b-vents Archives - Charles Buell Consulting LLC

As a Seattle Home Inspector I see a lot of damaged B-vent pipe. (B-vent is the typical type of vent pipe used to vent mid-efficiency gas furnaces.) Usually the worst of this damage is limited to the portions of the pipe exposed above the roof. On a recent inspection I found some of the worst deterioration of b-vent that I have ever found. All of this deterioration was in the attic space of the home.

Badly rusted B-vent from improper installation

It is also pretty rare for the interior liner of this double-wall type pipe to actually corrode all the way through. But this pipe had at least one area where venting into the attic space was obvious as condensate was building up on the surface of another b-vent pipe that was running above it.

Holes corroded all the way through the outer layer of the B-vent

Another hole though the outer liner of the b-vent

Nearly the entire run of this vent pipe was close to horizontal. There was about 16 feet of horizontal run of pipe in relation to approximately 8 feet of vertical run of pipe—-a combination not suitable for proper drafting of the exhaust gases.

You might ask how this could have ever been installed this way.

Simple—-form not following function.

In this case the “look” of the front side of the home was more important than the safe installation of the b-vent pipe (and thus more important than the safety of the home’s occupants). The vent could have gone straight up through the roof above the furnace without any horizontal run of pipe—but it would have been visible from the Front side of the home. When “aesthetics” rules over safety, there is something wrong with the aesthetic in my opinion.

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Charles Buell, Seattle Home Inspector

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As a Seattle Home Inspector I frequently find insulation tightly packed around B-Vent pipes that run through attics. (B-vent pipes are used to vent combustion by-products from gas burning appliances such as furnaces and water heaters.) Many builders seem to think that because materials such as rockwool insulation are used as fire-stopping, that it should be “good-to-go” right up against the hot pipes.

The manufacturer’s of these pipes typically require a 1″ clearance to combustibles to allow for air circulation so that heat is not conducted to those combustible materials.

So, even though rockwool is not considered combustible, it too must be retained away from the vent pipes because the insulation itself will conduct heat to things that can burn.

While this may or may not be a big deal, the air space also does something else—-it minimizes corrosion. Almost all B-Vent penetrations through the roof will leak over time. Wet insulation against the pipe can lead to corrosion of the pipe—-if it gets wet and stays wet. Sometimes this water that runs back down the pipe is condensate from the exhaust gases themselves. This water is slightly corrosive and makes the situation even worse.

This first picture shows insulation up against the pipe with some rusting apparent where the insulation covers the pipe. In this case a leaking flashing kept the insulation wet where it eventually rusted the pipe.

B-vent buried in insulation---with some corrosion — B-vent buried in insulation—with some corrosion

In this next picture the installer couldn’t run the B-vent through the space without touching the wood framing so it was changed to a single wall type pipe (this pipe itself is touching the wood) and then it was changed back to B-vent type pipe. The problem with this is that the single wall pipe requires even greater clearances (6″).

Furnace vent pipe too close to combustibles

A proper vent with insulation baffle looks more like this:

Insulation baffle around a B-Vent pipe in an attic

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Rusted B-vent.

B-Vents can Be a problem