My 1920s home currently has the furnace (from the 50s- 60s) and the hot water heater venting (I think that's the correct term) into the chimney.

I am having a new furnace and A/C installed this week. The furnace will now vent outside the house, leaving only the hot water heater going into the chimney.

The question is: do I need to downsize the chimney liner? I have researched quite a bit and it's about 50/50 - yes and no. The "no" votes say only a flue cap is needed, while the "yes" votes (all 'votes' are by professionals) have the same reason - to minimize gases, etc, from escaping, etc.

The other comment is that there is a small amount of water/condensation at the base of the chimney in the basement. I am having the roof checked for leaks in about a week.

Any advice on the chimney liner is welcome. Two last questions - 1) is stainless steel the way to go - or is aluminum okay? 2) Should I let the HVAC team install the liner or should I leave that for a chimney professional?

Thank you!