Of the numerous special applications of smoke and heat detection, a common one is smoke detection used for air duct applications. There are two types of duct smoke detection: passive and active sampling.

Passive and active sampling

Passive sampling simply means that tubes are placed inside the duct system with the smoke detector mounted in a box attached to the sampling tubes. The term “passive air sampling” refers to the typical air duct smoke detectors with rigid sampling tubes. The air passes over the tubes, and the holes in the tubes “sample” the air flow. If there is no air flow—in other words, the HVAC system is off—then no sampling takes place, and detection is unlikely to occur. As air passes over the sampling tubes, the smoke-laden air will filter into the box with the detector. When there is enough smoke, the detector will actuate and cause the fan to shut down. 

“Active air sampling” means that the tubes inserted into the duct are fed back to a detector/fan assembly that draws samples of the duct air into the detector. When the smoke density inside the assembly reaches its set point, the detector actuates, causing the fan to shut down. 

There are two important installation requirements to know. First, the passive or active air sampling system must be listed for air duct applications. Second, the inlet and exhaust sections of pipe installed inside the air duct are air-tight and must exhaust the sampled air in accordance with the manufacturer’s published instructions. It should be obvious, but the sampling tubes for the passive system must be installed in such a way as to obtain a representative sample of the air stream.

Air duct systems are protected with passive and active air sampling smoke detectors to prevent the recirculation of smoke from the fire area to the rest of the building. To prevent this recirculation, a detector approved for air duct use is installed on the supply side of air-handling systems as required by NFPA 90A, Standard for the Installation of Air-­Conditioning and Ventilating Systems. NFPA 72 also references this standard.

Air duct smoke detectors may not be used as a substitute for open area smoke detection and are not considered early warning smoke detectors. One obvious reason is the dilution of smoke in an air duct. In a passive system, when air is flowing, the tubes are only sampling about 0.5% of the air.

The enabling requirement to install duct-type smoke detectors in air ducts is found in NFPA 90A. Typically, it is necessary to manage smoke flow in buildings, and duct smoke detectors are generally used to shut down HVAC systems, close dampers or initiate smoke management.

As stated in Annex A of NFPA 72-2025, “Where duct detectors are used to initiate the operation of smoke dampers, they should be located so that the detector is between the last inlet or outlet upstream of the damper and the first inlet or outlet downstream of the damper.”

Filters affect the performance of duct smoke detectors. Due to a history of filter fires, the detector’s location relative to the filter and smoke source must be considered during the design process. Also, when the duct smoke detectors are installed on the supply side of the system, they will detect smoke that enters the system through the fresh air intake for the HVAC unit. The duct smoke detectors installed on the supply side of the air duct system cannot be expected to provide detection for the return side. Additionally, duct smoke detectors are not required where air distribution systems do not extend beyond the enclosing walls, floors and ceilings of the space.

Because the fire protection and life safety communities accept that duct smoke detection is primarily for control and not early warning, and the dust issues associated with air duct systems that cause false alarms, these devices are typically not allowed to transmit an alarm condition to the building or to an off-premises supervising station.

The IMC requires that smoke detectors be installed 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 and appliances. NFPA 72 allows the elimination of these duct detectors where there are spot-type smoke detectors installed in all portions of the building served by the air distribution system, with the spot-type detectors connected to a fire alarm control unit.

Knowing when duct smoke detectors are required and how to properly install them will help prevent failing a fire alarm system acceptance test. Ensure your installation team coordinates with the mechanical contractor.

Johnson Controls inc.