Stainless, Cast-in-Place or Clay Liner?

What a liner does

The liner is the passage inside the chimney that carries smoke and combustion gases out. It keeps heat away from the surrounding masonry and wood framing, and it resists the acids and moisture in those gases. For a fuller explanation of the terms, see flue vs. liner.

A chimney is usually relined for one of four reasons: the existing liner is cracked or deteriorated, there is no liner at all, a new appliance needs a different size, or the fuel is changing. CSIA identifies relining and appliance changes as situations that call for a Level 2 inspection, which includes a video or other internal examination of the flue.

Stainless steel liners

Stainless liners come as flexible corrugated pipe or rigid sections. They are tested to UL 1777, the standard for chimney liners. Different alloys and wall constructions are listed for different fuels, so the specific liner matters, not just the word "stainless."

• Fuel: Products are listed for wood, oil, and gas, sometimes all three. Check the listing for the appliance you have.
• Insulation: CSIA notes that liners serving solid fuel appliances must be insulated, while gas and oil liners often benefit from insulation for performance. One manufacturer's instructions require insulation above the first flue tile and recommend insulating the full length.
• Sizing: Manufacturers state that the liner must be sized at least as large as the appliance instructions require.
• Prep: Installation instructions call for the flue to be inspected and cleaned of glazed creosote, and for damaged masonry to be repaired first.

Cast-in-place liners

Cast-in-place systems place a former, often an inflatable tube, down the center of the flue, then pump a lightweight cement-based mix around it. When the mix sets, the former is removed, leaving a smooth, usually round passage. A building trade article describes the result as a cost-effective way to provide properly sized flues and notes that it can address deteriorated masonry and broken tile liners.

Because the mix fills voids around the former, cast-in-place lining can add some strength and insulation to an old chimney. It is a specialized process, and results depend on the system and installer.

Clay tile liners

Clay tile is the standard liner in most traditionally built masonry chimneys. It handles high temperatures well and lasts a long time when the chimney stays dry and free of chimney fires. Its weak points are the mortar joints between tiles and its vulnerability to rapid temperature changes.

Replacing broken clay tile inside an existing chimney usually means opening the chimney, which is why relining with stainless or cast-in-place is more common. Damaged tile joints can sometimes be repaired in place instead. See flue resurfacing vs. a new liner.

Comparison by situation

This table is general. The appliance manual and the liner's listing govern the actual choice.

Why sizing matters as much as material

A liner that is too small restricts draft. One that is too large lets gases cool and slow down, which can mean poor draft, more creosote with wood, and condensation with gas. The orphaned water heater problem is a common example. New Jersey's chimney verification form for replacement fuel-fired equipment asks whether the chimney is lined and sized appropriately for the appliance, and it records the liner's manufacturer, listing, and dimensions.

Signs a liner may need attention

Liner problems are mostly hidden, so homeowners usually learn about them during an inspection. A few clues can prompt one:

• Pieces of tile or flaky material falling into the firebox or the base of a furnace flue.
• Moisture, white crust, or rust at the bottom of a flue that serves a gas appliance.
• A strong or unusual odor from the fireplace, or poor draft. See why a fireplace smells.
• A known or suspected chimney fire. See signs of a chimney fire.

None of these confirms liner damage on its own. They are reasons to have the flue examined. If a carbon monoxide alarm sounds, leave the house and call 911.

How fuel changes the choice

Each fuel is hard on a liner in a different way. Wood produces creosote and, in a chimney fire, extreme heat; liners for wood must handle both, and that is why solid fuel liners are insulated. Oil produces soot and sulfur compounds; CSIA explains that these combine with moisture into acids that can spall clay tile and pit metal liners. Gas burns cleanly but produces a lot of water vapor; in an oversized or cold flue, that vapor condenses and attacks mortar joints and tile. A liner that suits one fuel may not suit another, which is why a change of fuel is one of the triggers for a new evaluation.

How we recommend a liner

We start with an assessment: the appliance and fuel, the flue size and shape, the condition of the existing liner seen with a chimney camera when the flue allows, and the masonry. You get findings with photos before any work is scheduled. From there we can explain which liner types fit your situation and why. Chimney 911 installs all three types covered here: stainless steel, cast-in-place and clay tile.

Next step

If you are adding or changing an appliance, or an inspection found liner damage, request an assessment. Read about our chimney liner work and inspection before a new appliance.

Questions homeowners ask

Sources

1. CSIA: Homeowner Resources and FAQ
2. CSIA: The Three Levels of Chimney Inspections
3. DuraVent DuraLiner installation instructions
4. Journal of Light Construction: Relining Old Chimneys
5. NJ DCA Form F370: Chimney Verification for Replacement of Fuel-Fired Equipment
6. NJ Department of Community Affairs: Significant Changes to Minor Work and Ordinary Maintenance (2018)

This guide is general information. It can't diagnose a specific chimney; an inspection can.