Duct smoke detectors are required by airflow. Under the International Mechanical Code (IMC), any return air system over 2,000 cfm needs a duct smoke detector in the return, upstream of filters and outdoor or exhaust air connections, and return air risers serving two or more stories in systems over 15,000 cfm need a detector at each story. NFPA 90A, where it is adopted instead, puts the first detector on the supply side of systems over 2,000 cfm, downstream of the filters. In both, the detector shuts the air handler down and, where the building has a fire alarm system, reports to it as a supervisory signal. The details below are taken from the 2024 IMC, NFPA 90A, NFPA 72 and System Sensor's duct detector application guide; always confirm the code edition your AHJ enforces.
What a duct smoke detector does
An HVAC duct detector samples air moving through the duct and detects smoke in it. Its job is to stop the air handler from spreading smoke through the building. It is not early warning for the occupants: it only sees smoke while the fan is running and smoke reaches the duct, and the dilution in a large duct means smoke levels are lower than at the source. System Sensor states plainly that a duct detector is not a substitute for area smoke detection.
When are duct detectors required?
International Mechanical Code (IMC Section 606)
Most US jurisdictions enforce the IMC, or a state code based on it. In the 2024 edition:
- 606.2.1 Return air systems: detectors are required in return air systems with a design capacity greater than 2,000 cfm, in the return air duct or plenum upstream of any filters, exhaust air connections, outdoor air connections, or decontamination equipment. Exception: not required where all portions of the building served by the system are protected by area smoke detectors connected to a fire alarm system.
- 606.2.2 Common supply and return systems: where multiple air handlers share common supply or return ducts or plenums with a combined capacity greater than 2,000 cfm, the return air system needs detectors per 606.2.1. Fan-powered terminal units of 2,000 cfm or less each can be shut down by those detectors instead of having their own.
- 606.2.3 Return air risers: where return air risers serve two or more stories and serve any portion of a return system over 15,000 cfm, detectors are required at each story, upstream of the connection between the riser and any ducts or plenums.
- General exception (606.2): not required where the air distribution system cannot spread smoke beyond the walls, floor and ceiling of the room where the smoke starts.
IMC 606.1 also requires duct smoke detectors to be listed to UL 268A, and 606.3 requires installation per NFPA 72, with access for inspection and maintenance.
NFPA 90A
NFPA 90A (Section 6.4.2 in recent editions) requires listed duct smoke detectors:
- In supply systems over 2,000 cfm, downstream of the air filters and ahead of any branch connections.
- In return systems over 15,000 cfm serving more than one story, at each story before the connection to a common return.
Return detectors are not required under NFPA 90A where the entire space served is covered by area smoke detection. The key difference to remember: the IMC starts with the return at 2,000 cfm, NFPA 90A starts with the supply at 2,000 cfm. Which one applies depends on what your state or city adopted, and some jurisdictions amend both.
| Requirement | IMC 2024 (Sec. 606) | NFPA 90A (Sec. 6.4.2) |
|---|---|---|
| Single unit over 2,000 cfm | Detector in the return, upstream of filters and outdoor/exhaust air connections | Detector in the supply, downstream of filters, ahead of branches |
| Return system over 15,000 cfm, multi-story | Detector at each story on return risers | Detector at each story before the common return |
| Area smoke detection exception | Return detector not required if all areas served have area detection on a fire alarm system | Return detectors not required if the entire space has area detection |
| On activation | Shut down the air distribution system, or go to smoke control mode | Stop fans; connect to fire alarm system per NFPA 72 |
Signal type: supervisory, not alarm
IMC 606.4.1 requires duct detectors to be connected to the building's fire alarm system where one is required, and to activate a visible and audible supervisory signal at a constantly attended location. Where the building is monitored by a supervising station, duct detectors report only as supervisory, not fire alarm. NFPA 72 says the same and permits an alarm signal only where there is no constantly attended location or supervising station. In buildings with no fire alarm system, the IMC requires a visible and audible signal in an approved location, and trouble conditions must be identified as air duct detector trouble.
Where to place a duct detector
- Straight duct. System Sensor recommends 6 to 10 duct widths of straight, uninterrupted run from a bend, inlet or obstruction, so smoke is mixed evenly across the duct instead of stratified.
- Supply side: downstream of fans, filters, coils, heaters and humidifiers, and ahead of branch connections. Keep at least 10 ft downstream of humidifiers to avoid condensation false alarms.
- Return side: upstream of filters, outdoor air intake and exhaust connections, so the detector sees room air before it is diluted or cleaned.
- Access. The housing must be reachable for testing and cleaning. If it is above a hard ceiling or more than 10 ft up, plan a remote indicator and test station.
- Environment. Duct air must stay within the detector's listed range. System Sensor InnovairFlex detectors such as the D4120 and DNR are rated -4°F to 158°F; use a weatherproof model or a housing such as the DH400OE-1 for rooftop units and outdoor ducts.
Air velocity and pressure
A duct detector draws a sample through its sampling tube using the pressure difference created by duct airflow. It must be listed for the air velocity in the duct. System Sensor InnovairFlex models (D4120, D2, DNR, DNRW) operate from 100 to 4,000 ft/min. The installation manuals require the pressure differential between the sampling and exhaust ports to be between 0.01 and 1.11 inches of water, measured with a manometer. For ducts under 500 ft/min, where a standard manometer cannot read accurately, the manuals give a low-flow test with a differential pressure transmitter or aerosol smoke. Measure velocity with an anemometer before choosing the detector.
Sampling tubes
The sampling tube is ordered separately and is sized to the duct width. System Sensor's DST tubes run from DST1 for ducts up to 1 ft to the two-piece DST10 for 8 to 12 ft, for example the DST3 for 2 to 4 ft ducts. Install the tube with the inlet holes facing into the airflow, fit the end cap supplied with the detector, and support tubes over 3 ft long at the far wall. System Sensor recommends the tube extend at least two-thirds of the way across the duct; check the exact minimum in your detector's manual. Simplex housings use their own tubes. See all sampling tubes.
Remote test stations and indicators
NFPA 72 requires a remote alarm or supervisory indicator where a detector is concealed more than 10 ft above the floor or its own indicator is not visible to responders. Most installers fit a remote test station so the detector can be tested and reset from the floor. The System Sensor RTS151KEY is a key-operated test station with an alarm LED; the RA100Z is an LED-only remote indicator. Addressable detectors need a remote-test-capable sensor head to work with a test station. Browse remote test stations.
Choosing a duct detector
- 4-wire conventional with relays: the System Sensor D4120 runs on 24 VDC/VAC or 120 VAC and has its own relays for fan shutdown, so it works with or without a fire alarm panel.
- Addressable: the System Sensor DNR housing takes the addressable head of your panel brand. The Simplex 4098-9756 is a 4-wire addressable housing with a relay output for Simplex IDNet systems.
For a head-by-head comparison of DNR, D4120, D2 and Fire-Lite D365PL, including which test station and tube each takes, read Which duct smoke detector do I need, or browse all duct detectors.
Testing and maintenance
Following NFPA 72 Chapter 14 as summarized in System Sensor's guide: visually inspect duct detectors semi-annually, functionally test annually, and check sensitivity within one year of installation and every alternate year after that. Testing should include an airflow (pressure differential) check and confirmation that the fan actually shuts down. Check for cracked sampling tubes, leaks at the housing, and dust on the tube and head.
FAQ
At what CFM is a duct smoke detector required?
Over 2,000 cfm. The IMC puts it in the return; NFPA 90A puts it in the supply. Multi-story return systems over 15,000 cfm need detectors at each story under both.
Does a duct detector go in the supply or the return?
It depends on the adopted code. IMC jurisdictions require the return (upstream of filters and outdoor air); NFPA 90A requires the supply (downstream of filters). Some AHJs ask for both.
Should a duct detector sound the building alarm?
Generally no. Where there is a fire alarm system with a constantly attended location or supervising station, it reports as a supervisory signal and shuts down the fan.
Can area smoke detectors replace duct detectors?
For the return air system, yes, if every area served by the unit has area smoke detection on a fire alarm system. That exception is written for return detectors; check the NFPA 90A edition your AHJ uses for any exceptions to the supply detector.
What air velocity does a duct detector need?
It must be listed for the duct's velocity. System Sensor InnovairFlex detectors are listed for 100 to 4,000 ft/min.
Do I need a remote test station?
You need remote indication when the detector is concealed above 10 ft or its LED cannot be seen. A test station is the usual way to meet that and make annual testing practical.