I am remodeling a bathroom. When I ripped out the shower stall I found behind the wall is my furnace vent pipe which is 7 in. diamter Type B gas vent pipe.. So rather having just a stud wall, the wall behind the shower was framed seperately 5 inches out from the framing of the other interior wall to allow enough space for the pipe plus the 1 inch gap on each side that is required clearance from combustable materials. I would really love to regain that 5 inches of space to allow for a larger shower. Can I just replace that section of vent pipe with an oval pipe that is designed to go inside stud walls?