The Importance of Flue Lining in Your Masonry Chimney

Parts of a masonry fireplace

Flue Lining

A flue lining of a 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." A wise homeowner will want to know about the condition of the chimney, including the flue lining, in the home he owns or plans to buy. This information can be supplied by a CSIA Certified Chimney Sweep. Although codes and building practices may vary from one state or locality to another, the installation of flue lining in chimneys has been recommended since the early part of this century. It should be noted however that many chimneys built since then have not included a flue lining.

Terra-Cotta Flue Tile The most common material used as a lining for masonry chimneys in new construction is terra-cotta clay flue tile. It is a relatively inexpensive and easily obtainable product. Lining products made of other materials, such as ceramics, metal, and cast concrete, are also available and may be used in new construction. However, these alternatives are most commonly used in upgrading chimneys built with improper linings, those with damaged or oversized linings, or those originally built without the benefit of any type of lining.

Performance Testing Of Chimneys In the 1940s and again in the 1980s masonry chimneys were tested by the National Bureau of Standards (NBS) for durability due to rising concerns about their performance and safety. These tests involved lined (using clay tiles) and unlined chimneys. The published results of these studies are revealing.

(1) Unlined chimneys allow heat to move through brick chimney walls very quickly. The unlined test chimney caused adjacent woodwork to catch fire after only three-and-one-half hours of testing. In Fact, the unlined chimney performed so poorly that testing of unlined chimneys was abandoned.

(2) Clay flue tiles were easily damaged during the testing as a result of thermal shock. All of the flue tile samples subjected to the heat shock test suffered damage in the form of cracking.

(3) 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 these components due to mortar deterioration. It is important to note that flue gases are acidic in nature and can be quite corrosive.

At the conclusion of the testing, the researchers recommended that flue linings, and the bonding materials that joins them, be capable of containing flue gases and be designed to minimize condensation in the system. In fact, the researchers felt so strongly about the need for proper lining that they characterized building a chimney without a lining as "little less than criminal!"

During all of the testing, it has been consistently shown that clay tiles crack quite easily from thermal shock. Testing has also shown hat clay tile does not crack from normal usage.

Thermal Shock and Liner Damage Clay tile is a ceramic product and as such it cannot rapidly absorb and evenly distribute heat during rapid temperature rises in the chimney. A rapid temperature rise ( as during a chimney fire) will cause the clay material to rise in temperature much more rapidly on the inside than on the outside of the tile. As the inside heats up, it will begin to expand very rapidly, much more rapidly than the cooler outside is expanding. 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 shatter instantly!

The exterior of a cracked clay flue tile liner used in chimney fire tests by the U.S. National Institute of Standards and Technology

Thermal shock results only during periods of rapid temperature rise. The conditions are met almost solely during what is know as a chimney fire.

Chimney Fire Characteristics

A chimney fire is defined as "combustion within the venting system of an appliance." A venting system will include the flue, the connector pipe of a heating appliance, the thimble area (the point where the connector pipe passes into the chimney), and the smoke chamber of a fireplace (the area between the firebox and the flue). None of these area were designed nor intended to accommodate combustion. Combustion in these areas cause temperatures to rise and heat the surrounding chimney structure quite rapidly. During a chimney fire, the flue gas temperature may well approach and at times even exceed 2000 degrees Fahrenheit. This is more than enough to produce thermal shock and crack the flue tiles.

A chimney fire may be either "free-burning" or may be classified as "slow-burning." A free-burning chimney fire is what comes to mind when we hear the phrase "chimney fire." Free-burning chimney fires are usually noisy and dramatic enough to be detected by the home occupants or by passers-by. These fires usually result in rumbling noises from the chimney, a tinkling sound from the connector pipe, and has even been compared to the roar of a large jet aircraft flying in the vicinity. Flames may be observed coming from the top of the chimney, and at times a virtual fireworks show will occur, often drawing the attention of neighbors.

A slow-burning chimney fire, on the other hand, does not always exhibit the sights and sounds commonly associated with a chimney fire. This fire burns slowly because the supply of oxygen or fuel in the venting system is quite limited, but is there no less. A slow-burning fire is potentially more damaging to the chimney and surrounding combustibles than a free-burning fire because the area where they occur are not vented. This lack of ventilation holds down the obvious signs of the fire while contributing to rapid heat build-up in the chimney.

When either a slow or a free-burning fire has occurred, certain signs of the fire may be observed. Contrary to popular belief, a chimney fire will not clean out a chimney. Signs that chimney fire has occurred commonly include one or more of the following:

(1) Ashes from burned creosote are usually present after a chimney fire has occurred. Creosote ash will be lightweight and fragile, with a tissue paper-like consistency. Often the creosote will exhibit a steel-grey color and have a peculiar odor.

(2) Clean areas may appear randomly in the flue. These clean areas are the result of extremely high heat build-up and possible air leakage into the chimney.

(3) Metal chimney components such as dampers, chimney caps, or metal smoke chambers may be warped and/or discolored. Antennas mounted near the chimney may also be damaged.

Looking down into a chimney lined with clay flue tiles after a chimney fire. Note that the lower tiles have been burned absolutely clean while the higher tiles show significant amounts of expanded creosote ash. Close inspection reveals cracks in the liner.	The exterior of the flue area of the chimney pictured at left.(above) The charred header and framing in contact with the chimney and concealed by a drywall covering were ignited by the heat generated by the chimney fire.

It is not unusual for the occupants of a home to be completely unaware that a chimney fire has occurred. Chimney sweeps find numerous cases where undetected chimney fires have occurred. Chimney fires may also occur while the occupants of the home are either asleep or away from the home. Continued use of a chimney system which has been damaged carries with it several hazards. These hazards include but are not limited to:

(1) A masonry chimney expands and contracts, as it heats and cools. This may cause pieces of a cracked flue lining to fall out. As a result, the chimney becomes unlined in these areas and presents all the hazards of an unlined chimney. Fallen pieces can also block the flue and cause flue gases to back up into the appliance and the home.

(2) Flue gases can leak during use of the heating system. One of the primary functions of the flue lining is to contain these products of combustion. When a flue lining is cracked, these cracks tend to open during the heating phase and will allow flue gases to pass to the chimney walls. These gases, being acidic, attack the masonry and will shorten the life of the masonry structure. In addition this can lead to carbon monoxide finding its way into the residence.

(3) Heat can be transferred to the vulnerable chimney walls. One of the important functions of the flue lining is to serve as an insulator between the hot flue gases and the chimney wall.

When the chimney walls are overheated, this heat can also transfer to surrounding combustibles.

Never use a chimney that does not have a liner or has a damages or improper lining!

The interior of a clay flue tile-lined chimney illustrates the impact of a chimney fire. Note the many cracks in the liner sections and a missing segment of tile which fell down into the smoke chamber/damper area.