Finger pointing, passing the buck, shifting responsibility… This seems to be a common theme when it comes to installing duct smoke detectors on new construction projects. Mechanical Contractors, Electricians, Fire Contractors… Who is responsible for making sure these detectors are installed correctly? Perhaps this confusion is due less to the detectors’ working scope and more to the uncertainty felt by contractors regarding the proper installation and placement of such fire protection devices. I wish I could say otherwise, but the answer regarding liability is probably not as simple as we’d prefer. Like many other efforts on a construction site, installing duct smoke detectors requires a coordinated effort among all parties. The mechanical contractor is usually responsible for mounting the detectors. Still, he would hope that he didn’t want anyone else cutting and connecting his ductwork. The electrical contractor is often required to run conduit to the detector location and is often required to manage the subcontract for the fire alarm company. In the end, the fire alarm contractor is responsible for making sure the device works properly and reports back to the fire alarm control panel. Coordinate effort.
NFPA 90A, where required
NFPA 90A is the standard for air conditioning and ventilation systems. This code states that a duct smoke detector is required on the SUPPLY side of any HVAC unit greater than 2000cfm. Those detectors should be located downstream of the air filters and before any bypass connections. If you can’t get ahead of any branch connections, you should have one at each branch. In addition to the supply side detector, NFPA 90A states that a duct smoke detector is required on the RETURN side of any unit greater than 15,000cfm. These detectors are required on each floor prior to connection to a common return and prior to any recirculation or fresh air intake. They are not required when the entire space is protected by smoke detection in the area.
NFPA 72, Installation Media
NFPA 72 is the National Fire Alarm Code, the standard for the installation of fire alarm system components. First, NFPA 72 reminds us that duct smoke detectors are NOT a substitute for open area detection. NFPA refers to the manufacturer’s published instructions for installation requirements. Manufacturer’s instructions advise that duct smoke detectors be located a minimum of 6 duct widths from a bend or other obstruction. This means that if you have 18″ wide duct, the detector should be located a minimum of 9′-0″ downstream from a bend or other obstruction. This is often difficult to achieve. The contractor should be aware that the 6 duct width guideline is based on the fact that the airflow stops when it reaches a curve. The duct detector requires air to flow through a 1/2″ diameter sample tube that protrudes into the duct. If the airflow bounces all the way through the duct, it is less likely to reach the sample tube as required for proper conditioning smoke detection Because the code states that it “should” rather than “shall” be located in a minimum width of 6 ducts, it is the responsibility of the contractor to use their best judgment to locate the detector as far As HVAC ducts are often located above the finished ceiling or high up in joists out of visible reach, the location of smoke detectors in the duct should be permanently identified and recorded and Firefighters and service personnel must be able to identify the location When duct smoke detectors are installed more than 10′-0″ AFF or when the detector is not visible to responders, indicators must be provided remotes to locate the device with ease. On occasion, when acceptable to the AHJ, remote indicators may be removed if the detector is specifically identified and clearly announced on the FACP and annunciators.
Alarm/supervisory shutdown and fire alarm
Once duct smoke detectors have been sufficiently installed, there seems to be an ongoing debate as to whether the detectors should announce a supervisory signal, which requires an investigation to determine if there is a fire, or whether the detectors should announce an alarm signal. Immediately evacuate the building and call the fire department to the site in response. Proponents of the supervisory signal argue that duct smoke detectors are a common source of false alarm signals. Often when heaters are first turned on at the start of winter, the heat blowing through the ducts burns up dust (we all know the smell of a heater being turned on for the first time), triggering an alarm. Neither the owners nor the firefighters want to evacuate the building or take trucks to a site that turns out to be a false alarm. On the other side of the argument, however, is the concern that if the detector goes off and is then doing its job, an alarm signal is required to ensure the safety of the occupants. No risk allowed. Because there are valid points on both sides of the argument, NFPA 72 has chosen to take the middle ground and allow this to be a local decision. The Fire Alarm Code states that duct smoke detectors may be EITHER alarm-initiating OR supervisory-initiating. The local AHJ will most likely have a preference. The only definitive action supported by the code, per NFPA 90A, is that duct smoke detectors must automatically stop their respective fans; and that any time a duct detector is provided, it must be reconnected to the building FACP.
Duct detectors and clean agents
Another question that often arises in connection with fire systems is how duct detectors play into clean agent suppression systems. Clean agent systems have their own control panel and require a means of fire detection prior to discharging a fire extinguishing agent. The question arises when a duct detector is provided in the CRAC unit installed inside a computer room that is protected by a clean agent system. Who monitors the status of the smoke detector in the duct and does its alarm activation influence the clean agent dispensing sequence? Let me answer the first question by explaining a bit about the second question… NFPA 2001 is the standard for clean agent suppression systems. This standard requires forced air ventilation systems to be shut down ONLY where continued operation would adversely affect the performance of the fire suppression system. In addition, the standard states that fully self-contained recirculating ventilation systems (ie, Liebert or CRAC units) will not be required to be turned off. This is because the recirculation of air within the protected space does NOT negatively affect the performance of the extinguishing system; in fact, it helps in the retention time of the agent in the atmosphere. The continuous recirculation of air within the protected space actually helps extinguish the fire and prevent re-ignition. That being said, duct smoke detectors play no role in the clean agent distribution sequence. Since the duct smoke detector is not part of the clean agent distribution sequence, it is not linked to the clean agent fire panel. The detector should now be connected to the base building fire alarm control panel. Whether or not activating the detector causes the unit to shut down is now an optional decision for the owner, although not recommended in this case.
In closing, I’d like to thank everyone for taking the time to learn a bit more about this often confusing point of construction coordination. The more educated all associated contractors are, the more likely we are to install a fully functioning fire alarm system, regardless of who owns it.
