Post-Mitigation Verification With a Seven-Hour Fan Outage
A sub-slab depressurization system is installed in a home with a pre-mitigation 48-hour average of 8.9 pCi/L in the basement family room. The contractor starts the fan Monday at 4:00 p.m. Your SOP requires the verification test to begin at least 24 hours after start-up and to run 48 hours with the system operating continuously under closed-house conditions. You deploy a CRM Tuesday at 10:00 a.m. in the same family room. The homeowner reports the system alarm sounded Wednesday from about 2:00 a.m. to 9:00 a.m.; the contractor says a condensate problem tripped the high-pressure switch and the fan was off for those seven hours. Hourly CRM data: hours 1–15 at 1.1–1.6 pCi/L; hours 16–22 at 3.6–4.4 pCi/L; hours 23–48 at 1.0–1.7 pCi/L. The 48-hour average is 1.7 pCi/L. The contractor wants the report to close the job, and the homeowner asks whether the house is 'fixed.'
1. Is this a valid post-mitigation verification test? Identify every defect and explain why each matters.
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Invalid, for two independent reasons.
- Early start. Deployment at 10:00 a.m. Tuesday is 18 hours after start-up, not the 24 your SOP requires. The sub-slab and the soil around it have not reached steady-state communication with the fan, so the early hours do not represent normal operation.
- The system was not operating continuously. The seven-hour outage falls inside the test. The data show the consequence: 3.6–4.4 pCi/L during the outage versus 1.0–1.7 while the fan ran.
A verification test answers one question: what is the concentration in this room when the system is working as designed? Here, seven hours of the record describe a different building state.
The 1.7 pCi/L average looks favorable, but averaging across a non-continuous period does not verify anything. The test must be repeated after the fault is repaired and after the system has run undisturbed for the required interval.
2. What does the fan-off interval tell you about the system and the house, and what should be checked before retesting?
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The outage is diagnostic, not merely an error. The rise to 3.6–4.4 pCi/L within hours of fan-off, then the drop back to about 1.3 when the fan restarted, indicates that depressurization is genuinely controlling entry and that the house reverts toward its 8.9 pCi/L baseline without it.
Before retesting, check:
- The high-pressure switch, condensate drain, and freeze protection.
- Actual suction at the slab: manometer reading compared with commissioning values.
- A communication test at the farthest test hole.
- Whether the alarm is audible where the homeowner sleeps. A seven-hour overnight outage with only a local alarm is a warning-device gap worth correcting.
Also confirm the homeowner understands the system must stay on. A system that works but fails silently provides no protection during the failure.
3. What do you tell the homeowner and the contractor, and what is your retest plan?
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To the homeowner: the numbers while the fan was running are encouraging, but the test does not verify the system, because the fan stopped and the test began too early. The house is not 'fixed' on paper yet; keep the system running.
To the contractor: the verification is invalid. The fault must be permanently corrected, the system must run at least 24 hours before the retest begins, and the 48-hour CRM must be continuous, under closed-house conditions, in the same location. Put the repair commitment in writing.
Retest plan: repair, verify continuous operation, wait the required interval, then run 48 hours with a CRM plus, ideally, a duplicate or second device. Record the manometer reading at start and end. If the retest meets the target, issue the report with the outage documented, so the client has a complete record of what happened and why the first test was rejected.