# IPC Section 916.2



## Mech (Mar 3, 2017)

2009 IPC

916.4 Multiple branch vents. Where multiple _branch _vents are connected to a common _branch _vent, the common _branch _vent shall be sized in accordance with this section based on the size of the common horizontal drainage _branch _that is or would be required to serve the total _drainage fixture unit (dfu) _load being vented.

If the common drainage branch required is 3" and I upsize it to 4", is the common vent size based on the required 3" drainage branch or the provided 4" drainage branch?

Thanks


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## fatboy (Mar 4, 2017)

I read it as it is based on what the dfu requirements are, if the 3" vent works for the dfu load, then I would accept it, even though the drainage was upsized.


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## north star (Mar 5, 2017)

*$ ~ $*

Mech,

FWIW, I agree with ***fatboy***........The total dfu's are
what is to be calculated and the correct vent sizing
for those dfu's........That you upsized the common
vent is a plus to the whole system.

Why did you or would you upsize from the 3" to a 4" ?


*$ ~ $*


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## Mech (Mar 6, 2017)

Thanks for the replies.  I am not sure if I described the scenario well enough.  What I was trying to say was a 3" Sanitary Drainage Branch will accommodate the dfu load.  The plumber wants to increase to a 4" Sanitary Drainage Branch.

Is the common branch vent 1-1/2", based on the 3" Sanitary Drainage Branch even though a 4" Sanitary Drainage Branch is installed or must it be 2", based on the 4" Sanitary Drainage Branch?


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## north star (Mar 6, 2017)

*# ~ #*


Mech,

The 1- 1/2" vent is based upon the 3" sanitary drainage
line.

It does not matter that the branch drainage line is a
4".......The vent sizing is based upon the total amount
of dfu's.

*(1)*  Why does the plumber want to increase to a 4"
drainage branch line ?......*(2)* Is this proposed 4" line
the soil stack [  i.e. - the main drainage line for this
number of dfu's, ] or will this proposed 4" drain line
gather more dfu's along the way ?

*# ~ #*


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## fatboy (Mar 6, 2017)

Right on north star.......


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## Mech (Mar 6, 2017)

Thanks again guys!

I'm not sure why the plumber is upsizing the line.  In some instances, the branch vent length exceeds 40 ft and will require the next nominal pipe size.  Going with a 3" wc sanitary drain allows a 2" vent rather than 3" for vent lengths over 40 ft.


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## FLSTF01 (Mar 13, 2017)

I agree with Fatboy and North Star.  Maybe he wants to upsize to 4 inch because he is close on the available slope?  3-6 inch pipe is the same 1/8 per foot, but nonetheless, maybe?


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## north star (Mar 14, 2017)

*# > # > #*

Mech,

I was thinking some more about your application
of the upsizing from 3" to 4" pvc drain pipe.

I was thinking along the "Old School" lines of thought
in that more air in to the drainage system would be better.
What I did not think about at the time was, the variable
of the "low flow toilet fixtures".......Because the "low
flow toilets" have less water being flushed, there is a
fine balance of moving the solids downstream with an
adequate amount of water........If the piping is too large,
then the water level is spread out inside the pipe, thereby
lowering the actual water level, and the ability of the
amount of water used to adequately transport the solids
downstream  [  i.e. - friction & loss of viscosity inside the
piping   ].

IMO, ...I would question the plumber as to specifically
why they want to upsize the 3" to a 4" drain pipe.


*# < # < #*


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