Standpipe overflow/ AAV problem

Hello ,
This is my first post. I joined purely to solve this problem. I have attached a crudely drawn, in Paint, layout of the washing machine standpipe inside of the house and out. This is an old house (50 years) and when I had it inspected to before buying, he said the standpipe needed to be repaired because it drained into an open sewer pipe on the exterior of the house. So it was fixed, and that is the drawing I have attached. These pipes were just installed and never used so I doubt it is a clog.
I went to do my first load of laundry and water began overflowing from the standpipe. The height of the pipe is 39" from the floor, so that seems correct. My friend and I did some tests and it seemed to drain just fine when the exterior screw cap (see drawing) was removed. As if it let air out of the system. So I screwed the cap back on while the washer was expelling water and it immediately backed up. SO I took the AAV of the line and it drained just fine.
However, this pipe, as previously mentioned, drains directly to the sewer line under ground. So removing the AAV may allow gases to come into the house.
My problem is that every test I have done shows that air needs to get out for the water to drain properly. The AAV only lets air in. And every article I've read shows this setup being correct.
Could you please help me?
Thanks

I don't get the useless termination thing. Why do you need the AAV¿ Can a proper vent be installed¿

the termination is just that. I think it went to something a while ago but has since just been capped off. A) is that causing the problem B) what is a standard vent?

A standard vent would be a 2" vent going up usually above the roof. What seems to be happening is your washing machine draining is causing pressure in your drains. AAV's (air admittance valve) only let air in, not out, hence the backup. You get positive pressure in the drain line.

What size is all that piping...

Cass said:

What size is all that piping...

Click to expand...

all piping is 2".

Eliminate the useless termination part and install a 2" san T...I will bet the water is being forced into the termination area and then back at the trap causing some type of a problem...also I have seen where the use of to much laundry detergent will cause to many bubbles and they can't flow like water...that will cause things to back up also...into what size pipe does the 2" connect to? once it is outside the house...

drain

Some "4 way fittings" have internal diversion baffles which reduce the opening to a fraction of the pipe size. In other cases, the AAV can cause the symptom if there are other problems in the drain line. Remove it and then run the washer. If it drains then the AAV is creating the backflow, BUT you have to find out what is happening elsewhere to make it happen.

There is no point in putting an AAV on a pressure drained fixture. They let air in, not out.

hj said:

Some "4 way fittings" have internal diversion baffles which reduce the opening to a fraction of the pipe size. In other cases, the AAV can cause the symptom if there are other problems in the drain line. Remove it and then run the washer. If it drains then the AAV is creating the backflow, BUT you have to find out what is happening elsewhere to make it happen.

Click to expand...

I have done this (removing the AAV and doing a cycle) and it works just fine. If I put my hand over the pipe where the AAV was, to create a seal, water immediately begins to fill up the standpipe. I can feel positive air pressure aganist my hand and upon letting go, the air escapes and the standpipe clears quite quickly.

Cass said:

into what size pipe does the 2" connect to? once it is outside the house...

Click to expand...

It remains a 2" pipe once outside until it enters what looks like a 3 to 4" pipe that goes into the ground.


Could this be a problem further down the line, i.e the sewer it enters into? If I open the exterior screw cap, I can see the water flowing just fine with no backup or hesitation. That leads me to believe that this problem is contained only within the standpipe/AAV setup.

pipe

SOMEWHERE, after the AAV, the pipe is sitting full of water so the air cannot flow out of the washer drain pipe, and thus the water cannot flow into it. A vent through the roof, would MASK the problem by allowing the washer to drain, but the real problem would still be there.

HJ is correct...the AAV should work fine, and do it's job, when the system is functioning correctly...

Cass, I like your idea about removing the termination. Upon thinking about it, I bet the intial surge of water is moving not down into the sewer but into that 1ft of dead end pipe. Therefore, when the water heads back to be expelled it meets more water, thereby causing turbulence that allowing air to escape somehow helps. I will remove this pipe and see what happens.
Could it be as simple as this?

possibly but....

you were saying. but...

My 1st initial thought was about that ugly 4-way fitting and useless termination too. I thought the same thing about the water being forced across the san-cross and causing a positive pressure to build up in the piping.