Looking to hookup a wifi (smart) thermostat that requires common constant low voltage power. The hookup of the old thermostat didn't use the blue wire as it was shoved into the wall but I traced it up to the air handler.

Below is the electrical diagram as well as an image of what I've already done.

The blue wire wasn't connected to anything in the air handler. I used the wire nut that was there already being used for a thick and thin blue. That is what I did to my other one in the house and it worked. This one did not.

I've written some notes on my 2nd picture circling the "c" wire that goes to the thermostat, how I plugged it into the wire nut, and then showing a trace of that thick blue wire and where it goes with a zoom of where it goes. It mounts to the metal with a screw as a ground. This did not work as the thermostat didn't light up.

I believe that in my 2nd photo the ORANGE arrow is where I am really suppose to tap into (blue coming off of the circuit board). I don't want to do anything without a little bit more confidence though.

Any help or suggestions is much appreciated.

UPDATE: I've now updated wires. I'm only referring to BLUE wires, but I use overlay color to describe.

A - Green B - Pink C - Orange D - Turquoise


enter image description here

  • Can you figure out where the blue wire from the 9-pin connector on the control board leads? Aug 6, 2016 at 17:42
  • The blue wire (that I labeled in orange "this one?" goes into that gray device that is (in the picture) above the most left wire nut. There is a lighter blue that goes into it (top left) and then right next to it is a darker blue (THIS GOES TO THE 9 PIN BLUE). Then next to that is a light blue (gray) that goes up and out the side of the equipment wall into another piece of equipment. I can't add another picture, so what I did was edit the original post. WHAT I DID WAS follow the 9 pin blue with ORANGE dots/line showing where it goes. Along with those other cables going into that 24V thing Aug 6, 2016 at 18:37
  • Are there any spare terminals for the blue wire on the 24V transformer? (i.e. the gray device) Better yet, can you get me a closeup shot of it? Aug 6, 2016 at 19:37
  • I have a close up of the 24V transformer. It is now in the bottom left of the 2nd picture of the original post. There are NO spare terminals for the vlue wire on the 24V transformer. That light blue/blue/light blue (gray) are all daisey chained into one terminal on the 24V transformer. NOTICE that with the zoom you can see the green dotted (blue) wire goes into a screw and that one of the 24V (light) blue wires goes into a nut next to it for grounds. Aug 6, 2016 at 19:57

1 Answer 1


The initial picture showing that I connected into the other blue nut was correct. The unit had a button that had to be pressed in to ensure that the cover was on. DOH!!!!

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