Adding a C wire to an Intertherm system

Question

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.

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!