First verify outdoor air provisions exist . Then verify they are working and actually allowing fresh air in . I have seen cases where plywood has been placed over these to reduce energy costs . I have also seen where the damper motor is operating and the energy management system is telling the damper to open , but the linkage to the damper is either broken or has been disconnected .
There are often protective devices that will close an outdoor damper if the outdoor air gets too cold to where a coil may freeze ( called Freeze Stats ). These are set up such that if the sensor or control fails , the damper will return to a closed position , i . e . “ fail safe .” Many times these controls have failed without maintenance being aware of it or lack of maintenance funds to repair . Failure of a damper control in one room ’ s unit ventilator but not in another room can easily account for significantly different radon measurements in these rooms .
There are also filters on the fresh air make-up and sometimes dehumidification coils . These filters need to be changed regularly and the coils cleaned . If not , the required air flow is reduced . If you are told they are working perfectly , take your smoke bottle and prove it to yourself . Many times the HVAC systems serving the elevated rooms are not allowing air in . In one case , I asked the maintenance person when was the last time the filters were changed . The answer was “ Never ” because the access to the filter ( from the outside ) had been compromised when they had repointed the exterior brick 20 years ago . After a little hammer and chisel work , the situation was fixed and new filters installed with two results : The radon went down and the occupants of the room noticed a significant improvement in air quality . All without a Pit .
Another example was an airport where one terminal was elevated and the other was not . Due to the size of the airport , dozens of ASD systems would have been required to fully treat the subject terminal . When talking with maintenance personnel they bemoaned how the HVAC system had been installed on the “ bad ” wing such that they could not clean the dehumidification coils like they could on the “ good ” wing . Again some modifications were made , coil cleaned and same result : Low radon , better air quality .
When determining a strategy , walk into the subject rooms and in addition to looking down searching for footings also look up at the ceiling vents . Are there both a supply and a return ? Sometimes a partition wall has been