Top floor bathroom sink in a 3 story townhouse.

When running the hot or cold water at a low flow rate, the sink drains normally. Running at high flow rate causes backup in the sink, i.e., the drain is slow.

Here's the mystery. If you run the faucet at low flow rate for one minute and then increase the flow rate as high as you like, the water drains normally. What's going on?

  • Sounded liked a semi clog, but last line, might add vent problem to the mix
    – crip659
    Dec 6, 2021 at 22:13
  • My first thought was a venting problem.
    – jwh20
    Dec 6, 2021 at 22:16
  • I had a similar problem in kitchen sink, turned out years of cooking discards plugged the horizontal run of the pipe (under the slab) bad enough the plumber had to take out big machine to snake through the stuff. Less likely in bathroom sink though.
    – Eugene
    Dec 6, 2021 at 23:52
  • I agreed with JWH20 that the issue is a venting problem. Use a straw and stick it in the drain and see if the flow rate improves Dec 7, 2021 at 17:01
  • 1
    Sometimes the sink overflow outlet to the waste pipe, which also serves as an air vent, is blocked or not connected correctly under the sink. If you pour water into the overflow opening a the top of the sink and it backs up, that's the first place to look for blockage. Dec 7, 2021 at 22:21


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