Do you need a chimney liner?
Old chimneys are subject to wear and tear, from chinks in the mortar and percolating moisture to the buildup of creosote inside the flue. While many of these issues can be addressed with proper maintenance, the enclosed nature of the chimney makes it impossible to see or anticipate every potential hazard. That’s why many chimney sweeps and building professionals often urge owners to install chimney liners as a safety precaution. Chimney liners perform three functions: They protect the building by insulat- ing any flammable material around the chimney from the hot gases in the flue.
They protect the masonry, brick or stone, as well as the mortar from the deleterious effects of the acidic chemistry of the by-products of burned gas, oil, or wood.
Finally, a liner can be sized to optimize airflow through the flue.
Chimneys constructed before the first decades of the 20th century were usually straightforward affairs of brick or stone. As the masonry was laid up, the interior of the chimney received a parging coat of mortar as a means of protecting the brick or stonework and mortar joints from the effects of the acids created during the combustion process.
Early in the 20th century, masons began to install rectangular tubes of terra cotta as the chimney was built. These clay flue liners were stacked and mortared as the chimney rose around them. When complete, the clay liners could last 50 years or more.
Problems arise when inspections find that one or more of the clay liner pieces have developed cracks or breaks. Repair made by replacing individual flue “tiles” is not only difficult, but also can be quite destructive. Usually an interior or exterior wall must to be opened to remove the broken clay flue liner and replace it.
Clay is durable and has a low material cost. Normally unaffected by heat and combustion acids, clay liners are sometimes prone to thermal shock, particularly when the outside air temperature is very low. Impurities and even minor defects during the manufacturing process can increase the chance of damage. Alternative liners include metal and cast-inplace masonry liners. Like clay, both have pluses and minuses.
Metal liners are commonly galvanized steel, aluminum, or stainless steel. For straight runs they can be supplied as bolt-together sections. For chimneys that present with turns, stainless steel (for example) is supplied in a corrugated tube to assist bending and produce a virtually seamless flue from the firebox opening or stove to the top of the chimney.
A new liner will undoubtedly decrease the diameter of the flue, which may affect how well a chimney draws if the fireplace is an open one. Decreasing the size of the flue can actually be a benefit if the liner serves a stove or insert. (Metal liners are required by code for both.) While aluminum and stainless-steel liners have shorter lifespans than either clay
UNLINED CHIMNEYS don’t retain heat as well as lined flues, leading to more condensation of flue gases and creosote buildup. Call the chimney sweep when creosote reaches the thickness of a dime—or once a season with regular use.
or masonry, they are far easier to replace. The cost is not insignificant, with most liner jobs running from about $1,200 to $2,400.
Cast-in-place liners are installed in one of three ways. In a system marketed by Thermocrete, a rotating spray nozzle can be lowered into any chimney to spray on a coating that can be layered over several passes. This process will maintain the diameter of the existing flue as it repairs cracks and chips in a clay liner or unlined chimney. The surfacing can be refreshed in the event of coating failure.
Another process, available from Supaflu, involves inserting a long rubber tube into the chimney flue. The tube is then inflated and a lightweight, heatresistant (refractory) cement is poured around the tube. When it hardens, the tube is deflated and pulled from the chimney, leaving behind a smooth flue suitable for use with any fuel. This permanent liner is also known to strengthen older chimneys. It is, however, nonreversible and about twice the cost of a stainless-steel liner. If the masonry fails, the alternative is to reline the chimney with a stainless-steel liner.
Finally, the Ahrens system employs a vibrating bell that is lowered into the chimney. As it is drawn up, a refractory cement is slowly installed around the bell, which subsequently packs it tightly to the walls of the chimney. The end result is a liner not unlike the lining described above, and at about the same cost.
TOP Clay flue liners made by Superior Clay. LEFT An old square flue before lining; the same flue, after being lined using the Thermocrete process. OPPOSITE A stainless-steel flue liner kit made by Duraflex.