A chimney liner — also called a flue liner — is the interior channel through which combustion gases travel from the appliance to the outside air. In residential construction built before roughly 1985, the liner is almost universally unglazed clay tile, installed in sections during the original masonry work. In newer construction or after a relining project, it may be stainless steel, aluminium (for gas appliances only), or a poured cast-in-place system. Each material has a defined set of applications, limitations, and Canadian code references that govern where it can be used.
Why Relining Becomes Necessary
The two most common triggers for relining a residential chimney in Canada are liner deterioration and appliance change. Clay tile liners develop hairline cracks through repeated thermal cycling — expansion and contraction with each burn. Over 30 to 50 years, those hairlines widen into gaps that allow combustion gases, including carbon monoxide, to migrate into wall cavities and adjacent living spaces.
Appliance change is the second trigger. Converting from an oil furnace to a natural gas appliance, or installing a higher-output wood stove in place of an open fireplace, alters the flue gas temperature and moisture content. An existing liner sized for the old appliance may not be the correct diameter or material category for the new one — a mismatch that a WETT assessment or gas appliance permit inspection will identify.
Clay Tile — The Original Standard
Unglazed clay tile (terracotta) was the default liner material in Canadian masonry chimneys from the late 19th century through the 1980s. The tiles are manufactured in rectangular or round cross-sections and stacked with refractory mortar joints. Properly installed clay tile performs adequately for wood and oil appliances burning at sustained temperatures.
The limitations are well established. Clay tile has low resistance to rapid thermal shock — the sudden temperature spike of a chimney fire causes tiles to crack or spall. Mortar joints deteriorate from exposure to condensate acids, particularly in flues connected to mid-efficiency gas appliances where flue gas temperatures are low enough for significant condensation.
Repair options for clay tile liners include repointing deteriorated mortar joints (accessible from the firebox or a cleanout), replacing individual cracked tiles where the chimney configuration allows access, or applying a brushed-on or poured sealant product to restore continuity to a cracked but structurally stable liner. Full tile replacement is possible but expensive and requires dismantling the chimney section-by-section — typically justified only during a major renovation where the chimney exterior is being rebuilt anyway.
Stainless Steel Flexible Liners
Flexible stainless steel liner systems are the most common relining choice for Canadian residential chimneys because they can be installed in a single day, navigate the slight offsets common in older masonry chimneys, and are available in sizes that match virtually any residential appliance output.
The liner is supplied as a corrugated flexible tube, typically in Grade 316L stainless steel for wood-burning and solid-fuel applications, or Grade 304 for gas. The grade matters: wood smoke and its condensate products are more corrosive than natural gas flue gases, and using a 304-grade liner in a wood application can accelerate liner failure. Canadian standards reference ULC S635 for the liner itself; the connector components must also carry appropriate ULC listings.
Installation Procedure
The liner is connected to a top plate at the chimney crown and a connection piece at the appliance collar, then pulled down through the existing flue from above. In most installations, the annular space between the new liner and the existing clay tile is filled with an insulating pour material — perlite-based or vermiculite — that improves flue gas temperature retention (which reduces condensation and creosote) and provides additional structural support.
The liner diameter must be sized to the appliance output, minimum cross-sectional area requirements from CSA B365, and the height of the chimney. Undersizing creates draft problems; oversizing slows flue gas velocity and lowers flue gas temperature, both of which increase condensation. Appliance manufacturers specify minimum and maximum connector sizes in their installation documents.
Cast-in-Place Liner Systems
Cast-in-place (also called poured liner or Supaflu) systems involve pumping a refractory slurry into the existing flue around a removable form, creating a monolithic seamless liner once the material cures. The result is a smooth, round, thermally stable liner bonded to the surrounding masonry.
The chief advantage over flexible stainless steel is that cast-in-place systems structurally reinforce the chimney — the cured liner becomes load-bearing in the sense that it ties cracked tiles together and fills mortar-joint gaps. For chimneys with significant liner deterioration but sound exterior masonry, a cast system can restore structural integrity that a flexible liner cannot provide. Cast systems are also well suited to non-standard flue shapes where getting a round flexible liner to seal properly is difficult.
Limitations
Cast-in-place systems are significantly more expensive than flex liner installations — material costs are higher, and the process is more labour-intensive. They also cannot be removed for inspection without destroying the liner, which means a camera scan is the only way to assess their condition over time. Not all formulations are approved for all fuel types; the product's ULC listing specifies whether it is rated for solid fuel, gas, or oil applications.
Aluminium Liners
Single-wall aluminium flexible liner is approved only for Category I gas appliances — low-temperature, non-positive-pressure applications such as conventional natural gas furnaces and water heaters. It is not appropriate for wood stoves, fireplaces, oil furnaces, or any solid-fuel appliance. Its corrosion resistance is insufficient for the more acidic condensates produced by those fuel types. This distinction matters because aluminium liner is sometimes encountered in older relining jobs that predate current standards, and a WETT or gas inspection may flag it as a deficiency in a solid-fuel context.
Selecting the Right Option
The decision between stainless flex and cast-in-place typically comes down to three questions: Is the existing masonry structurally sound? Is the flue reasonably straight? And what is the budget? For a sound chimney with a modest liner problem, a properly sized 316L stainless flex liner with insulation fill is the standard repair. For a chimney with significant structural concerns — loose tile sections, large gaps, substantial cracking across multiple liner joints — a cast-in-place system may be the more durable solution despite the higher upfront cost.
Both options require a permit in most Canadian municipalities. The work falls under the mechanical or gas sections of local building codes, and inspections are required before the first use of the relined system. Retaining the permit and inspection documentation is important for insurance continuity and for disclosure at any future property sale.
External reference: Natural Resources Canada — Heating with Wood
Last updated: May 4, 2026
