What is a Class A Chimney? | TSE Glossary

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A Class A chimney is a factory-built insulated metal chimney system listed to UL 103 HT, designed to vent solid-fuel appliances such as wood stoves, fireplace inserts, and pellet stoves at temperatures up to 2100 degrees during chimney fires. The triple-wall construction limits exterior surface temperature, allowing the system to pass through combustible framing with code-mandated air-space clearances.

Class A pipe consists of three concentric layers: an inner stainless steel liner contacting the flue gas, a middle layer of high-density ceramic insulation or air gap, and an outer galvanized or stainless shell. Sections lock together with twist-and-lock or band-clamp connections, and the system is supported at the floor line, ceiling line, and roof penetration with manufacturer-specified hardware. Storm collars and rain caps complete the termination.

Class A is the only chimney system rated for solid-fuel use other than full masonry chimneys with UL 1777 listed liners. B-vent and direct-vent systems are categorically not Class A and cannot be used for wood stoves regardless of how the homeowner wishes to economize. Sweeps verify Class A listing markings on every section during a CSIA Level 2 inspection">Level 2 inspection, and improperly mixed components are a frequent finding on DIY installations.

DFW workshop wood stove installations and freestanding hearth-room appliances rely on Class A chimney runs through unconditioned attic space and the roof. The 2-inch air-space clearance to combustibles must be maintained even where the pipe passes through dense blown-in insulation, which means an installed insulation shield is required. TSE permits and inspects every Class A install per IRC, with proper firestop spacers at every framed level.

• [UL 103 HT chimney](/glossary/ul-103-ht-chimney/)
• [B-vent](/glossary/b-vent/)
• [Direct vent](/glossary/direct-vent/)
• [Pellet vent](/glossary/pellet-vent/)
• [Chase](/glossary/chase/)

• UL 103 Standard for Factory-Built Chimneys
• NFPA 211 (2024), Section 9.2
• IRC 2021, Section R1005

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