New washing machine drain is backing up

[matt7891]

I recently moved my washer/dryer to a different room in the basement from its original location. It drained fine in its new location for about the first 10 loads but now the standpipe is backing up and I can't figure out the problem.

In moving the washing machine, essentially I did was extend the trap arm to a total of seven feet at 1/4" drop per foot. Two-inch pipe. It drained fine for about 10 loads and it had drained fine in its previous location for many many loads. When it started to back up, I assumed that the trap arm was too long and it wasn't venting properly, so I added a new AAV. But it still happens.

Please see the diagram and let me know your thoughts. Thanks!

The main drain becomes cast iron just before it enters the concrete floor in the basement. I'm wondering if this is most likely a clogged main drain or if there is something wrong how I plumbed it. The bottom of the p-trap is about 13 inches off the floor, and the standpipe itself is about 25 inches in length.

 

[matt7891]

I recently moved my washer/dryer to a different room in the basement from its original location. It drained fine in its new location for about the first 10 loads but now the standpipe is backing up and I can't figure out the problem.

In moving the washing machine, essentially I did was extend the trap arm to a total of seven feet at 1/4" drop per foot. Two-inch pipe. It drained fine for about 10 loads and it had drained fine in its previous location for many many loads. When it started to back up, I assumed that the trap arm was too long and it wasn't venting properly, so I added a new AAV. But it still happens.

Please see the diagram and let me know your thoughts. Thanks!View attachment 95976

The main drain becomes cast iron just before it enters the concrete floor in the basement. I'm wondering if this is most likely a clogged main drain or if there is something wrong how I plumbed it. The bottom of the p-trap is about 13 inches off the floor, and the standpipe itself is about 25 inches in length.

Also I'll add: upstairs above the basement on this line is the kitchen sink and dishwasher. This should be obvious from the diagram, but want to make that clear.