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An air handler moves a lot of air through a building, and during a fire it can move smoke just as efficiently. A duct smoke detector samples air passing through the ductwork and, when it senses smoke, shuts down the fan and signals the fire alarm system so the HVAC equipment does not spread smoke from one area to another. On rooftop units and commercial air handlers it is a code-driven safety device, and inspectors check it.
This category carries System Sensor InnovairFlex duct smoke detectors along with the accessories that complete a proper installation: sampling tubes sized by duct width, remote test stations, remote annunciators and strobes. Buyers are mostly HVAC contractors setting new rooftop units, service techs replacing failed or outdated detectors, fire alarm contractors, and facility managers keeping equipment ready for inspection.
A duct smoke detector is mounted on the outside of the duct, with its housing connected to two tubes that pass into the airstream. The sampling (inlet) tube has holes facing into the airflow. Moving air enters those holes, passes into the detector housing, flows across a photoelectric smoke sensing chamber, and returns to the duct through the shorter exhaust tube. The pressure difference created by airflow across the tubes keeps a continuous sample moving through the chamber.
When smoke particles enter the sensing chamber, they scatter light from an internal LED onto a photo sensor. Once the signal crosses the alarm threshold, the detector changes state. Depending on the model and wiring, it can energize or open relay contacts to shut down the fan, signal a fire alarm control panel, close smoke dampers, and drive remote indicators.
The InnovairFlex line is a modular platform. Detector heads and housings are configured for conventional four-wire installs, which power the detector directly and use onboard relays for fan shutdown, or for connection to a fire alarm control panel, with separate relay modules where needed. Models are rated for a wide range of duct air velocities, roughly 100 to 4,000 feet per minute on the InnovairFlex line, and the housing design allows installation on round or rectangular ducts. Check the specific model's installation manual for velocity range, wiring and mounting.
Decide whether the detector will be a stand-alone conventional unit powered locally, typically from 24VAC/VDC or 120VAC depending on the model, or connected to a building fire alarm system. If there is a fire alarm panel, the detector must be compatible with it, and the fire alarm contractor of record usually needs to be involved.
Sampling tubes are sold in lengths matched to duct widths. Pick the tube that spans the duct, generally the nearest length equal to or larger than the duct width, and cut it to fit if necessary following the manufacturer's instructions. On wide ducts the tube typically passes through the far wall and is sealed. The holes must face into the airflow.
Confirm the duct velocity at the mounting location falls within the detector's rated range. Mount the detector in a straight run of duct where air is well mixed, away from elbows, dampers and humidifiers, typically a distance of several duct widths downstream of disturbances. The installation manual gives exact spacing guidance.
Duct detectors are often installed above ceilings or on rooftops where they are hard to see or test. A remote test station lets an inspector or technician test the detector and check its status from an accessible location. Key-operated test stations limit access to authorized personnel. Remote annunciators provide an alarm indicator light, and strobes add a visible signal where required. Match the accessory to the detector model and wiring per the compatibility list.
| Component | Purpose | Selection Notes |
|---|---|---|
| InnovairFlex duct detector | Senses smoke in the duct airstream and shuts down the fan | Choose conventional or panel-connected model; confirm voltage and relay needs |
| Sampling tube | Delivers air from across the duct to the detector | Match length to duct width; holes face upstream; seal far end as instructed |
| Remote test station | Allows testing and status checks from an accessible location | Use where the detector is concealed or hard to reach; key versions limit access |
| Remote annunciator | Shows alarm status away from the detector | Confirm compatibility with detector model and wiring |
| Strobe or horn/strobe | Provides a visible or audible alarm signal | Required in some applications; confirm with the AHJ |
Duct smoke detector requirements come from mechanical codes such as the International Mechanical Code, along with NFPA 90A and NFPA 72. A widely used rule requires smoke detectors in the return air of air distribution systems with a design capacity above 2,000 CFM, with additional requirements at higher airflows and on systems serving multiple stories. There are exceptions, for example where the entire area served is covered by area smoke detection connected to the fire alarm system.
Codes differ by edition and jurisdiction, and local amendments are common. Always check the code adopted in your area and confirm with the authority having jurisdiction before you design or install. Duct detectors are not a substitute for area smoke detection in occupied spaces.
Duct detector installation involves line or control voltage, life safety wiring and, often, a building fire alarm system. It should be performed by a qualified HVAC technician or fire alarm contractor in accordance with the manufacturer's manual and local code.
HVAC365 carries System Sensor InnovairFlex detectors and their sampling tubes, test stations and indicators alongside the Supco replacement parts techs use every day, so you can order a complete rooftop unit install in one place. Product pages include specifications, with spec sheet information where available. Orders ship in 3-5 business days.
Not every unit. A common code threshold is return air smoke detection on systems above 2,000 CFM, with exceptions and additional rules at higher airflows. Requirements vary by jurisdiction, so check your local code and the authority having jurisdiction.
Measure the inside width of the duct where the detector mounts and choose the tube length that spans it, cutting if needed per the manufacturer's instructions. The tube must reach across the duct to sample representative air.
The usual causes are a dirty sensing chamber, condensation or humidity, mounting too close to a humidifier or coil, or duct velocity outside the rated range. Cleaning, relocating or correcting airflow typically resolves it.
No. Use a test method listed by the manufacturer, such as the detector's test function, a remote test station or listed aerosol test agent. Open flame is unsafe and can damage the detector.
It lets you test the detector and see its status from an accessible location, which matters when the detector is above a ceiling or on a roof. It speeds up inspections and routine testing.
No. Duct detectors protect the air distribution system and control fan shutdown. They are not designed or listed to provide area smoke detection for occupants.
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