null

Duct Smoke Detectors

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 Buyers are mostly HVAC contractors setting new rooftop units, service techs replacing failed or outdated detectors, fire alarm contractors, and facility managers keeping equipment ready for inspection.

How a Duct Smoke Detector Works

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.

Signs a Detector Needs Service or Replacement

  • Trouble or dirty indication. Many detectors signal a trouble or maintenance condition when the sensing chamber is contaminated. Dust and debris from the airstream build up over time.
  • Nuisance alarms. Repeated alarms without smoke usually trace to dirt, high humidity or condensation, installation too close to humidifiers or coils, or air velocity outside the rated range.
  • Failed functional test. If the detector does not alarm with a listed test method, or does not shut down the fan when it does, it must be repaired or replaced before the unit returns to service.
  • Cracked housing or cover. Rooftop exposure, UV and impact damage can break the seal between the housing and duct, letting outside air dilute the sample.
  • Plugged or missing sampling tube. A detector without a correctly installed tube cannot draw a proper sample, no matter how good the sensor is.
  • Older equipment past its service life. Manufacturers and NFPA 72 set testing and maintenance requirements. Replacing very old detectors is often more practical than chasing repeated failures.

How to Choose the Right Detector and Accessories

Detector type and wiring

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 tube length

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.

Air velocity and location

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.

Remote test stations and annunciators

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

Code Context: When Duct Detectors Are Required

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.

Installation and Safety Tips

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.

  1. Follow the installation manual exactly. Mounting location, tube orientation, wiring and testing procedures are model-specific.
  2. Coordinate with the fire alarm contractor. If the detector ties into a fire alarm control panel, the panel's contractor of record typically needs to make or approve those connections.
  3. Seal every penetration. Use the gaskets provided and seal tube holes so the detector samples only duct air.
  4. Verify differential pressure. The manual describes a method to confirm adequate airflow through the sampling tubes. Check it with the fan running.
  5. Test fan shutdown. Alarm the detector with a listed test method and confirm the air handler shuts down and any dampers and signals operate.
  6. Plan for maintenance access. Install remote test stations where detectors are concealed, and leave room to open the housing for cleaning.
  7. Document the test. Inspectors and building owners need records of installation and periodic testing.

Part Details & Shipping

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.

Frequently Asked Questions

Does every rooftop unit need a duct smoke detector?

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.

How do I pick the right sampling tube length?

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.

Why is my duct detector giving false alarms?

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.

Can I test a duct detector with a lighter or match?

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.

What does a remote test station do?

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.

Does a duct smoke detector replace room smoke detectors?

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.

Compare
Show: 12 24 48

Fast Shipping

Easy Returns

Warranty Coverage

Financing Available