Eric in North TX wrote:
I've found vents hidden inside walls, someone took a short cut and used
pvc one way valves in an attempt to make drains work better. That might
be alright at first (though I doubt it would pass code), but if the
rubber flap or what ever makes it one way fails you have an open pipe.


Vents like these are supposed to be on risers that take them up into the
attic so that they can be serviced.

In theory, you can bring all vent pipes to a single vent trap like this
and run a single penetration of the roof.