Reverse-Flow Radon System (RFS)
Radon enters a home mainly because the pressure in the basement is slightly less than the pressure in the soil. The house literally sucks air out of the ground. The typical radon reduction system operates by depressurizing the soil, creating a vacuum under and around the house that draws off soil gases into a duct before they ever reach the house.
Amazingly, it is often possible to achieve the same radon reduction by running the system in reverse, i.e. blowing fresh air into the soil, creating a flow from the duct through the soil and away from the house. Same airflow, just in the opposite direction. Think of it as a bubble of fresh air displacing the bad air in the soil under the house.
The significance of the reverse-flow system is that the ductwork carries fresh air, not radon-laden soil gas. This means that the fan can be located in the basement, because there is zero health risk from a leak. An interior fan, protected from the elements and accidental damage, can be expected to last much longer than one mounted outside.
We can also eliminate the piping that carries the exhaust to above the eave, because there is no exhaust! Instead, there is an air intake down close to the ground, essentially invisible (hence the nickname "Invisible Radon System"). This may be especially important if the sump is at the front of the house, or under a deck, where a radon exhaust stack would be simply unacceptable.
Here is an example of an RFS system on a home in the Northville/Novi area. Try to imagine what it would look like with fan and exhaust stack on the exterior.
Interior View
A Radonaway XP201 fan is mounted upside down, blowing air into a sealed sump. As with the conventional system, a pressure gauge (the red u-tube) is mounted between fan and soil to indicate its operation.
Exterior View
A picture of the same home. Where's the system intake? It's the small brown dryer vent just below the window, next to the white sump exhaust. Not quite invisible, but almost so. Maybe we should call this an AIR system: Almost Invisible Radon system?
The EPA technical manuals recognize the reverse-flow system as a legitimate means of radon reduction. Then with all those benefits, what's the catch? Why don't we install the RFS as the standard configuration? As you might guess, there is a downside:
- Sometimes it just doesn't work, and can actually raise radon levels if the pressure field does not cover the entire footprint of the house. We have enough experience with reverse flow systems to recognize some cases where they definitely will not work, but we can't make any promises for the other situations. Many people prefer the guaranteed success of the conventional design, especially if (as is usually the case) aesthetics are not a problem.
- Unlike the conventional system, the RFS does require some maintenance. You have to keep the intake screen free of leaves and other debris.
- In the winter, the pipe is carrying cold air. Condensation and frost may develop on the outside, and a small puddle may even form on the sump cover. If this happens, we recommend wrapping the piping in fiberglass insulation. Some homeowners prefer to flip the fan during cold weather, creating a conventional system except for the interior fan and ground level exhaust. If you do this, it's extremely important to have a tight seal on top of the fan, and we strongly recommend a radon-sensing alarm in case something goes wrong and you begin pumping 300 picoCurie/liter soil gas into the basement.
- If the fan is in the basement, there will be fan noise in the basement too. It's much less than (say) the typical furnace, but some people may prefer to install insulation or box the system in with drywall.
Bottom line: As in everything else in life, there are tradeoffs to be made.