I need to install a return air drop in a 14" x 5 1/2" wall cavity. The ideal location has a 1/2" cold water line and a 1 1/2" pvc drain pipe running through it. I've got plenty of square inches for my air-even with the pipes. My question is, would sharing return air with copper and pvc piping be a code violation? What if it were just the copper water line?