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**Pictures attached to go along with my rambling explanation

I have my Lennox G23Q3/4-100-2 natural gas furnace and my central air system that has been working since 1995 on a basic programmable thermostat and before my purchase of a new Ecobee4 thermostat, I verified that I had a connection on the C terminal. I have 4 wires running to my thermostat, I noticed that the C terminal is actually connected to the Red wire leading to my A/C Compressor and the White wire from the compressor connects to the Blue wire going to my thermostat and connected to the Y on my old thermostat, and my Y post at the furnace has nothing connected to it. So I am a bit confused as to why the C post is connected to the Y on the thermostat, and how I now wire it to my ecobee4.

My picture show the wiring that allows the Ecobee4 to manage the heating, but no A/C, and I'm afraid to start swapping the C and Y wires around out of fear for burning out the board/compressor/thermostat or just not providing the necessary power. Ecobee4 provides a Power Extender Kit (PEK), but with my current wiring, I am not sure how to use the PEK.

Can anyone offer any advice to get my Ecobee4 working with my Lennox furnace?

Old Thermostat Wiring

New Thermostat Wiring

Furnace Wiring

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  • Oh boy -- was the old furnace set for "fossil"/"gas"/"oil"/ or "electric" mode? – ThreePhaseEel Apr 25 '18 at 3:16
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It has hard to see that mess of wires. I believe what they did was wire nut the C to the wire that goes out to the outdoor unit contractor. That works fine as long as the blue wire is hooked to Y on the thermostat. The blue was your Y wire for cooling. But then what do you use for common?You are going to need 5 wires from your thermostat to your furnace. Just hook up R to R, W to W etc. then on the C and Y terminals at the furnace you will attach the red and white wires coming from the compressor so you will have 2 wires on each of those terminals.

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Sorry for the much later response. I called Ecobee, they sent me details on the exact wiring that should be performed, worked without issue and as expected.

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  • If you posted this exact wiring, then the answer would be useful for other users. – Chenmunka Jun 14 '19 at 18:05
  • If anyone would like it then let me know, but I did not add it because my situation may be different from someone else's wiring and I don't want someone to burn out their board or thermostat because they followed my configuration. The correct answer should be to contact the manufacturer for assistance as they can help with your specific wiring needs. – John Weidauer Jun 21 '19 at 20:00

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