What the liner actually does
The liner contains heat, keeps sparks and gases from reaching framing inside the wall, and gives combustion byproducts a smooth path out. A failed liner is the single most common reason a chimney is declared unsafe to use.
Signs your liner has failed
These show up on a camera inspection, and sometimes in the firebox itself.
- Pieces of clay tile falling into the firebox
- Visible cracks or gaps between flue tiles
- Rust flakes or holes in a metal liner
- A chimney with no liner at all — common in older homes
Why stainless relining is the usual answer
A stainless steel liner sized to the appliance is continuous, insulated and rated for the temperatures a fireplace or gas appliance produces. It also restores draft in oversized flues, which often solves smoking problems at the same time.