The Metal Gas Vent Pipe From Ceiling to Roof:
B-Vent, Rust, Caps, Bracing and Clearance

Single-wall pipe and B-vent

Two kinds of metal pipe carry exhaust from a standard gas appliance.

Single-wall pipe is the plain galvanized pipe that usually connects the appliance to the vent. It gets hot, so it needs a lot of room: commonly 6 inches from wood and other combustibles for a standard gas appliance with a draft hood.

B-vent is double-wall pipe, an inner pipe (usually aluminum) inside an outer galvanized one with an air space between them. It stays cooler on the outside and is listed for much less clearance, typically 1 inch, which is printed on the pipe. Once the vent goes through a ceiling or into a wall, it’s normally B-vent, with a metal firestop spacer where it passes through each floor or ceiling.

High-efficiency furnaces and tankless water heaters are different. They vent through plastic pipe or special stainless vent, and none of this post applies to them. A house that changed from an older furnace to a high-efficiency one often still has the old metal vent, now serving only the water heater.

Rust and holes above the roof

Rust on the part of the vent that sticks out of the roof was the single most common finding in this group. Most of it was surface rust and worn coating, which the reports note so it can be watched and budgeted for. In Vancouver, Portland, Hillsboro, Beaverton and other houses, the rust had gone further and opened holes in the pipe.

Some vent rust is weather. A lot of it starts inside the pipe. Gas exhaust carries water vapor. If the exhaust cools too much on the way up, that vapor condenses on the inside of the vent and runs back down. That happens more when the vent is too big for what it serves (a water heater left alone on a vent sized for a furnace and a water heater), when the vent runs a long way through a cold attic, or when it’s single-wall pipe where B-vent belongs. A Beaverton attic had white mineral deposits and moisture staining along the vent pipe, a sign of condensation inside.

The roof flashing around the vent corrodes too. Russ wrote up rusted vent flashing in Beaverton, Vancouver and Happy Valley. Some was still shedding water; one was severe enough to replace.

Caps, storm collars and bracing

The top of a gas vent has a listed cap that keeps rain and birds out and helps the draft in wind. Caps get bent by falling branches, roofers and snow. Russ found damaged caps in Portland and Vancouver. A crushed cap can restrict the exhaust.

Just above the roof flashing, a storm collar clamps around the pipe to shed water running down it. A Sandy house was missing one, which leaves a path for water under the flashing.

Tall vents need bracing. How tall a vent has to stick up depends on the roof’s pitch and on what’s nearby, so on steep roofs and next to taller walls, vents end up several feet tall. Vent manufacturers call for bracing above a certain height. A Gresham house had a tall vent with no bracing, and in Lake Oswego the support wire on a tall vent had come loose. In Canby, a vent had open holes where a brace had been removed. In one Portland house, the vent came through the roof at a sharp angle, sat too close to the roof surface, and was held in place with heavy sealant.

Stains at the ceiling and clearance problems

Where the vent goes up through the ceiling, there’s a metal collar. Stains around that collar showed up in 14 houses, often on a garage ceiling. They come from one of two places: a leak at the roof flashing that runs down the outside of the pipe, or condensation inside the vent that runs down and drips. The ceiling water stain post covers how Russ tells those apart. In a Beaverton garage, the collar at the ceiling had slipped down the pipe and left the opening uncovered.

Clearance problems are less common but more urgent. In one Beaverton house, foam pipe insulation on a nearby water line was touching the water heater vent and had started to melt. Anything that can burn, from insulation to framing to stored boxes, belongs outside the clearance printed on the pipe.

What a home inspection checks

Russ follows the vent where he can see it: from the appliance to the ceiling, through the attic when it’s accessible, and out the roof from the roof itself or the drone. He looks at the pipe type, joints, slope, supports, clearance to combustibles, the firestop collar, any stains, and on the roof the flashing, storm collar, cap, height and bracing.

He doesn’t run a camera inside the vent or test how well it drafts. Parts of most vents run through closed walls and can’t be seen. When the pipe is rusted through, damaged, too close to combustibles or missing parts, the report recommends a licensed HVAC contractor. When the gas furnace was replaced with a high-efficiency model and the water heater was left on the old vent, the backdrafting post explains why that combination gets a closer look. For the age of the appliances on the vent, the water heater serial number decoder and the HVAC serial number decoder read the data plate.

To set up an inspection, you can book online.