How it works

UL 103 HT chimney pipe is a triple-wall construction: an inner stainless liner, a layer of high-density ceramic insulation or air gap, and an outer galvanized or stainless shell. The configuration limits exterior surface temperature to safe levels even during sustained operation, allowing the system to pass through combustible framing with only a 2-inch air space.

The HT designation distinguishes Class A from lower-rated chimney systems intended only for gas appliances. Wood-burning stoves, fireplace inserts, and pellet stoves all require HT-rated venting because their flue gas temperatures exceed what gas-only systems can handle. Sweeps verify HT listing on every solid-fuel installation during a CSIA Level 2 inspection">Level 2 inspection because mismatched venting is a common code violation in DIY installations.

DFW context

DFW homeowners installing wood-burning stoves in workshops, outbuildings, or freestanding hearth rooms must use UL 103 HT chimney for the full vent run. The same applies to pellet stove conversions where the original gas-only B-vent must be replaced. TSE technicians never reuse pre-existing B-vent for solid-fuel applications, and they verify Class A listings on permit applications submitted to local jurisdictions.

Related terms

• [UL 1777 listed liner](/glossary/ul-1777-listed-liner/)
• [Class A chimney](/glossary/class-a-chimney/)
• [B-vent](/glossary/b-vent/)
• [Pellet vent](/glossary/pellet-vent/)

Sources

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