Section 17.7.5/Duct Smoke Detectors
NFPA 72 Section 17.7.5 covers duct smoke detector requirements including required locations, placement in the ductwork, testing access, and coordination with HVAC fan shutdown.
Duct smoke detectors must be installed in the supply duct of air handling systems with a capacity of 2,000 CFM or greater, downstream of any filters and upstream of any branch connections. Return air duct smoke detectors are required where the return air system serves more than one story and handles more than 15,000 CFM, or where the return air is recirculated from more than one smoke zone. The detectors must be listed for duct installation and must be installed per the manufacturer's instructions, typically in a duct-mounted housing that samples the air stream. Access panels must be provided at each detector location for inspection, testing, and maintenance. Upon activation, duct smoke detectors must shut down the associated fan and close smoke dampers, but they are not a substitute for area smoke detectors and must not be used for open-area detection. The IMC requires duct smoke detectors per Section 606.2, referencing the NFPA 72 requirements. Duct detectors must be connected to the fire alarm system and annunciated at the fire alarm control panel.
Why this section exists
HVAC systems can spread smoke throughout a building by recirculating smoke-laden air from the fire area to other floors and zones. Duct smoke detectors detect smoke entering the air handling system and shut down the fan to prevent smoke distribution. Supply duct detectors prevent the system from drawing smoke from a fire near the air handler and pushing it to every space served. Return duct detectors prevent the system from pulling smoke from the fire floor and delivering it to other floors through the supply. Duct detectors are NOT replacements for area detectors because the dilution in the duct airstream makes them much less sensitive than room-mounted detectors. Their purpose is HVAC shutdown, not early fire detection.
What plan reviewers look for
Plan reviewers check the mechanical drawings for air handling units with capacities of 2,000 CFM or greater and verify duct smoke detectors are shown in the supply duct. They check for return duct detectors where required (multi-story return systems over 15,000 CFM). They verify detectors are located downstream of filters and upstream of branch takeoffs. They check the fire alarm riser diagram for duct detector connections and verify the sequence of operations includes fan shutdown and smoke damper closure upon activation. They check for access panels at each detector location. They verify duct detectors are NOT shown as area detectors on the fire alarm floor plan.