N. Thornton wrote:

I wonder if the 2 pipes go different places, its quite possible. I'm
told that can be done to allow use of a smaller septic tank. If so
they'll need to be kept on the 2 separate flows.

Ah - good point. The house used to have a septic tank, but now it's on
the mains, so it could well have been designed like that.

Your question: I dont know. Certainly if you used an open drain you
wouldnt need a vent pipe.

Well, it's an open drain, so it looks like I'll be fine doing away with
the vent pipe. Thanks.

--
Steve Loft