radon problems, sub slab depressurization doesn't seem to help.

Hi. Here's my sceanrio. I live in Southern NH I have a new house, built in 2003. Colonial, 2 floors, full basement, attic.
My basement is approx, 30 x 70. The foundation is on a footing that goes around the entire house. Down the middle of the house, from 1 end to the other is a footing for the columns that support the center line of the house. Under the concrete floor is 2" crushed stone. My basement has 7' high concrete walls all the way around, the soil around the house is about 2-3 feet under the top of the basement walls.
When the house was built, I had them install a radon pipe. In Feb 2007 I did the ProLab test I got 68 and 70. The kit has 2 vials. I put them near my furnace in the basement for the test. I did the test again a month later and, same reading. I bought the digital saftey siren radon monitor and put it in the same location as my lab vials. The reading maxed at about 36.
I then installed Fantech inline radon fan on the mitigation pipe in the attic and vented through the roof. Summer came, house had open windows all summer, monitor reported usually < 4, sometimes as low as .6
Cold weather returned, windows closed and monitor started to go up. In a week after a memory clear on the monitor, it's gone from 18 to 39 Still in same location as original
First I caulked the entire basement perimiter, cracks and cuts the contractor put in for expansion. No improvement. So, I started to diagnose the fan, I made my own manometer, I shows about 1.5 - 2" of H2O suction when the fan is on a 2' long pipe and the pipe end is sealed. So, it's not the fan. I then cut the mitigation pipe open about 3" above the slab basement floor, looked inside, The bottom was clogged with rock and cement. I chiseled all that out and noticed 1/2 of the pipe was actually butted up on the footing. So, I chiseled that out, now the fan can actually pull air under the basement floor. It then read about 1" on manometer. A week later, the monitor still read Hi 30s, 38.x usually.
Some neighbors had pros do their homes, and they had a second vent pipe installed diagonally across the basement, that then hooked up to the same exhaust pipe to the attic. So, I figured, OK, go drill a hole over there and do the same. So, I put 1 1" hole through the floor and noticed that I could hear wind. If I hold a piece of paper over the hole, it gets sucked in. If I shut off the fan, no more wind and no more suction. Thus, it appears to me that the fan is sucking under the entire floor. I clogged the hole and waited 4 days, same reading 38. I opened the hole waited a few days, same reading 38ish.
This past weekend, I installed a secondary pipe from this new hole, all the way to the attic vent pipe and the number is now mid 39. I now have about 1/2" of H2O on the manometer.
Can anyone tell me how to make this number go down? I considered the impact of radon in the water, but even if we're gone for a week, the numbers don't drop. I've tested in rooms higher in the house, the bathrooms (1/2 the value) , everything has significantly lower readings than the basement.
So, can anyone offer any insight? Is there not enough vacuum? Is the problem my basement walls? Why does it seem like my vent pipe/fan makes no difference. What do you think the next step should be?
Thanks a million
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I work in multi-family and have only had 1 building needing radon prevention. We were required to put in two pipes -- on for air in and one for air out. The pipes went in a bed of crushed stone with a plastic layer on top.
View from top, with "i" being intake vents and "o" being vacuum lines.
iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii i i ooooooooooooooo i o iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii o i oooo <-- to vertical stack with fan. i ooooooooooooooo i iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii
This washed the entire sub-foundation with clean air and vacuumed out the radon.
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so its only a problem in the winter? what sort of heating system do you use? is there outside air intake as well as exhaust?
your furnace may be depressurizing your basement sucking radon in.
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On Mon, 12 Nov 2007 16:40:26 -0800, " snipped-for-privacy@aol.com"

That's a good possibility.
I would get an engineer in to thoroughly check the situation. It will be expensive, but can you put a price on peace of mind?
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Haller, Can you please explain how the depressurization happens? thanks.

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No outside air intake. The house was built in 2003, it has a large 2 zone furnace / ac unit. The air is pulled back to the furnace via numerous returns in the house floors.
There is an exhaust that blows outside. and a condensation drain that goes outside.
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yeah two issues..
make sure the there are no open RETURN ducts or other unexpected openings in the return ducts that will lower the pressure in the basmement.. this is due to the conditioed air system pulling air..
the second issue is the Combustion air, the furnace needs air to feed the fire, you may want to insatll a drey vent that will work in reverse to allow a source of outside air to flow in to feed the combustion air to the furnace.
Does running the furnane and or air handler impact hte pressure in your basement.
Mark
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How would I know if the furnace was changing the pressure? I'd probably need a pretty sensitive pressure gauge of some sort right?
I hear you on the pressure issues. I'll try to inspect.
Thanks for the advice.
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After some time, I decided to do some more testing
Dec 8 2007 I put the digital sensor in my unfinished garage, reset it, 2 days initial read 0.4 (great!) That day I put it in basement, reset, initial read after 2 days, 38.2, 2 hours later 38.8 Then I shut off my radon fan. wait 2 days, reset sensor 2 days later, initial read dec 23 8am 44.3 So, it looks like the radon fan is barely doing anything. Only getting about 6 point improvement.
So, I opened 2 basement windows, turned fan on and by 12:00pm 43.0 dec 24, got down to 33.7, reset sensor, put house fan in window blowing out at hi for 2 days. dec 26 12pm initial read 10.2 6pm 9.8 So, it looks like just exhausting via my window works 4 x better than the sub slab depressurization. I closed my windows then
Dec 12 27 7am 10.8 Dec 12 28 12pm 12.9
So, it's going back up with radon fan on.
Shouldn't that fan, with good vacuum pressure perform better than that?
Jan 2 I installed 2 HP220 fans 6" in the basement blowing into a Y and then up to the original fan in the attic. reset sensor Jan 4 initial read 16.7 Jan 5 17.4 Jan 6 20.4 Then I opened the basement windows again and with all 3 fans running, after a whole day the average only went down to 19.0.
Any suggestion?
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