In my one story house I'm removing an interior wall which houses an oval, probably asbestos cement vent for a natural gas furnace (80%, 1998) which sits below in the basement. I want to remove it and replace it with a newer type one and move it over the 6" that seem to be available for doing that. The old pipe is 3" + wide and just over 11" long. Is that called a 4" pipe? Will a new type one be the same length, or might it be shorter? What should I know ahead of time? I understand the asbestos issues and am prepared to deal with that part of the list. What else?