Every once in a while our bathroom sink will start filling up with water if you let the water run while brushing your teeth. It is really slow to drain. I used Drano, Liquid Plumber, I tried a plunger, I cleaned the "u" piece and probably tried some other things to no avail. Then a day or two later it would work fine. Ever since I tried all that stuff the drain slows down every once in a while and if I don't do anything in a day or so it will work just fine. What could cause it to back up and then a day later work fine without doing anything?

It sure sounds like an obstruction somewhere in the line- I would focus there first. Check venting also to make sure you're not creating a vacuum, preventing air exchange in your piping when the water is flowing. You could add an air admittance valve to that sink but if it's worked fine in the past an obstruction is more likely. Good Luck

Check the pop-up stopper, it may be clogged up with hair. I have to remove ours about twice a year and clean the hair out of it.

I would put an Air Admittence Valve, its an easy install

It was used too much when it was new.

How old is the plumbing, does the sink drain have a "S" shaped trap. If it is I would change it to a "P" shaped trap and see if that helps

Agree. Sounds like a venting issue.

Another thing not mentioned is the over flow hole at the top of the sink . Pour some drain opener in it also . Wait 30 minutes and close drain fill sink up and let water go out the overflow. Had to redo a sink in a house once the plumber used caulking to insert the drain pipe and put too much around it . After you do that , clean drain plug, it should be ok . But it sounds like there is a clog down line somewhere. If it's not used it lets everything drain out slowly. Then after it backs up during use is when you notice it. Don’t think it’s vent if toilet and tub drain ok . I’m sure they all use same vent .

All a plumber needs to know: Crap flows down hill and payday is Thursday.

My first thought when I read your post.

And don't chew your fingernails.

Not pop up stopper, that was the first thing I checked. If it is a clog, why would it be okay for a couple weeks and then become slow for a day and then work okay again?

It's also possible that your vent above the roof is frosting closed or nearly closed. Has your weather been cold enough for that ?

The weather was extremely cold for our area a week or so ago, but when this happened it was not very cold.

Well it might be something to check if it isn't too difficult to get up there and look.


You might also have a broken pipe going to the septic or the city sewer.....whichever you have. Do you have snow cover ? Do you drive across the area where that sewer line runs ??

White17, we have a septic system, no snow cover, no driving over it as it is in the back yard. We have two bathrooms. No problems with the other bathroom and no problems with the kitchen sink.

OK I'd still check that vent on the roof ,,,,,,IF the other systems are on a different vent run. If they are all on one vent line (unlikely) then that's not the problem.

Can you take the trap apart under that bathroom sink or is it all glued together? Usually they are threaded on one side.

If it’s down the line it could drain when not in use Then after it runs for awhile it starts backing up .

If there’s a section of the vent that is running horizontal over to the main vent going through the roof that’s not pitched away from the main vent. It could fill with water from rain and cause the problem. Have you noticed it happening after it rains