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.