I am raising the floor of a sunken living room to match the rest of the house.  There is a cold air return for the furnace that is low on the wall and will be covered by the new floor.  The size of the boot is 6X14.  There is also a return directly under the furnace that is 14X20.  The furnace was replaced a few years ago if that means anything.  I was wondering why there was a second return.  Wouldn’t the one under the furnace suffice?  The house isn’t that large, maybe 1500 sq ft.  Is there any reason why I couldn’t just cap this secondary one?  It sits under the sill plate of the main floor level, so I don’t see any way to just move it up like I’m doing with the electrical.  Any help is very appreciated!