Prior to my bathroom remodel, I had a toilet flange moved in my basement to a standard 12" rough-in, since the toilet was about 5" off the wall and looked silly. The problem I have now is a vacuum in the upstairs toilet lines, such that when I flush one, the other toilet (opposite wall) sucks down water and vice versa. I believe this to be related to the plumbing alteration since the toilets seemed to work fine beforehand.

When the plumber jackhammered up the floor he noticed a split in the pipe. Because of this split he couldn't move the toilet and said he had to remove the split. BTW, he said he never sees this sort of thing. The split seems to be where the vent pipe connects to the main plumbing.

enter image description here

Once he removed the split, he diverted the vent pipe (and bath sink) into the main wall pipe.

enter image description here

You can see how the vent pipe now tees into the larger pipe (upstairs plumbing) and then goes into the floor where it meets the new toilet line. He seems to have tapped off the old pipe

So, now to my question. Is it obvious from this plumbing change as to why I now have vacuum in the lines? Next question, is this plumbing alteration correct?

  • sounds like he cut off the vent
    – Steven
    Mar 28, 2014 at 11:56
  • Ditto. Plumbing installations often need more than one vent line. If the vent is on the down flow side of both toilets then when for sure when the toilet closest to the vent will create a suction toward the further toilet. I think that the ideal situation is that there be a vent stack by each bathroom toilet.
    – Michael Karas
    Mar 28, 2014 at 15:20
  • 1
    Why are you using couplers when going PVC to PVC? I am not even sure they are rated for that. I wouldn't trust them.
    – DMoore
    Mar 29, 2014 at 0:15
  • @DMoore I hired a plumber to do this work so not sure why he used couplers
    – Andrew
    Mar 29, 2014 at 2:45
  • @Andrew - I just asked because it isn't something that any of my plumbers (or even myself would ever do... like ever). First PVC is a smooth material and there isn't as much rigidity for the coupler to compress into. It can be a source of problems in the future - maybe 2 years maybe 10 maybe less. No problem using these and he used a good brand but I only use them when they can be accessed. Usually for PVC to cast iron main. And then second part is the cost. I could have put PVC FF link for about $2. The use of it in this case is suspect to me (unless he made you pay for parts).
    – DMoore
    Mar 29, 2014 at 20:49

2 Answers 2


Its a venting problem. Without enough volume of air to replace the volume of water moving down the pipe a suction is created. This is due to sizing or placement problems. Get a new plumber, a licensed one if your state upholds that standard. The license would separate those who say they are a plumber from those that are plumbers.


I can't be absolutely sure, but this is the best explanation I can think of. First of all, the existing venting system must be less than optimal for this theory to apply. No one can confirm this for sure without significant (and likely destructive) investigation.

Second, the side line coming into the original "split" (tee-wye actually) is actually the vent for the basement toilet. The tee-wye was installed incorrectly for a vent due to constrictions of floor level and adjacent plumbing. The side inlet should have been rotated up 45 degrees for a proper vent take off. Improper vent take offs like this are quite common and do not always present any practical issues. It might even be allowed by some codes.

The horizontal line in the wall that drained into the former vent made it a wet vent. Depending on what the horizontal line is draining, the wet vent configuration is possibly an acceptable installation.

The new configuration where the vent take off is at the main drain stack from upstairs is incorrect. This makes a short section of the main drain a very much improper wet vent. Additionally, the horizontal section of new pipe now becomes a horizontal wet vent, which is not allowed by some codes, but might be by others.

The plumber apparently thought this vent was actually a drain, in which case the new installation would have been fine. But how did he think the toilet was vented?

This basement toilet vent is connected into the vents of other fixtures upstairs. This is completely normal. However, now that it is improperly tied into the main drain stack, falling waste water in the drain induces significant air pressure fluctuations in the vent pipe that never occurred with the old configuration. It is these pressure fluctuations in a pipe tied to the upstairs toilet vent(s?) causing your observed suction in the toilets.

Even though wrong, this configuration should not normally cause the problems you observe if the venting system were working correctly. There has to be some constriction in the vent path through the roof for the suction you see to be possible. Even though the most direct path from the roof down may be clear based on your testing, the actual toilet vent portion is still obstructed or misconfigured.

The solution to this problem is to properly vent the basement toilet and remove the tie in to the main drain stack. Venting the basement toilet properly is easier said than done due to a lot of plumbing happening in the immediate area. It's impossible to suggest an exact solution without much more information about how everything is laid out. It may be necessary to reconfigure the entire bathroom to get the toilet away from this congested area.

You will still have a venting issue upstairs, but at least this correction restores the original situation where suction was not a serious issue.

  • I will try and figure out where exactly those two pipes are going upstairs but I believe neither is a vent. I'm still holding out for 2 theories: 1. the original toilets were not air-tight and so vented occurred through gaps in the wax seal (now they are beyond air tight) 2. they vented something into the attic and the r50 blown insulation I got clogged it up (I know it's not code to vent in attic but homeowners do crazy things. Lastly, the bathroom now is completely finished with about $4K into it so jackhammering is out of question
    – Andrew
    Mar 29, 2014 at 2:53
  • Unfortunately I didn't realize problem until I finished the last upstairs bathroom and had two toilets working, some 1.5 years later
    – Andrew
    Mar 29, 2014 at 2:54
  • Well, it was a vent, though someone may have very well drained into it though they shouldn't have. Or it was acting as a vent. It's really semantics in a way, it's certainly tied into the vent system either way. If you can't change the basement, you'll need to locate the obstruction above the toilets to solve this. The basement toilet is not properly vented, but I suspect it should continue to work OK.
    – bcworkz
    Mar 29, 2014 at 16:53
  • If we are defining a vent as a pvc pipe that is exposed to fresh air then this isn't a vent. The vent through roof is on other side of house. Of course it all connects to the main line, but indirectly
    – Andrew
    Mar 29, 2014 at 23:36
  • Ok, the pipe on the right is vented through the roof. I didn't see it before. Now I need to check if it's clogged
    – Andrew
    Mar 30, 2014 at 0:51

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.