Kitchen sink drain siphoning?

[Rowan809]

Hi all,

We’ve been having an issue with an intermittent mild sewer gas odor at our kitchen sink. The house was built in 2018, and we did not experience the issue until later in 2020. The issue seems to worsen in colder weather (below 50 degrees) and improve in warmer weather. I just recently realized that it seems to occur after running either the washing machine or the dishwasher, and resolves after running water in the sink. For this reason, I’m thinking the trap is being partially siphoned when either of these two appliances drain, but can’t figure out why.

Dishwasher is tied in above the p-trap for the sink, and washing machine is on the second floor but has its own trap and is vented. All other fixtures (including the washing machine) vent through a roof vent. The kitchen sink is on the first floor and is the only fixture in the house that is vented with an AAV. The kitchen drain is the last branch to join the main line before the main exits the house to the septic, which has a whole-house trap in the basement.

We recently changed the AAV from a Studor Redi Vent to a Studor Mini Vent, thinking that the original vent may have failed, or was not appropriately sized to handle a double sink and dishwasher. The smell resolved temporarily after changing the vent, but gradually returned (again, intermittently) after a few days.

The plumbing setup beneath the sink appears to be OK to me, but my knowledge is pretty limited. (See profile picture for reference.) We do not experience the smell at any other fixture, and don’t have any issues with slow drains. The only other “symptoms” I can think of are that the dishwasher is very loud when it drains, and one of the toilets upstairs (not often used) tends to lose water over a few days (almost as if it were being siphoned slowly as well).

Any ideas as to what might be causing the issue?

Thanks in advance for any help!

 

[Reach4]

Does the smell occur more in a sink bowl or under the cabinet? If siphoning the trap, that smell should be more in the sink bowl than under the sink.

Your picture is pretty small. I was wondering if you have a crown vent. Look that one up.

I am also wondering if you could put in a transparent p-trap temporarily to make it easy to see if your trap is being siphoned out.
https://www.homedepot.com/s/transparent%20p-trap?NCNI-5

Even with the white polypropylene traps, as you have, you may be able to see the water level by shining a light through.

 

[Sponsor]

 

[Rowan809]

Thanks for the reply! The smell is originating from the sink bowls, and is initially worse when running the water. (After running the water for a bit, the smell eventually dissipates). The space under the sink in the cabinet does not smell.

Regarding the crown vent, I think the space between the trap and vent is sufficient, but the perspective in the original photo makes it appear that it’s very short. I’ve included a couple better photos below.

That’s a good idea about the transparent trap. I’d like to give that a try to confirm that it’s being siphoned. Assuming it is being siphoned, do you think it’s worth reconfiguring the plumbing beneath the sink to try a different setup? Maybe cap that vertical stack and move the AAV to a horizontal branch after the p-trap? Also not sure if having the dishwasher discharge into the horizontal sink drain would be better than having it so close to the top of the trap?

 

[Reach4]

That’s a good idea about the transparent trap. I’d like to give that a try to confirm that it’s being siphoned. Assuming it is being siphoned, do you think it’s worth reconfiguring the plumbing beneath the sink to try a different setup?

Not sure.

Maybe cap that vertical stack and move the AAV to a horizontal branch after the p-trap?

The path to the AAV should not be horizontal after the santitary tee unless above 6 inches above the flood level of the sink.

Also not sure if having the dishwasher discharge into the horizontal sink drain would be better than having it so close to the top of the trap?

I was wondering that too.