Science (Plumbing) Problem

[Mark1]

I'm getting ready to perform a remodel of a counter top in a ½ bathroom, but before I do any work I would like to understand the science and solve a slow draining sink issue in this bathroom. I was hoping someone on the forum would have an idea of what is occurring with my sink. Any suggestions (or experiments to conduct) would be greatly appreciated.

The issue doesn't seem to be related to restricted piping, but instead seems to be related to some type of pressure restriction. For example, if the sink basis contains only a small amount of water (say <1”) then it usually takes an extended period of time to drain (i.e. minutes), However, if the basis is allowed to reach a higher level (say 3 inches) then the basin seems to overcome some pressure restriction and always drained very quickly (few seconds). The bathroom designed is a rustic outhouse, so the sink basin is essentially a galvanized bucket as shown in the attached picture. I'm willing to wreck out the wall to get at the plumbing to solve this problem, but I would like to first understand what needs fixing.

I have talked to my normal plumber concerning this issue during a visit to solve a different issue and his response was that “the sink doesn't have a overflow hole so there is no where for the air to exist such that the water can drain”. This answer was less than satisfying, since my master bathroom sinks don't have overflow drain holes and they drain just fine.

I tried installing an aerator under the sink and even added a flexible piping segment such that I could vary the height (see photo). It may have helped the drainage slightly, but the difference was negligible.

I also tried installing a bundle of stainless steel straws of varying lengths within the drain tailpiece piping hoping it would allow an exist for the air if that was indeed the problem. This too was unsuccessful in addressing the issue.

The house is only 7 years old in an area with lots of county building inspector involvement, so I would like to think the house was built right (i.e. to code), but who knows . I bought the house when it was ~ 2 years old so I don't have any personal construction history.

I guess I can start to bring out different plumber to look at the sink, but I really would like to understand the science involved before I start wrecking out walls, etc. Thanks for any thoughts you might have

 

[PaunchyPirate]

I know nothing about this. However, out of curiosity, I googled around. In all the examples pictures that I see, the air vent (you call it an aerator) is positioned so that it is higher than the bottom of the sink. Could that be it? Obviously, it appears that your setup would not permit this using your current configuration.

As for the master bath working fine, is it possible it is tied into the toilet air vent?

 

[mountainsoft]

Slow drains are most often caused by clogs, such as hair in the sink drain or P-trap, or potentially further down the drain somewhere.

Assuming you pulled everything apart and verified the drain is clear (did you run a snake down the line?), the next obvious cause would be blocked or non existent venting. Without a proper vent there won't be air coming in to let water go down the drain. Think of it like pouring liquid out of a can. If you only make one hole it goes glug glug when you pour it. But if you pop a second hole on top for a vent, the liquid will pour out freely.

You mentioned the house was only 7 years old, so I would think a proper vent would have been installed inside the wall. Especially if the installation was inspected. You might look on the roof to see if a vent comes out of the roof there, and if so check to see that it's open (no leaves, bird nests, or other debris blocking it).

It looks like you added an air admittance valve under the sink. These can work as alternative to a standard vent if necessary, but a real vent is still better. In your situation my guess is a low level of water doesn't create enough suction to overcome the spring or whatever inside the air admittance valve. So essentially no venting when draining small amounts of water. But fill up the sink and let it drain and the pressure would be enough to overcome the mechanism in the valve. Don't know for sure, just my guess.

I would check for a roof vent and make sure it's clear. If you have a roof vent, I'm not sure if you can also have the AAV in the drain also. I've only seen one or the other, never both.

Good luck!

 

[Ronstar]

2 possibilities- clogged drain or clogged vent.

 

[Jerry1]

I know nothing about this. However, out of curiosity, I googled around. In all the examples pictures that I see, the air vent (you call it an aerator) is positioned so that it is higher than the bottom of the sink. Could that be it? Obviously, it appears that your setup would not permit this using your current configuration.

As for the master bath working fine, is it possible it is tied into the toilet air vent?

Interesting. I have almost the same set up and exactly the same problem. However, I looked at my air vent and it is above the bottom of the sink.

I feel confident it’s an air pressure problem because if I make a whirlpool by swirling the water around, the water will drain fast. Generally, I’ve given up because my sink is a utility sink and it’s big enough that it would not overflow from even a couple loads of laundry and basically, it’s working. But you’re right, a small amount of water doesn’t drain as well as a larger amount.

My guess is that it’s the air vent. There must be some type of valve in there to prevent sewer gasses from entering the house and it must take a certain amount of pressure to open that valve. You could test this by removing the vent and see if that changes anything.

 

[Winemaker]

To troubleshoot your issue, remove your under the sink vent, place a bucket under the vent; then try to run your drain/sink with whatever water level you want. If the drain is plugged after your vent, you will get some overflow out of your under sink vent (and into your bucket.) If your drain runs fine, your vent is the issue. If your sink backs up and there's no overflow out the vent (it's lower than the sink drain) then your sink drain has an obstruction.

 

[Exchme]

There's a spring in the air admittance valve that opens on vacuum, closes on pressure. So it needs to be routed to above the water level in the sink in order to work, it's doing nothing for you right now.

Sounds like the roof vent isn't attached or is clogged. When you get enough water in the sink, it finds an alternate way to vent (may push air in the reverse direction up some other P-trap and out some other drain). The water then drains away quickly due to the same reason a toilet flushes - once the water is moving, the energy balance adds the kinetic energy of the water's motion to the static head of the liquid level and that keeps the water moving.

 

[Fermion]

What the heck is that? You can't vent a trap below the level of the sink!

 

[ERD50]

....

I tried installing an aerator under the sink and even added a flexible piping segment such that I could vary the height (see photo). It may have helped the drainage slightly, but the difference was negligible. ....

The house is only 7 years old in an area with lots of county building inspector involvement, so I would like to think the house was built right (i.e. to code), but who knows . I bought the house when it was ~ 2 years old so I don't have any personal construction history. ...

Is this bathroom a remodel? AFAIK, code would require a proper vent stack. Some codes might allow an air admittance valve (the proper term) for a remodel.

But you say you added the air admittance valve? Sounds like there is a proper vent stack, but maybe it is clogged?

What the heck is that? You can't vent a trap below the level of the sink!

It appears you can, why do you say you can't? This site shows the air admittance valve installed at about the same level as the OP shows. I don't think that's the problem. The air admittance valve solves the problem of a vacuum from the water flow of a different drain sucking the water out of the trap. Vent or no vent, the drainage should be the same.

https://garynsmith.net/venting-101-air-admittance-valve-pros-cons/

-ERD50

 

[Fermion]

It appears you can, why do you say you can't? This site shows the air admittance valve installed at about the same level as the OP shows. I don't think that's the problem. The air admittance valve solves the problem of a vacuum from the water flow of a different drain sucking the water out of the trap. Vent or no vent, the drainage should be the same.

https://garynsmith.net/venting-101-air-admittance-valve-pros-cons/

-ERD50

Yeah my wife corrected me, she knows all the code lol. She did say it is a pretty crappy way of venting but is allowed in some jurisdictions. The problem is, it is quite easy for a clog to travel up into the air admittance valve and jam it.

 

[Winemaker]

What the heck is that? You can't vent a trap below the level of the sink!

Since all new S traps have been outlawed by code, the above picture is the correct way by code to correct a pre existing one.

 

[H2ODude]

Eh, for having spent my career in water and sewer utilities the mysteries of plumbing venting always eluded me; I just had someone familiar with codes draw what I needed to do.

I recently completely rebuilt an addition and bath. Mistakenly duplicated what was there, including using the vent stack to roof I put in 15 years ago. Inspector comes out and informs me that nope, shower drain no longer 1-1/2, now 2" required. Sink drains larger too. Also told me best thing to do was completely lose the roof stack (WHAT?) and just put an air vent on both sinks in the vanity, exactly like yours is below the sink. Approved by inspector, works just fine. My guess is OP's problem is a restriction of some kind in the drain, the fact that greater hydrostatic pressure (sink full) speeds it up seems to make that probable. I'd think if it was a venting issue you'd hear burping and gurgling from drain system.

 

[RobbieB]

There should be a vent on the roof directly over the sink drain.

 

[finnski1]

There should be a vent on the roof directly over the sink drain.

Well there should be a vent thru the roof somewhere but not necessarily directly over the sink drain.
My guess is that exists and there is a clog in the sink drain or main drain or a clogged or partially clogged vent somewhere above the sink. Should get up on the roof if possible and make sure vent is free and snake the drains to be sure they are clear as well.
Also I would definitely not have his "normal" plumber back again. The answer he received about the sink needing an overflow hole is comical. Many bathroom sinks have no overflow and I know of no kitchen sink with an overflow hole.
Once he gets his vent/drain issue resolved I would remove that AAV as it is not necessary and in fact was not there originally which points to a clogged drain.vent issue.

 

[NW-Bound]

I believe the problem is caused by the strainer at the bottom of the bucket used as the sink.

I know, because I have a similar strainer for my bathtub to catch long hair to keep it from clogging the drain pipe. When taking a shower, I often have an accumulation of water that takes a long time to drain. If I lift up the strainer temporarily to allow the water to flow, then immediately put it back down, the increased water flow will continue.

I think the problem is caused by the small holes in the strainer not allowing air to escape upwards, to let the water to flow downward. A strainer with larger holes may not have the same problem.

The original bathtub stopper broke, and I put a strainer in place of it. The stopper did not cause any flow restriction, as it has a circular clearance gap and not tiny holes. However, the stopper did not catch any hair.

 

[H2ODude]

Codes vary by state and they change over time. My 2021 bath rebuild failed first rough inspections because I did it the "old way." I worked for the city and know the inspectors are all licensed, tested, and the one I had was very helpful and knowledgeable. The bath is about 60 ft from the existing house plumbing, has a toilet, shower, and two sinks. I had re-plumbed it to stack I put in 16 years ago. The inspector, who does nothing but plumbing, said it was best to abandon the stack (I've since had a new roof and welcome the loss of the roof penetration) and put two air inlets on the trap lines from the two sinks, just like OP. They're about 6' downstream of the toilet. Works great, passed inspection no problem. Nearest roof penetration 60' away. FWIW it's NC plumbing code. Frankly, it seemed strange to have a toilet that far away from a roof vent, but it works just fine.

FWIW the plumbing was a bitch because the crawl space is 18-24". The original run I did years ago did not have the fall for 1/4" per ft, so I was allowed to do it 1/8" to the ft, and holding to that over 60 ft was a challenge. Never say never, but I'll never do plumbing under this house again.

 

[6miths]

I believe the problem is caused by the strainer at the bottom of the bucket used as the sink.

I know, because I have a similar strainer for my bathtub to catch long hair to keep it from clogging the drain pipe. When taking a shower, I often have an accumulation of water that takes a long time to drain. If I lift up the strainer temporarily to allow the water to flow, then immediately put it back down, the increased water flow will continue.

I think the problem is caused by the small holes in the strainer not allowing air to escape upwards, to let the water to flow downward. A strainer with larger holes may not have the same problem.

The original bathtub stopper broke, and I put a strainer in place of it. The stopper did not cause any flow restriction, as it has a circular clearance gap and not tiny holes. However, the stopper did not catch any hair.

This is what I was thinking as well. Not just because air cannot vent up through the small holes (although this likely contributes) but because the surface tension of water needs to be overcome at each one of the small holes in order for water to start to drain. When the sink is fuller the water pressure above can break the tension and start the flow. When only a small amount of water in the sink the surface tension may be high enough that the water just 'drips' through the holes rather than flows. There might also be a build up of soap, grease, precipitate on the underside of the drain that further restricts flow.

 

[calmloki]

Your under-sink vent looks very fresh and new, so this is unlikely, but I have had the rubber flapper in an under-sink vent stick closed from grease/soap/volatile accumulation. The added weight of deeper sink water seems like it is opening the vent better. Also had a pigeon decide to die and slide down a vent stack, which fowled things up, but again, real uncommon.

 

[wmc1000]

Maybe it's just the angle of the photo but there doesn't appear to be any drop from the trap to the wall. Might even run slightly uphill which would lead to more back pressure from standing water to overcome

 

[NgineER]

I think the problem lies in the geometry of the vessel. Flat bottom vessels don’t drain well and the drain velocity depends on the water height in the sink. A round bottom or cone bottom sink are popular choices in industry if you are working with hard to drain solutions, suspensions or solids.

I would expect a house built within the last 7 years would have proper venting and the air admittance valve would help even if it was not properly vented.

 

[Out-to-Lunch]

Also had a pigeon decide to die and slide down a vent stack, which fowled things up, but again, real uncommon.

I see what you did there!

 

[Out-to-Lunch]

Maybe it's just the angle of the photo but there doesn't appear to be any drop from the trap to the wall. Might even run slightly uphill which would lead to more back pressure from standing water to overcome

I didn't notice that, but I have to agree!

 

[NW-Bound]

This is what I was thinking as well. Not just because air cannot vent up through the small holes (although this likely contributes) but because the surface tension of water needs to be overcome at each one of the small holes in order for water to start to drain. When the sink is fuller the water pressure above can break the tension and start the flow. When only a small amount of water in the sink the surface tension may be high enough that the water just 'drips' through the holes rather than flows. There might also be a build up of soap, grease, precipitate on the underside of the drain that further restricts flow.

Yes. I did not think of the word "surface tension" when making my post.

If the OP can remove the strainer like I can do mine, he will see the problem going away "tout de suite".


PS. It is really the air not being able to escape that slows down the water drainage.

Here's one easy way to prove it. Find a small plastic tube, something smaller than a drinking straw that you can push through one of the small holes in the strainer. Pour some water and see how fast the water goes down, if some air can escape upwards via this small tube.


PPS. It's not the strainer itself that causes the slow drain. It's also the presence of the J trap. Further venting downstream of the J trap will not help. It can all be explained simply, if I can make some simple drawings to post here.

What happens is that there is a column of air trapped between the strainer and the J trap. To push this air down past the J trap requires a higher head of water above the strainer. The surface tension of the water is what keeps the air from bubbling up through the small holes of the strainer. And it is this air that prevents the water from draining.

 

[RobbieB]

I agree. Pull the strainer, plug the hole with a rag, fill the sink and pull the rag.

 

[NW-Bound]

Interesting. I have almost the same set up and exactly the same problem. However, I looked at my air vent and it is above the bottom of the sink.

I feel confident it’s an air pressure problem because if I make a whirlpool by swirling the water around, the water will drain fast. Generally, I’ve given up because my sink is a utility sink and it’s big enough that it would not overflow from even a couple loads of laundry and basically, it’s working. But you’re right, a small amount of water doesn’t drain as well as a larger amount...

The reason swirling the water helps it drain is because when the water does not cover all the holes in the strainer, some air can escape up to exchange place with the water coming down.

I agree. Pull the strainer, plug the hole with a rag, fill the sink and pull the rag.

Remove the strainer, and use a rubber stopper.