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Unlined Fireplace Insert Chimneys

Unlined Fireplace Insert Flue Diagram


As wood exhaust passes through an unlined chimney, liquid creosote condenses onto the inner brick surfaces and attacks the bonding agent in the unprotected mortar joints.

Eventually, the mortar bond is destroyed, allowing the liquid to penetrate the mortar joints. In worst cases, this highly combustible liquid can soak all the way through to the outside of the chimney structure.

This soaked-in creosote can't be removed, and will likely ignite in the event of a chimney fire, resulting in open flames on the outside of the chimney.

Gas exhaust contains a mixture of water and nitric acid, which also attacks unprotected mortar joints, eventually dissolving the binder. When the binder is gone, the water saturates the mortar, turning it to mush.

The water saturates surrounding walls, causing dry-rot. Eventually, the chimney structure collapses.


A UL listed, stainless steel flue liner, the same diameter as the exhaust opening on the insert, is lowered down the chimney flue and connected directly to the insert's flue collar. A sealer plate at the top of the chimney creates dead-air insulation, keeping flue gases warmer and improving updraft, which reduces creosote formation. Ceramic blanket insulation may be added if warmer flue temperatures are needed. A top clamp, storm collar and rain cap complete the installation at the top.

Flexible stainless pipe is most often used for fireplace insert installations, as it is easier to snake through the damper area and any offsets in the chimney above.

For gas inserts, UL listed aluminum alloy pipe is used in place of stainless steel, due to its resistance to both water damage and nitric acid corrosion.

Water-soaked mortar joints will often dry out and resolidify after a gas flue has been relined.


For more info about installing a stainless liner, click here.

Note: Each Code Authority chooses the standards that regulate appliance and chimney installation and usage in their jurisdiction, and may modify code specs as desired. The above-referenced specification is from the Washington Association of Building Officials Woodstove Installation Code Book, and may not reflect the code requirements in your area. Contact your Code Authority for local regulations.

Manufacturers who submit an appliance to a recognized laboratory for safety testing may receive a specific listing for that appliance which may be accepted by your Code Authority in lieu of the standard code requirements. These listings are unique to each model, and can be found in the appliance's installation manual.

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