Recently, I encountered questions regarding the nonactuation of smoke detectors in an atmosphere the occupants reported as “smoky.” The discussions revealed a general misunderstanding of how a fuel-dependent fire grows, how smoke and heat detectors function, and how a designer should apply them to a space.


Learning these devices’ limitations is the best way to understand a fire alarm system’s detection capabilities.


The stated purpose of NFPA 72 2016, National Fire Alarm and Signaling Code is to “define the means of signal initiation, transmission, notification, and annunciation; the levels of performance; and the reliability of the various types of fire alarm systems, supervising station alarm systems, public emergency alarm reporting systems, fire warning equipment, emergency communications systems, and their components.”


In the same section, the code states that it “establishes minimum required levels of performance, extent of redundancy, and quality of installation but does not establish the only methods by which these requirements are to be achieved.”


The Annex A material for this purpose section of the code states, in part, “Fire alarm systems intended for life safety should be designed, installed, and maintained to provide indication and warning of abnormal fire conditions.”


However, the code does not define how the design and placement of the detection devices should meet the owner’s life safety goals. Understanding the applications of various detectors and their limitations falls to the contractor or fire protection engineer.


Knowledge is fire power


Do you know how the smoke detectors that you use work? How much smoke must enter the detector to initiate an alarm signal? If you install a spot-type smoke detector on the recommended spacing of 30 feet on center, but the ceiling height is 30 feet, should you expect early warning? Under such circumstances, early detection of a small fire is unlikely.


When a fire begins, it generates heat and smoke. The rising hot air forces the smoke up until the plume reaches the same temperature as the air surrounding it. At this point, upward movement of the smoke plume stops. As the fire forces additional smoke into the plume, the plume spreads out horizontally in all directions.


Fire researchers call this phenomenon “stratification.” Until the fire gets hotter, becomes larger or involves more fuel, the smoke will not rise any farther. Thus, the smoke plume may never reach a smoke detector located on the 30-foot high ceiling.


Since building codes provide for generalized minimum protection, they cannot guarantee the exact detection needs of every space or environment. For example, installing smoke detectors in corridors will not provide early warning in a closed-off apartment or office. Also, environmental issues could negatively affect smoke detector operation, for example, causing false alarms. Many contractors have learned how to ensure a stable, false-alarm-free environment for smoke detectors.


Contractors should also be familiar with the environmental impacts of high air flow, high ceiling and beamed ceiling construction, and other conditions that affect the operation of various types of smoke detection.


In the discussions I mention at the beginning of this article, a similar question arose from an owner who had the heating, ventilating and air conditioning (HVAC) system serviced. The owner thought the duct-type smoke detector should have actuated when the HVAC motor generated an oily mist the first time it operated after the service. This question raises some important issues.


First, in response to changes in the code requirements, duct-type smoke detectors are now designed to initiate a supervisory signal, the same type of signal we usually assign to the unauthorized closure of an automatic fire sprinkler system gate valve. This is done for two reasons: No one cleans duct-type smoke detectors regularly, and the fire-protection industry recognizes that duct-type smoke detectors do not provide early warning but simply intend to prevent the recirculation of smoke from one area served by the HVAC equipment to another area.


In the first case, uncleaned detectors can produce all-too-frequent false actuations. If each actuation initiated an alarm signal, it would produce an unacceptable increase in false alarms. In the second case, the normal duct-type smoke detector samples approximately 5 percent of the air in the duct. Because of this low sampling rate and the effects of the airflow in various parts of the duct system, the duct essentially needs to become almost fully charged with smoke before the detector will actuate.


Upon actuation, duct detectors have the primary function of shutting down the associated HVAC unit’s fan. The supervisory signal indicates that such an action has taken place.


In addition, the code states in Section 17.7.5.2.1 that “Detectors that are installed in the air duct system … shall not be used as a substitute for open area protection.”


The annex explains, “Smoke might not be drawn into the duct or plenums when the ventilating system is shut down. Furthermore, when the ventilating system is operating, the detector(s) can be less responsive to a fire condition in the room of fire origin due to dilution by clean air.”


In addition, a dilution of smoke-laden air can occur due to air entering from other air intakes in the building or from outside air drawn into the supply. This can cause a room to contain large amounts of smoke with no detectable smoke in the air duct at the detector location. Of course, if the air conditioning system has stopped operating or the ventilation system has shut down, the HVAC system will not draw smoke from the rooms into the duct system. The owner’s duct-type smoke detector referenced in this question likely did not “see” enough smoke to operate. And even if it had actuated, based on the current code requirements, the detector would not have initiated a fire alarm signal in the building; it would only initiate a supervisory signal.


Apply the methods


I have just scratched the surface of what you should know when designing and installing fire alarm systems, but understanding these basics will help toward providing useful, reliable and false-alarm free fire alarm systems.


Using this knowledge of how smoke travels to a detector and how much smoke it takes to actuate that detector, as well as the understanding of detection capability limitations of these devices, you can now advise your customers why they need more detection to meet their fire-protection or life-safety goals and why a smoke-detection device may not have actuated in an instance where a smoky odor is present.


Understanding how all fire-detection devices work, especially smoke detectors, is as important as knowing how to install those devices properly.