Correct interpretation of DFUs - IPC Table 710.1(2) vs UPC 703.2

[jamesc]

Looking at IPC 710.1(2) for a 1.5 inch pipe, if I'm reading it correctly, it's telling me I can carry 3 DFUs horizontally but only 2 DFUs if it's vertical (per interval). The DFU capacity on a single branch interval is lower when used as a stack than for a horizontal run.

Under UPC, table 703.2 tells me I can carry only 1 DFU horizontally but then 2 DFUs vertically. i.e. higher drainage capacity in vertical orientation vs lower capacity per the IPC table.

Makes me think I'm reading it wrong. Anyone able to clarify? Thanks.

 

[wwhitney]

Looking at IPC 710.1(2) for a 1.5 inch pipe, if I'm reading it correctly, it's telling me I can carry 3 DFUs horizontally but only 2 DFUs if it's vertical (per interval). The DFU capacity on a single branch interval is lower when used as a stack than for a horizontal run.

So, the definition of horizontal branch allows some vertical portions. And a stack is a vertical drain that extends through at least one full story. A 1-1/2" stack serving multiple horizontal branches would be pretty rare; if there's only one horizontal branch, then while that could include a stack, I would think the horizontal branch limit of 3 DFUs would apply.

In other words, I see the righthand 3 columns in table 710.1(2) in the IPC as covering the case of a multistory stack with multiple horizontal branches connecting. Then the stack can carry more than 3 DFUs, but each branch interval (story) can only bring in 2 DFUs.

Cheers, Wayne

 

[Sponsor]

 

[jamesc]

ok, that's helpful, thanks Wayne.
I see now in the definitions the "extends through not fewer than one story" definition for a stack.

In my situation I have a 1-1/2 inch vertical drain line (and vent) for softener regen waste water that is located in a garage -- so it does not extend through more than one story.
The fixture / softener drain pipe is 0.5 inch PEX, air-gapped into the 1-1/2 inch drain.
I wanted to tap into that drain line with a san-tee to drain a utility sink which would be 2 DFU.

I was trying to calculate the DFU for the softener but can see a case for counting it as:
a) 1/2 DFU per 709.4.1 as non-potable clear water waste
b) 1 DFU since it's 1/2 inch fixture drain (using table 709.2)
c) 2 DFU since it's 1-1/2 inch trap leading to the drain pipe (also using table 709.2)

So, it's unclear to me whether the total is 2.5, 3, or 4 DFU.
It's a bit academic as the softener will back flush ~1x per week at 2am but would be nice to have an argument as to why 1-1/2 inch DWV piping is appropriate.
(This is under IPC)