I recently had my gas forced hot air heating furnace replaced. I used to have an 80% furnace that vented through a chimney. It has been replaced with a 97% furnace that is sealed combustion and vents through the basement wall with PVC pipe. My gas domestic hot water heater hasn't been changed. It still vents through that chimney. The contractor who installed the furnace never mentioned it, but someone else said that having only the water heater vent through that chimney raises issues: specifically that there's now maybe not enough hot exhaust gases going up chimney so the chimney could be damaged/corroded, etc. over time. But even in the past there was at least six months a year where the furnace was off and only the water heater was using the chimney so if that was OK what's the problem now? If it's really a problem, the solution would seem to be to change out the water heater to a direct vent model and no longer use that chimney. That's an expense I'd rather not incur if unnecessary. Current water heater (Rheem 50-gallon) is over 12 years old, but works just fine. Never any problems. I drain it once a year to remove sediment -- just enough so that water runs clear, a few inches into a small bucket.
What do folks think I should do?