I presently have a 81% mid efficient furnace (132k btu/hr input) & a 42k btu/hr draft hood hot water tank common vented up a common 5" B-Vent (approx 30' total length). It is a code installation where i live. I am hoping to soon start a ~500 ft2 renovation / addition which may require upsizing my furnace or adding an additional furnace. The easy solution is go high efficiency (hence separate pvc vent) but i am curious if it would be possible to vent an additional (say 60k btu/hr max) furnace up the same b-vent? Dont worry, i will have the work hired out / permitted appropriately. I am just in the cost estimating / scoping phase & figured i would post here for response prior to calling a local qualified contractor. Note: the addition is incremental 2nd story square footage hence i may be ok with existing furnace (per my own heat loss calculations on local degree heating days on house).