detectors

Duct Detectors and New Construction Coordination

Finger pointing, passing the buck, 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’s responsible for making sure these detectors get installed properly? Perhaps this confusion is less about whose scope of work the detectors fall under, and more about the uncertainty contractors feel regarding the proper installation and placement of such fire protection devices. I wish I could say differently, but the answer regarding responsibility is probably not as cut and dry as we would prefer. Like many other endeavors on a construction site, the installation of in-duct smoke detectors requires a coordinated effort between all parties. The mechanical contractor is often responsible for mounting the detectors. I would expect he wouldn’t want anyone else cutting in to and attaching to his ductwork anyhow. The electrical contractor is often required to run conduit to the location of the detector and frequently required to manage the subcontract for the fire alarm company. In the end, the fire alarm contractor is responsible for making sure the device functions properly and reports 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 an in-duct smoke detector is required on the SUPPLY side of any HVAC unit greater than 2000cfm. Those detectors must be located downstream of air filters and ahead of any branch connections. If you can’t get ahead of any branch connections – you must have one provided in each branch. In addition to the supply side detector, NFPA 90A states that an in-duct smoke detector is required on the RETURN side of any unit greater than 15,000cfm. These detectors are required at each story prior to the connection to a common return and prior to any recirculation or fresh air inlet. They are not required where the entire space is protected by area smoke detection.
NFPA 72, Means of Installation
NFPA 72 is the National Fire Alarm Code, the standard for the installation of fire alarm system components. This first of all, the NFPA 72 code reminds us that in-duct smoke detectors are NOT a substitute for open area detection. NFPA defers to manufacturer’s published instructions for installation requirements. Manufacturer’s instructions advise that in-duct smoke detectors be located at a minimum of 6 duct-widths from a bend or other obstruction. This means that if you have an 18″ wide duct, the detector should be located a minimum of 9′-0″ downstream of a bend or other obstruction. This is often difficult to accomplish. The contractor needs to be aware that the 6 duct-width guideline is based on the fact that airflow is disrupted as it comes around a bend. The duct detector requires the conditioned air to flow through a 1/2″ diameter sample tube that protrudes into the ductwork. If the airflow is bouncing all over the ductwork, it is less likely to make it …