New washing machine waste pipe overflowing

I'm the opposite of knowledgeable when it comes to plumbing however I have done a fair amount of searching through forums to find an answer to my problem and unfortunately still have questions.

To make it easier, I have taken a picture of the pipes as well as the actual waste hose behind my machine.

The hose from the machine sits in what I believe is called the standpipe on the left and goes approximately half a foot down.

I have taken apart the p bend (or u bend - still don't know the difference) and confirmed that there is no blockage.

I have run a snake down and around all the pipes with no issues and also used some soda crystals just incase but water flows through the pipes easily.

What I believe is the issue is that the output from the machine is producing water faster than it can travel through the pipes which are 40mm in diameter.

I believe that if I removed the p bend and just replaced it with a 90° corner, it would work fine but obviously it's there for a reason and particularly with the pipes only a 6 feet from the drain, I'm very reluctant to do that.

One more thing to point out is that on the right hand side, there is a pipe which I have capped off. I was wondering if that trapped air could be preventing the water flowing down as quickly and if I need to find some kind of air valve which may allow air out?

As it stands, I have just put the waste hose into the sink next to the machine and that has let me at least use it for now.

I appreciate any advice people can offer.

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The hose diameter looks nearly as big as the standpipe. Is it larger than your previous machine's hose? The hose can handle a bigger flow than the standpipe, even though it is smaller, because it is pumping the water out under pressure. But once it empties into the standpipe it is under atmospheric pressure and the flow rate drops. That said, 40mm is a standard size for a washing machine drain, so it should work. A 6" length of hose in the standpipe is normal too.

I think your suspicion about an airlock in the capped-off length of pipe may be right - any air in there can't get out through the p-bend trap, nor downwards. Was that capped off pipe stub there with the old machine? Can you remove the pipe stub and cap it off at the T instead, or replace the T with a straight connector? Might be worth a try, unless anyone has a better idea. I don't like the idea of an air vent, it'll probably leak sooner or later.

TheBag (author)  Yorkshire Lass3 months ago

So last night, I took the cap off the pipe on the right and the water at least flowed through without instantly filling up the standpipe but then of course flowed straight out the uncapped pipe which was to be expected especially since it is lower than the top of the ubend. It makes it seem like the airlock caused by capping that pipe is what was causing the overflow.

I'm considering adding a ubend to the now uncapped pipe and then putting another standpipe on the end just to raise the level temporarily and still prevent smells coming back into the house.

I will need that other standpipe in a month or so anyway once I order my dishwasher although then there is the question of whether or not having both of them on at the same time will cause them to overflow - I'm happy to cross that bridge when I get to it.

If anyone can think of a reason why this won't work or is a bad idea, please let me know!

Also - based on it looking like an airlock, wouldn't capping off the pipe at the T cause the same issue?

Toga_Dan TheBag3 months ago

review posts by Jo Murch, QuadriF, and myself. I've worked as a plumber, I've studied the uniform plumbing code, and these are the 3 people posting who show the most understanding of how these systems work. If you don't understand a term one of us uses, Google it. Google images is especially useful.

bottom line is minimizing the number of times water changes direction. Also, those pipes should all be 2 inch. Whoever put this system together didn't do a good job. Learning to cut and glue pipe will help. It's not too hard.

I don't know if you want to go that route but at work we have to wash quite filthy pieces of cloth on a regular base.
Two problems:
Oil/grease and contamintes havier than water.
Food grade oil/grease and food residue before you are concerned about the enviroment :)
Anyways, the old system used a standard trap as used for literally all sinks aorund here.
Once a week it was blocked by the food build up or someone was smart and used cold water only so the grease blocked it.
The way out was to instally a custom piece of 100mm PVC downpipe with some caps and connections.
The water goes in through a side connection at a flat angle to the outside radius but sligthly facind downwards, causing the water to spin rather than to flow in.
The outlet is about 10cm higher and just a straigh connection of the same diameter as the drain pipe in the wall.
With this all heavy debris collects at the bottom while all oil or grease that remains will be at the top and flush the next time hot water is used.
As it as a fully closed system there is no problem with air and the resulting vortex makes sure everything heavy stay in the pipe while the rest is flushed out.
You could basically cap your open standpipe and and connect the new one to the t-piece on the side without the existing plastic elbow.

TheBag (author)  Downunder35m3 months ago

Thanks for your reply!

Forgive me if this is a stupid question but if I cap the current standpipe and put in a new on on the T, without the corner elbow, won't the standpipe be horizontal? I'm sure I'm just misunderstanding completely so I apologise in advance!

You don't really need the standpipe, that's why I said to put a cap on it.
To avoid too much hassle the existing elbow with the cap is removed - instead the outlet connection for the sediment filter pipe we made goes on.
But unless you have really filthy washing in the terms of sand, soil or similar you should not anything but a hose connector for your washing machine instead of the existing and capped elbow.
To avoid the overflow you simply cap the existing standpipe as well.

Quadrifoglio3 months ago

One of the things you are having to overcome is a 180 degree P trap, two 90 degree bends, a T with a dead leg, and another 90 degree bend in rapid succession. It depends on the reference but this is equal to roughly 40+ linear feet of pipe in about three feet. If it meets code, moving the stand pipe to the right and connecting the P directly to the drain line would minimize the pressure drop.

Also, the source of the backup can be way down the drain line. This is usually revealed by the washer draining for a while and then backing up after it has filled the drain line. Mine does this and I have to periodically jet the drain line Jetting a Slow House Drain (Residential) https://www.instructables.com/id/Jetting-a-Slow-House-Drain-Residential/.

+1 on resistance to flow from changes in direction.

rickharris3 months ago

This is a standard way to plumb a washing machine in the UK and should be no problem> Possibly your extension pipe - capped - is causing some kind of back flow and I would remove it.

This is a picture of mine. Side on but it has a U bend trap at the bottom.

Toga_Dan3 months ago

I would throw out the 2 abs (black pipe) 90s. Then put the vent (the white thing on the right) on a T which faces upward. I'll draw a sketch later.

Toga_Dan Toga_Dan3 months ago

I just re-read your words. I think you've capped off the vent, which is supposed to let air in, but not out. Unless there is a vent in the wall, this means that a slug of water will suck the p trap dry. Then sewer gases can get into the house.

Isn't the fact that the hose is just stuck (unsealed) in the top of the standpipe vent enough?


I'm lookin at this now on a real computer. Sometimes I can be over-hasty in reading on my tiny phone. I misread, and responded to what I had misunderstood.

No. that isn't adequate vent because air coming from above can push the water out of the trap. then there is nothing to keep the sewer gasses out of the house. this is essentially a siphon.

A vent is downstream of the trap, and allows air from above the roof to follow the slug of water down. Alternately, a check valve can allow air from inside the house to follow the water. Either of these prevent the trap from emptying.

standard way of doin it (unsealed). If it were sealed, the washin machine pump might blow the trap dry, or even suck dirty water back into the machine when it stops pumping.

Toga_Dan3 months ago
steveastrouk3 months ago

what about the fitting that connects a dishwasher to a line ? They seal and pump directly into the drain.

Downunder35m3 months ago

No offense but with this design it can't work.
Modern washing machines pump out the water quite fast and an open drain with such a bad bend in it won't take it.
Around here are a lot of old houses with similar problems and most people hand the pipe into the laundry sick as a buffer.
The best option would be a closed pipe and a connector for the pipe.
No bend or so required as the machine filters out the worst bits anyway.
You also get under the sink traps that already have this pipe connector, offering you both a trap and the proper connection.

Your problem is that the bend will always have water that needs to be pushed up.
If the inrush is quite fast this backflow can cause overflowing.
Try to remove the bend and use a direct pipe connection with a hose clamp to be sure.

The water trap, U bend is mandatory in the UK for any pipe that connects to the sewer system. It is very common for all drain pipes that exit the house as a way to stop smells returning to the house from the drain.

The open 2 inch plastic pipe is also the normal way to drain a washing machine, either that or a connection to the pipe below the sink. The pipe should be able to take the water from the washer drain assuming it is clear and vents into an open drain or clear sewer pipe.

I can see at least three things possibly seven slowing down your flow.

Every 90 degree bend in your pipe will mess up your flow.

You have two 90 degree bends you don't need and the white 90 degree bend should be replaced with a more rounded bend.

It looks like there may be a forth and a fifth 90 degree bend in the wall you may be stuck with unless you are willing to take the wall apart.

For the best flow your pipes should look like this pic.

Two other things that can slow down your flow is no air vent or a blocked air vent preventing air from getting behind the outflow. (RED ARROW) The air vent works best near the drain.

And the horizontal pipe travelling up in the wall instead of down, or going a long distance without a vent. (GREEN ARROW)

These are common problem in homes where there have been a number of different amateur plumbers doing home hacks.