Duct Smoke Detector Requirements
When an air-handling system needs a duct smoke detector, and what the detector does. The threshold is the same number — 2,000 cfm — under two different codes, but they put the detector in different places: NFPA 90A on the supply, the IMC on the return. On detection, the detector shuts down the air handler so it cannot spread smoke. Confirm which code your jurisdiction enforces.
Duct detector requirements
| System | Code | Threshold | Detector location | Action |
|---|---|---|---|---|
| Supply air | NFPA 90A | > 2,000 cfm | Supply duct downstream of the filters | Stop the supply fan(s) / shut down the air handler |
| Return air | IMC 606.2.1 | > 2,000 cfm | Return duct upstream of the filters and outlets | Shut down the air distribution system |
| Return riser, 2+ stories | IMC 606.2.3 / NFPA 90A | > 15,000 cfm | At each story, upstream of the riser connection | Detector at each story; shut down the system |
Supply vs return — why the two codes differ
The reason the same 2,000 cfm threshold lands in two different places is that NFPA 90A and the IMC approach the problem differently. NFPA 90A watches the supply air downstream of the filters, catching smoke about to be pushed out to the space; the IMC watches the return air upstream of the filters, catching smoke being drawn back from the space. Neither is wrong — they are different adopted standards, and where both are enforced you may end up with detectors on both sides. The one rule they agree on is the return riser: a riser serving two or more stories with any portion over 15,000 cfm gets a detector at each story.
Supervisory signal at a constantly attended location (IMC 606.4.1) — not, by itself, a general evacuation alarm. Whether it also triggers a building alarm is set by the fire alarm design and the AHJ.
Common questions
When is a duct smoke detector required?
It depends on the code and the air-handler capacity. NFPA 90A requires a duct smoke detector in the supply air, downstream of the filters, on systems over 2,000 cfm. The IMC (606.2.1) instead requires one in the return air, upstream of the filters, on systems over 2,000 cfm. And a return riser serving two or more stories with any portion over 15,000 cfm needs a detector at each story.
Is it 2,000 cfm on the supply or the return?
Both — but under different codes, which is a common source of confusion. NFPA 90A puts the 2,000 cfm detector on the supply side (downstream of filters); the IMC puts it on the return side (upstream of filters). They are two different standards with two different locations, and some jurisdictions enforce both. Confirm which code your AHJ has adopted.
What does a duct detector do when it activates?
It shuts down the air-handling unit — stopping the fans so the HVAC system does not distribute smoke throughout the building — and may activate a smoke-control mode. It generally produces a supervisory signal at a constantly attended location rather than a full building evacuation alarm; whether it also triggers a general alarm is set by the fire alarm design and the AHJ.
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