Venting Problems - How to Diagnose Positive vs. Negative Pressure

[ngmtl]

From what I understand, it's generally safe to say that gurgling from a drain signals a venting issue. I'm looking for ideas to help diagnose which of two possibilities is in play. Feel free to suggest other possibilities that I have not considered.

Possibility 1 - Negative pressure but with insufficient air available from the roof vent and/or any air admittance valves. DWV system starts to siphon out P-trap water or pulls air through it resulting in gurgling.

Possibility 2 - Positive pressure but an insufficient release of air out of the stack vent (possibly a clog in the stack). This condition also closes any air admittance valves in the system so they are of no help. Air and water need a way to escape and find their exit through the P-trap water also resulting in gurgling.

I wanted to ask if anyone knows of any simple tests or things to look for to determine with a high degree of confidence which of these two possibilities is the culprit? On a related side note, one feature strangely missing from the AAVs I've used is some kind of mechanical indicator that will make it easy to see visually when it is open and when it is closed.

I wanted to note that this is a follow-on question to an earlier one in which I've been troubleshooting gurgling and slow drainage from a lav I recently converted to a double in a high rise in which I employed the use of AAV traps on both lavs due to an inability to modify in wall to give each lav a dedicated path to air. The AAV traps were a proactive additional precaution I took due to the long horizontals (still with good slope) and the building does have external venting - so was surprised to have gurgling when the lavs are in use. It also gurgles when the apartments above are using their sinks for that matter. I am questioning whether there is some kind of blockage somewhere in the building stack. In any case, I wanted to break out the positive vs negative condition as a stand-alone question as it may be more broadly applicable to the community here.

 

[Reach4]

Feel free to suggest other possibilities that I have not considered.

Those vessel sinks in a bathroom can be a problem. They can gurgle all by themselves.

I wanted to note that this is a follow-on question to an earlier one in which I've been troubleshooting gurgling and slow drainage from a lav I recently converted to a double in a high rise

Did you make an open manometer?

 

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[ngmtl]

Just thinking about this a little more I think I answered my own question with a simple experiment. The experiment worked beautifully. Photo at the bottom of this message is where the right lav p-trap ties in (disconnected).

• Problem is absolutely positive pressure. Turn the water on in the left lav, results in outward pressure inflating the glove. AAVs will close in this condition.

• I believe the same thing happens with the apartment above uses their sink.

Question now is whether this is likely a clog in the building stack or an issue with my under lav split?

 

[Reach4]

You could snip the tip off of one of those fingers, attach a piece of clear tubing, and thrust the open end into a glass of water. The depth that the water in the tube goes below the surface of the water in the glass would let you quantify the pressure. If you do that, how about a photo of the indication...

You would want to find how much pressure you sometimes get when you are not using water.

The major effect that running water in the other bowl caused would tend to make me suspect that the problem is nearby.

 

[ngmtl]

I'm going to see if I can rig up a 5' - 10' snake this afternoon to check for any obviously clogs between the tie-in and the stack.

 

[Reach4]

See if flushing the toilet affects the glove.

Asking the upstairs neighbor to run water to the lavatory while you watch the glove could be informative also.

 

[ngmtl]

• Will try and run a flush and pressure test today - I think, but am not 100% sure, the toilets are on their own stack thankfully. Have not been having a problem there and don't recall any gurgling associated with flushing.
• With that said, I have things disconnected at the p-trap with a bucket under it. Nearly a gallon of water from our neighbors above and almost overflowing the bucket. Is this enough of a smoking gun to say with confidence that there is a blockage or venting issue in the stack rather than our branch or anything we've done outside the wall? Neighbors below us say their sink drains slowly but doesn't gurgle (may or may not be related).
• I should note we're on the 15th floor of a 17 floor building.

 

[ngmtl]

• Will try and run a flush and pressure test today - I think, but am not 100% sure, the toilets are on their own stack thankfully. Have not been having a problem there and don't recall any gurgling associated with flushing.
• With that said, I have things disconnected at the p-trap with a bucket under it. Nearly a gallon of water from our neighbors above and almost overflowing the bucket. Is this enough of a smoking gun to say with confidence that there is a blockage or venting issue in the stack rather than our branch or anything we've done outside the wall? Should we ever see water coming out of the tie-in from upper floors under normal operation? Neighbors below us say their sink drains slowly but doesn't gurgle (may or may not be related).
• I should note we're on the 15th floor of a 17 floor building.

 

[Reach4]

• With that said, I have things disconnected at the p-trap with a bucket under it. Nearly a gallon of water from our neighbors above and almost overflowing the bucket. Is this enough of a smoking gun to say with confidence that there is a blockage or venting issue in the stack rather than our branch or anything we've done outside the wall? Neighbors below us say their sink drains slowly but doesn't gurgle (may or may not be related).

• I should note we're on the 15th floor of a 17 floor building.

It's a smoking gun that the drain has a blockage. I wonder how the neighbor below you is doing. I don't know how far such stacks go. Sounds scary to be on a lower floor.

I think maybe your building could use a drain cleaning program.

 

[ngmtl]

• Didn't do a glove or balloon test with the toilet but with the pipe open I didn't hear any noise resonate from it after a flush, so it's likely a separate stack. Confines the problem but perhaps a toilet line would have had a wider pipe making a blockage less likely to begin with.
• More importantly, went ahead and executed a colonoscopy on the pipe. Tie in goes in about 6-8 inches then bends to the right, then goes about another 8-12 inches of additional horizontal before hitting some kind of junction. This is probably where it meets the vertical stack riser but I didn't have much luck piloting the scope any deeper.
• Photos below are in the horizontal branch leading up to that junction. Pipe is dry so a little tough to tell which way is up, but I presume the build-up is likely on the bottom here.
• Is this build-up in itself enough to be causing problems of the magnitude we are seeing or would we expect something worse elsewhere in the stack? This build-up is prior to joining any wastewater coming from the apts above or the one adjacent to us.
• I did run a whole bottle of Drano gel yesterday (it did speed up the drainage but didn't solve the positive pressure) and am thinking it would be worth it to run another. Thoughts there?

 

[Reach4]

Is this build-up in itself enough to be causing problems of the magnitude we are seeing or would we expect something worse elsewhere in the stack? This build-up is prior to joining any wastewater coming from the apts above or the one adjacent to us.

I think that what you are seeing would not explain your symptoms. I suspect some blockage below where your scope ends up. What in your pipe could explain getting backup water from the upstairs neighbor?

If your scope is waterproof, it would be interesting to have the neighbor run a lot of water from above.

I did run a whole bottle of Drano gel yesterday (it did speed up the drainage but didn't solve the positive pressure) and am thinking it would be worth it to run another. Thoughts there?

It would be a shame to damage the pipes with the strong chemicals.