A chimney liner is a clay, ceramic or metal conduit installed inside of a chimney, intended to safely contain the products of combustion and transport them to the outside atmosphere. An intact chimney liner protects the chimney walls from heat transfer to combustible materials nearby and the masonry from the corrosive products of combustion. It also provides optimum efficiency of the fireplace and chimney.
This picture shows an intact chimney liner when looking down from the top of the chimney.
Chimney liners must be free from perforations, cracks and damage, otherwise health-hazardous products of combustion can pass into the living space of the home or the chimney liner can overheat and ignite combustible materials nearby such as framing, walls, ceilings, insulation and floors.
This picture shows gaps between the chimney liner components. These gaps allow flue gases or condensate to enter the interior of the chimney which can cause premature deterioration of the structure or reduce the chimney's drafting performance.
This picture shows a cracked chimney liner caused by a severe storm or a chimney fire. If left unrepaired, the cracks in the liner can obstruct the flow of flue gases to the outside atmosphere.
In the long run, this may cause failure of the appliance to operate properly or pollute the interior of the house with health-hazardous flue gases like carbon monoxide.
This picture shows a chimney liner that has come apart. The broken liner is blocking the flue gas exhaustion which is a serious hazard for your family and home. This condition needs either repair or replacement.