I am installing a new Washing machine hook up in my basement. I am tying it into my existing down stack from the upper floors. I know I will need an inline vent, however I am worried about sewer gases since a washing machine isn't a closed connection to the sewer line. Is a S trap needed? Or will it cause problems when the washer drains by slowing down the discharge flow?

Hi Leathem...

You'll want to install a vent (as youmentioned) and a PTRAP (straps are illegal)... see images.

You can install a mechanical vent (called an AAV), but they are not code legal in most places so then you would be better off installing an 1.5" vent and running that back to connect into another 1.5" vent.

The drain size would be best at 2" and the standpipe out of the PTRAP should be between 24-36". OK?

See images below for ideas on this, OK?

Mark