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jskirwin 12-05-2009 11:01 PM

Septic Tank Plumbing Vents Covered With Filters in Basement
I recently moved to a rural area and am trying to understand our septic system. This system seems to be relatively new with PVC pipe in the basement. I notice that the plumbing has two vents in the ceiling of the basement covered with charcoal filters. These filters are made of PVC and are glued to the system.

After draining a bath these vents produce sewer odors in the basement that rise up through the floor into the house. I've checked the roof and it appears to me that there is no venting above the roof as I would expect with a city type sewer system.

Does anyone have any info on these vents? What are they called - and should I look into getting a vent cut through our first floor and roof?


meboatermike 12-06-2009 12:29 AM

There should at least be one main stack (a good sized pipe) that would normally vent your plumbing fixtures and it would usually go up through the roof. I have lived on septic systems my whole life and used to inspect new and existing houses for a government agency from about 1975-1992 and have never seen any filters on pipes venting into the cellar.

You really need to have a master plumber look at your plumbing system and make sure it is up to code -- you should not be smelling gases inside the house.

Ishmael 12-06-2009 10:55 AM

meboatermike is correct...
...I'd be willing to bet that "new" plumbing you're looking at doesn't meet the accepted plumbing code in your area (or any other for that matter), and has never been inspected. Get a professional in there to make it right as this can be a very dangerous situation. Methane gas can build up in the basement just like natural gas or propane can and cause an explosion. Did you not get a home inspection before you bought the house?

jskirwin 12-06-2009 10:26 PM

I had the house inspected before I bought it. The home inspector reported "The piping within the waste and vent system is good quality." He doesn't say anything about the sewer vents in the basement ceiling.

Since the ceiling is not finished, you can easily see the vents and most of the plumbing quite easily. I've attached a picture here:
The pipe leading from the right comes from the bathtub which has a trap. It slopes down the left and joins the main line that exits the house to the septic tank.

Ishmael 12-07-2009 12:31 AM

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.

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