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Radon remediation getting complicated

Bruce_Davis | Posted in General Questions on
Living in my new construction since May. Been doing Radon testing with two Airthing devices in various locations.  Usually get readings in the teens; on rare occasions, in the single digits.  In the attached picture, you can see the two 4″ passive radon pipes, one by the black arrow; one by the red arrow.  Both Y pipes were drilled with small holes by the plumber.
I did two pipes because the yellow line is a footer preventing full ventilation under the slab between two halves of house.  There is one small opening in the footer in the lower right corner.  Dotted purple lines are interior walls.  Sump pump is in upper left corner.  Slab is 3500 sq ft.  I think the size alone also warrants two passive pipes.  The interior footer/foundation is to support a 4-car suspended garage to the right of the yellow line.
The neighbor on the right side has been there 20 years with no sump pit.  He just bought an Airthing and in the first week or so was in low single digits.  My neighbor on the left, was built the same time as mine.  After installation of a single fan with two input pipes and a sealed sump pit, they’re still high single digits with a 48-hour test.
We put a RadonAway RP265 fan on the red arrow pipe which provides 140 cfm at the 1.5″ static pressure shown on the manometer (if I understand it all correctly).  Radon numbers did drop immediately, but went back up to double digits a few weeks later and have stayed there.  Pressure has remained steady at 1.5″.
A week ago, I sealed the sump pump with the Radon Dome.  Immediately dropped about 10 to mid-single digits.  It’s creeping back up to double digits.  I do have to fill the hole under the bathtub as mentioned in my other recent thread.  It remains to be sealed.  There is a HVAC system on each of the two levels with an ERV hooked to both air handlers.
My question concerns the second passive pipe with the black arrow.  There is minimal room in the attic at the exit point for another fan.  May or may not fit.  What is my current situation with one active exhaust and one passive exhaust.  Does it negatively (no pun intended) affect the active fan exhaust pressure?  Is it best left as is, one active, one passive; or should both have fans or should I cap the passive exhaust.  I don’t remember my physics for liberal art majors in college.  Thanks.

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  1. Expert Member
    Dana Dorsett | | #1

    With an active slab depressurization it may be drawing most of the air from the passive stack, and only taking a lesser amount of soil gases. With a smoke pencil at the top of the passive stack on a calm wind-free day you may be able to determine if it's back drafting. If it's clearly back drafting, capping the passive stack should improve overall effectiveness.

    In most houses installing a reasonably sized dedicated HRV for the basement would cut the radon levels roughly in half, even if it's just a single pair of Lunos e2 or similar.

  2. Bruce_Davis | | #2

    My theory (I didn't get a great grade in physics) was my setup would be like having windows open on both sides of the house to get that strong cross-breeze. I thought the back draft would force more air through the slab and out the other side. Of course, my passive stack is about 30 ft off the ground so I'm not keen about climbing up there. What do you think about having both stacks active, does that double my exhaust capability? I think RadonAway says their fans are only meant for about 2000-2500 sq ft slab. Thanks.

  3. Jon_R | | #3

    Your active fan should operate at ~100x more pressure than a passive vent. So plug the passive vent that is almost certainly back drafting and reducing the effectiveness of the whole system.

    The goal is to maintain negative pressure (measure this) and *reduce* flow - not ventilate the radon away.

  4. MattJF | | #4

    Another vote for plugging the open passive stack.

    It's really important to plug all the holes in the slab. Use a polyurethane caulk to seal the joint between the wall and floor all the way around. I think sealing everything well should get the improvement you want. The goal is to get the slab under vacuum as Jon said.

  5. user-5946022 | | #5

    @Bruce Davis: Did you ever solve this, and if so, how?

  6. Bruce_Davis | | #6

    @C L, thanks for checking back. I've been doing long-term testing with two Airthing monitors, one at each end of the basement. See the original post for details of my layout.

    My roofer was kind enough to plug the second vent (passive) about 10 months ago. Can't say that made a noticeable difference. I'm waiting to hear back from my neighbor who recently added a second fan and the initial reading was still poor. That was a month or so ago. The radon company has been out since, but don't know the status.

    I've found the biggest factor is the weather, primarily the temperature. In warm weather, my numbers are very nice. Whether on the side with the radon fan or no fan, I get daily numbers below 1 or 2. Conversely, in cold weather (cold edge of Zone 4), the numbers with the fan are fair; daily range typically 4 or 5 or so. The area without the fan is poor; daily range typically high single digits in late fall and in the teens during winter. In the coldest parts of this winter, I'm up to mid 20s. That's with the unusually sub-zero weather this past week.

    I did fill the bathtub hole with some hydraulic cement, but it was too thick and the result is porous as it was hard to pour. A bucket of water drained through it pretty quick. Think I need to pour some very thin cement in there.

    In the 10 months since I reset both monitors with no other changes, I'm getting 2.78 in the area with the fan and 6.2 in the area without the fan.

    It was lazy of me not to caulk the floor/wall seam before framing. I'd say about 20% or so is now inaccessible due to being finished, but I could do the other 80%. Seems the 15 mil vapor barrier was a waste of money as they didn't seal the seams very well nor did they tape to walls; just laid material on the footers. I do have the slab overlaying 6 inches of footer. I was hoping that would provide sufficient sealing in itself.

    I'm thinking I do need the other fan. I may wait until after I seal the bathtub drain & walls. Where should I put the fan?

    1. On top of the existing 4" passive pipe. Y-pipe is already in there and extending under slab in gravel (see lower left side of original photo). No room in attic for fan. Fan would be on backside of roof and not visible from street. ABS pipe has lead flashing over it so not sure how fan would mount to that. Or . . .

    2. Run new pipe from sump pit (upper left corner of picture). Unfortunately, side exit from house would be near other inlet vents. Pipe would have to go to rooftop. Pipe and fan would be visible from street.

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