Why do I need to vent this washer/sink drain?

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AllenEllis

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Hi all,

I'm re-doing a laundry area and I think I have a clever trick to venting my washer and sink, but I want to see what you guys think:

washer%20sink%20vent.jpg


As I understand it, the purpose of venting is to allow air to escape from the pipes as water tries to flow down. But because of the way the washer's drain hose rests inside the drain pipe, there is an opening, and therefore, two-way airflow.

So as water drains from the sink, air can be pushed out through the top of the washer's drain, and vice versa. The p-trap under the slab prevents sewer gases from coming up either opening. So in this case, is there even a need for a vent through the roof?

The complications I forsee are:
1) If the sink is full and plugged, the washer may not be able to drain and may overflow
2) Draining water from the washer might flow into my sink a little
3) If you drain a lot of water down the sink at once, might there be enough pressure for it to come out the washer's drain? Would a sanitary T help prevent this?

Any other thoughts?

Thanks.

I don't see a vent on the p-trap underground, that would be sure to siphon dry
Terry
 
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TristanP

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Posting this before Terry does

washer_rough_b.jpg


Maybe an AAV above the T would make things less awkward. You need a P trap at the bottom of the washer drain before it goes horizontal.
 

AllenEllis

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Lol, yep I've seen that image posted in lots of other threads I found. I see lots of diagrams like that with roof vents, but I'd like to know whether I can get away without that given that there is airflow in this case.

You need a P trap at the bottom of the washer drain before it goes horizontal.

Why is it necessary for the washer to have a P-trap before the T? Isn't its function just to prevent sewer gases? And if so, why doesn't the p-trap in the slap accomplish that?

Thanks again.
 

Nukeman

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You are not allowed to double trap a fixture (like your sink in the diagram).

Too much height from the trap to the top of the standpipe can cause the trap to siphon. This is why they want the washer trap above the floor and why there are limits on how tall the standpipe can be.

The washer discharge will pull the water out of the sink's trap

washer_rough_1.jpg
 
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AllenEllis

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You are not allowed to double trap a fixture (like your sink in the diagram).

I was under the impression that you can't double-trap in a closed system, but if there is airflow between the traps (as in this case), it's ok. That's not true?

Too much height from the trap to the top of the standpipe can cause the trap to siphon. This is why they want the washer trap above the floor and why there are limits on how tall the standpipe can be.

Right, 18-48" is what I've read the standpipe height should be. The p-trap is about 8-10" below the concrete, so my total height would be 20-25".

The washer discharge will pull the water out of the sink's trap

What about an AAV above the T as Tristan suggested? I know those are considered cheap workarounds, but would it work?

Thanks again, I appreciate all your help :)
 

Redwood

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The reason for that picture is that it is done correctly...
I would suggest getting your drawing closer to that picture...
 

hj

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It is NOT a "clever trick", you have merely created a convoluted trap riser. The trap is NOT vented, and because of where it is located, it can be subjected to other forces besides siphonage. The trap under the sink is useless, even though your washer pipe would vent it. Air would NOT flow through the sink trap, unless it had lost its seal from siphonage, which would only happen if your drain did not vent it. The reason for the vent is NOT because the washer riser will not allow air and water to flow in both directions. The hose does not fit so tightly that it could not happen, but that has NOTHING to do with the requirement for a vent anyway, so it is irrelevent. You are NOT a plumbing hydraulics expert, so do not quit your day job, and leave piping designs to the people who know how to make them work.
 

hj

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quote; Thanks everyone for your help.

I hope that is sincere, and not sarcasm. We try to keep people from making mistakes when they try to do tasks they have absolutely no idea what they are doing.
 
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