Draining a washer tub in the basement through 25' of 2" pvc connected to the soil stack that is also connected to a floor drain. An AAV on a standpipe is placed 1.5' away from the tub's p trap. But water backs up through the floor drain opening. Solution: put in a directional valve on the floor drainage line. But now water doesn't backs up, but tub drains very slowly and may overflow with two closely spaced rinse cycles. There is a vent stack at other end of the 25' line about 5' away from where the 2' pvc is connected to the soil stack. QUESTION: should I connect to the vent stack from this point?

Overflowing floor drain tells you you have clogged sewer pipe down the line. Installing "directional" valve doesn't solve anything. Snake your sewer.

Installing vent: I would suggest that you run new vent all the way through the roof or connect with vent stack in the attic. Or, since you are in the basement, and have single story house, you can connect with vent about 48" above the floor on the 1st floor...

I am not sure whether I understand your reference to AAV correctly: AAV shouldn't be installed on stand pipe - but on the drain pipe after the trap...

Thanks, Milo. I just snaked the sewer a few months ago. I wasn't clear in my original post -- I installed the "floor drain" and washer drain together just now and they share the same 2' line. I put the former in quotes, because it's not really a floor drain, it just has an opening on the floor to collect and drain the AC condensation and furnace humidifier run-off. The opening is about 8ft "upstream" from the "Y" junction that connects the washer drain to the line. The AAV is about 5 ' above the floor at a point before the Y junction and after the point where the 1.5' washer tub drain is T'd into the 2' line. Connecting to the vent stack on the first floor/attic could be a problem -- I have concrete floors. The line on the vent stack I plan to tap into doesn't seem to be used by anything upstairs that I can see -- an old basement bathroom sink was connected to it, and when I removed the sink and left it open, no water came out of it. This makes me think I should be able to use it -- does this sound about right?