My home currently has a central a/c unit which is controlled from an 3-wire old mercury switch thermostat. I want to replace it with a newer Honeywell that requires 4 wires.
The existing thermostat a Green, White and Red wire. The new thermostat requires and additional Yellow wire.
I tried lots of combinations, but nothing worked properly, yet with the old thermostat, I can turn on the a/c, or just the circulating fan.