I located this below-slab sewer drain access pit just inside the front basement door of a home. Inside the pit there was a cast iron pipe with one clean out access, at least I could only see one (not two like with a U trap / house trap), and a PVC pipe was connected to it, which appears to be a vent. This appears questionable, and so I’m looking for some other opinions on it.
If it is a vent it has inadequate clearance to that door and it’s not properly installed as it should terminate above the roof line, the only acception that I can think of would be is if they somehow installed an AAV that is not visible within the wall.
Also the wood to earth contact below the slab is a tasty invite for termites.
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Did you put your nose up to the exterior vent to see if you could detect sewer gasses?
This piece you labeled cap, was it actually attached to something? Looks to be just laying there discarded.
It appears that the cap used to be covering the access point that this PVC pipe is connected to.
I looked into this a bit online, and it’s called a “fresh air drain inlet” which seems to be common in the NYC area (not sure of other areas). You’ll have your vent at the roof as usual, but then also will see these at the front wall of the home or coming up out of the ground or sidewalk just outside a building. I’ve seen them before, but not connected to the cleanout in an access pit like this, which is why it stood out to me. So I guess the real question is about the location more than the purpose.
Here is an article that mentions it: https://www.balkanplumbing.com/find-house-sewer-line-sewer-clean-out/
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Interesting. I’ve never heard of these. I found this article that seems to explain both the location and the purpose pretty well.
"Digging in old plumbing manuals, here’s the only reason I’ve found for an FAI (Fresh Air Inlet):
The FAI is installed at the lowest part of the plumbing system, either just before the building drain exits the building or just after. The FAI’s purpose is to allow fresh air to enter the plumbing system and exit the vent through the roof, in order to prevent the premature corrosion and failure of ferrous plumbing materials such as cast-iron, galvanized and copper piping."
It is indeed a fresh air inlet for a house trap. A common thing in my area. The termination is just a foot or so above the grade. The vent is on the house side of the house trap, not the sewer, so unless the trap goes dry, you will never smell it.
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Interesting. Thanks for locating and sharing that article.
So the main purpose of the fresh air inlet (FAI) is to protect horizontal runs of cast iron pipe (or ferrous materials / materials that contain iron) from premature corrosion which happens at the inside top of horizontal runs due to sewer gases combined with condensate which forms corrosive carbonic acid.
The house trap and special conditions is still a bit vague. But if you have a FAI then you should also have a house trap so sewer gases from the street side don’t exit the FAI.
So in this case it appears the FAI is connected to the house side clean out of the house trap. Which I guess would do what it’s intended to do, but I don’t like that this connection would have to be disconnected any time someone wanted to access the house trap clean out. It’s blocking easy access. And from a photo I saw in the article I shared further above, it appears it should ideally be connected before the house trap clean outs. See photo below.
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This is a good educational thread on the subject and I learned something new.
Thanks for posting!! 
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