I've discovered that there is a return air duct opening in the ceiling of an unfinished, unheated basement. This is connected to a furnace/AC in the attic that serves the second floor. It seems counterproductive to be drawing very cold air into the system. Also, the gas water heater and boiler for first floor heat are down there so am concerned that the draft may interfere with their operation. Would it make more sense to block off the basement opening and replace it with one above on the first or second floor? The basement return is the only opening on that duct branch.