I noticed in the VisionPro 8000 thermostat installation manual there is a 24v common wire that needs to be connected to avoid battery reliance. Where does this wire usually come from? I'm getting a Trane TEE air handler installed and want to make sure the new stat is not battery reliant. Does this 24v common wire come from the air handler or does it require a special transformer, or is it merely a "ground" from which the thermostat draws relative voltage from the other wires?
Just wanted to get a quick layman's explanation so I can make sure the installer hooks it up correctly, or that I at least know in advance what to request so there are no unpleasant surprises/excuses at installation time.