Case Study
US Courthouse and US Postal Office 

US Courthouse and US Postal Office

The U.S. Courthouse was our most interesting radon mitigation project. This structure is nine stories in height. The sub basement is beneath the below grade parking garage, leaving the sub basement slab over 20′ below grade. With the parking garage directly over the sub basement and the exhaust requirements of a parking facility, this situation created substantial negative pressure in the structure. The pressure differential between the sub basement and the sub slab environment were over 1/3″ w.c. The normal stack effect of a nine story structure would be approximately .050″ w.c. The amount of negative pressure in this particular structure is almost never encountered under normal conditions. Several methods for radon reduction were utilized to reduce the radon constrictions in the sub basement.

Active soil depressurization Repair of several HVAC Systems Sub basement pressurization

Step 1: The sub slab depressurization system has eighteen floor penetrations and long duct runs. The numerous existing systems (HVAC, Steam, Plumbing, Electrical etc.) made routing of the radon system complex. We decided to run the radon mitigation system through the steam tunnel into the U.S. Post Office instead of up through the nine stories of the courthouse.

Step 2: The complete HVAC control system was upgraded at the same time as the installation of the mitigation system. This repaired several defects in the HVAC system that affected the radon levels in the sub basement. For example, the parking garage exhaust fan (44,000 cfm) was running continuously with no make up air. Even small bathroom exhaust fans running continuously will elevate radon levels. One can imagine what a large parking garage fan can do.

Step 3: Sub basement pressurization was used to reduce the radon levels even further. The chiller room has two cabinet exhaust fans (4500 cfm and 6500 cfm) to remove heat. We rebuilt one of the fans and turned it around to pressurize the chiller room instead of depressurize it. The amount of air remained constant, but at a positive pressure not a negative one.

U.S. Post Office The U.S. Post Office had a radon mitigation system that was designed by our parent company, Thomas J. Gerard & Associates, Inc., and installed by the HVAC contractor during a complete remodel several years earlier.

Knowing the capabilities of the U.S. Post Office system we tied the two radon systems together and changed the fan from a 2 horsepower belt driven roof top blower to a 5 horsepower belt driven inline blower. The combined systems have 48 floor penetrations and approximately one city block of slab area. This is one of the largest radon mitigation systems in the U.S.