B-Vent Type B Gas Vent Pipe Uses, Repair, Replacement

A B-vent is a Type B double-wall metal vent pipe that carries combustion gases from gas-fired appliances safely to the outdoors.

B-Vent (Type B Gas Vent Pipe) diagram — labeled parts, dimensions, and installation context

What It Is

A B-vent, often called Type B gas vent, is a factory-built metal vent system used with natural draft and some fan-assisted gas appliances. Its double-wall construction helps the vent warm up quickly, which improves draft, while the air space between walls keeps the outer surface cooler than single-wall pipe.

In homes, B-vents are commonly connected to gas furnaces, atmospheric water heaters, and some boilers. They are designed only for listed gas-burning equipment and are not interchangeable with chimney liners, pellet vents, or vents for condensing appliances that produce corrosive moisture.

Types

B-vent systems are sold as straight sections, adjustable lengths, elbows, tees, firestops, roof flashings, storm collars, and listed caps. Round sections are most common, though ovalized sections may be used where framing clearance is tight.

The vent type stays the same even when the fittings change. What matters is that every component is listed as part of a compatible Type B vent system and installed with the required clearances.

Where It Is Used

B-vents are used where gas appliances need to vent hot combustion byproducts vertically through the house and out the roof. You will often see them above older non-condensing gas water heaters in garages, basements, and utility closets, and above mid-efficiency gas furnaces in mechanical rooms or attics.

They are not used on high-efficiency condensing furnaces that vent through plastic pipe, and they should not serve wood-burning or oil-burning equipment unless specifically listed for that purpose. Proper appliance matching is critical because the wrong vent can spill carbon monoxide or deteriorate prematurely.

How to Identify One

A B-vent is usually a round metal pipe with a visible gap between the inner and outer walls at the cut end. It commonly has twist-lock joints, a metal firestop where it passes through framing, and a listed cap above the roof.

Look for labeling on the pipe that says Type B gas vent or shows a listing mark from the manufacturer. Rust streaks, loose joints, crushed sections, missing support straps, or signs of backdrafting at the appliance draft hood are all red flags.

Replacement

Replace a B-vent when it is rusted through, dented enough to restrict flow, disconnected, poorly supported, or no longer matched to the appliance it serves. Replacement is also common during furnace or water heater upgrades when the vent size or appliance category changes.

A damaged or improperly sized vent is a combustion safety issue, not a cosmetic one. Replacement should follow the appliance manufacturer instructions, local code, and the vent manufacturer's listing requirements.

B-Vent (Type B Gas Vent Pipe) — FAQ

What does a B-vent do?

A B-vent carries combustion gases from a gas-fired appliance to the outdoors. It is designed to maintain draft while keeping the outer wall cooler than single-wall vent pipe.

When should a B-vent be replaced?

Replace it when the pipe is rusted, crushed, disconnected, undersized, or no longer appropriate for the appliance. If you are replacing a furnace or water heater, the vent should be evaluated at the same time because the new equipment may require a different venting setup.

Do I need a permit to replace a B-vent?

Usually yes, especially if the vent is being altered as part of a furnace, boiler, or water heater replacement. Local code officials often want permit review because venting errors can create fire and carbon monoxide hazards.

How much does B-vent replacement cost?

Cost depends on vent height, diameter, roof access, and how many fittings or firestop penetrations are involved. A short, simple replacement may be a few hundred dollars, while a full vertical re-vent tied to appliance replacement can run much higher.

What is the most common B-vent failure?

Corrosion at joints or at the top termination is one of the most common failures, especially where moisture has been condensing in the vent. Improper slope, loose connections, and using the vent with the wrong appliance are also common problems.