Chimney deterioration is primarily due to the moisture present in the exhaust of natural gas combustion. When venting into a cold chimney the moisture can condense – especially in winter above the roofline and creates a freeze/thaw cycles that break down the masonry structure.
Often when having a new natural gas or propane water heater installed, a new chimney liner is recommended, or even mandated by code. This requirement is a critical safety consideration for your property and addresses several issues.
Information provided by Lindemann Chimney and Fireplace 2018