The flue is the passageway inside a chimney that carries smoke and other combustion products out of the building. A chimney liner is the material on the inside of the flue which keeps combustion products from the fireplace or other appliance safely contained as it flows up the chimney. Whether it is an integral part of the masonry fireplace, a separate feature added during construction of the fireplace or added later as a retrofit, the chimney liner is one of the most important parts of the chimney system.

Chimneys built before the 1940′s did not have liners built in separate from the masonry chimney. The inner surface of the bricks and mortar inside the flue served the function of a chimney liner. Starting in the 1940′s, most brick chimneys were built with terra cotta clay tile liners built inside the flue. A special mortar called parging was applied between the tiles as the chimney construction proceeded in order to form an airtight seal between the inner surface of the flue and the rest of the chimney.

In addition to chimney liners that are built in as the chimney is constructed, a chimney liner can be inserted into an existing chimney using several different materials and techniques. Special mortar mixtures can be pumped around an inflatable rubber bladder centered inside the flue to form a solid mortar cylindrical chimney liner when the bladder is deflated and removed. A similar process builds a mortar liner by pulling a round tool up through the mortar as it begins to set to form a round liner inside the flue.

A metal chimney liner can also be inserted inside the flu. Metal liners are manufactured from various grades of stainless steel and aluminum. The conditions the liner will be used in determine which alloy is appropriate for the application. Metal liners can be made from rigid material or from flexible material. Flexible metal liners are the easiest type to install in chimneys where the flue is offset, meaning that it is not completely vertical from top to bottom, but has one or more angles in its construction. Rigid metal liners are the least expensive choice, but they are suitable only for installation in straight flues. A clay tile chimney liner can be replaced with a new one, but this requires extensive work that involves removing old tiles through openings in the brick masonry. In appropriate situations, a metal liner is the simplest and often the cheapest choice for a new chimney liner or for replacing an existing one.