A flue lining in a masonry chimney is defined as “A clay, ceramic, or metal conduit installed inside of a chimney, intended to contain the combustion products, direct them to the outside atmosphere, and protect the chimney walls from heat and corrosion.” Although building codes vary from one state or locality to another, the installation of flue lining has been recommended since the early part of this century, and indeed most fire codes now mandate liners.
Liners in chimneys serve three main functions:
1) The liner protects the house from heat transfer to combustibles. In the NBS tests, unlined chimneys allowed heat to move through the chimney so rapidly that the adjacent woodwork caught fire in only 3 1/2 hours.
Types of Chimney Liners
Chimney liners come in three main types: Clay Tiles, Metal, and Cast-in-place.
The first is that, being a ceramic product, they
cannot rapidly absorb and evenly distribute heat
during the rapid temperature rise that occurs during a chimney fire. This uneven heating produces an unequal expansion which in turn causes the flue tiles to crack and split apart. This is similar to immersing a cold drinking glass in very hot water. It will instantly shatter. A chimney with cracked chimney liners must be repaired before use. The second disadvantage is that tiles
cannot adequately contain the liquid combustion byproducts produced by modern gas appliances.