Everyone who installs or tests fire alarm systems has a few complaints about duct detectors, which are usually located where the accessibility is limited, making them also hard to test. These detectors provide one of the two most commonly controversial issues with fire alarm system installations (the other is smoke detectors in elevator hoistways). There are two conflicting sets of requirements. Which requirements you use will depend on the applicable code in your jurisdiction. If your authority having jurisdiction (AHJ) enforces NFPA 101, the Life Safety Code, it references NFPA 90A, the Standard for the Installation of Heating, Ventilation and Air Conditioning Systems. If enforcing the International Building Code (IBC) or International Fire Code, the AHJs reference the International Mechanical Code (IMC).
NFPA 90A requires the first duct smoke detector to be installed on the supply side of air handler units of more than 2,000 cubic feet per minute (cfm) and requires one on the return side of units of more than 15,000 cfm and serving more than one story. According to paragraph 126.96.36.199.1 (2010 edition) of NFPA 72, the National Fire Alarm and Signaling Code, the purpose is “to prevent the recirculation of dangerous quantities of smoke …”. NFPA 72 also offers explanatory information in Annex A.188.8.131.52.2: “In almost every fire scenario in an air-handling system, the point of detection will be some distance from the fire source; therefore, the smoke will be cooler and more visible because of the growth of sub-micron particles into larger particles due to agglomeration and recombination.” For these reasons, photoelectric detection technology has advantages over ionization detection technology in air duct system applications.
Section 606.2 of the 2009 IMC requires smoke detectors to be installed in return air systems of more than 2,000 cfm and upstream of any filters or exhaust or outdoor air connections. Both the IMC and NFPA 90A require the detectors to be installed in accordance with NFPA 72 and be connected to a fire alarm system, if one is required. Both also have exceptions that allow the deletion of the return duct smoke detectors if the area served is supplied with area smoke detectors and they are configured to control the HVAC system in the same way that the duct detectors would have. NFPA 72 also states that a duct smoke detector is not allowed to replace an area smoke detector, mainly because if the air-handling unit (AHU) is not running, the detector would not detect smoke. Therefore, duct smoke detectors are typically considered mechanical equipment devices instead of life safety devices. Both the IMC and NFPA 90A state that, if multiple air handlers serve the same area and the combined air volume is greater than 2,000 cfm, detectors are required. Both also have exceptions that would not require the duct smoke detectors if the air handler is only serving one room, since smoke could not be distributed to other areas.
So why is there such a difference in requirements? There are two schools of thought concerning the purpose of duct detectors. There have been numerous unsuccessful proposals over the last couple of I-Code cycles to change the IMC requirements to match the NFPA 90A requirements. The primary purpose of the duct detector on the supply side is to shut down the air handler if fire is detected in the fan or filter. This would protect the equipment and keep smoke from being distributed to the air conditioned space. Some would argue that detectors on the supply side are susceptible to outside air or smoke, which could affect the operation of the supply side smoke detector. There also is the problem of false alarms due to dust accumulation on heat strips in the AHU burning off when heaters are turned on and activating the smoke detector. For this reason, both the IMC and NFPA 72 allow duct smoke detectors to initiate a supervisory signal rather than an alarm signal.
Duct detectors located on the return side and upstream of any filters or introduction of outside air detect smoke coming from an air conditioned space. This eliminates interference caused by outside smoke or heat-strip dust-accumulation burnoff. However, it takes time for smoke to travel through the ductwork in sufficient quantities to -activate the detector. If the purpose is to detect smoke coming from a room in the building, would it not make more sense to require a smoke detector to be installed in that area rather than waiting for enough smoke to reach a return duct smoke detector?
In addition to the above requirements, the IBC and NFPA 101 require smoke detectors to be located at smoke damper locations to control the dampers. That is a topic for another article.
HAMMERBERG is currently the president/executive director of the Automatic Fire Alarm Association Inc. headquartered in Jasper, Ga. He serves on a number of NFPA committees, including the NFPA 72 Technical Correlating Committee and the Protected Premises Technical Committee. He can be reached at TomHammerberg@afaa.org.