The flue liner traditionally is made of terra-cotta, a type of clay. It is formed into short sections in either a square, rectangular or round configuration, usually being 2′ long. The short sections are inserted into the chimney as it is being built, with the brick or stone being laid around it. The liners are cemented to each other with mortar, but should not be cemented to the brick or block. Usually about 3/4″-1″ thick, they will generally stand up to use for many years.
In chimneys where there are multiple flues, they are typically arranged so each is about 4″ higher than the adjacent flue. This helps ensure proper draft, and no down draft from one flue to another.
Changes in your heating appliances, such as installing a high-efficiency furnace can have a detrimental effect on the liner. While a high-efficiency furnace is good, it is designed to extract more heat by recirculating and burning gasses. This causes a lower temperature for the exhaust. This is where chimney problems begin to develop. Rather than venting hot gasses, the gasses emitted by high-efficiency furnaces are cooler. This causes condensation to form near the top of your liner. When this happens, water mixes with creosote to form an acidic compound that begins to deteriorate your liner. In time, the liner will crack, crumble, and pieces may drop out and actually clog your chimney.
This is why periodic cleaning and inspection of your chimney is critically important to the health and safety of your heating system. Once cracks or crumbling develop, you have a couple of options.
One is to have a mason break out all the terracotta lining with special tools, and remove all the residue. Then new clay liners can be inserted into your chimney. This method is expensive, time consuming, and dirty.
Another option is to install a stainless steel chimney liner system. These often come with lifetime warranties, technical support from the manufacturer, and all the pieces and fittings necessary to install them and reconnect your heating appliance.