Maximum length Type b vent

Is the maximum length of a 6" b vent, 9 feet? That’s according to a table of values found here, Chapter 24: Fuel Gas, Residential Code 2018 of Nevada | UpCodes. Is that the correct code?

The attached photo is from a custom built, new construction home that has passed final code inspection and will be on the parade of homes this weekend.


No. That’s the maximum length of the vent connector.


I see this all the time, city inspectors don’t go into attics. I would call out the lack of supports. There should be some type of support at every elbow. I would also call out lack of sheet metal screws. It is recommended by every B vent manufacture and may be in your code.


Martin, is any of this vent system considered a “connector?” If so, where do we say it starts and ends? Start at furnace and end at 90 degree turn up to roof?

Also, do we need to consider the ratio of horizontal run to vertical run in this installation?

That whole monster is a true thing of beauty…just sayin…


I was thinking the same thing Jim. “Holy Octopus Batman!!!”


The entire B vent system that I see in that attic space is composed of connectors and fittings.

guy was obviously charging by the fitting…

You’re looking at the wrong table, you should be look at this table (you have a fan-assisted cat 1 furnace using b-vent connector) :

The horizontal part (lateral) of the vent before it turns vertical is the vent connector. The sizing is appliance BTU input dependent.

Take a look at page 11 of this PDF, it is easier to read than the code:

hart-cooleytypebgasventall-sizing_guide.pdf (2.5 MB)

Definitely needs more support. Find out if screws are required in your locale, the manufacturers do not require them on the smaller size b-vents and it’s not a UL listing requirement.

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These are from the installation instructions for B vent 6 inch and under. Two separate manufacturers both recommend sheet metal screws. :slightly_smiling_face: I call it out every time I see it installed without sheetmetal screws. High winds in my area (or any area) can push on a B vent and twist poorly supported fittings and connectors in the attic space.

I noticed in the tables that the laterals given are never longer than the height. Judging from the OP’s picture, his lateral may be significantly longer than the height. This seems like a concern to me.

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I agree Ryan I noticed that too. The best thing for the OP to do is to just call it out for repair by a licensed HVAC contractor. Lack of proper supports, recommended sheet-metal screws are all of a concern.

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Your first pic from DuraVent say “the connections maybe be further secured” This is not a requirement, you know this.

Your second pic is either outdated or came from larger b-vents installation instructions. This is the current b vent instructions for Metal Fab sizes (3"-14"):

If the b vent in the OP pic was properly supported, I would not call out missing screws – we don’t have 100mph winds inside our attics here. If tornado makes a landing inside that attic, the screws won’t do much good anyway. As you know, the liability here is high, if it was common for these b vents to come loose, they would all indeed require it.

Yeap, if the rise was shorter, it wouldn’t have enough draft to overcome the friction/restriction of the lateral connector. It is the difference in height that creates draft.

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Funny looks like the same photograph I supplied :rofl: Looks like I have current installation instructions. This is a screenshot from the installation instructions you provided :slightly_smiling_face:

It’s just hard to tell from the picture how much “rise” is in the lateral.

This is why I said recommended and not required. I do know this, I’ve been in the business for 35 years :slightly_smiling_face:

If that’s where you got it, then refer to the instructions. The diagram you posted shows you where and how to do it if needed. It is your choice to believe all b-vent manufacturers recommend them, I’m not here to stop you :slight_smile:

Can you show me where anybody said that the manufactures requires this? You need to read posts carefully Simon before you try to jump somebody :kissing_heart:

They neither recommend nor require them.