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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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Visu
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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:https://diy.stackexchange.com/questions/103825/how-to-add-c-wire-to-a-very-old-2-wire-lennox-g16-gas-furnace – Visu Dec 10 '17 at 21:12
  • What was your old thermostat, a mechanical unit? – ThreePhaseEel Dec 10 '17 at 21:21
  • Yes, it was an old Honeywell one (the one with the mercury). Also, I found the diagram and I am going to edit/update my post and the diagram momentarily. – Visu Dec 10 '17 at 21:23
  • I just now posted the wiring diagram an the Ecobee wiring that I am trying to do. Thanks a lot for your help! – Visu Dec 10 '17 at 21:27
  • Can you post a better photo of the junctions to the thermostat cables please? – ThreePhaseEel Dec 10 '17 at 21:35
  • Added just now, please verify. If this is not good enough, please let me know if there is any particular cable(s) wiring you want to take a look at and I will try to upload a specific picture – Visu Dec 10 '17 at 21:50
  • Just now added another picture. – Visu Dec 10 '17 at 22:02
  • Is there another junction of thermostat wiring involved in this mess? – ThreePhaseEel Dec 10 '17 at 22:21
  • No, there is no other junctions that I am aware of. However, there are two thermostats - one for heating and another for cooling. I am replacing the one for heating. – Visu Dec 10 '17 at 22:31
  • Where does the two-wire thermostat cable at the furnace head? – ThreePhaseEel Dec 10 '17 at 22:49
  • I see some problems with the way the control wires are wired now or we don't have enough information. I think you need a rework. Let's see what others see? – Paul Logan Dec 10 '17 at 22:58
  • @PaulLogan -- no kidding -- my bet is there's a junction hiding somewhere in his house that's causing all this derangement. – ThreePhaseEel Dec 10 '17 at 23:03
  • Hi ThreePhaseEel, I am not sure I understand your question. But, I just uploaded another image where I see two more wires (similar to the ones going out from that junction) going up. Hi @PaulLogan, so there is no way I can find out which one is my C wire and how to connect it so I can power up my Ecobee? – Visu Dec 10 '17 at 23:04
  • Can you follow the two-wire cable from the furnace to where it leads? – ThreePhaseEel Dec 10 '17 at 23:06
  • You are right...the wires are going around the duct and and there are some joints and connections made at least in two other places, similar to what is seen inside the furnace. Also, looks like there was a humidifier there before but not anymore. So, I guess I will get a HVAC guy to get it sorted out. Thanks for your time and thanks for trying to help. – Visu Dec 10 '17 at 23:24
  • @Visu -- post clear, close-up photos of the junctions you found, and we can continue helping you here :) – ThreePhaseEel Dec 11 '17 at 01:31
  • It this is true about these other splices, then if it is at all possible install an all new, unbroken 5+ conductor cable from furnace to this stat location. – Paul Logan Dec 11 '17 at 05:20

3 Answers3

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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.

Fred Hirsch
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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

user76730
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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.

Machavity
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BobSage
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