My laundry machine doesn't work properly in my new flat (the wash cycle never ends), and the instructions specify that the common cause of my problem is that the drain is too low (my drain is indeed at 15" above the machine bottom, whereas the instructions recommend 30").

I'm quite curious as to why this is the case; common sense dictates that too low a drain shouldn't be an issue, doesn't it?

Does somebody have an explanation for this requirement?

  • I believe that height helps ensure against sewage being siphoned back into the machine.
    – keshlam
    Apr 30, 2016 at 18:02
  • Respectfully, there is a trap, a "J bend" in the wall, that serves that purpose. Also the inherent bend the drain pipe of the washing machine has, allows water to set in it as well. The drain pipe connection to the wall is not a sealed fit, so the J bend in the wall does the task of sealing out the sewer fumes.
    – Jack
    Apr 30, 2016 at 18:29
  • Having the drain arch at least that high keeps a leaky valve from emptying the whole washing machine. Apr 30, 2016 at 21:30
  • 2
    Actually, I'm not sure there's even a valve on the drain of most machines; there may just be a pump that is either on or off. When off, the water is retained by the height of the water column left in the drain (which should arch higher than the water level in the tub). Apr 30, 2016 at 22:38
  • ... Ah. I was right about syphon risk, misremembered the direction. @Jack has it right.
    – keshlam
    May 1, 2016 at 2:36

1 Answer 1


The water in the wash tub will drain out if the drain in the wall is lower than the water level in the washing machine. Simple physics, water seeks its own level. The drain located at the recommended height will allow the water to remain and allow the pump to remove the water at the end of the cycle.

  • That's right. I imagine that the cycle never completes as it's always trying to fill up to the required level.
    – handyman
    May 3, 2016 at 19:05
  • I just learnt this today as well while taking apart our washing machine. I expected there be to be some valve at the drainage, but there isn't.. so the water just flows out if the drain is installed too low.
    – stmax
    Aug 5, 2020 at 20:49

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