Some US apartments have a washer connection to the common waste system similar to the picture here: enter image description here

How is one supposed to prevent backflow back to the tub in this system? Basically, everything that is in the waste hose and is higher than the tub bottom exit will flow back into the tub, that is all the crap goes back because the "crap" settles in those hoses.

Is there a trick to avoid it other than cutting the hose in two pieces and installing backflow preventing valve?


Since some are baffled why am i bothering, this is what the washer looks like, the inner and outer tubs.

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  • 1
    Have you found this to be an actual problem you're experiencing? Considering how many modern American apartments are probably set up exactly like that, it seems unlikely that there would be a major issue with the installation method.
    – FreeMan
    Feb 28, 2023 at 13:16
  • @FreeMan Yes, I am dealing with an old amana washer that is disgustingly dirty. I guess most people do not care, i do. I cleaned the inner tub and all parts. To clean the outer tub, i disconnect the pump from the motor and use drill to pump out water from the outer tub after cleaning. But it keeps coming back from the drain hose with all the dirt. Cleaning the drain hose, I cannot imagine how. btw Modern apartments does not mean anyone actually put any thought to that, unless there is any backflow prevention in the pump itself, there will always be a backflow, unless one drains to the floor.
    – atapaka
    Feb 28, 2023 at 14:47
  • That crap in the washer has nothing to do with drain backflow.
    – kreemoweet
    Mar 1, 2023 at 8:28
  • I say that washer box should be even higher, so that folks can reach the valves without having to move the washer. Of course, that would result in even more backflow, so ...
    – kreemoweet
    Mar 1, 2023 at 9:00

2 Answers 2


The pump is at the bottom of the washer and has its own backflow prevention. Water may sit in the hose that hasn't been siphoned out with a discharge, but it cannot flow back into the tub.

  • I pump out the water from the tub, when i turn off the pump, the water literally jumps back into the tub from the pump with all the crap that is in that drain hose (i am using a drill connected to the pump). The washer is an old Amana ntw5245tq1 with this type of pump: walmart.com/ip/… There is no backflow prevention inside the pump, at least i am guessing from the video here youtu.be/LHTaehXjb1c.
    – atapaka
    Feb 28, 2023 at 14:43
  • So you use a drill to empty the washer? That info would have been very valuable in the original question. We are not psychic and know exactly what you are working with. A pic of your set up would be very helpful.
    – RMDman
    Feb 28, 2023 at 15:18
  • I only use drill to power the pump to extract water during cleaning, otherwise the pump is connected to the 2 speed motor of the washer. The washed does not have drain only mode, so I disconnected the shaft between the pump and the motor and just power the pump by a drill, it makes no difference other that RPM though.
    – atapaka
    Feb 28, 2023 at 19:04
  • You take the pump out and use a drill? That is just bizarre.I'm out.
    – RMDman
    Feb 28, 2023 at 19:13
  • I need to clean the outer tub. When I clean it, water is obviously in the tub and i need to pump it out. So I can either run a wash drain - spin cycle (actually this washer does not even have that) which will try to spin as well as drain (via pump) or pump it manually. I am not sure what is so bizarre that i actually care about hygiene... I added some pictures, the outer tub gets filled with water and dirt from cleaning, how am i supposed to remove the water? If I run the washer, i) all is more exposed, ii) there is not drain cycle, and the motor will try to spin the center shaft.
    – atapaka
    Feb 28, 2023 at 22:19

This is a variant of the usual setup that I have seen. The usual setup is that a hose goes from the pump outlet at the bottom of the machine but instead of being a continuous hose connected to a pipe in the wall, it is an open ended hose that is either inserted into a larger pipe (so not sealed in any way) or into a laundry tub.

The basic concept is the same as a dishwasher. With a dishwasher you have either an air gap (hose from the dishwasher goes into a little plastic/metal contraption on top of the counter and water flows up out of that hose and down into another hose which goes to the drain) or a high loop which is what you appear to have here.

Since this is a variant of the standard connection method, I would not expect any significant amount of water pushing back into the washer tub. Since you are having that problem, it points to a problem with your washer and not with the plumbing.

There is one exception. If you have a sewage backup on the main line and this hose is the lowest exit point then sewage could either get into the washer or burst the hose. But that is an extreme situation. In most cases there is a toilet or floor drain as the lowest exit point and not the washing machine.

  • The first paragraph describes exactly the setup (which is also visible in the picture of the original post. Most water in the drain hose that is between the bottom of the outer tub and the top of the hose before it falls into the pipe in the wall, will always return back to the tub that is the water in the drain hose and in the tub have to be at the same level. Now, in my case, the drain hose is old and full of crap and all that stuff will wash back into your laundry after every drain cycle. It is utterly disgusting. A solution might be a new drain hose but they are quire expensive.
    – atapaka
    Feb 28, 2023 at 19:11
  • The only proper solution that is clean and hygienic and i can think of is to have a backflow preventing device but I cannot find anything that would be suitable for these drain hoses...
    – atapaka
    Feb 28, 2023 at 19:12
  • 1
    A new drain hose should be very inexpensive. $15.50 right now at Home Depot. This is a standard (or very close to standard) setup, whether for apartment or single family home. Since I haven't heard of this problem before, that tells me either (a) your hose is so messed up that it causes more problems than usual (in which case, replace it) or (b) there really is a problem with your washing machine (which will probably cost more than $16 to fix...) Feb 28, 2023 at 19:16
  • I added some real pictures. Whenever the pump stops pumping, as the water falls from the height, it literally jumps out of the bottom hole in the outer tub. This will happen whenever you have this setup and the pump has no backflow prevention. In my case, the drain hose is full of crap that gets washed back into the tub. Not sure which hose you refer to for 15USD but the drain hose is WPW10096921 and anywhere from 40-60 dollars depending on shipping, the tub to pump is 285871 and anywhere from 20-30. Starting to thing: the best is to take a hammer and ... tell landlord washer stopped working.
    – atapaka
    Feb 28, 2023 at 22:32
  • 1
    The machine pumps (or should) water out with considerable force and velocity. Anything that that flow can't flush out is certainly not going somehow jump out into the small amount of backflow that happens at the end of the drain cycle. The filth in that tub is caused by former tenant abuse, e.g. grossly excessive amounts of soap, overloading, etc. resulting in dirty soap scum world in there.
    – kreemoweet
    Mar 1, 2023 at 9:00

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