I have a drain in the front vestibule that I've turned into a sump pit since I have stormwater backup that I need pumped out of it when the backyard floods and overflows into the front drain through the combined house storm/sewer. When it overflows, there is nowhere for the water to get out of the enclosed back yard other than either this front drain or the first floor bathtub - and my preference obviously is to overflow that water out into the front yard out of the vestibule pit instead.

How can I create a seal on the horizontal 3" pipe so that sewer gas and mosquitos are blocked, meanwhile keeping the 2 way flow/drainage of water working during a flood or backup. Ideally it stays closed until there is a light amount of pressure from the overflow so that the sump pump can pickup the overflow. We also would want to allow drainage if there is a flood in the front yard that drains into the vestibule so that it doesn't go into the house front door. Maybe some sort of light flapper of some sort?

Note that it is a combined storm and sewer system and it is connected right at the junction between the house and city storm/sewer system.

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  • There should be no sewer gasses from anything that isn't connected to a sewer or septic system
    – keshlam
    Aug 10, 2023 at 20:57
  • it's connected to the sewer Aug 10, 2023 at 20:57
  • Is it connected to the house sanitary sewer or an in street storm drainage system?
    – Josh King
    Aug 10, 2023 at 21:01
  • 1
    Ah. Not what I think of as a sump pit. Personally I'd change that design. If you must retain it, you need a trap.
    – keshlam
    Aug 10, 2023 at 22:59
  • 1
    Put an elbow that gets the 3" turned down and going below the water level. Sumps can feed into a sewer like that but there's supposed to be an upside down damn over the outlet.
    – Mazura
    Aug 11, 2023 at 2:23

2 Answers 2


Put in what's called a "running trap", which is just a U-shaped dip in the pipe. It would be good to have access to it to be able to remove sediment from time to time.

  • That would be the right way of doing it - unfortunately I'd have to break up a large amount of concrete to expand the pit, and I don't think I have the space or clearance in front of the pit without breaking up my front door threshold. I wonder if there is a hack I can do instead? Aug 10, 2023 at 22:43
  • 2
    If you can't put a trap in the 3" pipe, can you seal the top of the pit, add one or more drain grates nearby to collect flood water, connect those grates to the pit with PVC pipe and put traps in THOSE pipes? IE in pipes TO the pit instead of from it?
    – jay613
    Aug 11, 2023 at 0:05
  • That could work - that would also involve chopping a lot of concrete but it seems possible. I also found where it connects to the house sewer in the basement and it is in front of the house trap - maybe I can ask the plumbers to connect that behind the house trap instead... which would also involve a lot of concrete chopping :( Aug 15, 2023 at 11:48

I would think that something like a pop-up emitter could be used where the pipe enters the pit:

enter image description here

This BobVilla.com article (source of the image above) describes their use for gutter-collected rain water, so I'd think that it would work in your case, as well.

I'm not 100% certain that it would seal well enough to prevent all sewer gas from escaping, but, since you're in NYC (IIRC), the small bit escaping would mix with the normal garbage smell* and probably wouldn't be noticeable. You may have to check/replace the seals on the pop-up more often than those living in suburbia who use them for their intended gutter-to-lawn purpose.

*Source: My grandparents lived in midtown Manhattan for decades. Everywhere I went in Manhattan had a general garbage smell. I don't recall it changing much on my occasional forays into the Bronx or Brooklyn.

  • I think that's a good idea, though I think I have to enlarge the pit to make room for that since I jury rigged a normal yard drain's 12" box into a makeshift sump pump pit. FWIW, the garbage smell is only on garbage days - but I'm in Queens so it's not bad. Aug 15, 2023 at 11:50
  • Well @MonkeyBonkey, to the best of my knowledge, I've never been to Queens, so I guess I wouldn't know... :)
    – FreeMan
    Aug 15, 2023 at 13:26

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