Hey! Great site and I have found it quite useful. Thanks to everyone that provides their knowledge. I don't have any venting in my old house so I have added AAVs to my kitchen and island sinks ( past the traps and pretty much counter height ) However, when I pull the plug in my full kitchen sink, the water in my island sink trap gets sucked out. My drains are all 1.5". Do I need 2 AAVs in the island sink system or perhaps go with a 2" AAV? The island sink drain connects to the kitchen sink drain under the floor ( in the crawlspace ) about 5 feet from the island sink and this line continues for about 15 feet a few degrees off horizontal.

All the water drained and left your trap dry is probably because you have an S-trap. The AAV prevents the trap from being siphoned when another fixture's flow drains past it. Post a picture of your current drain setup so the pros can advise.



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remove the AAV under the kitchen sink and then drain the island sink. what happens when that pipe is open?

sorry...what do you mean by remove it? cap it off so no air enters? or remove the AAV assembly so the vent is wide open? it will get stinky if I remove it.



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Get rid of the s traps.

is there a vent anywhere on the septic tank/ leeching field?

is there a vent anywhere on the septic tank/ leeching field?
yes



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Got it. Thanks. Get rid of the s traps

OK, I will try to do that. What is the science behind that? Cheers!



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As soon as the pipe starts turning down as it exits the trap, it's pulling water out of it and the vent needs to be at that point there to break that siphon.

An S-trap siphons the entire column of water as it is expelled from the drain when you have full flow. Full flow is when the entire pipe is filled with water like when you flush the toilet, drain a filled bath tub or kitchen sink. It doesn't happen when you have partial flow because of the presence of air in the pipe. When you turn on a faucet to wash your hands it will not siphon the trap dry.

Hi
Thanks for the suggestions

I have replaced the s traps with the p configuration as recommended. Both sinks drain nicely when using water with no issues. However, when I have a full kitchen sink and pull the plug air is forced UP through the trap in the island sink. I assume that because there is so much water that it creates enough pressure to force some air back up through the island trap.
I am thinking that I could either create a longer trap in the island that would hold more water making it harder to push back, or I could install a check valve on the island sink drain that would stop anything from being forced back up into the trap.
I understand that these are likely not the proper solution but as this is an old house I have run out of options to vent anywhere or change plumbing in the crawl space that goes out to the septic but in your opinion which might be the best solution?






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When the kitchen sink drains in "full flow", it creates a positive pressure in front of the water column being drained, and at the same time negative pressure in the back of the water column.

The positive pressure pushes forward and upward to wherever there is space. It goes up to the AAV and the AAV doing it's job not letting air out, then it pushed into the p-trap and bubbles up into your island sink.

The way to resolve it I can think of two ways. One is to replace the AAV with an atmospheric vent which resolves positive and negative pressures, but you can't do it because it's an island. The second method is to not allow the drain to be in full flow once it reaches the horizontal section, typically this is done by upsizing that pipe to say 3". What size is the horizontal section now? 1-1/2" or 2"?

Another possibility to consider, any chance you have a partial blockage or a belly downstream? If so this can act as a dam and slow the flow, causing the air ahead of the wastewater column to seek an easier path like up the island piping.

IMHO I don't think making a steeper P-trap will do anything but making your island drain to not function well, and adding a check valve that anytime you fill your kitchen sink drain up you have to close that valve, and if you forget your island drain will back up and all it takes is for a watermelon seed to lodge in your check valve to prevent it to close 100% and you still have that bubble coming up anyway.

Just out of curiosity, if your plug your island sink with a stopper is it strong enough to push it out? If not that could be the "check valve".



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Thanks "SUM"

The stopper in the island sink isn't quite strong enough to hold back the bubbles. I could look for a different style drain fixture that would be stronger. That is a good suggestion thank you!

What I envisioned with a check valve would be a one way valve that prevents backflow. Like they make for pumps.
I was thinking that if I installed it between two slip joints it could be removed and cleaned if a watermelon seed ever found its way into it. I do have a very fine screen on my island sink drain which I hope would help.

You could be correct about an issue downstream, but I would have no idea where to start cutting and it is a long distance.

I like the idea of the larger section of pipe, but if it were on the horizontal it could trap debris and there is very little room to put it on the vertical.

But before I start cutting into my lines, I am going to try something I just thought of, putting something in the sink drain so that it drains much slower and see if that slows the flow enough to stop pushing air back up my drain.

Thanks for your suggestions and insight. I appreciate it.
Cheers from Canada



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do a search for "floor drain backup preventer".

they are devices that you insert into a floor drain that makes them one way.

there might just be something you can fit in your sink drain ???

thanks will do



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doctorgrafix (Canada) wrote:

> "I don't have any venting in my old house so I have added AAVs to my kitchen and island sinks"

That seems so odd. Are you sure there is not a vent poking through the roof or siding anywhere, or into the attic space, or even inside a wall near an upstairs bathroom? Without any venting, there would likely have been consistent drainage issues throughout the decades.



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Small house, one floor, one 1 1/2 drain that collects all the drains as it goes right out into the septic system, separately from the toilet drain. The AAVs on the sinks in the kitchen do the trick. Thanks

SUM!
Got the check valve at Home Depot and it works like a charm.
It is easily removable too - in case I get one of those pesky watermelon seeds

Cheers and thanks again for your input.





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The check valve stopped the island sink from gurgling, but I'll bet the main sink drains a lot slower now....you fixed the gurgling symptom but not the main cause of the air backpressure. There has to be (or should be) an atmospheric vent further down the line to allow pressure out as water comes down, and also serve as a vent for gasses from the sewer or septic system. I had a similar problem with a 100-yr old house where the previous owner installed a second whole-house trap in a branch of the main drain without adding a vent at that location. Air couldn't push past the trap as water came down from upstream bath fixtures.

The AAV is only meant to protect the trap of a sink or fixture from siphoning, it's not a substitute for zero atmospheric vents in the system.



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