Greetings.

Academic studies like the one below show that ventilation with a HRV or ERV can effectively mitigate radon:
http://www.wseas.org/multimedia/journals/environment/2012/55-340.pdf

I tried to achieve a similar result in my house but found that ventilation with a HRV had no effect. By contrast, passive ventilation with unconditioned air had a strong effect. The details are below. I would be interested if anyone sees a reason for this puzzling outcome.

The house was built around 1870. The basement is small, about 5,300 cubic ft. The basement floor is one-third dirt and two-thirds thin, cracked concrete. When I first moved in, radon on the 1st floor measured about 5.0 pCi/L when windows were closed.

I then sealed air leaks in the basement -- covering drafty windows with plastic, applying spray foam to the rim joists, and covering the doorway leading to the Bilco door. After these changes, radon levels in the living area doubled.

Next, I installed a HRV in the basement that provides 0.7 ACH. I positioned a single supply inlet in one corner. I configured two exhaust intakes like the arms of a Y and located them in the opposite corner. I assumed that this layout would create a constant movement of air diagonally across the basement, forcing bad air toward the exhaust intakes. I used a mamometer to check that supply and return are balanced.

The air in the basement is noticeably fresher. However, radon levels in the house have not changed (as measured over months with a Corentium continuous gauge).

If modest passive ventilation cut the radon in half, why does forced ventilation at 0.7 ACH have no effect? Apparently, there are factors in the physics of radon or of ventilation that I’m overlooking...

Your supply and return are balanced, I wonder what the pressure differential between basement and main floor is?

I would think that you would want the basement slightly negative compared to the main floor to counter the stack effect and to induce a slight downward flow to at least keep the radon in the basement where it can be ultimately exchanged to the outside by the HRV.

I was taught that it is best to remove interferences at their source. ALong that line, I would probably be inclined to bag/tape the basement floor and walls and provide some ventilation of the sealed space(soil/earth side of the sealing envelope) to place it under a negative pressure and vent that outside to try and intercept the radon as it escapes from the earth before it ever gets into the main basement air...

Not sure how your basement is connected with the rest of the house, but a slight negative pressure would be beneficial. Any gas appliances in the basement? Depending on draft, they could upset your balance. It may be (if your basement allows it) best to add a heavy vapor barrier and a 2" rat slab in the entire basement with sub-slab radon mitigation.

There could be some additional sources of radon gas in the home which might not be addressed by basement ventilation. Well water is one possible source, slight possibility natural gas supply has some. If your 1870 home has significant amounts of granites and marbles or other stonework, that is another potential source. These are usually smaller sources, but could be problematic if the home is really tight.

Why not an HRV in the main floor. Bring the fresh air into the living space?

I suspect that your "modest" passive ventilation was actually many more ACH than the HRV. Or maybe your tests don't account for different seasons (changes stack effect) and/or heating system operation.

An airtight barrier with negative pressure vented to the exterior will work.

Thank you all for these thoughts. You make me realize that the original passive air leaks may have flowed mainly outward. They may have drawn air down from the house and sent it outside. In that case, the passive leaks would keep the radon out of the house no matter how minor they were.

I'll try increasing the airflow in my HRV's exhaust in an effort to re-create that effect. Will report here if successful.

I know that sealing the floor and installing subsoil suction would be more effective, but that option would be expensive and disruptive in this basement.

The heating system is a geothermal heat pump with forced air. The air should be circulating only in the living space, unless there is a duct leak in the basement. The system does add another variable to the equation.

I'm late to this discussion but I would note that:
1) Seasonal variations might be significant, because the stack effect varies in size and direction by season. An electronic monitor lets you keep track all the time and keep tabs on those variations.
2) Even crude partial sealing of the dirt floor without suction could make a difference.