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Building codes require that residential masonry chimneys with walls thinner than 8 inches (nominal) be lined with a fire clay flue lining, fire clay brick, or a material that resists corrosion, softening, and cracking from flue gases as hot as 1,800 degrees Fahrenheit. Chimneys usually are lined or relined with one of four types of liners: clay tile, cementitious precast, cast-in-place, or metal. Some proprietary systems are combinations of these systems. CLAY TILE Clay tile is the most common way of lining residential chimneys. According to ASTM C 315, Standard Specification for Clay Flue Linings, clay tiles must be made of fire clay, shale, surface clay, or a combination of these materials. CEMENTITIOUS PRECAST LINERS One proprietary precast flue liner is made of volcanic pumice aggregates and refractory cements. The sectional liners are about 2 feet long and come in 6-, 8-, and 10-inch inner diameters. A 2-inch space between the chimney and liners is filled with a proprietary mixture of expanded clay aggregates, portland cement, and water. CASTING LINERS IN PLACE WITH INFLATABLE FORMS One way of casting liners is to lower a long inflatable rubber form into the flue, inflate it to the proper flue size, center it in the chimney with spacers, then fill the space between the form and the chimney with a cementitious mixture. The next day after the lining has cured, the form is deflated and removed. STAINLESS STEEL LINERS Stainless steel flue liners come in sections or one piece. In general, they are lightweight and easy to install, and they can be used in both new and existing construction.