Chimney Liners: Every Type, How to Size One, Code Rules, Installation and Costs

A liner has three jobs:

1. Protect the house from heat. Masonry conducts heat. Without a liner, a long or intense fire can raise the temperature of nearby wood framing over time. The National Bureau of Standards' 1987 report NBSIR 87-3515 measured the temperature rise on the enclosure around test chimneys during normal log fires: about 119°C with a fire clay lining, 50°C with stainless steel, and 20°C with a poured masonry (cast-in-place) system. The report found that normal and overfire conditions brought temperatures close to limits in national standards and recommended code updates, particularly for relining.
2. Protect the masonry from corrosion. Combustion gases contain water vapor and acidic compounds. When they condense, they attack mortar and brick from the inside.
3. Size the flue to the appliance. Draft depends on flue size, height and temperature. A flue that is too large lets smoke cool and slow down, producing creosote (wood) or condensation (gas and oil). One that is too small restricts the exhaust and can cause smoke spillage.

The code rule. The 2021 International Residential Code requires masonry chimneys to be lined (R1003.11). The approved liner materials in R1003.11.1 are:

• Clay flue lining complying with ASTM C315 (or equivalent)
• Listed chimney lining systems complying with UL 1777
• Factory-built chimneys or chimney units listed for installation within masonry chimneys
• Other approved materials that resist corrosion, erosion, softening or cracking from flue gases and condensate at temperatures up to 1,800°F

For gas appliances, the International Fuel Gas Code requires masonry chimneys to be lined with approved clay flue lining, a UL 1777-listed lining system or other approved material, with an exception for lining systems listed only for draft-hood and Category I gas appliances (IFGC 503.5.3). The IFGC also points to NFPA 211 for lining existing chimneys.

1. Clay tile (terra cotta) liners

Definition. Fired clay flue tiles, square, rectangular or round, about 2 feet long, stacked inside the chimney as it is built. The IRC requires them to be laid in medium-duty, water-insoluble refractory mortar conforming to ASTM C199, with an airspace or insulation between tile and masonry not exceeding the thickness of the liner (R1003.12).

How to identify it. A square or rectangular tile, often reddish or tan, projecting a few inches above the crown. A camera shows tile sections with a joint every 2 feet or so.

Pros. Inexpensive when installed during construction; handles wood-burning temperatures in normal use; long-lived when intact and dry.

Cons. Cracks from the sudden heat of a chimney fire and from thermal shock; mortar joints erode; square or rectangular shapes are less efficient for draft than round; cannot be replaced in an existing chimney without major demolition. Clay tile is not a UL 1777-listed system.

Typical cost. Replacing clay tile in an existing chimney is labor-intensive: HomeGuide lists about $2,000–$3,500 installed, and removal of old tile about $500–$2,500. Most relining uses metal or cast-in-place instead.

When it applies. New masonry chimneys; existing chimneys where the tile is intact.

2. Stainless steel liners (flexible and rigid)

Definition. Round (or oval) stainless steel pipe run down the existing flue, connected to the appliance at the bottom and to a top plate and cap at the top. Systems are listed to UL 1777 as a complete kit.

Sub-types.

Alloys.

• 316Ti. Stainless with titanium added. The Copperfield liner-sizing reference lists 316Ti as its preferred alloy, with 316L, 304L and 304 as alternatives. 316Ti liners are widely sold for wood, oil and gas.
• 316L and 304/304L. Other common alloys. Some sources describe 304 as suited to wood and oil; always follow what the liner listing and warranty say for your fuel.
• AL29-4C. A high-chromium, high-molybdenum ferritic stainless alloy marketed for gas and oil appliances, especially high-efficiency gas appliances, because of its resistance to acidic condensation. It is generally not the choice for wood burning; check the listing.

How to identify it. A round stainless pipe emerging from a flat stainless top plate on the crown, with a cap directly attached.

Pros. Can be installed in one day in most chimneys (HomeGuide lists installation at about 4 to 8 hours on average); sized exactly to the appliance; round shape drafts well; UL 1777 listing; usually carries a manufacturer warranty.

Cons. Needs insulation for wood and most solid-fuel applications; must be matched to fuel; flexible corrugated liners can be damaged by the wrong brushes; will not strengthen deteriorated masonry.

Typical cost. HomeGuide 2026: about $900–$3,800 installed for stainless, materials about $20–$90 per linear foot; national survey anchors put full relining projects at $1,500–$5,000, up to about $7,000 with insulation, tile removal and complex chimneys.

When it applies. Most relining for inserts, wood stoves, oil and gas appliances; the default choice in most US relining.

3. Aluminum liners

Definition. Flexible aluminum pipe run inside an existing chimney.

Where allowed. Only gas appliances where the liner is listed for that use; the IFGC exception allows lining systems listed only for draft-hood and Category I gas appliances. Aluminum is not for wood, coal, pellets or oil.

Typical cost. HomeGuide: about $625–$2,250 installed; materials about $5–$30 per foot.

When it applies. Gas water heaters and standard-efficiency gas furnaces in masonry chimneys, especially when a flue has been orphaned by a furnace upgrade.

Caution. Never connect a solid-fuel appliance to an aluminum liner. When the house changes fuel, the liner must change too.

4. Cast-in-place (poured) liners

Definition. A lightweight, heat-resistant cement mix poured or pumped into the chimney around an inflated form (a rubber bladder) that is removed after the mix sets, leaving a smooth, seamless round or oval flue. Some systems are listed to UL 1777.

How to identify it. On camera, a smooth, continuous interior with no joints; often the flue opening at the top is round inside a square masonry opening filled with the liner material.

Pros. Fills voids and cracks and adds structural integrity to old masonry; very good insulation (NBSIR 87-3515 recorded the smallest enclosure temperature rise, about 20°C, for poured masonry); suits irregular and historic chimneys.

Cons. Highest cost; requires specialized equipment and trained installers; reduces flue area; less common, so fewer contractors offer it.

Typical cost. HomeGuide: about $2,000–$7,000 installed, materials about $40–$120 per foot, and labor about $800–$3,000.

When it applies. Unlined historic chimneys, chimneys with loose interior masonry, and chimneys where a smooth masonry-type liner is preferred.

5. Ceramic resurfacing systems (e.g., HeatShield and similar)

Definition. A two-part or spray-applied ceramic (sometimes called cerfractory or refractory) coating applied inside an existing clay tile or masonry flue to repair open joints, fill cracks, or resurface the whole liner. HeatShield is a widely marketed product in this category; HomeGuide also references Thermocrete.

How it is applied. The sweep cleans the flue, then uses a specialized tool pulled up the flue to fill joint gaps (joint repair) or to coat the entire interior (resurfacing).

Pros. Keeps the original tile; avoids reducing flue size as much as a metal liner; can be less expensive than full relining when damage is limited.

Cons. Only suits flues with a basically sound tile structure; product listings and warranty terms vary, so ask for the manufacturer's documentation showing the system is tested or listed for your application and fuel.

Typical cost. HomeGuide lists ceramic/Thermocrete liners at about $1,000–$5,000, or about $50–$200 per linear foot.

When it applies. Clay tile liners with open or eroded mortar joints or limited cracking, often found on a Level 2 inspection after a small chimney fire or during a home sale.

6. Insulated vs uninsulated liners

Definition. Insulation around a metal liner is either a ceramic fiber blanket wrapped around the liner, a pre-insulated liner, or a poured insulating mix between the liner and the old flue.

CSIA guidance. CSIA states that liners for gas- and oil-fired appliances do not require insulation because of their lower temperatures, but that liners serving solid-fuel appliances do need to be insulated, and that it makes sense to insulate every liner serving a wood-burning appliance. The Copperfield sizing reference also calls insulation required for all solid-fuel applications.

Why insulation matters. It keeps flue gases warmer (stronger draft, less creosote), reduces heat transfer to nearby framing, and is often a condition of the liner's listing for reduced clearances.

Typical cost. HomeGuide: blanket insulation about $8–$15 per linear foot; pre-insulated liners about $26–$40 per foot.

Liner comparison table

Liner size is not a matter of preference. The appliance or fireplace decides it, and the code sets limits in both directions.

Open masonry fireplaces

The 2021 IRC sets minimum flue area as a fraction of the fireplace opening area (width times height) (R1003.15.1):

Worked sizing: a fireplace opening 36 inches wide by 28 inches high has 1,008 square inches. A round liner needs at least 84 square inches (1,008 ÷ 12). A 10-inch round liner has about 78.5 square inches (too small); an 11-inch has about 95 square inches (acceptable). A round liner of that size may not fit inside an older rectangular tile flue, which is one reason fireplaces are sometimes relined with oval liners or cast-in-place systems, or the opening is made smaller with a listed insert. Chimney height also affects draft; the IRC includes a chart relating flue size, opening size and chimney height.

Wood stoves and inserts

• Match the liner diameter to the appliance's flue collar, following the manufacturer's instructions. The Copperfield reference says not to reduce below the appliance outlet collar size when unlisted.
• The International Mechanical Code limits the flue serving a solid-fuel appliance to no more than three times the cross-sectional area of the appliance's flue collar or outlet (IMC 801.7).
• A 6-inch collar has about 28 square inches of area; a 6-inch liner is typical. An 8-inch collar is about 50 square inches.

Most wood inserts vent into a flue sized for an open fireplace, far bigger than three times the collar area once you account for real interior dimensions. That is why a full-height liner connected to the insert is standard practice and, for inserts listed to UL 1482, commonly a condition of the listing.

Gas and oil appliances

Gas appliance venting is sized from the fuel gas code's venting tables (in the IRC these appear in Chapter 24; the Copperfield reference cites IRC G2428.2–G2428.3 in the 2024 edition), using the appliance input (BTU/hr), the chimney height, the lateral connector run and the number of appliances. Oil appliances follow the manufacturer's instructions and the applicable mechanical code. When an appliance is removed from a shared chimney (for example, a furnace replaced with a condensing unit vented in plastic), the IFGC requires the chimney to be resized as necessary to control condensation and provide the remaining appliance with the required draft (IFGC 501.15.1).

Common sizing mistakes

• Oversized liner for an insert or stove: smoke cools, creosote builds, draft is weak.
• Liner sized to the old tile rather than the appliance: same result.
• Undersized liner for an open fireplace: smoke spills into the room.
• A water heater left alone in a big furnace flue: condensation damages the flue and draft may fail, allowing carbon monoxide spillage.
• Connecting a new, higher-efficiency appliance to an existing liner without rechecking the tables.

States and cities adopt these model codes on their own schedules, sometimes with amendments. Massachusetts, for example, has its own amended version in 780 CMR. Before relining, ask your local building department whether a permit is required; HomeGuide lists liner permits at about $100–$150 or more.

A sweep will typically recommend relining in these situations:

1. The chimney is unlined. Common in houses built before about the 1950s.
2. Clay tiles are cracked, shifted or missing, often after a chimney fire or earthquake, or from settlement.
3. Mortar joints between tiles are open or eroded beyond what a joint-repair or resurfacing product can fix.
4. A new appliance is being connected: a wood insert, stove, pellet stove, oil or gas appliance whose flue collar is much smaller than the existing flue.
5. A furnace was removed or replaced and the remaining water heater now vents into an oversized flue.
6. The fuel changes, for example from oil to gas or from gas logs to wood, and the existing liner is not suitable for the new fuel.
7. Condensation damage shows as stains, white residue, damp spots or crumbling mortar at the chimney base or inside the house.
8. An existing metal liner failed, corroded, or was the wrong alloy or wrong listing for the fuel.
9. A home sale or insurance inspection finds liner defects on a Level 2 camera inspection.

Signs you can notice yourself: flakes of tile in the firebox or cleanout, smoke smell in other rooms or in the attic, stains on walls next to the chimney, a water heater that spills exhaust at its draft hood, or a carbon monoxide alarm. See carbon monoxide and chimneys.

The steps below describe a typical professional installation of a stainless liner for a wood insert or stove. Details differ by liner system; the manufacturer's instructions govern.

1. Level 2 inspection. The Copperfield reference states a Level 2 inspection is mandatory under NFPA 211 before relining. The sweep sweeps the flue and runs a camera to measure the flue, find offsets, and confirm the masonry can support the liner.
2. Choose the system. Alloy and wall type for the fuel, diameter from the appliance collar and code limits, length from the measured height plus connections, insulation for solid fuel.
3. Permit. Pulled where required by the local building department.
4. Prepare the flue. Sweep thoroughly. Remove obstructions such as damper plates or broken tile; some flues need old tile broken out ("flue busting") to fit an insulated liner, which HomeGuide prices at about $500–$2,500 for tile removal.
5. Prepare the liner on the ground. Unroll and straighten the liner, attach the insulation blanket (with its protective mesh), seal the seams per the instructions, and attach a pulling cone or nose cone.
6. Lower the liner. Working from the roof, the crew feeds the liner down from the top while another person guides it at the bottom. Flexible liners follow offsets; rigid liners need a straight flue.
7. Make the bottom connection. For a stove or insert, connect to the appliance collar with a listed connector or adapter; for a tee-style installation, fit the tee and cleanout cap at the base.
8. Seal the top. Cut the liner to length, install the top plate on the crown (sealed with high-temperature silicone), clamp the liner, and fit the listed cap.
9. Seal the damper area. For inserts, the damper area is usually sealed with a block-off plate to keep warm room air from escaping and to keep the liner space closed.
10. Test and document. Test fire or check draft, confirm clearances, photograph the installation, and leave the warranty paperwork and a record of the system and alloy used.

HomeGuide puts typical installation time at about 4 to 8 hours on average.

Most stainless liner systems carry a manufacturer warranty, and installers often offer a separate workmanship warranty. Terms differ widely, so read them before signing. Ask:

• Who issues it? Manufacturer (for the product) vs installer (for workmanship).
• What fuels does it cover? Some warranties exclude coal or treat it differently.
• What conditions apply? Common conditions include installation by a qualified professional, registration within a set time, the correct alloy for the fuel, insulation where required, annual inspection and sweeping, and burning only approved fuels.
• Is it transferable? This matters at home sale.
• What is covered? Material replacement only, or labor too?
• What voids it? Chimney fires, burning trash, the wrong fuel and skipped maintenance often do.

Keep the warranty card, the installer's invoice showing the liner brand, alloy, diameter and length, and your annual sweep receipts together.

Estimated national ranges based on 2026 HomeGuide and Angi surveys. Local labor costs, chimney height, access, roof pitch and the need to remove old tile change these numbers.

Relining pre-job checklist

• Level 2 inspection completed, with video and measurements
• Appliance make, model and flue collar size recorded
• Fuel confirmed; liner alloy and listing match the fuel
• Liner diameter checked against collar size and code limits (3x rule for solid fuel; fireplace ratios; gas tables)
• Insulation included for any solid-fuel appliance
• Offsets and flue dimensions confirmed (flexible vs rigid; oval needed?)
• Tile removal needed? Priced separately?
• Top plate, cap, and bottom connection (tee, connector, block-off plate) included
• Masonry repairs (crown, mortar) scheduled before or with relining
• Permit responsibility in writing
• Manufacturer and workmanship warranty terms in writing
• Photos of the finished job promised

These are illustrative scenarios, not real customers. Costs are estimated ranges based on 2026 national surveys (HomeGuide/Angi) and will vary with local labor rates, height and access.