I am trying to install a new smart thermostat that requires a C connection. The previous thermostat used a 4 wire connection but there is a blue wire that's unused right behind the current thermostat. Problem is that I looked in the air handler and there are no terminals like most units, just a bunch of wiring. So trying to see if anyone may know where I have to connect the other end of that blue wire. Here are some pictures:

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I am assuming the skinny blue wire at the bottom of the handler is the one routes to the thermostat but is not connected to anything. This blue wire is best visible in picture 1 and bottom of picture 3 (not the thick blue wire that has a white cap in the middle of picture 3).

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1 Answer 1


Your best clue is to observe that the 24VAC output transformer is labeled as follows:

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The secondary is indicated to be the BRN and RED wires out of the transformer. One of these is the one that provides the power connection to the thermostat and is most likely the RED one that exits from the wiring box and connects to what looks like the skinny RED off to the thermostat.

The BRN (brown) wire from the transformer is what you are going to want to connect to the skinny blue wire to the thermostat. It is a bit hard to see from the pictures that you provided, but it looks like the BRN wire is connected out of the wiring box as also a BROWN wire that is connected a wire in the smaller control wire cable (which appears to be supporting part of an air conditioner unit).

Carefully check and trace the transformer secondary wires to see if they are as I described above. If so you should be well on your way to getting your smart thermostat operational.

  • First of all thank you for the reply. You are correct on the red wire meeting up with the red to thermostat. The brown does go into another section where it does connect with other brown wires as you have said. I added the pictures in the 1st post. So i need to run/extend the blue wire up to reach the brown wire where it connects to the others or splice into the brown and add another wire twist correct? Just making sure as most of my knowledge is auto based so not to sure on house wiring.
    – Sergey
    Nov 28, 2018 at 3:34

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