I have an ancient furnace/AC combo that came with my apartment that originally came with a very simple battery powered thermostat, which I replaced with a modern Honeywell programmable wifi thermostat.
As the original thermostat did not have a c wire connection, which the new one needed, I needed to figure out where on the unlabeled low voltage panel to connect the c wire to (there was an unused wire in the existing cabling that was disconnected on both ends)
My best guess for the c connection was the common wire for the connection to the outdoor unit, which was wired thus:
R wire -> thermostat -> Y wire -> outdoor unit -> common terminal.
On powering back on the furnace, the thermostat powers on, the fan can be turned on, but the heating system seems to only turn in the fan, with no heat coming. Being winter, I didn't fully test the cooling system.
Did I break it? Or just wire it wrong?
I haven't had a chance yet to test again with the original thermostat, due to giving back the small screwdriver I borrowed before testing it.