I have a split AC at home, the water outlet pipe from that was extended and kept into a bucket so that the water doesn't make puddles on the floor . The AC mechanic told when the bucket fills with water and the pipe gets submerged the water would stop flowing and it would start leaking from the split AC unit from the top. I argued against it and told gravity would take care of it even if the pipe gets submerged since the water is coming from an upper level (atleast 8 feet difference) , the water would flow down from pipe to the bucket and the bucket should eventually overflow. Turns out the mechanic was right , as soon as the tip of the pipe got submerged , water started overflowing from top. I still don't know why it would happen. Can someone explain . Crude image for the situation https://i.stack.imgur.com/9PjTF.jpg

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    $\begingroup$ I think the bore of the AC outlet is quite narrow. In a narrow pipe air-locking is common and water won't flow anymore. It is a bad design of the AC water vent rather. See Wiki on Air lock. en.wikipedia.org/wiki/Air_lock $\endgroup$
    – AChem
    Aug 4, 2021 at 3:46
  • $\begingroup$ Just out of interest, what good is having a bucket if you intended to let it overflow? How did you intend to dispose of the water? $\endgroup$ Aug 4, 2021 at 15:00
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    $\begingroup$ @M.Farooq that does seems right , but wouldnt it happen without the bucket also ? $\endgroup$
    – Rahul
    Aug 5, 2021 at 6:10
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    $\begingroup$ @CarlWitthoft , there are some plants in the balcony ,i just use the water collected to water them . A little bit overflow if bucket fills up is ok , but i didnt want it to puddle up all the time. $\endgroup$
    – Rahul
    Aug 5, 2021 at 6:13

1 Answer 1


Basically, when the bottom pipe is submerged in the bucket, the pressure of the water surrounding the mouth of this pipe will be $$\tag 1 P=P_0+\rho gh$$ where $h$ is the depth of the pipe, $P_0$ is atmospheric pressure, $\rho$ is the density of water and $g$ is the acceleration due to gravity.

Even though at the top point where the water is released by the AC outlet, the water there is being pulled down by gravity, there will be a point when even the pressure in the pipe there will be exceeded by the pressure given by equation (1), as $h$ in equation (1) increases.

The point when this happens does depend on the height of the AC unit, but when $P\gt P_{AC}$ (the pressure at the top), the water in the pipe will stop flowing downward, and the AC unit will leak at the top.

  • $\begingroup$ This doesn't make sense unless your $\rho$ is an indicator of friction inside the pipe. Can you elaborate? $\endgroup$ Aug 4, 2021 at 14:59
  • $\begingroup$ @Carl Witthoft $\rho$ is the density of the water. How did you get “friction”? $\endgroup$
    – joseph h
    Aug 4, 2021 at 20:04
  • $\begingroup$ @CarlWitthoft since the water column in the pipe is at a much higher height , its pressure would be more than the bucket . $\endgroup$
    – Rahul
    Aug 5, 2021 at 6:09
  • $\begingroup$ @Rahul Correct. And when the depth of the water gets to a certain point, then the pressure will be greater than at the top. $\endgroup$
    – joseph h
    Aug 5, 2021 at 6:12
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    $\begingroup$ @Rahul " i just added a rough image of the situation " Please add this image to the question. It really needs it to understand what is happening. Even though it is a crude image, it would be very helpful $\endgroup$ Aug 5, 2021 at 9:47

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