Question on adequate toilet venting

[IPDQKWID]

I am wondering if a 1.5" vent is adequate for a single water closet. The vent is in the wall immediately behind the water closet, goes vertical into the upstairs story and turns above the ceiling to tie in with a 2" vent before traveling over to the 3" stack. I know the code says 1/2 the size of the drain, and this is the only fixture on this vent before it ties in to the 2". The MN state code has a note in the table where a 1.5" vent capable of 8 units, is noted with 'except water closets'.
The upstairs wall travel and turning above the ceiling upstairs would be so much less difficult with a 1.5", and I could make a 2" work. My question is really about the adequacy of the venting. I want the flow out of that water closet to be optimized - so would a 1.5" versus a 2" vent make ANY difference? Thank you.

 

[Terry]

Code calls for 2" and most trapways on toilet bowls are also 2".
I have not tried the downsizing of a vent to see it works the same.

 

[Sponsor]

 

[IPDQKWID]

I take it that is what the 'note' in the vent sizing table is for - not to be used as venting a water closet. Thank you, I'll go 2" - gosh there's a lot of misinformation online regarding the "1/2 the diameter of the drain pipe" statement in the code.

 

[Jeff H Young]

Code calls for 2" and most trapways on toilet bowls are also 2".
I have not tried the downsizing of a vent to see it works the same.

I was wondering if IPC allows 1 1/2 " ventI seen a bunch of referances on this site talking 1 1/2' vent ipc code. Ive never put anything smaller than2 and wont unless code changes. But I belive youd never know the differance but its just a guess. gosh we are only talking about a gallon to a gallon and a half. we used to flush 5 even more on a 2 inch vent so its 1/5 the amount of water . I too wonder if it truley make any differance whatsoever?

 

[Reach4]

I was wondering if IPC allows 1 1/2 " ventI seen a bunch of referances on this site talking 1 1/2' vent ipc code. Ive never put anything smaller than2 and wont unless code changes. But I belive youd never know the differance but its just a guess. gosh we are only talking about a gallon to a gallon and a half. we used to flush 5 even more on a 2 inch vent so its 1/5 the amount of water . I too wonder if it truley make any differance whatsoever?

MN has its own code.

IPC would allow 10 drainage fixture units (DFU) to be vented by up to 100 ft of 1.5 inch pipe, if I read correctly. See SECTION 916 VENT PIPE SIZING
https://county.mckenziecounty.net/usrfiles/2009_International_Plumbing_Code.pdf page 75. They allow 1.25 venting for 2 DFU up to 3o ft.

Page 63 says a Water closet, private (1.6 gpf) is 3 DFU.

 

[TheOverThinker]

I want the flow out of that water closet to be optimized - so would a 1.5" versus a 2" vent make ANY difference? Thank you.

The 1.5" would make no difference whatsoever in the function of the toilet.

The issue is code, you have to follow it.
You're probably better off with a straight shot 1.5" rather than a convoluted 2",
but code does measure actual effect, it's prescriptive.

The size of vent piping shall be determined from its length and the total number of fixture units connected thereto, in accordance with Table 703.2. The diameter of an individual vent shall be not less than 11/4 inches (32 mm) nor less than one-half the diameter of the drain to which it is connected.