Some background: About 11 years ago, replaced my original (1964) gas furnace with heat pump; no problems. About 10 years ago, I replaced my gas water heater myself, no problems.
Yesteday, gas utility replaced my meter; wouldn't re-light the water-heater as vent is corroded, not good! I thanked them for the free inspection and heads up.
I've been reading on the "orphaned" vent from going from two to one exhaust sources, but since this has all worked OK for years, I'm confident the venting worked well, but may have caused condensation that took out the original galvanized pipes. Still, they are 47+ years old, so not surprising.
So reading your site, about single wall (it is 6 inch, water heater is 3 inch "y'd" into the 6 inch just above the water heater. From there it goes about 3' out the basement, into crawl space (problem 1), is bent in a bit with no rise to clear a heat vent in about 8'(problem 2 and 3) then goes about 6' to the chimney with slight rise. It is about 2" from the floor joists in places (problem 4).
Tactically, I don't want to tackle the chimney immediately. Does it make the best sense to replace the 6" single wall with 3 or 4" double wall for this horizontal section, so I could fix all the problems above? I'd have to cut the single wall a couple inches from the chimney and use an adapter to get from the new smaller double wall to the 6". I know the whole chimney section should be evaluated by a pro, but to keep some hot water going until then, does this seem sound?
Very nice site you have here. Regards, ADS