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So for context: we had radon in the house so we had a fan installed to suck the air underneath the basement slab. When they drilled the hole, they found water under our slab. The guy said that it was probably going to be okay since there was still some space for the air to go through. Fast forward a couple days after the installation, the fan made a really high-pitched noise and when I removed it the water level was really high it was actually touching the slab so there was no space for the air.

We decided to install a sump pump to remove the excess water, and the water level definitely has lowered. (The sump pump is about 3 feet next to the radon fan) What happens now when we start the radon fan, for a couple of hours it will be fine, no unusual sound, but after 4-5 hours a gurgling sound appears, like some bubbles of air are moving under our slab. When I remove the radon fan to inspect the hole, I can see the water is a little bit high, but there is still space for the air to move (maybe 2 inches between the water and the slab) . When that happens, I turn off the radon fan for a couple of hours, the water lowers a little bit more, and then I turn on the radon fan again, and after a couple of hours the water level increases and the gurgling sound comes back.

I suspect that the radon fan actually sucks the water from the ground over time and creates a pressure differential strong enough to prevent the water from going to the sump pump. That would explain why turning it off removes the problem for a couple of hours, but I'm no expert. I talked to the plumber that installed our sump pump, and to the guy that installed our radon system, but both of them don't know enough about radon or plumbing to help us. In my area there's no other radon mitigation companies... That's why I'm posting here because I have no idea how to fix this. Any ideas?

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  • Was just a pit added for the sump pump or drain lines also? Do imagine the fan is reducing air pressure under the slab.
    – crip659
    Commented May 6 at 13:32
  • That is its job. But now there's a big new air leak into there from the improperly vented sump.
    – Ecnerwal
    Commented May 6 at 14:21
  • Do you have a radon monitor, or are you depending only on test results? Turning the radon fan off and the way this sump was installed will decrease the effectiveness of the system at removing radon. Connecting the sump vent to the radon system should fix that. Radon monitors have become fairly affordable devices in the past 5 years or so.
    – Ecnerwal
    Commented May 6 at 14:35

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That was poorly done. The radon system should have a connection to the sump.

Well, as a new sump, it really should have been installed where the radon system went into the floor anyway, but too late for that now.

The apparent vent pipe where the sump is connected (unusually for a sump - most places don't allow connecting a sump output to the sanitary sewer, which is what that appears to be) to the sewer pipe could be cut, the sewer connection of it capped, and that sump vent connected to the radon pipe.

The sump pump won't have any problem overcoming the slight pressure differential when it runs, but it will eliminate the pressure differential between the sump and the radon system. Right now it's sucking sewer gas from the pipe, into the sump, and under the floor to the radon system. Presumably/hopefully that's a properly sealed sump cover for use with a radon system, so it needs the vent, but the vent does NOT go to the plumbing stack, it goes to the suction side of the radon piping.

Not directly related to this problem, but worth noting: It's not to present standards to have the radon fan inside the house, because the section of pipe above the fan is pressurized, so any leak there will blow radon into the house.

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  • Just to be sure, do you think we should connect the sump vent to the radon pipe before or after the fan? If I understand correctly the radon fan should also suck the air inside the sump? And yes the sump cover is properly sealed.
    – dude777
    Commented May 6 at 14:46
  • Before (suction side of) the fan. That fan really should not be in your living space at all - installation guidelines put them outside, or in unoccupied attics, so that all radon pipe in living spaces is under suction. Here you want to suck on the underside of the floor from the sump area as well as over there in the corner.
    – Ecnerwal
    Commented May 6 at 14:52
  • Thank you! I think I understand the issues with my system. There is however one thing that still bothers me. The water level is still very high, do you know if connecting the vent to the radon pipe will lower it enough to remove the gurgling sound. Because that is the real problem in my opinion because when I drain the water with a wet vacuum, the system works, For a while it lowered my radon levels from 550 to about 70-80
    – dude777
    Commented May 6 at 15:08
  • I can't say for certain, because I don't know the details of the soil under your slab and it seems that the sump was probably installed without any drain pipes leading to it being installed. But by depressurizing the sump the natural flow of water to the sump from the radon fan area should no longer be impeded, so it improves the heck out of your odds. I hope those numbers are Bq/m3. If need be you can probably retrofit a pipe from the sump towards the radon system to move water to the sump faster.
    – Ecnerwal
    Commented May 6 at 15:12
  • Huge thanks for your answers. We will try connecting the vent to the radon pipe and if that's not enough, retrofitting a pipe between the sump and the radon system is a good idea! Big big thanks to you!!
    – dude777
    Commented May 6 at 15:36

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