How does water sink works?

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I tried to do the same thing but the water just stayed: enter image description here

  • $\begingroup$ Keep the extra length of pipe on the right pointing up - then fill it up and it'll work. $\endgroup$ Apr 2, 2017 at 17:47

2 Answers 2


This is a simple case of communicating vessels:

example of communicating vessels

When water enters your sink, it raises the water level on the right side of the kink all the way up to the sink. This means that the water pressure on the right side of the pipe is greater than on the left side of the pipe. This imbalance is "not allowed", so the water level on the left side rises (therefore increasing the pressure on that side). If it could, it would rise up to the level of the water on the right side of the pipe. However, since the left side connects to the sewage column, the water merely drains away.

Once you close the faucet, the water on the right hand side will drop until it reaches an equilibrium with the left side.

What you tried with the flexible tubing didn't work because both the entrance and exit levels are the same. If you are putting water in from the left (as would seem to be the case), then lower the right side just a bit. When you add the water, it will fill the tubing up on both sides until it reaches an exit, which will be the now-lowered right side.


The purpose of the kink in the line is to keep some water in the dip. The reason being that any smell in the rest of the line will get trapped in the top part and not flow back through into the house.

  • $\begingroup$ Yes, I get that, I've read an article about that... my question is, how does water keeps moving in an elevated pipe (see the first image)? $\endgroup$
    – m0Onfang
    Apr 2, 2017 at 17:24
  • $\begingroup$ I think what you're asking about is "head pressure". When the column of water on the sink side is greater than the pressure (air + water height) on the drain side the system will equalize by draining water. Your second image system didn't work because the ratios of the heights weren't sufficient. Increase the height on the sink side and it will work. $\endgroup$
    – Diesel
    Apr 2, 2017 at 17:42
  • 1
    $\begingroup$ I'm assuming it'd either be a pump on the non elevated side forcing the water up the pipe or there's a lower pressure on the top half that allows the high pressure to be attracted to the low pressure top half, which would cause it to flow upwards. $\endgroup$ Apr 2, 2017 at 17:43

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