Do you need a chim­ney liner?

Old House Journal - - Restore - —Ray Tschoepe

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.

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