I've read a bunch of threads on problems people have with washing machine water backing up around the drain stack. In particular the posts by hkstroud and speedball1 have been very informative.

I have a 1 1/2" pvc drain stack that is above the height of the washer. It also has a trap before going under the floor. It runs through floor then horizontal where it connects to an older cast iron drain about the same width. That runs outside to an old drywell. A kitchen sink drain (also pvc) runs almost straight down and connects with a T to the same line at the point where the washing machine drain connects to the cast iron. There is no vent for any of this. The sink drains normally and so does the washer most of the time. I just replaced the washing machine today (it was almost 20yrs old) and snaked the lines. I want to prevent any future backups by using a compression fitting and having a closed system.

My question is do I need a vent or an AAV valve or can I just close the system without it? That would mean the only air in the line would be from the open sink drain or the washing machine itself. The drain does get partially clogged up every once in a while. I'm sure from food particles and such. I snake it out and it's fine again but water coming out of the washer drain isn't a very good early warning system to tell me when the line is clogged. :) I'm thinking the closed system might actually pressurize the drain line and keep it cleaner.

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I'm thinking the closed system might actually pressurize the drain line and keep it cleaner

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WRONG! Rule of thumb, if it has a trap it MUST be vented.
This vent may be a AAV or a vent through the roof but if you close off the vent you slow down the drainage process and the washer will just attempt to vent through another fixtures trap. Your drainage system sounds exactly like the one in my 54 year old home. If you have a problem with backups out of the stand pipe consider installing a compression fitting on the stand pipe, (see image) and this will make your system a closed one with the exception of the vent. The added pressure from the pump will help scour your drainage pioes. Good luck, Tom

Wow, Thank you for your very fast answer, Tom.

You got me a little confused or my first explanation was confusing. There is no vent at all in my system. It sounds like you're saying I do need to add ventilation or an AAV valve before using a compression coupler? Is that correct? If I do need one, where on the drain line would you recommend I put one? The cellar where the drain lines run is all accessible. It's actually the room the well pump is in. The washer drain goes through the floor and runs horizontally about 12feet before it connects to the other drains then outside.

I noticed your other posts where you mentioned your own home and yeah, I thought it sounded like my situation. I never notice whether you said your system was vented though. This is a very old farmhouse probably about 100yrs old and it's had various things added on to it over the years.

For a long time I've just had a sponge wrapped around the washing machine hose where it went in to the drain line to prevent any backups. From your information about ventilation, I probably shouldn't have done that since that was the only place air was getting in?

My washer vents through the kitchen stack. If you have the room install a AAV just downstream from the trap and continue to close off the hose and washer stand pipe with a sponge, duct tape or a compression fitting. Good luck, Tom

I feel stupid. I'm reading your advice but not understanding it or it sounds like it's conflicting with advice in other threads.

I found this picture in another thread you posted in and this is what has me confused. You say here to mount the AAV just downsteam from the trap? I thought it had to be above it? I believe hkstroud posted this diagram originally. (see attachment) Would this example in the picture work with my present 1.5" washer drain? That is could I just adding a sanitary tee or wYe connector to it then the AAV valve on top? In my case there is an S-trap about where the shielded coupling is in the picture. The drain runs through the floor then angles down till it connects on to the older drain to the drywell. My kitchen sink also has an S trap.

Sorry for my confusion but I'd like to get this right the first time. I'm not even sure how the AAV valve works in this case. I thought they let air in but it seems like my problem is the air in the pipe needs a way out so the water can flow more easily.

Hi all...

EGK...you caught me midpost (this responds to your last post below me...;)) Read on

I had a couple minutes and a drawing already drawn up for this one so thought I'd post it up.

This is going to be the correct method to vent the washing machine using an AAV... see image. You will want to keep the AAV at about the same height as the top of the standpipe and be sure to order an 1.5" PVC x 1.25" tubular compression fitting. Depending on where you live, these are also called 1.5" PVC x 1.25" PVC desancos... also called 1.5" PVC x 1.25" TRAP adapters.

Keep the standpipe out of the trap as high as possible, but not to exceed 36" tall.

Use the shielded clamp to transition over from the old piping to new PVC.

Don't forget to snake the drain line just after you cut out the old STRAP.. won't be a better time than when the trap is removed. In the future, when needed, just unscrew the AAV to access the drain line to snake it out again.

Let us know if you have any more questions...

MARK

Thanks a lot for the drawing Massplumber. I'm sure this is what speedball1 was saying about putting the AAV after the trap and I just wasn't getting it. I was thinking of it as just connected to a T after the trap without the vertical stack.

This looks like a good idea. I just got done snaking the drains and doing a load of laundry and it works fine as is. I just like the idea of using the compression adapter to connect the machine's hose. My aim is to minimize the chance of a mess in the future.

Exactly...

Glad all clear now... :)

MARK

I do have one more question if you don't mind? It's more in general because I'm curious. I mentioned my house is 100yrs old. The bathroom and kitchen sink are both S-trap drains. The person who installed this washing machine drain apparently just kept with that.

My question is why did they use S-traps back in the day? It sounds like they simply don't work as intended if they can be siphoned dry. Was it just they didn't know any better back or that water drainage was much slower in sinks at the time?

I can clear that one up. I broke into plumbing at the time that most of the older homes weren't vented and had "S" traps throughout Will "S" traps work? Sure they will. Gravity will drain everything. Vents help the process because they relieve the suction caused by a unvented trap. In a few isolated 'S" trap cases the suction lowered the trap seal allowing sewer gas to escape. That was enough for the code framers. " trap cases the suction lowered the trap seal allowing sewer gas to escape. That was enough for the code framers. " traps were outlawed and vents were in.

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it seems like my problem is the air in the pipe needs a way out so the water can flow more easily.

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Not quite so. Vents let air in as the drainage occurs.
Regards, Tom

Thanks again to both of you for your help. Now I know how to fix it the right way.

[QUOTE=EGK;2095217]Thanks again to both of you for your help. Now I know how to fix it the right way.[/QU
Please let us know how you made out. A picture would be nice. Good luck, Tom

Hey thanks for having this info available. It helps to clear my problem up (in my head at least) Roy

Hi Roy and Welcome to The Plumbing Page at AMHD.com
. Thanks for the update and results. That's why we're here to help people just like you have a good day, Tom