Septic Tank Plumbing Vents Covered With Filters in Basement
That's called an Air Admittance Valve (AAV) or a mechanical vent. It's not a charcoal vent, rather it's a spring loaded vent that is supposed to let air into the pipe when waste water washes past it. Here in Massachusetts, they're illegal unless you've got prior permission from the State Board of Plumbers and Gasfitters (the local/town inspector can't even grant permission), and they'll only allow it if you can prove there was no other viable option for running a proper vent (which is almost impossible in a single-family residence).
Having said that, they've been used in europe for decades, and many states do allow them, BUT there has to be at least one vent stack rising up through the house and out the roof. The reason being is that there still has to be a way for sewer gases to exit the piping/septic system.
So...they're either allowing sewer gases (under pressure) to escape each time they open, OR...since there's no vent exiting the roof, there may be an open vent pipe sticking up in your attic - maybe even hidden under some insulation. I would guess the latter because if there's no other way for sewer gases to get out, you'd occassionally hear gurgling at various fixtures as the gas forces its way past p-traps. Either way, you should see if you have any recourse against the home inspector.
Last edited by Ishmael; 12-07-2009 at 01:35 AM.