I've read many posts on this, and believe I know what to do, but want to make sure. My furnace (a Lennox G14 Series) does not have a C terminal which needs to be connected to the C terminal in my new wifi thermostat. In the attached picture of the terminal strip in the furnace, the red wire on the T terminal and the white wire on the Y terminal go to the air conditioner. The other 4 wires go to the thermostat. I've measure across the R and T terminals and get 28vac. Based on this I believe I can use the T terminal for the C connection...is that right?

Eventually I'll run a 5 wire cable but for now I plan on stealing the green wire to make the C connection if I can use the T terminal.



UPDATE: I've added the wiring diagram from inside the furnace.


2 Answers 2


Note: the solution here is the querent's -- I'm simply writing it up in a CW answer to get this off our plate as the querent has not returned to migrate it from his question-edits to an answer himself.

SOLVED (01/04/15): I decided that the T terminal was exactly what I needed so gave it a try and everything is working perfectly. I also pulled in some 18-5 thermostat wire. Here is my setup, in case it might help someone else:

Furnace: Lennox G14Q3-60-13

Thermostat: Honeywell RTH6500WF

I've attached pictures of my connections.

enter image description here

enter image description here


R wire is power supply hot
C wire is power supply common
W wire is furnace heat ignition relay (heat)
G wire is furnace fan on/off (heat)
Y wire is air conditioner fan on/off (cool)

If using an air conditioning compressor unit,

  • add 1 wire from furnace terminal block label Y to the a/c compressor relay hot,
  • add 1 wire from furnace terminal block label T to the a/c compressor relay common

Lennox terminal block inside the furnace uses T label on the terminal block for common, as shown in your electrical wiring diagram.

Older thermostats did not need a constant source of power for the thermostat display, so the C wire was typically not used.

Modern thermostats which have backlit displays, wifi etc., require a constant source of power and therefore require the C wire to be connected to the power supply in the furnace terminal block, to allow the flow of constant power to the thermostat.

Thermostat C Wire: Everything you need to know about the “common” wire

Don't waste your time calling Lennox customer service. It is a call center whose only mission is to send a repair technician to your home.

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