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g/fire_alarm • Posted by u/suppression_sue •

Duct smoke detector placement — common mistakes that fail inspection

We've started seeing more rejections on duct detector placement from our local AHJ. They're citing NFPA 72 section 17.7 more aggressively and rejecting installs that were passing 5 years ago.

For those working with AHJs that are scrutinizing duct detector locations more closely — what are the most common installation mistakes you're catching or that inspectors are flagging?

💬 3 replies

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u/panel_programming_paul •

One I see in existing buildings: duct detectors wired in the fire alarm system but not interfaced to the HVAC controls to shut down the air handling unit on alarm. NFPA 90A requires duct detectors to shut down the associated air handler — if it's just connected to the panel for notification with no AHU shutdown, that's a code violation.

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u/fire_code_fred •

Top five that I see flagged: (1) Distance from branch duct connections — sampling pipe within 18" of a branch can pull from the branch, not the main duct. (2) Sampling tube not sized for duct width — too short and you're not sampling the full cross-section. (3) Detector installed on a return duct downstream of recirculation from contaminated spaces. (4) Inaccessible detector location — no service access panel. (5) Detector installed downstream of the air filter — you want it upstream so it sees actual air content.

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u/smoke_detector_steve •

The inaccessible location rejection is the most preventable. NFPA 72-17.7.3.3.1 requires accessible location. Coordinate with mechanical contractors before the ductwork is insulated and covered. Once insulation is on, retrofitting an access panel is an argument with the GC.

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