Ceramic clay flue tiles are most commonly found in chimneys venting an open, wood burning fireplace. Many times the chimney contains a second or even third flue which is frequently used to vent the furnace and water tank. Clay tiles come in a variety of square, rectangular, and round configurations and stand 24 inches tall. They are stacked on top of each other inside the chimney and must be given space to expand and contract between the brick walls of the chimney. Often, the damage to these flues is contained in the top 2 or 3 sections making replacement relatively easy and inexpensive. However, a chimney fire or other thermo generated shock may cause the liner to crack much lower in the chimney. If this is the case it often makes more sense to install a factory built and warranted stainless steel liner or a complete cast in place system.
Factory built chimneys are currently the standard in new construction. Double wall galvanized chimneys, Type-B vents, have been used for decades with gas fired boilers and furnaces. This type of pipe is also used for direct vent gas fireplace. Stainless steel chimney components are frequently used to vent wood burning fireplaces and stoves in addition to oil fired furnaces and boilers. Lighter weight Stainless Steel components can be used for high efficiency gas furnaces and water tanks. These liners are inexpensive and often required when venting gas appliances into a masonry chimney.
A single wall brick chimney is extremely hazardous. The purpose of a liner is to protect the house from heat transfer to combustibles. An unlined chimney allows enough heat to transfer through and ignite wood structures in just a few hours. Liners also protect the chimney structure from the corrosive byproducts of combustion. If the flue gases penetrate to the brick and mortar the usable life of the chimney decreases. Flue gases are acidic and literally eat away at the mortar joints from inside the chimney. Heat transfers more rapidly to the nearby combustibles and dangerous gases such as carbon monoxide can leak into the living areas of the home. Liners allow for a correctly sized flue, improving the performance and safety of the appliance being vented. An under or over sized liner can allow the production of carbon monoxide which then leaks into the structure.
Your current liner could be damaged due to incorrect sizing, lack of cleaning, thermo shock, chimney fire, or just general deterioration from acidic flue gases. Just as a car muffler needs periodic replacement, so does your chimney liner. Your flue should be visually inspect at least once per year by an equipped and experienced professional. If there is any reason to suspect cracked or damaged liners this should be confirmed by a video camera inspection of the entire flue.
If your structure was built more than 20 years ago there is a good chance you could improve the safety and efficiency using modern methods. The most common improvements are to fireplaces and furnaces venting through masonry chimneys. New stainless steel and aluminum liners are relatively easy upgrades and in many cases can be professionally installed in just a few hours.