The flue is the internal passageway within a chimney that channels smoke and combustion byproducts safely out of the building. A chimney liner is the protective material installed on the inside surface of the flue, keeping those harmful gases and intense heat contained as they travel upward and out of the chimney. Whether the liner was built as an integral part of the original masonry, added as a separate component during construction, or installed later as a retrofit, it remains one of the most critical elements of the entire chimney system.
Chimneys constructed before the 1940s typically had no liner separate from the masonry itself — the interior surface of the bricks and mortar served as the only barrier between combustion gases and the chimney structure. Beginning in the 1940s, most brick chimneys were built with terra cotta clay tile liners set inside the flue during construction. A specialized mortar application called parging was used to seal the joints between tiles, creating an airtight barrier between the flue's interior and the surrounding masonry.
Beyond liners that are built in during original construction, chimney liners can also be installed into existing chimneys using several different materials and methods. One approach involves pumping a special mortar mixture around an inflatable rubber bladder centered inside the flue. Once the mortar sets, the bladder is deflated and removed, leaving behind a solid cylindrical liner. A similar technique forms the liner by pulling a shaped tool upward through the wet mortar as it begins to cure, creating a smooth, round flue channel.
Metal chimney liners offer another popular option for relining an existing flue. These liners are manufactured from various grades of stainless steel and aluminum, with the specific alloy chosen based on the type of appliance being vented and the conditions the liner will face. Metal liners are available in both rigid and flexible configurations. Flexible metal liners are the preferred choice for chimneys with offset flues — those that are not perfectly vertical from top to bottom but include one or more angles along their length. Rigid metal liners are typically the most affordable option, though they can only be used in flues that run straight without any bends. While a deteriorated clay tile liner can be replaced with new tiles, the process is labor-intensive and requires cutting access openings through the brick masonry to remove the old material. For most homeowners, a metal liner provides the simplest, fastest, and often most cost-effective solution for installing a new chimney liner or replacing a failing one.