So I have a Nest Gen 3 thermostat and I am in need of a C wire. I have a Lenox G12D2-82 furnace. I have 4 wire set up. The Venstar makes 4 into 5 wires. I have a fan heat cool and power wires. There is no control board since my furnace is so old? All I really need to know is where the C terminal is or the C connection. enter image description hereenter image description here

  • Can you post the wiring diagram for your furnace please? – ThreePhaseEel Jan 3 '19 at 1:34

You want the wirenut with the 3 yellow wires in it

Apparently, your installer used non-standard thermostat wire, and as a result, your Y wire is actually the blue wire in the thermostat cable. This is nutted to the blue wire going off to the compressor (outdoor unit), whereas the yellow wire from the compressor, which must be the C wire, is nutted with two thick yellow wires in front of the transformer in your pictures.

Furthermore, we also know that that clump of yellow wires is our C wire because the wiring diagram says that the yellow wire from the low-voltage control circuits goes off to the compressor, while the other wire to the compressor is connected directly to the Y wire from the thermostat with no connection to the furnace control.

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  • So following venstars YouTube video for add a wire. The red venstar wire will go to the clump with the red thermostat wire. Brown common venstar wire goes to the clump with the clump of 3 yellow wires. Green theonstat wire will also go to that clump of 3 yellow wires. Remove blue thermostat wire and replace with yellow venstar wire. Venstar green wire will go where the original green thermostat wire was which was connected to the white wire. Then lastly blue thermostat to blue venstar. And this is all with the diode at the nest. I had to draw lines to remember this haha – A. Krall Jan 5 '19 at 17:15

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