Section 5.3.1/Duct Smoke Detection
NFPA 90A Section 5.3 requires duct smoke detectors in air handling units exceeding 2,000 CFM to shut down fans and prevent smoke distribution.
Duct smoke detectors must be installed in the return air system of air handling units with a capacity exceeding 2,000 CFM, upstream of any filters and downstream of any return air connections from the served spaces. Supply duct smoke detectors are required downstream of the air handler when the unit serves more than one floor. When activated, the duct smoke detector must shut down the air handling unit to prevent distributing smoke throughout the building.
Why this section exists
An operating air handling system can distribute smoke from a fire to every space served by the system within minutes. Duct smoke detectors monitor the air stream and shut down the fan when smoke is detected, limiting the spread. Return air detectors are more common because smoke from a fire in any served space is drawn back to the air handler through the return system, concentrating at the detector location.
What plan reviewers look for
Plan reviewers check the mechanical drawings and fire alarm riser diagram for duct smoke detectors at each air handling unit exceeding 2,000 CFM. They verify the detector location (return duct, upstream of filters). They check that the detector is connected to the fire alarm system and programmed to shut down the associated air handler. They also verify supply duct detectors on multi-floor systems.
Common violations
Related NFPA 90A requirements
Section 4.3 covers fire damper and smoke damper requirements. NFPA 72 Section 17.7.5 covers duct detector installation details. IMC Section 606 covers duct smoke detector requirements in the International Mechanical Code.