When bidding on fire alarm system installations, you will look for all the obvious items for the pricing take-off, such as manual pull stations, heat detectors, spot-type smoke detectors, necessary control equipment and audiovisual appliances. You may even be aware of the high ceiling issue in certain spaces and plan for a linear beam smoke detector, knowing that open area, spot-type smoke detectors are not acceptable on any ceiling height above 15 feet.

Special applications

However, many bid specifications will have requirements for duct-type smoke detectors buried in the mechanical specification, and many times, these devices are not shown connected to the fire alarm control unit. In some occupancies, there can be many of these devices throughout the building.

There are numerous special applications of duct-type smoke detection. One of the more common is smoke detection used for air duct applications, either to shut down air handler systems or start up exhaust fans.

The reason that air duct systems are protected with passive and active air-sampling smoke detectors is to prevent recirculation of dangerous quantities of smoke from the fire area to the rest of the building. To do so, 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. This is also referenced by NFPA 72, National Fire Alarm and Signaling Code.

Air duct smoke detectors are not allowed to be used as a substitute for open-area smoke detection and are not considered early warning smoke detectors. 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, if the HVAC system is off—then no sampling takes place and detection is unlikely to occur. When air is flowing, the tubes are only sampling approximately 0.5% of the air. This is the obvious reason these detectors are not considered early warning smoke detectors.

Code requirements

The requirement to install duct-type smoke detectors in air ducts is also found in the International Mechanical Code (IMC). 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 72-2022, “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.”

NFPA 72 also states that the preferred signal annunciated on the fire alarm or signaling system for actuation of duct smoke detectors is a supervisory signal—not an alarm signal—to minimize nuisance alarms. This correlates with the requirements in NFPA 90A and the IMC.

Filters affect the performance of duct smoke detectors. Because there is also a history of filter fires, the location of the detector relative to the filter and the source of smoke must be considered during the design process. Also, when the duct smoke detectors are installed on the supply side of the duct 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 serve the purpose of providing detection for the return side of the system.

In general, employees do not reference the IMC during the bid process, but it is important to know how it affects the fire alarm system. The following key points relate to the fire alarm system installation. The requirements are found in Section 606, Smoke Detection Systems Control. An overview of the requirements includes the following:

• Duct smoke detectors shall comply with UL 268A.
• Duct smoke detectors shall not be required where air distribution systems are incapable of spreading smoke beyond the enclosing walls, floors and ceilings of the room or space in which the smoke is generated.
• In facilities required to be monitored by a supervising station, duct smoke detectors shall report only as a supervisory signal, not as a fire alarm signal.
• Access shall be provided to duct smoke detectors for inspection and maintenance.

The key takeaways here are that duct smoke detectors are not considered an early warning device, and because of that, they will only provide a supervisory signal when actuated. These types of smoke detectors are primarily installed to provide HVAC control during a fire condition.

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