We moved into this house in New Hampshire a year ago. Tons of radon off-gassing in this area due to granite rock substrate . Home inspection showed that the mitigation system was functioning properly, long-term test results below 2 pc/L. Recently I noticed that the manometer was showing equal readings on both sides of the u-tube. I don't recall what the level was during the home inspection. There are no marks I can see on the gage. When I turn off the breaker powering the fan, there is no change on the manometer. The fan is running; I climbed up and stuck my face it the exhaust; it smells wet and musty like basement air. It inflated a plastic shopping bag immediately. The sump pit looks to be well-sealed with silicone caulk. But I also see a 3" diameter hole through the concrete slab that is covered with a thin piece of cardboard. I can hear it whistling, and when I put my hand in the hole I can feel suction. Is there anyway for me to gain confidence in this radon mitigation system without hiring a contractor?

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3 Answers 3


Our u-tube shows a 2" WC. IMO, the hole is the problem: the pump is sucking air from the basement, making both sides of the tube at the same (basement) pressure. Hence, 0 pressure difference. This will obviously reduce the suction from the subsoil; at the same time, it will be pulling radon-contaminated air from the basement. I have no idea what the net effect would be.

You could get a continuous radon monitor. (Amazon, $130) Not cheap, but I find ours very reassuring.

FYI, a couple of things, from experience with our system. (It doesn't seem like these apply to your problem, but just in case.)

  1. We've had the outlet ice up in winter. (Moist air from sump, cold pipe.) When this happened, we showed 0 pressure.
  2. We've had problems in summer, where a water bubble developed in the u-tube. This affected the reading, but I don't remember how much. At the time, we were drawing outside air into the basement, and getting a lot of condensation. So, warm air, u-tube in contact with the cold pipe. We now keep the cellar mostly closed in summer, and I spaced the tube out from the pipe.

Systems typically read around 1" WC. The open hole that is whistling is curious. How far from the exhaust pipe is this hole? With that much air being pulled the WC should be higher than 1/4". WC is the difference in column heights not the reading on the positive scale. Add the below zero value to the above zero value.

  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know the details of contributing here. Commented Aug 8, 2020 at 23:36

U tubes are quite simple devices. Make sure the tube connection is not plugged. The hole may be the problem, try plugging it then if the pressure Increases you have found the problem if there are no restrictions for air to enter the pressure will be the same on both sides and no difference in the reading.

  • I plugged the hole as best I could. It actually extends underneath the sill plate, so I may need to come back with spray foam to completely seal it. That did not change the gage reading. Then I pulled out the flex tube and re-sealed it with silicone. This gave me a manometer differential of 1/4" WC. Is 1/4" WC enough to ensure that my system is working properly, or should I expect a higher reading?
    – twd000
    Commented Sep 12, 2016 at 19:26
  • I am not sure what reading to expect but now you are pulling a slight vacuum. I would think with any vacuum it is doing something. At least you know the shaft is not venting into your home. It is possible that some fresh air allowed to be pulled into the system will help exhaust the radon.
    – Ed Beal
    Commented Sep 12, 2016 at 19:56

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