I have a couple of Duct Booster Fans that I would like to power from my thermostat using a relay. There are two wires connecting my thermostat to the furnace. One carries 24V from the furnace (Red) and goes to the thermostat. One carries 24V from the thermostat (White), when the thermostat switch is closed and goes to the furnace to turn it on.

For a return or ground connection, it looks like I have two choices for connecting the relay. I can either connect to the "C" terminal on the furnace circuit board or I can merely connect to ground through the sheet metal on the furnace. My ohm meter shows that there is virtually no resistance between the "C" terminal and the sheet metal.

I don't have any strong preference for connecting the return wire; one method is about as easy as the other. However, if I connect to the sheet metal, when the day comes that a technician installs a new furnace, the connection will be automatic and the installer won't have to deal with it. If I connect to the "C" terminal on the circuit board, the person who installs a new furnace is going to have to connect the wire to the "C" terminal on the new furnace's circuit board.

My inclination is to connect to the sheet metal in order to avoid the problem of the booster fans not working after a new furnace is installed and all the associated problems of trying to figure out why they don't work anymore.

Does anyone have any thoughts or advice on how to make the ground connection?