Is it possible to add a Common wire to an old furnace? I have a Rheem RGAA-125C furnace, along with AC, and don't see a Common terminal on the unit. My new thermostat seems to require it, wondering if a Common can be added. I do have extra wires (including blue) available at the thermostat. Any suggestions? Thank you very much -

Call a professional. You are to far out of your element .