Chimney Repairs

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.

In the 1940s and again in the 1980s, masonry chimneys were tested by the National Bureau of Standards for durability due to rising concerns about their performance and safety. The tests revealed that unlined chimneys were so unsafe that researchers characterized building a chimney without a liner as “little less than criminal.”

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.

Liners protect the masonry from the corrosive by-products of combustion. In the tests, it was determined that if the flue gases were allowed to penetrate to the brick and mortar, the result would be a reduction in the usable life of the chimney.

The flue gases are acidic in nature and literally eat away at the mortar joints from inside the chimney.

As the mortar joints erode, 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 provide a correctly sized flue for optimum efficiency of appliances. Modern wood stoves and gas or oil furnaces require a correctly sized flue to perform properly.

Clay tiles are the most common type of masonry chimney liners. They are inexpensive, readily available, and perform quite well for open fireplace chimneys that are properly maintained.

There are 2 disadvantages to clay tiles. 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 by-products produced by modern gas appliances.