Post-Mitigation Verification With a Weakening Pressure Field
Six weeks after a sub-slab depressurization system was installed in a 1965 ranch, you return to verify performance. A pre-mitigation 48-hour CRM test in the basement averaged 12.6 pCi/L. The system has a single suction point, 4-inch PVC pipe, and a U-tube manometer that read -1.8 inches of water column at startup. You place a calibrated CRM in the same basement room, confirm the fan is running, and start a 48-hour test under closed-house conditions. Hourly values settle at 1.2, 0.9, 0.7, and 0.6 pCi/L, and the 48-hour average is 0.8 pCi/L. At the end of the test the manometer reads -1.2 inches of water column. The homeowner mentions a musty smell near the floor drain and asks whether the system is "done." The house has an atmospherically vented natural gas water heater next to the basement stairs.
1. Is 0.8 pCi/L enough to call the mitigation successful? Explain what else must be verified.
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0.8 pCi/L is a good result - below the 4 pCi/L action level and below the 2 pCi/L post-mitigation goal that EPA recommends - but the number alone does not close the job. Post-mitigation verification also requires confirming that the system is operating as designed: fan running, no unusual noise or vibration, manometer showing adequate vacuum, labeled system, sealed suction point and floor penetrations, and pressure-field communication under the slab. Check combustion appliance safety because a depressurization system can backdraft an atmospherically vented water heater. The test conditions should represent a worst case (closed house, heating season), and the CRM must have a current calibration. Document the pre- and post-mitigation values together so the reduction is defensible.
2. The manometer changed from -1.8 to -1.2 inches of water column. What are possible causes, and what do you do?
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A drop from -1.8 to -1.2 inches of water column means the pressure field weakened. Check the simple causes first: U-tube fluid level (evaporation or a tilted gauge), tubing kinks or disconnected fittings, a cracked or opened test port, condensate in the tubing, and the fan itself (debris, ice, failing motor). Compare the U-tube against a digital manometer. Then inspect the suction point and slab for new cracks or unsealed openings, and check whether the homeowner added a floor drain or changed a sump cover. If the fan is healthy and the gauge is accurate but the vacuum is still low, the sub-slab communication may be marginal; a second suction point or additional sealing may be needed. Report the finding to the mitigator and do not call verification complete until the pressure field is adequate.
3. What combustion safety checks apply, and what do you do if you find spillage?
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Use a smoke pencil or match at the draft hood of the water heater and observe for spillage at the start of the test and after the system has been running, ideally under worst-case conditions with bath fans and the dryer running. Check for rust, soot, or backdraft marks, and confirm the water heater has a proper draft hood and vent slope. If you see spillage or the flue does not establish draft, shut down the appliance or advise the homeowner to do so and have a qualified HVAC or plumbing contractor evaluate it before the system is left running. Document the test, conditions, and referral. A radon system that lowers radon but creates a combustion safety hazard is not a successful installation.