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I need to run new thermostat wire since I don't have a C wire to power my smart thermostat, but need some guidance on where to hook it up. I have five wires at my thermostat, two going back to my furnace in the basement and the other three going to my attic where the A/C is located.

thermostat furnace furnace furnace furnace

  • Can you take the Nest base off and show us a closeup of the wiring coming out of the wall at the thermostat location, as well as a photos of the wiring at the air handler? – ThreePhaseEel May 19 at 23:19
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On dual transformer systems, Nest expects the common wire to come from the cool side (Nest Pro Guide, bottom of page 20); look towards your A/C not your boiler.

If there is a common terminal on your A/C control board and

  • 18/5 wire bundle - use one of the extra wires to connect the common terminal to your Nest
  • 18/3 wire bundle
    • Use Venstar Add-A-Wire (or equivalent) or
    • Run a new common wire (might make more sense to just run a new 18/5 or more bundle)

If there is not a common terminal on your A/C, you'll need to find the wiring diagram and locate the 24v transformer. Pull a common wire off the negative side of the secondary side (24v) of the transformer and connect to common terminal of Nest (or use Venstar Add-A-Wire). You'll need to verify that the added load of the Nest won't exceed the VA rating of the transformer (it shouldn't). If this is over your head - call an electrician.

  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. – Daniel Griscom Jul 19 at 19:56

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