There are several indicators of a faulty chimney liner. A visual inspection can identify masonry debris and deterioration in the structure of the chimney. Exterior signs also can be seen such as efflorescence (a white powdery looking substance on the exterior of the brick). It is recommended that a chimney liner be evaluated by a trained professional to determine its integrity.
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