Pressure Field Extension Fails in One Zone of a Subslab Depressurization System
A radon mitigation system was installed in a basement home with a subslab depressurization system: one suction point, a 4 inch PVC riser, and a 3.2 inch water column fan. The post-mitigation test reports 1.9 pCi/L, below the 4 pCi/L action level. The QC inspector measures pressure field extension at four probe holes: -18 Pa near the suction point, -9 Pa at the far wall, -3 Pa in a corner behind a footing, and -1 Pa in an adjacent slab section separated by a grade beam. The fan is running and the system has no obvious leaks at the riser. The homeowner reports the system was tested only once, two days after startup.
1. Evaluate whether the pressure field extension data support the reported result and what the inspector should conclude.
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Pressure field extension shows whether suction reaches the soil under the slab. Readings of -18 and -9 Pa indicate communication near the suction point, but -3 Pa behind the footing and -1 Pa in the adjacent slab section are weak or negligible, meaning those areas may not be effectively depressurized. Radon can still enter where the field does not reach, so a single post-mitigation test of 1.9 pCi/L does not prove the system controls all entry points, especially with a separated slab section behind a grade beam.
The inspector should conclude that the system is not demonstrated to be effective across the footprint. The inspector should not accept the 1.9 pCi/L as sufficient on its own. The appropriate next steps are to identify why communication is poor in the weak zones (a grade beam interrupting the field, a footing acting as a barrier, or insufficient suction), and to require design changes such as an additional suction point or a crossover in the separated slab. A verification test after correction should be performed under closed-house conditions.
2. What does the single test two days after startup tell you, and what testing conditions should be required?
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A single test two days after startup is weak evidence. Radon concentrations vary with weather, stack effect, and season, and a short post-installation sample can read low during a period of high ventilation or favorable weather. Two days also may not represent stabilized operation, and one sample gives no sense of variability. The result could be accurate or could be a low snapshot that does not reflect worst-case conditions.
Testing should follow the program's protocol for closed-house conditions, typically with windows and doors closed for the specified period before and during the test, and with the mitigation system operating normally. A long-term or multi-sample approach, or at minimum a confirmatory short-term test placed in the lowest livable level and in a room above the weak zone, is more defensible. The inspector should require a confirmatory test after the pressure field is corrected, and should document the test conditions, device placement, and dates. The report should not present a single snapshot as proof of long-term performance.
3. The installer proposes increasing fan size to fix the weak zones. How should the inspector respond?
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A larger fan may increase suction, but it is not automatically the right fix and can make things worse. If the weak zones are isolated by a grade beam or footing, more suction at the existing suction point may simply pull more soil gas and outdoor air through the path of least resistance near that point, raising flow and noise without extending the field. A stronger fan also increases negative pressure under the slab, which can increase backdrafting risk for combustion appliances and can draw more soil gas if the field is uneven.
The inspector should require a diagnostic basis for any change: the pressure field data, a communication test, and an assessment of the slab and footing layout. The likely correct fix is improved field extension, such as an additional suction point in the separated section or a crossover duct, sized to the actual subslab permeability. Any change should be followed by re-measurement of pressure field extension at the same probe locations and a confirmatory radon test. The inspector should also verify that combustion appliance safety was checked after the original installation and again after any change.