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Air‑quality monitors show no hazardous readings; positive sewer pressure traced to municipal main at FHN
Summary
SM Wilson engineers reported that air monitoring at Francis Howell North detected no hazardous hydrogen sulfide levels and that positive pressure spikes in the building traced to the municipal sewer main can be mitigated by a powered vent system being coordinated with the City of St. Peters.
SM Wilson representatives told the Francis Howell R-III board on Aug. 21 that air monitors at Francis Howell North (FHN) have not recorded hazardous hydrogen sulfide, oxygen or lower‑explosive‑limit readings and that positive pressure in the school is originating in the municipal sewer main, not the building.
“Air's been good since April,” said Matt Frank, an SM Wilson representative, describing continuous monitoring. He said a manometer — a device that reads air pressure — recorded high positive pressure in the building and at sewer manholes hundreds of feet away, indicating the source is the municipal sewer system.
SM Wilson tested a temporary relief by removing a manhole cover; that test reduced measured building pressure by roughly 50 percent, Frank said. The firm has proposed installing a powered relief vent — described by SM Wilson as similar in function to a radon mitigation vent — with an inline fan and roughly an 8‑inch pipe rising about 30 feet to discharge sewer gas to the atmosphere. The fan would be selected for continuous operation and will require routine maintenance, SM Wilson said.
“We were able to test a fix … The test basically reduced the pressures by 50 percent,” Frank said. The district’s plan is to work with the City of St. Peters on permit drawings and implementation; SM Wilson said final costs and the question of which party will pay remain to be negotiated.
Board members asked how the system would be monitored and what would happen in a power outage; SM Wilson representatives said the vent system could include alarms or notifications to indicate fan failure and that, even without power, the vent would remain open as a passive relief.
SM Wilson said the district will leave manometers installed after the vent installation to verify that the building pressure returns to 0 or negative values. The roof's temporary one‑way valves would be removed once data confirm the fix.
Asked about timing and price, SM Wilson said final design, permitting and subcontractor pricing are needed before the district can estimate a cost, and that discussions with the city and legal counsel were underway.
