Radon mitigation using air to air heat exchangers requires the building have low radon levels and a low natural ventilation rate. These restrictions mean air to air heat exchangers have limited use for radon mitigation.
Soil gas radon enters structures through openings in the foundation. The radon gas concentration in any building is determined by the radon concentration in the soil, soil permeability, and the pressure differential between the soil and the building.
The EPA suggests the radon level in the water should be tested if there is a private well and an elevated radon gas concentration is found in the home. This is good advice for a home owner but not necessarily good advice for a home buyer.