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I have a NEST thermostat. My heating and cooling system is in the attic.I need to add a "C" wire to the control board, but I can't find out where I need to open the unit in the attic to see where the existing thermostat wires run from my hallway thermostat to the attic?

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Sometimes air handlers/furnaces have lift off covers that allow you to access the wires, but often you will have to remove some screws or cut some duct tape (if the install was messy) to remove the proper panel. Pictures help here, but you want to look for any covers or panels that will slide or lift out of place.

If you find one, open it and see whats inside. If it only provides access to the blower or the heater portion of the unit and not the control board, you will most likely have to remove a few screws to remove another panel (panel can be large, like the whole half of the unit).

Again, this is general advice, but I think that many people might not think they need to start pulling out sheet metal screws and expect a pop-open panel. If you're not sure, post some pictures. I wouldn't want you taking apart the wrong thing!

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  • Thanks so much for replying. I will take some pictures tonight when I get home. There are 2 (large) panels on the side of the unit with screws I removed all of the screws on both panels. The one panel I was able to remove only shows the copper coils and no circuit board. The other panel has some black press-on adhesive that is very strong and holding the panel in place. When I tried to pry it apart the panel actually started to bend or crimp, so I stopped. I thought the black adhesive was a sign that the panel was not intended to be removed.That's why I decided to write and ask for some advice
    – kenneth
    Commented Dec 18, 2019 at 17:21
  • That adhesive could have been an overzealous sealing attempt by the installer, but pictures would help. Don't force anything open - there's a chance it's not a panel that should be removed.
    – JPhi1618
    Commented Dec 18, 2019 at 19:15
  • Additionally, if you can find a make and model number, do a google search for that + "Owner's manual." You'll probably get a searchable document, and you may be able to find the OEM's documentation on where the control board is.
    – NegativeFriction
    Commented Dec 19, 2019 at 16:20

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