Why does one of my bathroom sinks drain slowly when the water is running, but drains quickly when filled and drained?

Question

One of my bathroom sinks drains slowly with the water running ( like when brushing your teeth ) but drains quite quickly if I plug the sink, fill it all the way, and release the drain plug.

When the water is running and the drain is open, the sink drains slower than it fills, but using a stopwatch, the sink drains from full, when the stopper is removed, much faster than filling it took.

I think the drainpipe is pretty simple. It goes out from the bottom of the sink, through the tap, into the wall, and straight down to the main drain line. It was snaked a few months ago, but aside from relieving the immediate clog, did nothing to improve the speed of the drain.

One thing to add, when I release the plug and allow a full sink to quickly drain, just before it finishes draining, the drain makes a somewhat loud "sucking" noise before the last 1/4 of the basin drains out. Not sure if that's relevant, but I don't hear it under normal operation, etc.

Is this some kind of venting issue or having to do with the pitch of the pipe from the trap to the wall ?

What might cause this ?

Answer 1

Warning, wild speculation ahead, but I have had this problem.

I've always thought that the aerated water causes some kind of "bubble dam" that plays with the way the water is able to go through the relatively thin space that the popup creates. Blocking the bubbles with my hand and letting pure water start going down the drain seems to stop the bubble phenomenon and water will drain faster.

I think removing the bubbles and starting a good flow lets a siphon effect take over and suck the rest of the water down quickly. The bubbly water never lets a large enough volume of water to drain to start a good siphon.

I have had luck with raising the pop-up drain to allow a bigger gap for water to drain. Of course, I have also completely removed the pop-up as well because I never need to stop up the drain.

Answer 2

The sink drain recently clogged up bad. (I spent the best part of a weekend trying to snake the pipes...I broke a 25' hand-crank snake in the pipe and eventually was able to pull that back out.)

I bought an electric snake and ran the "snot" out of it until it basically went 35-40 feet and came back "clean" several times. Which is quite interesting as the sink is roughly 5-6 feet from the toilet stack...

The problem seemed to be just "accumulated bathroom sink drain sludge" which was plowed through by the electric snake. It could also be that on one of the trips down the pipe, the snake went up into the vent and removed a (perhaps partial) blockage. It drains quickly in all situations and no longer makes the "sucking" sound as it finishes.

Answer 3

It probably creates a whirlpool when full sucking the water down quicker.