Which terminal should the brown wire be connected to the RadioThermostat CT80? The original thermostat was wired as depicted:

enter image description here

The brown wire is connected to terminal E (not F).

As I understand it:

  • BRN => ? Terminal CT80 Radio thermostat
  • YEL => Y Terminal CT 80
  • GRN => G Terminal CT 80
  • WHT => W Terminal CT 80
  • RED => R Terminal CT 80
  • BLU => C Terminal CT 80
  • ORANGE => O Terminal CT 80

As I understand it, the brown why controls the Emergency heat relay. Not one of the seven diagrams has an E-Wire.

enter image description here


  • Is it possible to select one of the seven schemes based on the first photo?
  • Where terminal should the brown 'E' be wired in the CT80?
  • Can you post photos of the wiring at the air handler end? Mar 1, 2019 at 5:17
  • I'd have to return to the site to snap photos. The first photo is of the very old thermostat and I would think that the labeling of the colors is correct? Maybe the Brown wire should be innervated by terminal W2 on the CT80?
    – gatorback
    Mar 1, 2019 at 5:43
  • I hate to say this, but you really need to look at the documentation of the old thermostat and find out what "E" does. The way thermostat wiring is labeled isn't perfectly standardized when it comes to more obscure functions of heat pumps. Find out what E meant to the old 'stat. Mar 1, 2019 at 13:47
  • @Harper I think you are right. Unfortunately for me this project is inherited and the gear pre-dates the internet. If I can find more on the old thermostat I will update the post.
    – gatorback
    Mar 1, 2019 at 13:51


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