We'll be converting from oil to gas heating. Now we have steam heating system. We were told we need to intall liner by two plumber, one said no. We're going to have water heater as well. Any thoughts would be greatly appreciated.

As I understand it. As explained to me by a pro. That the newer highest efficiency furnaces, don't have enough heat in the exhaust to create a proper draft with a larger flue. Otherwise it cools off and condenses in the flue itself before it exit sand is acidic. That will degrade and destroy the chimney over time. I am not sure what exact point that is... but it applies to the 95+% efficiency units.