I have a situation where the P trap to the washing machine is in the crawl space instead of in the wall. There is no vent.
I thought about adding an Air Admittance Vent in the wall (see diagram--dotted lines indicate places I'm thinking of adding an AAV) but I realized that would be before the P trap, so it may not do much there.
To put the AAV after the P trap, it would have to be down in the crawl space, several feet below where the washing machine drain starts. Would this cause problems?