Chimney liner
The protective channel inside a chimney that keeps heat and combustion gases off the masonry and out of the house.
A chimney liner (or flue liner) is the channel running up the inside of a chimney that contains the heat and combustion gases from a fireplace or appliance and directs them safely outside. It protects the surrounding masonry and framing from extreme heat and keeps carbon monoxide and other gases out of the living space. Liners are clay tile, cast-in-place, or — most common for repairs — stainless steel sized to the appliance. A cracked or deteriorated liner is a safety issue, not a cosmetic one: it can let heat reach combustibles and gases leak into the home, which is why a [chimney repair](/guides/chimney-repair-cost) often centers on relining. A Level 2 inspection with a camera is the way to confirm a liner's condition before paying for a replacement.