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  • Largely to protect the main masonry units of the chimney -- e.g., brick, block, stone and mortar —  from deteriorating.  A liner keeps out harmful acids and moisture, and  acts as a heat buffer.  The liner itself may suffer some, but as long as  it is, itself, intact, then the exterior masonry can be counted on to  do its job.
  • A  liner also helps to prevent heat transfer, which could threaten wood  that is close by (often actually touching the chimney in old or poorly  constructed houses — whereas modern Codes require that combustibles  should be no closer than 2 inches).
  • Some liners actually strengthen and rebuild the exterior masonry components.
  • A tight venting system with a liner, moreover, will perform better than one that has some of the elements of "Swiss cheese!".



Clay  tile liners are usually found in original construction by masons after  1950.  These liner components are usually square or rectangular, but are  also available in a round, cylindrical shape.  Each tile is 2 feet long  and has been fired in a kiln at 1800 degrees or so.  They are pretty  durable in the face of acids and heat, but are vulnerable when cool  venting gases result in condensation of water-vapor with a resulting wet  acid bath.  They also are subject, quite easily, to sudden rising or  falling temperatures, as found in "chimney fire" occurrences.  Masons  often fail to use an acid and heat-resistant "refractory" cement between  the tile joints, and, furthermore, they frequently do not smooth off  the "snots" that squish out at these joints during installation.  Tile  liners also need to have a minimum 1/2" of airspace between the tile  walls and the main masonry units to account for different rates of  expansion (this gap should never be filled).  Multiple tile liners in  one chimney are suppose to be separated by 4" thick solid masonry  "wythe" walls.
Clay tile  liner sections often have to be removed to make room for adequately  sized and insulated liner systems during any relining process that takes  place after damage. 


Types of stainless steel liners:

Rigid sections, screwed  or riveted together — a little less expensive, but also of lower  quality with a shorter warranteed life — we, ourselves, don't use  "rigid" very often. 

Flexible, one-piece liners — available  in a wide range of shapes and custom lengths — installed by pulling  them up or down a chimney using a rope or winch and cable — much more  versatile where there are bends or offsets or where narrow passageways  require some ovalization.  Many flexible liner types are much thicker  and more durable than rigid liner pipe, and there are few joints or  screws.  Stainless steel liners are often insulated for greater safety  and performance.  They usually  have a lifetime warranty.

Alloys of Stainless Steel Liners:

304 — the common alloy used for wood venting — valuable for providing high heat resistance, particularly in times of chimney fires.

316 —  a different alloy whose strong point is acid-resistance, especially  appropriate for oil, gas and coal venting — fuels which are much more  acidic than wood, and have lower flue gas temperatures.

AL 294C —  an even more acid-resistant alloy, used especially when high efficient  oil and gas appliances result in lots of condensed water vapor, which  activates acids even more intensely.  Expensive and inappropriate for  wood venting.

Shapes of Flexliners:

Round is  always the best shape for smooth, efficient venting — a vast  improvement over squared-off tile liners and preferable to other shapes  of stainless steel if room allows.

Ovalized liners  allow for passage through some narrow, constricted spots.  Stainless  steel is malleable enough to be ovalized either at the factory or in the  field.  Oval drafts better than rectangular, as a general rule.

Square and rectangular liners  — usually fashioned from heavier duty stock.  Used for special  applications where maximum cross-sectional area is needed in a limited  space, most often for fireplace venting, or large woodstoves with an 8"  collar.  More expensive than round and oval shapes.

Gauge and Surface of Stainless Steel Flexliners:

  • Lighter  weight gauges are most often used for oil and gas (rather than wood)  applications, where chimney fire heat and burnout are not an issue.   These liners are one-ply thick, usually 316 alloy, and have a corrugated  wall, which results in a 15% to 20% reduction in flow capacity (because  of resistance) for oil and gas applications, compared to smooth-walled  alternatives.  Easy to work with and relatively inexpensive, however.  A  two-ply, smooth-wall version is available where flow capacity must be  maximized, but this innovative liner (also 316 alloy) is somewhat more  expensive and much more difficult to work with, especially with offsets  or constrictions.
  • Thicker  gauged, "heavy wall" liners are usually for wood applications and are  of a 304 alloy.  They often are of a 4 ply construction, extremely  rugged and resistant to chimney fire damage.  They can be winched up  through tight places without tearing apart easily.  Their smooth wall  construction allows for maximum flow, with minimum resistance and with  resulting reduced creosote accumulation.  Though much heavier than the  lightweights, these liners can be "punished" and shaped to accommodate  tough configurations in the field.  A 316 alloy and more expensive  version is also available in special circumstances.

Installation of Stainless Steel Liners (flex)

  • Oil  and gas liners do not require insulation unless installed in an  outside-the-house-chimney in a cold climate.  Insulation used in any  chimney, however, will maximize draft and minimize troublesome  condensation or soot formation.
  • Wood  liners always require insulation, by Code (unless absolutely impossible  or impractical) — primarily as protection against high heat transfer.  An exception to the insulation requirement applies when providing a  downsizing wood liner inside an otherwise intact clay tile liner.
  • Pellet liners need insulation only in cold, outside chimneys, for performance reasons only.

Two Options for Insulation:

  1. Foil-backed ceramic wool wrap,  either 1/4" thick or 1/2" thick.  In most applications, especially  wood, the 1/2" thick ceramic wrap is better, especially since, even in  wood venting, it meets what's called "zero-clearance" specifications,  which means that there is no problem even if wood or other combustibles  are right up against the chimney, contrary to contemporary Code (as in  the case in many new houses and almost all older houses!)
  2. Granular perlite cement mix,  available in several brands, but well-known by the Thermix® brand.   Mixed with water to a damp consistency, this insulation is poured in  around the liner and qualifies for "zero-clearance" at 1" thickness.   Requires spacing and some finesse to achieve proper and even  distribution, but can sometimes be a simpler and less expensive way to  achieve appropriate insulation.  The firmed-up cement component prevents  settling and ensures even peformance.


Thermocrete™ Spray Process ceramic flue sealant is by far the most revolutionary and existing system on the market for rehabbing old or damaged chimneys.  Click on Thermocrete™ for an entire website devoted to this unusual product.

When to Use ThermocreteCeramic Flue Sealant

1)  To Repair Damaged Tile Liners

  • When thermal shock from a chimney fire has cracked the clay tile liner.
  • When extreme acid erosion from oil, gas or coal venting has eaten away a large part of the clay tile liner walls.

The Thermocrete™ Process
            • We clean the tile liner interior thoroughly first, as needed.

             • Then, working from the roofline, we drop down a spinning spray head,  driven by compressed air, and lay down successive,
​               thin coats of ceramic mix over a period of several hours, with about a  half hour drying/curing time in between coats.
            • The resulting coverage is about 1/4" thick.
            • Specifications:

                    *Good to 3000°

                    * Impervious to water and acids 
                    * Repaired tile is as good or better than new

                    *Tested and listed to UL 1777 specifications
                    * Lifetime warranty

             • Fireplace smoke chambers are usually then enhanced and protected with hand-sprayed Smoktite™ insulated ceramic mix.

2)  To Repair, Enhance Unlined Brick Chimneys (the primary application for Thermocrete™)

  • This usually means chimneys built before 1950.
  • These  unlined flues, most often serving fireplaces, frequently are  constructed with soft "lime" mortar, before the advent of Porland  cement, and the mortar joints are often badly eroded (even all the way  through).  Basically, unlined fireplace flues are not to be trusted, and  using them, even only with "small", "occasional" fires, is a risky  activity.
  • Often  the wood framing surrounding unlined chimneys is touching the chimney  (contrary to today's codes) and, moreover, has a drastically lowered   kindling point due to many years of subjection to hyperdrying from  constant moderate heat (a process called "pyrolysis").

The Thermocrete™ Process
              • Many old fireplace flues are rectangular and do not lend themselves  readily to installation of other types of (round or even rectangular)  liners                              having a large enough
capacity  for properly, successfully venting fireplaces, which have large  openings by design, allowing a high volume rush of air and venting  gases,                      thus requiring large flues. 
               Enter Thermocrete™........!
              • The spraying process is remarkably simple and straightforward and,  therefore, cost effective compared with many complicated alternatives.
             • We clean the flue thoroughly first, as needed.
              • Sometimes we videoscan the flue to pinpoint any observed or suspected  holes (e.g., missing bricks or sections of walls).
             • Holes or gaps get repaired.  Shaky walls get shored up.
              • We spray Thermocrete™ ceramic flue sealant from the top down, laying  down successive coats over several hours with about a half hour

               drying/curing time in between coats.
             • The resulting coverage is roughly twice the thickness required for tile liners — about 1/2".

              • Extra mix can be sprayed on areas that are particularly weak or  vulnerable, in conjunction with other hand-applied solutions.

             • Specifications are the same as described earlier, for tile repair.

              • Fireplace smoke chambers (right above the damper throat) are always  sealed, enhanced, reshaped and protected with a
               hand-sprayed application
               of Smoktite™ insulated ceramic mix.
             • Raincap protection is required to validate the lifetime warranty.

              • Thermocrete™ by itself, in an otherwise unlined flue, is not  technically classified as a "liner", except when used for gas and oil  venting, for which

                 it has been tested, listed and certified as an appropriate liner by  Guardian Fire Testing Laboratories, Inc.  It is, however, by any  definition, a
                solid "lining" inside the old  brickwork and as thick and as effective as the clay tile liners mandated  as part of current code requirements for

                chimney construction.

•  Bob and Merrie Warner have used Thermocrete™ to line and seal the three  unlined fireplace flues in their own old 1830's cape home.

              • Thermocrete™ ceramic flue sealant has been successfully utilized at  the Wheelwright House cook fireplace at Strawbery Banke Museum.

•  Thermocrete™ is not appropriate, by itself, for woodstove venting.  A  woodstove in an unlined flue requires an insulated liner tested to UL 1777

                zero clearance specifications (usually stainless steel or poured masonry).

             • Unlined  brick flues venting gas and oil-fired appliances are usually lined most  efficiently and cost effectively with lightweight, flexible stainless  steel  rather than Thermocrete™ ceramic flue sealant, although Thermocrete™ may  be a necessary option in flue situations with narrow or odd types of

               configurations, or when multiple gas and oil appliances are straining the capacity of the existing flue.
             • Check the Thermocrete™ website at, and view their short video.
             • Thermocrete™, as we've said, is formally listed as a Codeworthy liner for oil and gas venting.

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