I recently bought a Sensi Thermostat with Wifi to replace my vintage one. I thought it would work with my 2-wire heat only system because it has batteries , but found out that using the Wifi feature with batteries only with drain them in only a couple days. After searching forums and looking at the Sensi website, I will need to add a C wire.

I read that I can repurpose the fan wire (G) as a C wire by connecting it to the C terminal in the circuit board. I looked at the thermostat wire and found the fan wire (G) unused and stuffed into the wall. However, my system is so old that it doesn't have a circuit board, only a mess of wire nuts.

No circuit board.

Close up.

Where I should connect the C wire to on my old system?


According to the wiring diagram for your system (i.e. Figure 2 in the linked PDF) -- the yellow wire on the 24VAC side is the C wire (it's the wire that goes to the compressor but not to the 'stat in the diagram). Just incorporate the green wire from the thermostat cable into the nut with the yellow and green wires in it on the 24VAC side, and you'll be set!

  • WOW you responded so quickly and your advice worked perfectly! Thanks for taking the time to help me out. I really appreciate it! I cant seem to upvote your comment because my reputation score is too low on this forum, but when I get it past 15 I'll be sure to upvote you. Thanks again!
    – LennoxG16
    Dec 4 '16 at 22:53

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