First time doing this, don't want to burn house down. The brown bundle is coming from thermostat, the white bundle I suspect is coming from A/C unit. Do I follow these steps?

  1. Find a scrap piece of 18 gauge wire
  2. Unplug furnace
  3. Strip the end of clipped blue wire in furnace
  4. Twist blue wire to scrap wire and cap with wire nut
  5. Wrap scrap wire around C screw on the left
  6. Plug in furnace

Furnace enter image description here

  • The white wire from the white bundle is being held by the insulation the red is just out of shot but double check that, the white from the black bundle isn't being held at all. – ratchet freak Jan 3 '19 at 12:53

Yes, you can use the blue wire the way you have described.

Instead of all of that work to attach an extension to the blue wire, I would just cut back some of the brown sheath. That's assuming that there is enough slack in the cable for that to work.

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