JoeZeppy412

HI, I could use some expert opinions. I need to replace my heating ducts due to age and corrosion to the point of giant holes in them. They existed for 50 years concealed behind a false wall in my basement. The furnace was replaced in the mid nineties but it appears they just connected to the existing flue. When I tore the wall out for remodeling, I discovered the corroded flue pipe. I'm looking at a 100,000 BTU Category 1 fan assisted Rheem furnace. The furnace outlet is 3 inches, which expands immediately to 4", does a dual 45 bend around the side of the furnace, into a 90, then expands into a 6" pipe, where you can already see signs of corrosion. It then does a 3 foot run, a 90 degree bend, then a 4 foot run to another 90 degree bend into to what was then an unlined masonry flue. The chimney flue rises around 20 feet. This is a single story ranch. The run past the 90 where it was behind the false wall is heavily corroded with numerous holes. It looks like heavy guage aluminum. Parallel to the furnace flue is a 40,000 BTU natural drafted water heater with a 3" flue that runs in a straight line 9 feet, then turns a 90 into the flue. I've just had chimney work done and they installed a 6" stainless steel flue liner terminating in a wye at the base of the flue. I now need to install replacement ductwork to attach to it. To summarize, I need to vent: a 100,000 BTU Category 1 fan assisted Rheem furnace, which rises 3 feet 9 inches in a 9 foot run to the flue, with two 45s and a 90 bend. a 40,000 BTU natural draft water heater, which rises 2 feet in a 10 foot run, than a 90 into the flue. My "in stock" furnace flue options at Lowes or Home Depot are. 4" B vent 4", 5" or 6" C vent. I'm not against special ordering or going to an HVAC warehouse, if I know what it is I need. the surrounding material is concrete block on the walls, with wood floor joists and a rim joist overhead. I plan to enclose the vents in a finished bulkhead, which will be removable for inspection and service. I do not have 6" clearance to the joists. I could install a firestop on the joists if that's acceptable. I'm confident I can do the physical installation, but two or 3 days of reading through the NFPA code, with help translating from ChatGPT, and I'm not convinced I'm understanding any better what the code requirements for the flue pipe size are. The chimney guys were nice enough to temporarily connect the existing ductwork to the new wye, but I need to get this replaced asap.