is empty and I have a wire in the C slot on the furnace even though there wasn't one at the old thermostat. I've since discovered that the A/C compressor is wired in at the furnace as well, with the Y from the thermostat and the Red cable from the A/C compressor twisted together with a wire nut, and the white cable from the A/C compressor going to the C slot on the furnace. I spoke with Honeywell support on the phone and they said I'd need to have an HVAC contractor come out to resolve this issue, saying I would not be able to use the new thermostat with the current wiring situation. (I've included a diagram of the current wiring setup.

My idea is that I can run new 18/5 thermostat wire (current installed wire has only 4 wires) and then wire up the new thermostat from thermostat to furnace as R-R, W-W, Y-Y, G-G, C-C and then instead of twisting the Y from thermostat with the red wire from the A/C compressor I can just put two wires on the Y on the furnace and also have two wires on the C on the furnace, with the second of each coming from the A/C compressor. I've included a diagram of this hopeful configuration. I'm not sure if this is the proper method and I haven't been able to find anyone with the exact same setup to verify so I thought I'd ask for anyone's assistance on this. Does that sound like the best solution? I appreciate any guidance with this. Thank you very much for your time.

Old Thermostat Wiring New hopeful Thermostat Wiring

1 Answer 1


Yes, that should work just fine.

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