IPC Common Vent - Double-pattern fitting requirement

[NeverEnoughTime]

Hello, and thank you in advance for your time! Pardon my "newbie" status... I've been a lurker for a while but have never posted.

A question I have is regarding something I read in the book "Plumbing Venting: Decoding Chapter 9 of the IPC", by Bob Scott.

In a case like what is shown below (image I grabbed from the web), the book stated that this installation is a code violation due to the use of the Y fitting. It stated that this is a common mistake, and that a double-pattern fitting must be used instead. Can someone tell me what the code violation (with reference) is by using the Y? I think I know why, but would appreciate some more experienced feedback...

Thanks!

 

[Sylvan]

Personally I see nothing wrong with this BUT the possible reasoning for requiring a double wye is to allow snaking of the line which makes no sense

 

[Sponsor]

 

[WorthFlorida]

Other than the offset to the first trap, that is a very common connection in Florida with double bowl sinks.

 

[Cjlambert]

In your photo, when you drain the sink on the left, the first opportunity the drain has to call for air, is from the sink drain on the right, which could siphon the trap. By using a double-fitting, the first opportunity for the drain to pull air is from the continuous vent off the top of the double-fitting, ensuring the trap on the right is not siphoned.

 

[Sylvan]

If the vent is right behind the wall (within 2 feet of the trap) I doubt the trap seal would be syphoned out as the waste tailpieces are 11/2" and the drain line is 2"

The 2" waste line will not be flowing at full capacity even if both sinks were to discharge at once

The left vertical drain line is close to the trap so it will not cause any noticeable back pressure on the adjoining trap and the garbage disposal causes a lot of restriction

Considering there is a 90 deg fitting then the trap 180 deg and a 45 Deg that is a lot of friction loss to cause aany

 

[hj]

Codes seldom operate on the basis of "common sense" when it comes to venting and siphonage. They operate on THEORY, and theory says one trap could siphon the other under full flow. And to prove it in the Chicago Cabrini Street laboratories, we would use vacuum pumps to simulate the effects of flows and venturi effects.

 

[Stuff]

So far no one stated what code this violates. Book is for IPC. Maybe it is a UPC code that the author thought was also in IPC?

 

[Reach4]

So far no one stated what code this violates. Book is for IPC. Maybe it is a UPC code that the author thought was also in IPC?

I find it as an IPC proposal in page 92 of https://www.iccsafe.org/wp-content/uploads/Master-proposals-document_Updated-7DEC2014.pdf

and page 2 of https://www.iccsafe.org/wp-content/uploads/Master-proposals-document_Updated-7DEC2014.pdf