How it works

The aluminum inner liner resists corrosion from gas combustion byproducts at the flue gas dew point, while the air gap between liner and shell limits exterior surface temperature. Natural draft drives flue gases upward through buoyancy alone, requiring proper sizing and minimum vertical run for reliable operation. B-vent terminates above the roof with a UL-listed cap and is supported at floor, ceiling, and roof penetrations with manufacturer hardware.

B-vent’s clearance to combustibles is typically 1 inch, much tighter than masonry or Class A chimneys. This allowance is engineered into the listing and depends on every component being installed per manufacturer instructions. Sweeps verify proper offset connectors, transitions, and termination height during inspections, particularly when a B-vent has been modified during HVAC equipment changes.

DFW context

DFW homes built between the 1970s and the early 2000s commonly use B-vent for gas water heaters and gas-log-only fireplaces in attic chases. When homeowners convert from gas logs to a wood-burning insert, the B-vent must be ripped out and replaced with Class A chimney pipe rated for solid fuel. TSE inspects every conversion to ensure the existing vent matches the new appliance fuel category before any work proceeds.

Related terms

• [Class A chimney](/glossary/class-a-chimney/)
• [Direct vent](/glossary/direct-vent/)
• [Vented gas log](/glossary/vented-gas-log/)
• [Power vent](/glossary/power-vent/)

Sources

• UL 441 Standard for Gas Vents
• NFPA 54 (National Fuel Gas Code), Chapter 12
• IFGC 2021