Post-Mitigation Failure: Short-Circuiting Through a Sump and Floor Drain
A contractor installs a sub-slab depressurization system in a 1965 ranch with a full basement. The design uses one suction point in the slab, 3-inch PVC, a 4-inch suction pit, and a 1/3 hp fan rated at 60 cfm at 2 inches of water column. At commissioning the manometer reads -1.8 in WC at the fan and -0.4 in WC at a test hole 6 feet from the suction point; pressure-field extension is measurable only about 5 feet from the pit. The basement has a sump pit in the opposite corner with a loose, unsealed lid and a floor drain connected to a foundation drain that daylights at the curb. A post-mitigation test three days later reads 5.2 pCi/L in the basement. The contractor proposes adding a second suction point on the other side of the basement.
1. What is the most likely cause of the poor pressure-field extension and the failed post-mitigation result?
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The pattern of strong suction at the fan, weak suction at the test hole, and limited pressure-field extension points to short-circuiting rather than tight soil. The sump pit with a loose lid and the floor drain tied to a daylighted foundation drain give the fan a low-resistance path to outdoor air. The fan is pulling air through the drain tile and out the sump opening instead of drawing soil gas from beneath the slab.
The -1.8 in WC at the fan is consistent with a fan near its rated operating point but moving air through the wrong path. A second suction point would add cost without fixing the short circuit and could worsen it by increasing total flow through the open drain path. The post-mitigation result of 5.2 pCi/L confirms that the sub-slab field is not being treated effectively. The diagnosis should be confirmed before any new suction point is added.
2. What diagnostic steps would confirm it?
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Temporarily seal the sump lid and the floor drain with a gasketed cover or plastic sheet and tape, then re-read the test-hole vacuum and pressure-field extension. If vacuum at the test hole increases and extends, the short circuit is confirmed.
A smoke test at the sump and drain with the fan running can show air movement and the direction of flow. Check the pit pressure and the fan's actual flow against its curve. Inspect the foundation drain for a direct connection to the sump and confirm whether the floor drain trap is wet. If sealing does not improve pressure-field extension, evaluate soil communication with a second test hole at 12 feet and consider a second suction point or a larger pit.
Record before-and-after readings at the same test holes so the comparison is defensible.
3. What repair would you recommend, and how would you verify it?
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Recommend a gasketed, sealed sump lid; seal the floor drain with a trap primer or a one-way drain cover; and if the sump is a radon entry route, tie it into the system with a sealed suction pipe or a dedicated sump suction. Re-test pressure-field extension at 6 and 12 feet, confirm the manometer reading at the fan, and run a new post-mitigation test no sooner than 24 hours after the repairs.
The target is a confirmed pressure-field extension across the slab and a result below 4 pCi/L, ideally below 2.7. If the sump and drain sealing alone do not achieve that, add a second suction point only after the short circuit is eliminated. Document the before-and-after readings, the sealing details, and the test conditions. Advise the homeowner to keep the sump lid sealed and to maintain the floor drain trap.