I am upgrading my thermostat (Have AC & Heat) and found that it doesn't have the "C" wire that I need to power my new thermostat. The furnace does have a terminal for C, but only condensation pump is connected. Can I simply run an 18 gauge wire from the furnace C to the thermostat C. to have a fully functioning, powered thermostat? There are already 5 wires connected. I have attached a picture of my furnace wiring in case more information is needed.
Technically, to do it properly, you should run a new cable with more wires.
But yes. If you connect the
C terminal on the thermostat, to that
C terminal on the furnace control board. That should power your new thermostat.