DWV Vent Sizing

[Michael.L]

I'm in Colorado under IPC 2015.

I have a floor drain with a 2" trap (2 DFU). I can vent this with a 1-1/2" vent. However, I want to add a wall cleanout for the downstream sanitary sewer using this vent pipe. So I plan to install a 2" combination wye & 1/8 bend with a 2" riser to the cleanout tee in the wall. Above the cleanout tee, I was planning to reduce the pipe size down to 1-1/2" for the lateral vent line.

But I've recently been told by someone that once a vent is started, the pipe size cannot be reduced. In other words, I must continue the vent from the cleanout tee with 2" pipe. This doesn't make sense to me because the 2" pipe from the wye up to to the cleanout tee is not a requirement for venting, it's simply to facilitate mechanical access to the drain pipe in the event that it ever needs to be cleaned. I maintain that it's acceptable to step down to 1-1/2" vent pipe after the cleanout because the plumbing code only requires 1-1/2" pipe to vent this fixture.

Which one of us is correct?

 

[classicT]

I would agree with you Michael. I believe that your colleague is misquoting IPC 704.2 (below), which only applies to drainage piping.

704.2 Change in Size
The size of the drainage piping shall not be reduced in size in the direction of the flow. A 4-inch by 3-inch (102 mm by 76 mm) water closet connection shall not be considered as a reduction in size.​

 

[Michael.L]

I also think he has confused the code. His claim is illogical as the flow of vent air is not restricted by the pipe size decreasing in diameter, as long as the pipe size is not reduced to less than what's required by code.

In fact, what I'm doing by reducing my vent pipe above the cleanout is no different than what is typically done with a common vent, wet vent, or circuit vent.