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In our basement there's a laundry sink that connects to the main 4-inch DWV pipe below the concrete floor. There's a 10-ft sloping horizontal length of 1-1/2 copper pipe that turns down ~90 degrees and disappears down into the concrete floor over near the main 4" vent pipe. So it is connecting to it below the trap weir, as in the diagram. I imagine there's a 4x4x1-1/2 tee in the concrete floor where the copper line meets and is sealed with lead and oakum. Other drains in the house use that approach.

I don't know if there's a siphon happening, but I am considering retrofitting an AAV mid-run on near-vertical pipe in case there is, roughly per this diagram which shows a vent preventing siphon. My AAV would be perched at the top of a near-vertical pipe so it's sitting an inch or two above the top of the sink. Is there any reason that would be a bad idea?

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Drain vents are commonly required to be within a short distance of the fixture. 10 feet certainly violates that principle (if not specific local code), so additional venting would be appropriate.

Instead of an AAV, though, which is a bit of a hack, consider boxing the vent to the 4" stack from near the sink, or run a new vent all the way up. (I've done so in a corner of a coat closet.) If an AAV is your only option, give it a try. Worst case you have to cap it.

All that said, you haven't indicated that there's an actual problem. I'd wait until you detect sewer gas leakage (or monitor the trap) before spending time and treasure on a maybe problem.

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  • The vent is iron pipe so I can't really tie into it, and there's no access above. An AAV is the only feasible hack.
    – mr blint
    Commented Mar 26 at 13:06
  • I see relatively inexpensive gas detection devices. Can they detect sewer gas?
    – mr blint
    Commented Mar 26 at 13:10
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    You can tie into iron pipe. I've done it myself for a basement laundry similar to yours. The right saw and a pair of shielded no-hub couplers and you're in business.
    – isherwood
    Commented Mar 26 at 13:11
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    Your nose will detect sewer gas. It should be easy to check the trap and do some testing, though.
    – isherwood
    Commented Mar 26 at 13:12
  • This 4" iron pipe goes up from the basement and out the roof (two-storey house). Isn't that a lot of weight to transfer to the horizontal pipes attached to it when a section is cut out and replaced with rubber?
    – mr blint
    Commented Mar 26 at 13:24

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