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It smells like really musty water, but the water itself does not smell or taste bad at all. Only happens when I run the water, so it doesn't make the room smell, just when you fill something up or wash your hands.

I tried drain-o and it didn't do anything and there's GREAT flow so it's not clogged at all anyway.

Only thing that has worked (temporarily) is pouring pretty much everything I could think of into it. Lemon juice, draino, bleach, etc.

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Is there a proper trap under your sink? –  Steven Mar 8 '12 at 20:59
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NEVER pour bleach and Drano down the sink at the same time. The two will mix to form poisonous chloramine gases, in a similar way as bleach and ammonia will. –  KeithS Mar 9 '12 at 15:50
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It's the overflow drain. I cleaned that with various things and it fixed the problem. Now the problem is that it's nearly impossible to get it really clean (at least as far as I know) and the smell comes back in about a week. –  Matt Aug 22 '12 at 15:50
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5 Answers

I'll echo the comment and say that you should make sure you have a proper P-trap installed under the sink. This trap holds water and provides a seal against sewer gases getting up into the bathroom. Without one, gases will leak in constantly, and will be displaced by water down the drain which can force the gases up into the bathroom even if normally it's not detectable.

I'll also ask if there is a proper vent for this sink. Even with a P-trap, if there's nowhere for back-flowing gases to go, they'll bubble up past the trap.

One more thing it could be is the overflow drain. Depending on the design of sink, the overflow can hold a small amount of water at the bottom where it tees in to the main drain, which can become stagnant. Run water down the drain and you'll force some air up the overflow (to make way for the water coming down), which will have that stagnant smell. To diagnose this, plug the sink and begin filling it; you shouldn't get any musty smell at first because there's no air movement. Once the water level hits the overflow drain, you will start smelling the musty smell for a while because the water is displacing the gas, which wants to rise above the water and so will move up into the bathroom. If this is the problem, you can ameliorate it with some foaming pipe snake; pour it down the overflow drain and it will clean out any caked-on gunk which contributes to the smell, and which may be trapping the water. The real fix is to make sure there's no "damming" effect of construction defects at the bottom of the overflow drain (a lip of porcelain, issues where the overflow meets the metal drain downpipe, etc).

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+1 for the overflow drain. I thought I was going crazy when I had this problem because I knew my trap was fine (I took it apart). –  uncle brad Mar 9 '12 at 22:23
    
I'm leaning towards the idea of it being a supply problem and not a drainage problem. Although @KeithS answer is also feasible with the possibility of displacement. –  ShoeMaker Apr 30 '12 at 16:54
    
It's the overflow. Anything other than bleach that would clean this out? –  Matt May 3 '12 at 15:30
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Foaming pipe snake is designed to clear out the odor-causing bio-slime that accumulates, and it will usually do a better job because it foams up to contact all surfaces of the line. Other than that, see if there's something that can be done to make sure there's no standing water in the overflow. It may not be possible or feasible, but it's certainly worth a look. –  KeithS May 3 '12 at 15:34
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Does it smell when you turn on the tap and catch the water in a bowl (so it doesn't go down the drain)?

If so, it's something in the faucet. Take off the aerator cap and look for gunk inside, and/or look in the barrel of the faucet if you can to clean it out. You can also consider replacing the faucet. (I am assuming that since this only affects the bathroom, it is not a water quality issue or bad house pipes)

If not, it's something in the basin, drain, trap, or overflow drain. KeithS has some good answers, and I also recommend the following:

  • Take out the drain stopper and look down the drain pipe for gunk. Clean it out with a snake or unraveled coat hanger if you can.
  • Spray bleach down the drain and overflow drain to kill anything nasty in there. You can bleach the basin too for cleanliness.
  • Get a large bowl or 1-5 gallon bucket. Fill it with water. Quickly but carefully pour all the water down the drain. The goal is to fill the basin and displace all/any water in the overflow drain and trap. Sometimes there can be nasty things in there that float so they don't go through the trap, and this forces them out the drain pipe.
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It's the overflow. Anything other than bleach that would clean this out? –  Matt May 3 '12 at 15:30
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I like bleach because it disinfects and kills all sorts of nasties. But you can use any spray cleaner that's safe for pipes, or even try just using forced water. –  The Other Steven May 3 '12 at 15:43
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From what I can ascertain, it's when the water is running. It's not that common, but we have had water that smelled like rotten eggs. The cause was sulfur in the water.

If you're on municipal water, check with your utility. If you're on a well, have the water tested by a lab. They'll tell you what the story is.

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We tried all that stuff, including flushing out the vent pipes with a hose to check for blockages; nothing worked. I decided to crawl under the house to see if there was a sewer leak - there was a dead cat under the house! If all else fails, check your crawlspace. On the plus side, we now have what are probably the cleanest pipes and septic tank in town. :-)

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Instead of using expensive garbage disposal deodorizers I just mix one pack of Lemonade flavor Kool-Aid (10 cents a pack) with a small amount of water. Then pour that into the sink and let it sit for a while. I also use a long handled brush to clean the bottom of the rubber guard in the sink because junk gets under there.

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I'd worry that a sugary drink mix in an overflow drain would result in mold growth. It may smell better, but the result might be much worse. –  BMitch Oct 19 '12 at 23:35
    
Note that the question was about a bathroom sink, not a kitchen sink. –  Niall C. Oct 19 '12 at 23:38
    
@BMitch kool-aid is sugar free by default. –  wax eagle Oct 21 '12 at 2:12
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