I need to vent a new furnace from my basement to the roof of a two story house. It’s a power vented furnace but is not a condensing furnace. The house is old and has an 8 inch inside diameter vent pipe which currently runs from just above the basement ceiling up through the roof. The pipe is a cementitous/asbestos material with thick walls and is in good condition with no blockage. Can I just run B vent from the furnace to the just inside the lower opening in the large pipe as opposed to running the B vent completely through to the roof (inside the large pipe or not)? The pipe bottom would be sealed shut with a suitable material. I could probably fabricate a sheet metal transition from the B vent to the pipe and seal any remaining openings that might leak with a fire rated foam or other appropriate sealent. What about venting a second appliance into the same pipe? Either a second power vented tankless non-condensing water heater or my existing natural draft tank water heater? Being able to do so would reduce venting costs and complexity considerably especially for a tankless water heater which used very expensive 3”/5” double wall stainless material from Metal Fab Inc. Is any of the above allowed? I am aware of the issues with asbestos but except for the lower access to the pipe it is sealed inside walls and is not accessible except in the attic and I intend to wrap that to seal it soon. Thanks.