I am trying to add an Ecobee smart thermostat to an older Intertherm furnace and A/C. I have extra wires running to my thermostat, but can't figure out what to connect to in the furnace wiring. Any help would be greatly appreciated.



Wiring connections

Wiring diagram

1 Answer 1


Use the existing black wire

From the wiring diagram for your unit, and the way your A/C appears to be wired, the light grey wire your unit provides for thermostat connection is the C wire. This, in turn, is connected to the black wire in your thermostat cable. So, simply use the black wire coming out of the wall at your thermostat as your C wire, and enjoy!

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.