Short definition
B-vent is a double-wall galvanized metal flue used to vent atmospheric (natural-draft) gas water heaters and furnaces. The air gap between inner and outer walls keeps flue gases warm enough to draft naturally and lets the flue pass through framing without a fire hazard.
What it is
Standard atmospheric gas appliances rely on the buoyancy of hot flue gas to clear combustion products through the roof. B-vent makes that physics work safely. The inner wall carries the flue gas; the outer wall keeps surrounding wood framing cool. Sections twist-lock together with a small offset and terminate above the roof in a UL-listed cap.
Residential B-vent runs 3 inches or 4 inches in diameter, matched to the heater’s draft hood collar, and is governed by UL 441. The whole point is that no fan is involved — draft is buoyancy-driven, so any obstruction, missing section, or competing exhaust appliance can pull flue gas back into the house.
B-vent only works for non-condensing atmospheric appliances. Condensing tankless water heaters and condensing boilers cool the flue gas below its dew point and need a different vent — usually CPVC, polypropylene, or stainless concentric pipe.
Why it matters to a homeowner
If your gas water heater has a sheet-metal flue running up through the roof, that’s almost certainly B-vent. When a plumber quotes a heater replacement and includes “vent verification,” they’re checking joint integrity, cap condition, and clearance to combustibles on this pipe. A disconnected B-vent joint in the attic is a carbon-monoxide hazard most homeowners never look for.
Switching from atmospheric to a sidewall power-vent or a condensing tankless usually means abandoning the existing B-vent and routing a new PVC or CPVC vent through a sidewall — which is a permitted job, not a like-for-like swap.
When you’ll encounter this term
- A water-heater replacement quote lists “re-use existing B-vent” or “new B-vent termination cap.”
- Your CO alarm trips during a windstorm and the plumber points to back-drafting at the flue cap.
- You’re switching from an atmospheric gas heater to a condensing tankless and the installer says “the B-vent has to be abandoned.”
- An inspector flags a B-vent joint separation in the attic during a pre-purchase inspection.
Common variants and what B-vent is not
- B-vent vs. direct-vent. B-vent uses room air for combustion and pushes flue gas up through the roof. Direct-vent uses sealed concentric pipe — outside air in, flue gas out — through a sidewall.
- B-vent vs. power-vent. Power-vent uses an electric fan to push flue gas through PVC pipe out a sidewall; no buoyancy needed.
- B-vent vs. condensing flue. Condensing appliances make acidic, low-temperature flue gas; B-vent will corrode through. Condensing units use CPVC, polypropylene, or stainless.
Common failure modes
- Down-draft / spillage. A kitchen exhaust hood, dryer, or whole-house fan competes for combustion air and pulls flue gas back into the house. Symptoms: melted plastic on the draft hood, soot streaks, CO alarm trips.
- Disconnected joints. Twist-lock sections can separate over decades, dropping flue gas into the attic — usually invisible until a CO alarm trips.
- Rusted termination cap. Water entering the flue corrodes the inner wall from the top down; eventually you get perforation and spillage at the heater.