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 '19 at 23:19

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.

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

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.