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 1940's and again in the 1980's, masonry chimneys were tested
by the National Bureau or 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
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.
2) Liners protect the masonry from the corrosive byproducts 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.
3) 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. The chimney is responsible for not only allowing the products of combustion
a passage out of the house, but the draft generated by the chimney also supplies
the combustion air to the appliance. An incorrectly sized liner can lead to excessive
creosote buildup in woodburning stoves, and the production of carbon monoxide with
Types of chimney liners
Chimney liners come in three main types: Clay Tiles, Metal, and Cast-in-place
1) 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 two 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 byproducts produced by modern gas appliances.
2) Metal chimney liners, usually of stainless steel or aluminum, are primarily used
to upgrade and repair existing chimneys. These liner systems are U.L. tested and
listed, and if properly installed and maintained are extremely safe and durable.
Stainless steel is suitable for woodburning, gas, or oil applications, while the
aluminum is an inexpensive alternative for certain medium efficiency gas applications
only. It is usually required that high temperature insulation be used in conjunction
with the liners for safety and performance considerations.
3) Cast-in-place chimney liners are lightweight, castable, cement like products
that are installed inside
the chimney forming a smooth, seamless, insulated passageway for the flue gasses.
They can improve the structural integrity of aging chimneys, and are permanent liners
suitable for all fuels.
Considering the dangers of old unlined or damaged chimneys, and the many cost
effective options now available to make these chimneys safe components of the home
heating system, may we suggest you have your chimney professionally inspected to
be sure it meets modern safety standards.