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I bought a 3rd generation Nest. It doesn’t turn on after I hooked it up. I did research saying I need a common wire as power supply.

I have closely look all the wires in my system and drew a diagram. Can someone take a look see if my solution going to work? There is a spare wire in place already, it’s just not connected on either end(in thermostat and in the furnace)

There are two common wires inside the furnace, one return itself, the other connects to the condenser (this is the one I am thinking connect to the thermostat).


I think you are right. From what I remember once I did something similar as their was no c to the thermostat. Luckily for me the unit was right next to the thermostat so it wasn’t hard to run.

You could use this instead if you don’t want to run the cable?

Venstar ACC0410 Add-A-Wire Kit https://www.amazon.com/dp/B008PLWT8C/ref=cm_sw_r_cp_api_i_OFbeDbAE61VDQ

  • Thx for looking at it. There’s spare wire in place already, just not connected on either end. – Yalin Jun 24 '19 at 0:55
  • Before you plug it into your Nest, verify that you get 24VAC across R and C. – Matt Jul 24 '19 at 22:06

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