Post-Mitigation Test Interrupted by a Six-Hour Power Outage
On March 3 an active soil depressurization (ASD) system is installed in a split-level home with a basement and a crawl space. On March 5 you start a 48-h post-mitigation CRM test in the basement, 20 in above the floor and 3 ft from the exterior wall. The system's u-tube reads 0.8 in w.c. and the fan is running at the start. At 2 a.m. on March 6 a neighborhood power outage stops the fan for 6 h; the CRM keeps logging. The homeowner restarts the fan at 8 a.m. when power returns. The 48-h average is 3.8 pCi/L; hourly values are 1.2–1.8 before the outage, 6.9–7.4 during the outage, and 1.3–1.7 after. The homeowner wants a clearance letter showing the home is below 4 pCi/L before a March 10 closing. The installer says the outage "doesn't matter because the average is still under 4."
1. Is the 3.8 pCi/L average valid for a post-mitigation clearance? Explain.
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No. A post-mitigation test must show that the system was operating normally throughout the test. Here the fan was off for 6 h, and the hourly data show the radon concentration rising to 6.9–7.4 pCi/L during that period. The 3.8 pCi/L average masks that failure because the pre- and post-outage hours were low. Averaging is not a substitute for continuous system operation. The test also should not begin until the system has operated normally for at least 24 h after installation or repair; that condition was initially met, but it was broken by the outage. The result cannot support a clearance letter.
2. What should be done before a new clearance test is started?
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Verify that the fan is running, the u-tube shows a normal pressure difference, and the system has no obvious failure. Then allow the system to operate normally for at least 24 h before starting a new 48-h test. Maintain closed-house conditions for 12 h before and during the test. Record the outage and the restart in the file. If the fan stops again during the new test, invalidate it again rather than averaging through the interruption. The contractor should also confirm that the system is designed and installed appropriately, not merely that the fan turns on.
3. How should you respond to the homeowner and installer about the closing deadline?
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Tell them that the current data cannot be used for a clearance letter, even though the overall average is below 4 pCi/L. The installer's argument is technically wrong because the system did not operate continuously, and the hourly peak shows why that matters. Explain that a compliant new test can start after the system has run normally for 24 h, so the March 10 closing may need to be delayed or handled without a radon clearance. Do not issue a letter based on the 3.8 pCi/L average. Document the outage, the hourly data, and the decision in writing so the reason for the delay is clear.