What is causing a smell from under my bathroom sink?

Question

(I'll preface this by saying I'm not an experienced DIYer, especially with plumbing, but this seemed to be the best place to post this question. Hoping someone can help!)

I've had a sporadic smell, like something rotten, from under my bathroom sink for several years. I've cleaned out the p-trap several times, and it hasn't made a difference. I have to use baking soda & vinegar and/or boiling water to clean it out (after that, there's usually still a little bit of an odor if you stick your head under the sink, but it doesn't permeate the whole room).

I had a plumber take a look, and his best guess was that I've got a crack in the septic vent pipe behind the toilet, and the smell wafts over and through the hole around the pipes under the sink. He suggested, short of taking down the entire wall and replacing the pipe, that I stuff the hole with cloth to block the smell (which I did, but this hasn't been effective).

The weirdest part of this is the trigger. When only a little water is run into the sink, the smell starts within an hour. If we flush the sink out every time we use it, the smell generally doesn't build up. This is why I'm not convinced that the septic vent pipe is the issue - if it were a crack, it would presumably be a problem all the time.

A few notes about setup: this bathroom is upstairs, with a second bathroom directly below it (which does not have a smell issue). Second bathroom is used regularly, except for the shower. We have a septic tank, which has been emptied during our time here and which did not affect the smell. Town water, so it's not something in our well. Running the bathroom exhaust fan increases the smell, but that makes sense, since it pulls the air up through the room. Opening the window is very effective, but not so practical year-round.

So my main question is this: why does a little bit of water going down the drain cause a funky smell, but lots of water does not? How do we resolve this? Does this sound like the sink is the problem, or the septic vent, or something else entirely?

Answer 1

Your line of reasoning is good, I'd say. I'd want to know several things...

• Is the odor coming through the back of the cabinet or out the sink drain? A fresh nose should be able to tell.
• What's the level of the water in the trap after testing with "a little" and a lot of water? Does the trap get siphoned out in either case? A flashlight in the drain should tell you.
• Is the vent open to the atmosphere, or is there a blockage such as from an animal nest? A look and listen from the roof might tell you.

Once you know that you can narrow the possible causes further. It's not rocket science. Just keep investigating and eliminating possibilities.

Answer 2

Consider that the odor may not be coming from under the sink but from the sink's overflow hole. Funky stuff can build up in there, even if your sink is never filled to the level of the overflow.

Pour 1/4 cup of bleach into the overflow hole and another 1/4 cup into the drain. Leave if for 15 minutes, then flush with water.

There's one bathroom sink in my house where I have to do this about once a month or the bathroom smells funky when left with the door closed. The bleach eliminates the problem.

It can't hurt for you to try this remedy and you might be pleasantly surprised, so it's worth a shot.

Answer 3

The weirdest part of this is the trigger. When only a little water is run into the sink, the smell starts within an hour. If we flush the sink out every time we use it, the smell generally doesn't build up.

That's actually typical of a clogged vent, or improperly vented drain, siphoning the trap dry (need not actually be dry, just low enough to defeat the trap seal, but dry is the usual term.) Running the water more the trap gets refilled.

Answer 4

The drain in my sink wasn't letting the the overflow water drain out completely.

I filed the hole a little deeper and problem solved! 5 minutes!

See my YouTube video with my solution:

https://youtube.com/watch?v=LVX5zl2NDA4&si=TyxcXrp47qfcRmqK