Got a wifi thermostat. Behind the old thermostat was a blue wire. The other side of the blue wire in the equipment isn't plugged into anything. I need to get the 24v common hooked up to this blue but don't know where it goes. Any help is greatly appreciated.




You don't show where all the thermostat wires are connected in the unit, so I can't see how it's currently connected. But according to the schematic, the blue wire heading off to the left of the first photo is the C wire.

C wire labeled


Fortunately, in your case, the fat blue wires in your first photo are the transformer return -- connect the skinny blue wire from the thermostat cable to the fat blue wires. Since the air-handler maker used quick-connect terminals for the internal wiring, you can crimp one of these onto the end of the thin blue wire and slide it on in place of one of the fat blue wires somewhere, then slide the fat blue wire onto the stacked end the linked terminal provides.

  • For testing purposes I could simply just expose some of the copper in the blue skinny wire and attempt to push into one of the blue wires? The digikey link looks useful for a long term solution! To crimp one of those digikey products do I simply use pliers or need some special tool? OUT of curiosity, what lines are the fat blue wires in the electrical diagram? Is it the BL on the right side of the diagram? I see that ground symbol to the right of it. – user3330299 Aug 5 '16 at 5:18
  • @user3330299 -- while it's possible to crimp these with pliers, you'll get somewhat better results out of a $10 crimper from the hardware store. (I use a more sophisticated ratcheting tool that costs ~$50, but that's probably not worth it for a one-off job). The fat blue wires are indeed the wires marked BL on the right side of the diagram. – ThreePhaseEel Aug 5 '16 at 11:34

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