Oddbean new post about login | logout @e7cebe76 are you referring to the C wire? I think one is needed to run most all of these. Are you saying you were able to work around not needing that wire?
@5a9ad3e3 It was years ago so I don't know which wire wasn't being used. It could have been the "C" wire. I mentioned the 4/5 wire issue because you may run into it and need an adapter or a jumper. My old Hunter thermostat uses a jumper with a 4 wire setup. Whoever installed the cable had cut off the 5th wire at both ends. There was enough slack to use the 5th wire with the Ecobee instead of the adapter.