A chimney liner is the material installed along the inside walls of your flue to contain combustion products as they travel up the chimney. It helps prevent smoke, gases, and other byproducts from escaping into the surrounding chimney structure or entering your home before they can be safely vented outside.
If you live in a home built before 1940, your chimney may not have a dedicated liner. Older chimneys often relied on the brick and mortar itself to contain flue gases. While this can work well when a chimney is new, years of water infiltration, corrosive combustion gases, freeze/thaw cycles, and other stresses can deteriorate the masonry. Over time, bricks and mortar can crack, deteriorate, or become displaced, allowing dangerous flue gases to escape.
If you can see smoke escaping through the bricks of your chimney before it reaches the top of the flue, the chimney may need a new liner or immediate repairs. Even when there are no visible signs of smoke leakage, regular chimney inspections are important. A professional inspection can identify hidden damage and determine whether repairs or a new liner are necessary to keep the flue operating safely.
If your chimney needs a liner, or if the existing liner needs to be replaced, there are several different materials and installation methods to consider.
New homes with brick or masonry chimneys almost always have a built-in clay tile liner inside the flue. These liners can last for several decades but may eventually become damaged or deteriorate. A new liner can be installed inside an unlined chimney or, in some cases, inside an existing liner using clay tiles, specially formulated mortar, flexible metal, or rigid metal. A poured mortar chimney liner is installed by placing specially formulated cement around a removable bladder or similar tool, which creates a smooth, cylindrical flue after the form is removed.
Rigid metal chimney liners can be installed inside a chimney when the flue is not offset, meaning it runs vertically from top to bottom without bends. If the flue has offsets or bends, a flexible metal liner may be a better option. Flexible metal chimney liners are available in aluminum and several grades of stainless steel. The appropriate material depends on the type of appliance and application. Flexible aluminum liners, such as the M-Flex Aluminum Chimney Liner, can be used with certain lower-efficiency gas appliances, including boilers and furnaces.
Lower-efficiency appliances typically produce hotter flue gases, so condensation inside the flue is generally less of a concern. High-efficiency appliances produce cooler exhaust gases, which can lead to condensation of water and corrosive compounds inside the flue. For these applications, a corrosion-resistant stainless steel alloy liner is necessary to help withstand the conditions inside the chimney. If you are installing a wood-burning insert in your fireplace, you will need to choose a flexible stainless steel chimney liner that meets the insert manufacturer's specifications.