If I plumb each fixture inside my house using an air admittance valve (AAV), my understanding is that they will drain/flush properly since air is being let into the system.

My question concerns the sewer gases that will need to be removed from the system. Instead of venting the fixtures in the traditional manner, resulting in a roof penetration, can I simply vent the main DWV 4" line just before it enters the house? Seems to me that the gases are going to rise at the first opportunity.

So, would the combination of AAV's and a single exterior vent suffice?

  • I had never seen or used the AAV in the way described after googling I would not one in my home. The P traps or U bends prevent toxic and or explosive (methane) gasses from entering the living space. The AAV valve could allow gasses into the living spaces.
    – Ed Beal
    Commented Aug 4, 2016 at 1:03

1 Answer 1


That's an interesting question, with a logical thought process applied to the term 'vent'. With that being said:

The wastewater system contains noxious, flammable sewer gases. These gases are kept out of the house by means of a P-trap at each fixture that seals the end of the pipe with water When wastewater is released from the fixture, it displaces water held in the trap. A vent on the sewer side of each trap supplies air so that draining water cannot create a suction to pull water out of the trap.


Vents aren't used to let the noxious gas escape out of the house, they'll always be there so it's impossible to get rid of them. The vents are used to avoid siphoning of the water seals which prevent the gases from leaking in to the house(per example, why you aren't allowed s-traps anymore as they had a risk of siphoning). Because of this you are required to have most fixtures within a certain distance of a vent or vented stack.

My post is more for information rather than an answer.

I believe this post will be of use to you, however.

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