I’ve noticed an odd behavior over the years I’m at a loss to explain. Looking for a factual answer, but this probably doesn’t rise to the level of FQ; anecdote and surmise is probably where we’ll end up.
I’m going to describe a phenomenon I’ve seen in many sinks over many years in many residences, hotels, and offices. It’s not universal, but it is very commonplace. So I think any explanation needs to go beyond e.g. “your pipes are clogged.”
Imagine a typical modern bathroom sink with the plunger drain stopper and the usual P-trap below. Turn on the water (hot, cold, or both) to a fairly high setting with the drain stopper in the fully open position. Which is still somewhat restrictive.
What I observe is the water is entering faster than it’s exiting. And so the sink will slowly fill to a depth of 1" to 1.5". There’s no obvious flow pattern or vortex in the water; the stream of water entering from the faucet is making lots of turbulence where it hits the water already in the sink near the drain.
About the time the water gets to be 1-1.5" deep, the rate of flow out the drain abruptly increases a bunch and the water quickly drains almost completely. The drain time is maybe 20% of the time it took to fill even though the input stream is unchanged. Which suggests the “after” flow rate is ~5x the “before” flow rate. There is never any sort of visible vortex. The turbulence from the input flow is apparently as dominant as ever.
Once the sink finishes draining, now water is pouring into the sink, spreading thinly around the bottom of the basin, and running down the drain at the same pace it’s flowing in. Just as it had been in the early seconds of you having just turned the water on. This situation will remain for as long as you leave the water running.
So what causes the initial backlog and what is the “phase change” that suddenly opens the drain path to greater capacity or efficiency or ???
Wild ass guess: Could it have to do with the air and water trapped in the p trap? Once you add enough in that initial stream, eventually it adds enough to overcome the pressure and break the air seal? And then it stays that way as long as you have water still flowing, but if you leave it off long enough, air fills back up?
I would guess it has to do with the creation of an air stream from the vent stack.
Once the air starts flowing, it doesn’t resist the draining of the water.
Magic, of course. The little gremlins that live in your drains notice that their air supply is being cut off, so they flip a switch to let more water out.
They are also the ones that control which direction the water spirals out of the drain depending on your hemisphere
Bought this used house about 2.5 years ago. Bathroom was remodeled within the last 10 years by previous owner.
When we open the faucet of the bathroom vanity sink, with the faucet fully open (i.e., water coming out fairly quickly, which is how I typically use it to brush teeth or wash hands), the sink basin fills up and the water doesn’t seem to drain (it probably does, but very slowly?). Then, after a minute of the sink being full and the water still running, all of a sudden the sink drains really fast, as if a blockage/lever were released somewhere.
This has been happening ever since we bought the house. I’ve cleaned the drain and also took the trap off a year or so ago and checked to see if any debris was in it but it was clear.
You did an excellent job describing the issue. My first guess is laminar flow. The turbulence gradually adjusts to the flow because of the depth and once a clean flow starts in is somehow able to maintain it.
It doesn’t happen to every sink every time. But a sink that does it will do it often, bordering on every time.
Where I differ from most folks is having spent a career using multiple unfamiliar different sinks every day. My sample space is much larger than most people’s. The fact my current bathroom sink does it is just the icing on the cake that led me to actual bother to post about it.
As the sink fills there’s more pressure pushing water down the drain. Don’t know why it would fully drain with the water still running except maybe because a clog was pushed out. Also, noted above, there may be inadequate venting and an air bubble has to work it’s way to the vent or get carried down the drain.
I’ve never noticed this happening. The water draining faster after the sink begins to fill I’ve seen, but I don’t recall all the water draining with the faucet open. Haven’t looked for it before now either so I’ll keep an eye on it.
Yeah I wonder if it is that simple. Beard hair, soap, and debris gets tangled on the plunger drain. Enough pressure and it is soften and dislodged … enough to at least no longer impede flow. Then flow is better until it dries up again.
Proposed experiment:
Fill the sink to and get to the point of good flow maintained. Turn it all off and return in three minutes. Any laminar flow stuff will be settled by then but the hair hanging from the plunger drain, if present, will still be wet and soft. Turn water back on.
If it drains well right off it is just the hair hanging off the plunger drain. If not then more interesting explanations may be at play.
Then for good measure Drano the sink and see if the behavior persists.
Plumbers hate chemical drain cleaners because they cost less than a plumber. There are some dangerously caustic chemical cleaners they have good reason to avoid but Draino is not in that category. It’s not going to cause serious corrosion on modern pipes. Definitely not in plastic pipes.
