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 linersavailable 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 www.thermocreteusa.com, and view their short video.
             • Thermocrete™, as we've said, is formally listed as a Codeworthy liner for oil and gas venting.



Stainless steel liner usually used to retrofit older, unlined chimneys or to repair chimneys whose (tile) liners are damaged or in disrepair


Why do we need liners?

  • 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.