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I am trying to install my Ecobee3 now. After finishing the connections, Ecobee did not power up. I did some reading and I believe the C-wire, though it is available is not connected to the power at the furnace-end (see the picture where you can find the blue standalone wire). Can you please suggest where/how I connect the C-wire in my furnace (model # lennox g12q4-110) to supply power to the Thermostat? Thank you very much for your help in advance.enter image description here

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    Can you post the wiring diagram for your furnace? It should be posted on the furnace somewhere... – ThreePhaseEel Dec 10 '17 at 21:03
  • Also, do you have an air conditioner? – ThreePhaseEel Dec 10 '17 at 21:07
  • Yes, I have an air conditioner, connected to the same system. I found the diagram on the internet but I can't find the upload button. – Visu Dec 10 '17 at 21:09
  • The diagram is the same as the one posted as link to the following post:diy.stackexchange.com/questions/103825/… – Visu Dec 10 '17 at 21:12
  • What was your old thermostat, a mechanical unit? – ThreePhaseEel Dec 10 '17 at 21:21
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I have this exact same furnace (30+ years old) and very similar messy nest of wiring. As user76730 noted the yellow on the 24V side of the transformer is the key. In your photos, this is the left most wire nut that is pointing up with 2 yellow wires and a green attached. I had a spare blue thermostat wire from my old Honeywell mercury blob setup that was completely unused and just wrapped back in the wall. Hooking that up to the yellow bundle and then the C terminal on the thermostat is working like a charm. I have a Nest E with just the R and W, and it was power stealing badly. Now it runs very smoothly.

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The pictures are a little difficult to follow but according to the electrical diagram there is a blue and a yellow wire coming off the low voltage side of the transformer. Connect the C wire from the thermostat to the yellow wire from the transformer

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I also have the Lenox G12Q4-110 furnace (heating and cooling) system with a four conductor thermostat wire. I found the instructions posted here of tremendous value in confirming that I could update the thermostat to an ecobee3 lite using their five wire to four wire adapter that was included in the box. I was able to make all of the wiring connections, have the ecobee installed and the system up and running in less than 30 minutes. FYI, the Lenox internal wiring to the thermostat that I found is:

  • Blue wire = R
  • White wire = G
  • Yellow wire = Y
  • Brown wire = W

As noted in the earlier posts, the yellow wire running from the 24VDC transformer inside the furnace is a key connection for using the ecobee adapter. This specific info allowed me to update/upgrade my 30+year old heating/cooling system into a smart-home controlled via Apple’s HomeKit.

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