I had some work done recently where I think they moved the fresh air inlet to after my whole house storm trap - thus sewer gases and mosquitos coming up where the fresh air inlet comes up in my semi-outdoor vestibule.

Should the Fresh Air Inlet go before the last cleanout trap or after?

Looking up the codes it says: https://up.codes/s/fresh-air-inlets


Building Traps Building traps shall be provided with a cleanout and a relief vent or fresh air intake but in no case less than 3 inches (76 mm) on the inlet side of the trap.

703.7.1 Fresh Air Inlets

Every sanitary or combined building drain shall be provided with a fresh air inlet pipe connected to the building drain immediately upstream from and within 4 feet (1219 mm) of the building trap.

I live in NYC so we have a combined storm sewer line and I have separate storm and house sewer 4" lines, each with its own trap before it combines and connects to the combined city sewer.

As a twist, it turned out the FAI ended was actually a front yard drain. Does that change the answer at all?

Here's a diagram and pictures with my proposed change to move it behind the trap.

enter image description here And the original plans before they did the work to move the FAI/yard drain. Note the original plans had a separate yard drain which looks like didn't happen and instead we just have the combined yard drain FAI under the stairs shed off the vestibule.

enter image description here

Inside the shed underneath the stairs off the vestibule - thus semi-outdoors. You can ignore the sump pump in the catch basin in the yard drain inside... this is what we thought was the FAI and, the sump pump is there to pump out stormwater backup from the backyard since it overflows into this area when it does backup.

enter image description here

Vestibule entryway

enter image description here

  • If I'm understanding you, you have a "fresh air inlet" inside your home that is tied to the plumbing waste line?!! That should not be so...not at all.
    – RMDman
    Commented Sep 1, 2023 at 16:24
  • Every outlet from all drainage to the sewer is required to have a trap, (U bend), it's used to trap fluid, thus preventing sewer gases, insects and the like from entering inlets. If you are getting sewer gases and insects from any inlet, fresh air, (vent), I believe you may have incorrect work done, you for sure should not be getting any form of smell or insects. Commented Sep 1, 2023 at 16:50
  • I added a photo showing the existing and what I am proposing in terms of moving the Fresh Air Inlet/Yard drain Commented Sep 2, 2023 at 11:03

2 Answers 2


There should be a vent after a whole house trap, if a whole house trap is being used.

As with any trap, that prevents the line after the trap siphoning it dry. As with any plumbing vent, it should be run up above the roof level, or somewhere on the sidewall with adequate clearance from windows and air intakes, or have an AAV on it.

It should be located a minimum of two pipe diameters from the trap weir.

  • but outside the home, correct? Not in a vestibule!
    – RMDman
    Commented Sep 1, 2023 at 16:58
  • Other than the worst of acceptable options (the AAV) yes - above the roof level is generally not inside.
    – Ecnerwal
    Commented Sep 1, 2023 at 17:02
  • Yes,.the OP states that it , "...comes up in my vestibule." That is the shocking thing.
    – RMDman
    Commented Sep 1, 2023 at 17:10
  • The OP's alleged plumbing is remarkably quaint for a supposedly modern location like NYC, yes. Of course it's an old city, in parts, but the extent of odd things done over various questions does make one happy to live in the boonies where we don't vent plumbing inside the house. We also haven't permitted whole house traps for decades.
    – Ecnerwal
    Commented Sep 1, 2023 at 17:14
  • They do manufacture auto air intake valves for internal termination., but obviously one is not used here Commented Sep 1, 2023 at 19:05

A fresh air inlet should only allow fresh air into your sewer line upstream of the house trap. Air is pulled in and up out the roof vents. They shouldn't smell, hence their location nearly always being coming out of the foundation often near windows and doors. The house trap separates this intake from the municipal sewer. The only sewer gases would come from your own sewer line upstream of the trap.

I've seen many houses with this intake capped and even temporarily blocked mine even though it didn't smell. I was worried it could smell. Doing some 'digging', I read this air intake is intended to reduce acidity in the near horizontal run of cast iron pipe under the house to reduce corrosion and increase the life of the system.

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