Upstairs and downstairs toilets shar the same stack which is 3". the upstairs toilet works fine (brand new). the downstairs toilet is getting suction. i snaked the vent from the roof all the way down. no obstructions. can i reduce the down stairs suction by reducing the roof vent from 3" to 1 1/2". Or can I stop the suction by forming a trap in the basement which is under the problem toilet. ( the toilets are not defective because I tried 6 different ones (4 kohlers, 1 american standard, and 1 older style 3.5 gpf) . Also rooter rooter came out and awgering from downstairs toilet flange all the way to the street. (again no obstructions) the gravity sewer main can't be causing that much suction. This suction problem sucks literally)

What is minimum size vent for toilets?

Hi again...

I can't be 100% sure of course, but here's what I think is going on...

As you mentioned, there are two different toilets attached to the same 3" waste stack and I'm willing to bet that the toilet on the first floor doesn't have its own vent and therefore when the upstairs toilet is flushed and passes by the unvented toilet on the lower floor it SUCTIONS (siphons) the toilet.

If there is a lavatory drain that comes off at or below the toilet drain on the lower floor then it may be that the lavatory vent acts as a WET VENT for this toilet and perhaps that drain/vent is clogged enough to flow but not to allow enough air into the system...

Anyway, minimum size vent for a toilet is 2"...even if wet vented by a lavatory vent. But here the idea will be to either install a new 2" vent at the lower floor toilet and connect that into the other vents OR unclog the lavatory drain/vent and see if that improves thing.

Let me know what you think.. and again, please post your reply HERE by clicking at the POST REPLY BUTTON at the upper left corner of this page.

MARK

Please simply add on to the thread that you started instead of starting a new one. To answer your question, two inch. Mark nailed the solution before I could post. Add a vent on the downstairs and I think your problem will vanish. Just follow Marks advice. Good luck, Tom