CASE STUDY: How SL-701 helped a suburban Chicago middle school identify a safety issue.


This case study focuses on a scenario where the proper functionality of duct Carbon Monoxide (CO) detectors was pivotal in identifying a safety issue. The incident demonstrates how the SL-701 CO detectors effectively sensed, leading to the discovery of a potential health hazard and the implementation of corrective measures.


A technical support request was initiated by a contractor re

sponsible for installing APC duct CO detectors in a middle school located in Chicago. The contractor reported that one of the detectors was sporadically alarming during specific times throughout the school week. This unexpected behavior prompted concerns about the detector’s functionality and reliability.

Challenges Faced:

Intermittent Alarms: The duct CO detector exhibited alarms at irregular intervals, around the periods of 8 am and 3pm on weekdays. The unpredictable nature of these alarms raised doubts about the sensor’s accuracy and raised the possibility of false alarms.

Sensor Reliability: The contractor suspected that the detector might be malfunctioning, casting doubt on its capability to accurately sense CO levels. It was imperative to address this issue promptly to ensure the safety of the school environment.

Technical Support’s Solution:

In response to the contractor’s concern, the technical support team devised the following strategy:

Diagnostic Approach: Instead of assuming a sensor malfunction, the technical support team advised the contractor to temporarily implement a point CO detector. This device would be used during the times when the duct CO detector was alarming to verify whether the alarms were stemming from installation issues or operational anomalies.

For an example of point CO detector visit: Clip SGD | Industrial Personal Gas Detector | Crowcon

Validation through Point CO detector: By using the point CO detector, the contractor detected trace amounts of carbon monoxide at low parts per million (PPM) levels during the time periods when the alarms were being triggered. This confirmed that the alarms were warranted and not the result of sensor malfunction.

To find out more about our product’s reaction time to different PPM levels please visit our SL-701 Installation Guide.

Discovery of Safety Issue:

The accurate CO detection provided by the duct CO detector led to the identification of a significant safety concern:

Bus Idling Exposure: It was discovered that the point CO detector’s readings corresponded precisely with the times when the alarms were activated. Investigation revealed that a classroom, situated at the back of the school, had windows frequently opened by a teacher. This room was in proximity to an area where diesel school buses idled during student pickup and drop-off, resulting in CO intrusion in the classroom and in the HVAC system.

Outcomes and Resolution:

Mitigating Health Risks: The incident emphasized the importance of accurate CO detection. By promptly identifying the exposure to carbon monoxide from the idling buses, the school administration was enabled to take action to mitigate health risks for both students and staff.


This case study underscores the critical role of duct CO detectors in accurately sensing and indicating carbon monoxide accumulation in building’s ductwork. By ensuring the proper functioning of the detector, a potential health hazard originating from bus idling emissions was promptly detected, leading to corrective actions. The incident highlights the importance of not only relying on technical support for troubleshooting but also leveraging the capabilities of advanced detection equipment to enhance safety and well-being in indoor environments.


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