Air Is Flowing In A Duct As Shown In Duct Detectors and New Construction Coordination

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Duct Detectors and New Construction Coordination

Pointing the finger, paying, shifting responsibility… this seems to be a common theme when it comes to installing in-duct smoke detectors on new construction projects. Mechanical contractors, electrical contractors, fire contractors… who is responsible for making sure the detectors are properly installed? Perhaps this confusion is less about whose scope the detector falls into and more about the uncertainty contractors feel about the proper installation and placement of such fire protection devices. I wish I could beg to differ, but the answer to responsibility is probably not as cut and dry as we’d like. Like many other endeavors on a construction site, the installation of in-duct smoke detectors requires a coordinated effort among all parties. The mechanical contractor is often responsible for installing the detectors. I hope he doesn’t have anyone else cutting and adding to his ductwork. An electrical contractor is often required to run conduit to the location of the detector and is frequently required to subcontract for the fire alarm company to manage. Ultimately, the fire alarm contractor is responsible for making sure the device is working properly and reporting it to the fire alarm control panel. Coordination efforts.

NFPA 90A, where required

NFPA 90A is the standard for air conditioning and ventilation systems. This code states that any HVAC unit larger than 2000cfm requires an in-duct smoke detector on the supply side. They should be located below the detector air filter and next to any branch connections. If you cannot go beyond any branch connection – you must have one provided in each branch. In addition to supply side detectors, NFPA 90A states that any unit larger than 15,000cfm requires an in-duct smoke detector on the return side. These detectors are required on each story before connecting to the common return and before any recirculation or fresh air intake. They are not required where the entire space area is protected by smoke detection.

NFPA 72, Installation Tools

NFPA 72 is the National Fire Alarm Code, which is the standard for the installation of fire alarm system components. First of all, the NFPA 72 code reminds us that in-duct smoke detectors are not a substitute for open area detection. The NFPA ignores the manufacturer’s published instructions for installation requirements. Manufacturer’s instructions advise that in-duct smoke detectors be located at least 6 duct-widths away from bends or other obstructions. This means that if you have an 18″ wide duct, the detector should be located at least 9′-0″ below a bend or other obstruction. This is often difficult to accomplish. 6 Duct-width guidelines are based on the fact that the contractor must be aware that bends around bends disrupt airflow. Duct detectors require conditioned air to flow through a 1/2″ diameter sample tube that exits into the ductwork. If the airflow bounces all over the ductwork, it is less likely to pass into the sample tube as required for proper smoke detection. This is because the code states “shall” instead of “shall.” provided that there is a minimum 6-duct width, it is the contractor’s responsibility to use his own best judgment to locate the detector as far away as possible. Since HVAC ducting is often located above the finished ceiling or outside the visible range in the rafters, permanently and clearly identify the location of the in-duct smoke detector. and must be recorded. Fire officials and service personnel must be able to identify the location. of these detectors. Where in-duct smoke detectors are installed more than 10′-0″ AFF or where the detector response is not visible to personnel, the device A remote indicator must be provided for easy locating. On occasion, where it is acceptable to the AHJ, remote indicators may be removed if the detectors are specifically identified and clearly announced in the FACP and annunciators.

Turn off alarms/supervisory and fire alarms

Once in-duct smoke detectors are adequately installed, there is a continuing debate over whether the detectors should emit a supervisory signal, an inspection to determine whether a fire has started, or whether the detector should emit an alarm signal, prompting an immediate evacuation of the building. The fire department was called to the scene in response. Proponents of supervisory signals argue that in-duct smoke detectors are a common source of false alarm signals. When the heaters are first turned on in early winter, the heat flowing through the ductwork burns the dust (we all know the smell from running the heater for the first time) causing the alarm to go off. Owners or fire officials do not want to evacuate a building or drive a truck to a site that proves to be a false alarm. However, on the other side of the argument, this is a concern if the detector is activated and does its job and 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 debate, NFPA 72 has chosen to take a middle ground and let this be a local decision. The fire alarm code states that in-duct smoke detectors can be either alarm-initiating or supervisory-initiating. The local AHJ will probably have preference. According to NFPA 90A the only definitive action recognized by the code is that in-duct smoke detectors must automatically stop their associated fans; And any time a duct detector is provided, it will be connected to the building’s FACP.

Duct detector and clean agent

Another question that often comes up in relation to fire systems is how duct detectors play to clear agent suppression systems. The clean agent system has its own control panel and requires a fire detection device before discharging the extinguishing agent. The question arises when a duct detector is provided on a CRAC unit installed in a computer room protected by a clean agent system. Who monitors the status of the in-duct smoke detector and activates the alarm during the clean agent delivery sequence? Let me answer the first question by explaining a little about the second question… NFPA 2001 is the standard for clean agent suppression systems. This standard requires that forced ventilation systems be shut down only where their continued operation would adversely affect the performance of the firefighting system. Furthermore, the standard states that fully self-contained recirculating ventilation systems (ie Libert or CRAC units) do not need to be closed. This is because air recirculation within the protected space does not adversely affect the performance of the extinguishing system; In fact, it aids in keeping the agent in the environment. Continuous recirculation of air in the protected space helps extinguish the fire and prevent re-ignition. That said, in-duct smoke detectors play no role in the clean agent delivery sequence. Since the in-duct smoke detector is not part of the clean agent distribution sequence, it is not connected to the clean agent fire panel. The detector must now be tied to the base building fire alarm control panel. It is now the optional owner’s decision whether activation of the detector will cause the unit to shut down, although this is not recommended in this case.

To wrap it up, I’d like to thank you all for taking the time to learn a little more about this often confusing point of construction coordination. The more educated all relevant contractors are, the better able we are to install a fully functional fire alarm system, regardless of whose jurisdiction it falls under.

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