Hoping someone can help me solve for powering a 24v Common wire to what will be my new smart thermostat. Currently my downstairs thermostat controls a boiler as well as the central air unit for downstairs. The thermostat wire does have an unused blue "C" wire but when I checked in the attic where the thermostat wire gets pigtailed into the wires coming out of the AC handler I can see that the blue wire is NOT actually connected to anything up there so I know it won't power anything. The wire from the outside AC condensing unit has 2 wires (one red and one white) that I believe always carry 24v (but maybe I'm wrong?) and the red is pigtailed into a brown wire that seems to be going into the control board of the AC handler and the white wire is pigtailed into a blue wire that is coming out of a light brown shielding that also has a green wire that is pigtailed into a gray wire coming out of the AC handler and a red wire that is pigtailed into a red wire coming out of the AC handler. The wires are then connected to the wires coming from my indoor thermostat using pigtails. Green is connected to green, red to red, and blue is connected to yellow (the yellow going to the thermostat). The thermostat wire does have two unused wires that are wrapped around the shielding of it, one white and one blue. My questions is, can I connect the blue wire that is currently unused, to the same pigtail that currently has the white wire from the outside AC compressor connected to the blue wire (which later is pigtailed into the yellow wire going to the thermostat)? AC compressor wires connected to AC handler wires thermostat wires connecting with wires going to AC handler

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    Can you post a wiring diagram for your air handler please? Apr 28, 2022 at 3:33

1 Answer 1


I was able to answer my own question here. By knowing what wire from the outside compressor is connected to the wire going to the "Y" on the thermostat (in my case it was the white wire from the outside compressor that was pigtailed to a blue wire which in turn was pigtailed to the yellow wire from the thermostat) I was able to figure out that the other wire from the compressor (in this case the red one) was the common wire (and looking at my AC handlers wire diagram confirmed it), so I simply added another wire to that pigtail junction (turn off all electricity to the AC first!) and pigtailed that to the unused "blue" wire going to the thermostat and everything worked perfectly.

  • It looks like you inadvertently created a second account -- you'll want to merge them so you can use the site fully May 7, 2022 at 14:08

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