Vent Developed Length Confusion, Table 906.1

[andredelattre]

Hi all,

I'm confused about Table 906.1 and developed lengths in general. I've attached a diagram below of a venting system to try to understand.

The 2inch vent here is only 130ft long in total (80ft + 50ft), but then connects into this 4inch vent and travels another 100ft (40ft + 50ft + 10ft) before exiting through the roof. Does this 2" vent then satisfy the 150ft rule imposed by Table 906.1? Does this connection to the 4" pipe essentially "reset" the developed length?

I'd appreciate any input on this!

 

[Jeff H Young]

I see no issue you are under 150 feet

 

[Sponsor]

 

[andredelattre]

Got it, just wanted to check if developed length stopped at a connection with another vent. Thanks for the help!

 

[wwhitney]

I don't have a complete answer or understanding of the nuances here, but my thoughts from rereading IPC 906:

IPC 906 distinguishes between (a) stack vents (the extension of a drain stack above the highest fixture served) and vent stacks (a stack that carries no drainage and serves like a relief vent for drain stacks over 5 stories (904.2)) and (b) individual vents, branch vents, circuit vents and relief vents. The vents in category (a) always terminate outdoors or to a stack type AAV (904.3), while those in category (b) can also terminate by connection to a vent in category (a). The developed length is measured from the vent connection to that termination in either case.

So I would say the answer depends on the rest of the vent system that you haven't shown. If the 2" vent is indeed a stack vent, then it appears to me it violates 906.1, as the developed length is 230'. You would need to upsize the drain stack it connects to 3" (if it isn't already) and upsize the vent to at least 2-1/2".

If the 2" vent is instead what I called category (b), then table 906.1 doesn't apply. Rather, the vent is required to be at least half the diameter of the minimum size drain that could carry the fixtures served, and if over 40 feet in developed length, one size larger than that. 18 DFUs requires a 3" drain, and the developed length is 130', so it is required to be one size larger than half of 3", or 2", which you have.

Cheers, Wayne

 

[wwhitney]

Got it, just wanted to check if developed length stopped at a connection with another vent. Thanks for the help!

For that part of the question, see Footnote (a) to Table 906.1 for vent stacks and stack vents, versus 906.3 for "individual, branch, circuit and relief vents".

Cheers, Wayne

 

[andredelattre]

Wow that clears things up a lot thanks Wayne. The 2" vent isn't a stack vent but just an individual (and combination of individual) vents. In this case you're right it would work because of 906.3, and especially because that 4" pipe doesn't' serve any more fixtures directly below that 10" section (since it looks like a vent stack is only allowed an offset in that scenario where after the offset it serves no more fixtures, so that whole 4" pipe seems to be the vent stack).

Thanks again for the help!

 

[Jeff H Young]

good catch Wayne !