My energy star house is very air tight. I am considering making an air exchanger. What if I ran a 4" vent pipe inside a 6" furnace duct pipe from the outside through the unfinished basement and terminate the 4" pipe into the cold air return and the 6" pipe ending in the unfinished unheated basement. That way any air that the furnace draws in would pass through the heated air that is free to exhaust outside. What do you think?