So I'm looking for some help in identifying where I can attach a C/Common Wire to my HVAC system, but unfortunately my system doesn't have a control board that I can easily attach a powerkit to. For reference I'm planning to install an Echobee3.

I've uploaded some photos here: http://imgur.com/gallery/LPzoq/

Any advice would be greatly appreciated, probably adding some circle/arrows so I know what you're talking about ;)

After looking at Tester101's answer, here's an annotated picture. Notice how they're two free wires(red and black) annotated


Based on the new information... It looks like the blue wire from the secondary side of the transformer is the R, which would make the yellow secondary wire C.

I should have read the transformer label, that clearly says BL (Blue) is common, and yellow is 24V.

enter image description here

  • Thank you :) How would you personally recommend me accessing that wire? Splitting it, etc. – Icestorm Mar 28 '15 at 0:26
  • @Icestorm Not sure, I can't really tell how it's terminated now. – Tester101 Mar 28 '15 at 0:28
  • Oh that comment was from when we thought the yellow wire was "C". If the black wire is common then it's easy. It just has a plastic cap on it. Thanks for the help :) – Icestorm Mar 28 '15 at 0:35
  • NO! BL stands for BLUE. Your system is wired up so that the blue secondary wire is connected to the red R wire, so the yellow secondary wire is going to be your C. – Tester101 Mar 28 '15 at 0:37
  • Ah! Then you previous comment about not knowing how it was terminated makes sense. But I now know what wire I need. – Icestorm Mar 28 '15 at 0:43

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