(I have very little plumbing knowledge, so please forgive my ignorance.)
We just moved into a new home and cannot figure out how to install the drain hose for our washing machine. The laundry room has an Oatey Eliminator washing machine outlet box (which is recessed into the wall) and there is a piece of pvc extending from the top of the box down into the opening of what I believe is the standpipe. I'm guessing this piece of pvc is being used to drain something else (maybe the ac condensor?). In any case since there is already a pipe extending down into the opening, we have no idea where to put the washing machine drain hose. Help!

 

Can you post a picture?

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If you have a drain from a air condition system you can take that out and run it just about anywhere you want, any drain or out the wall, it dosen't need to be in your stand pipe. Once you have removed it you have your connection for the wash machine drain, luck.

 

I'd assume that the PVC pipe is to drain the AC as you suspected. The prior homeowners probably jammed the washing machine hose into the same pipe so both could drain into the same standpipe.

As long as the washing machine hose isn't kinked too badly, it shouldn't be a problem sharing the standpipe.