Maximum vent length

[The Blur]

I have 2 toilets back to back on the east side of the house... (1st floor). that suck the water from each others bowl... when flushed. I went into the attic, and looked at the venting. 2" vent is in the attic above the 2nd floor bathroom. Therefore; 3 bathrooms connected to one 2" vent in the attic.

Now, the vent runs 70' across the attic (horizontally) to the west side of the house, where it meets the main stack.. and then goes through the roof. So the vent is excessively long, and doesn't have the greatest pitch.

Would it be legal & wise to run another vent through the roof on the east side of the house? .. so I would effectively shorten the vent run for all the 3 east side bathrooms. And each side of the house would have it's own vent stack..

Would 3" do it, or 4" where it penetrates the roof ??

 

[Terry]

2" works for three bathrooms.
If the horizontal is more than 1/3 the vertical you would go up one pipe size.

Some of the newer toilets will push water out of the opposite bowl when flushed. It's not a siphon, it's a push. When the water gushes out of the toilet, it pushes water across the sanitary cross and air with it. The water in the opposite bowl moves upward and then escapes down the trapway.
It's not venting, it's the back to back fitting that needs changing.

 

[Sponsor]

 

[The Blur]

1 toilet is indeed the newer power flush style, and the other is the older slow flusher. and the problem is indeed when the new toilet flushes.

However, when I drain the bath tub, the water in both bowls move around. It doesn't suck water out, but the bowl water certainly moves when the shower or bath is running. That's why I figured the vent was excessively long... and sagging horizontal.

 

[Terry]

You may have water in the vent if it's sagging. That needs to be plumbed with grade to let it drain downward.

 

[The Blur]

I went in the basement and looked; and I have a double sanitary tee. So I feel your graphic hit the nail on the head. And to re-do 4" PVC is just a Royal PITA, in between joist, with copper running every which way.

While I feel the vent is excessive in length, everything drains properly, and the problem started when we went to a new power flush toilet.