Trouble with P-Trap Backdraft

[Dvid]

Master bathroom, two sinks. One has a backdraft of sewage smell daily, the other is fine. It's definitely not mold, but sewage.

-The one that smells is used regularly, several times a day. It's not dried out.

-The sink drains perfectly fine, not slow.

-Smell comes when the water is hot or cold so it's not the hot water heater. This sink is probably closest to the softener and hot water heater though. But again, will emit when cold water is used and after a few seconds the smell is gone so I'm convinced it's residual gas in the p-trap not the water itself. Water from the tap in a glass does not smell.

-There is no leaking underneath the sink and the basement below has an exposed ceiling with no signs of leaks. So I don't believe it is the water level being too low in the p-trap UNLESS there is some sort of vacuum effect in the configuration. (Not sure that's possible, but it's odd this phenomenon is worse sometimes of the year that others.)

-I have cleaned the sink repeatedly including the overflow area, down the drain with a brush and baking soda and vinegar.

-I would guess if it was a venting issue that the other sink a few feet from it should smell as well. Right? Last year when we had this issue I went up on the roof to that vent and put down an electrical wire snake and it did not come into contact with any bird's nest or debris. When the toilet flushed you could hear it clearly. That was the extent of what I could check with that.

Is my next step to remove the drain and p-trap and inspect? (I hate doing that, it smells!)

If there is no visible clog or debris where it connects to the wall, any advice? Every plumber website I go to tells me to do the things I have already done and I'm not sure all plumbers are skilled at looking into venting issues.

Super, mega thanks for expert advice. I'm not a fan of smelling sewage. I'm surprised even if the water level was too low in the p-trap it would smell that bad.

 

[Reach4]

1. How about a photo of the trap area? You may be able to switch the trap from a 2 inch water seal to a 4 inch water seal.

2. Rather than vinegar and backing soda, I would try chlorine bleach. For sanitizing I would add a little vinegar, but I would make sure that is diluted before the bleach is added. I think I would stopper the sink, and let the water rise to the overflow. Then add 2 T of vinegar, stir, and then 2 T of bleach. Stir. Then run the faucet at a dribble, and let the treated water fall down the overflow holes.

Acid plus bleach, without diluting the acid, can generated chlorine gas.

 

[Sponsor]

 

[Dvid]

Thanks. Images per your request.

As far as cleaning.

1. I have a septic system. Will that bleach be an issue?
2. I actually have used a squirt bottle to clean the overflow area behind the sink several times. I can see it come out the holes in the drain.

It doesn't seem like bacteria smell or mold but sewage. Not saying I won't try your suggestion, but I've really cleaned the drain and overflow area multiple times.

 

[Reach4]

That may be a reversible trap. See if you can reverse the U of the trap. If so, then you will need a part to fill the gap. Also, while you have that off, see if that is very smelly. Maybe something is growing in there. Of course, put something under the trap to catch spillage.

The upper white thing is a tailpiece extender. Get a new one that is longer, and cut it to fill the gap. Lavatory tailpieces are often 1.25 inch OD. At some point, this may expand to 1.5 inches by the wall, but not always. In your case, I think that expansion happens where the metal meets the tailpiece extender. There should be a reducing washer to make the transition. If that reducing washer is missing or not the right part, that could explain some smell. But that washer would be on the upstream side of the trap, so it would not be leaking sewer gas-- just local smells.

So if that works out, you will have a trap with a water seal that is twice as deep.

 

[Dvid]

Thank you. Let me see if I understand. You're saying get a longer piece that attaches to the metal drain piece in the front? If so won't that push the p-trap lower and necessitate a longer vertical tube in the back too to connect to the curved piece that goes to the wall? Sorry I'm not a plumber.

Also, I just checked the second sink and now that is smelling as well.

Is it normal for sinks to smell like sewage? Maybe this is a venting issue?

 

[John Gayewski]

Take a very very bright flash light. Turn off the light on the room. Put the very bright flashlight up against the plastic tailpiece. Run the sink. You should see if the traps leave any opportunity for gas to get out or partially siphon below the weir.

I'm thinking it's not the traps but a leak somewhere else.

 

[Reach4]

Thank you. Let me see if I understand. You're saying get a longer piece that attaches to the metal drain piece in the front? If so won't that push the p-trap lower and necessitate a longer vertical tube in the back too to connect to the curved piece that goes to the wall? Sorry I'm not a plumber.

Nor am I. The trap U would sit lower. The piece that the trap connects to, and curves into the wall would not be longer. The U is asymmetric. Reversing the U would put the longer part of the U toward the wall.

I don't know if your trap is reversible. It looks like it, but you would have to try it to be sure.

 

[Dvid]

Take a very very bright flash light. Turn off the light on the room. Put the very bright flashlight up against the plastic tailpiece. Run the sink. You should see if the traps leave any opportunity for gas to get out or partially siphon below the weir.

I'm thinking it's not the traps but a leak somewhere else.

Flashlight. Why didn't I think of that. I can't see through the bottom curve p-trap, but I can see through the top curve piece that goes into the wall. It has gunk in it, but not like a total blockage. I can observe the water level going all the way up to that curved piece, so there seems to be no air cap in the p-trap curve below. I will say though that when I ran these tests I didn't smell sewage so they might not be valid tests. I will try again later in the day. It's hard to see the water level mark until I run the sink, so I might have my wife watch the level as I run the sink and smell.

As far as a leak, wouldn't I have water under the sink if there was enough of a leak to make the p-trap water level not high enough?

I looked through the cabinet hole at the connection to the wall pipe and there is no indcation of a leak there, nor downstairs in the exposed ceiling.

I will report back on testing with a flashlight again later. Thanks.

 

[Dvid]

Nor am I. The trap U would sit lower. The piece that the trap connects to, and curves into the wall would not be longer. The U is asymmetric. Reversing the U would put the longer part of the U toward the wall.

I don't know if your trap is reversible. It looks like it, but you would have to try it to be sure.

Oh, I missed the part about you suggesting I reverse the U shape at the bottom. Hmmm. Would that cause any pressure or vacuum issues? Thanks.

 

[wwhitney]

Is my next step to remove the drain and p-trap and inspect? (I hate doing that, it smells!)

If you remove the p-trap (all the white parts in your picture), then the only ongoing smell should be from the remaining black pipe at the wall. And if you securely cap that opening and leave the sink disconnected for a couple days, there should be no smells.

So if your p-trap itself smells, removing and cleaning it would be advised. And if you want to rule out any leak downstream of the p-trap, you could do the test of capping the wall opening while leaving the sink trap off. If the smell is somehow associated with running water in the sink, you could even stick a bucket under the tailpiece while the trap is off, so that you can try running the water.

I imagine it is possible to have a connection that is water tight in normal usage, but not gas tight. Any such connection downstream of your trap would be consistent with the info provided.

Cheers, Wayne

 

[John Gayewski]

Flashlight. Why didn't I think of that. I can't see through the bottom curve p-trap, but I can see through the top curve piece that goes into the wall. It has gunk in it, but not like a total blockage. I can observe the water level going all the way up to that curved piece, so there seems to be no air cap in the p-trap curve below. I will say though that when I ran these tests I didn't smell sewage so they might not be valid tests. I will try again later in the day. It's hard to see the water level mark until I run the sink, so I might have my wife watch the level as I run the sink and smell.

As far as a leak, wouldn't I have water under the sink if there was enough of a leak to make the p-trap water level not high enough?

I looked through the cabinet hole at the connection to the wall pipe and there is no indcation of a leak there, nor downstairs in the exposed ceiling.

I will report back on testing with a flashlight again later. Thanks.

Try using other fixtures while observing the p trap with a flashlight. Maybe flushing the toilet is causing the ptrap to loose prime slightly.

I'm still betting a loose connection in the piping somewhere not under the sink.

My bathroom is improperly piped and I haven't had the will to change it yet. When I flush a nice sized bolus it siphons my tub trap. I hear it every time it happens and refill it. My gf isn't as perceptive to these things and doesn't notice. It bathroom stinks to high heaven every once and a while.

 

[Reach4]

Oh, I missed the part about you suggesting I reverse the U shape at the bottom. Hmmm. Would that cause any pressure or vacuum issues? Thanks.

I don't think so. Reversing traps are made that way with that in mind.

The venting should take care of any pressure or vacuum issues.

If the p-trap does not have enough water, the symptom could be that smells would make their way thru to the bathroom.

 

[Robert Gift]

Problem resolved?
What was the cause?
Did water rush out so fast that not enough remained in the trap?
Or water running down the line created a vacuum pulling some water out of the trap so no longer a water seal?
Thank you

 

[Tuttles Revenge]

My money is on the overflow area being fouled.

 

[Dvid]

Forgot I had this thread. Update here: https://terrylove.com/forums/index.php?threads/sink-flange-smell.106841/