Duct Smoke Detectors and LED Test Plates

Duct smoke detectors are a common device found in most fire alarm systems from the past and present. Whether the duct smoke detector is addressable or conventional, they both work the same way. A sample tube is inserted into the duct work and samples the air for smoke. If smoke is detected then the photoelectric smoke detector head within the duct smoke detectors sends an alarm signal to the fire alarm panel and shuts down the air conditioning unit.

With that said, most of these duct smoke detectors are located in areas that are not very easily accessed. These areas could include areas such as above t-bar or hard lid ceilings, attic spaces or roof tops. If the actual duct detector is located above a ceiling then access hatches or panels must be provided for obvious reasons. Also if the duct smoke detector is located in an area out of sight then you must provide a detailed description of the device location within the fire alarm control panel program as well as a remote LED plate on the ceiling below the device. This will assist the fire department, end user and technician locate the device. In fact, some jurisdictions such as L.A. County are requiring the remote LED plate to be installed any time you have a duct smoke detector above a ceiling and out of plain view.

With all of this information it is hard for me to understand how some of these local fire alarm installation companies are getting away with installing the remote LED plates on the wall directly above the thermostat controls for the particular HVAC unit. This picture was taken at a local restaurant in the Valencia, CA area. Now with these remote LED plates installed on the wall it makes it easy for fire alarm testing but it totally eliminates their true purpose. If there is a fire reported at the building with a signal coming from a duct smoke detector then the fire department is going to want to know exactly where it's location is the minute they enter. The LED on the wall does not exactly help.

System Sensor Duct Detector Applications

System Sensor Duct Detector Applications

NFPA 90A2, “Standard for the Installation of Air Conditioning and Ventilating Systems”, specifies that “Smoke detectors listed for use in air distribution systems shall be located as follows:

(1) Downstream of the air filters and ahead of any branch connections in air supply systems having a capacity greater than 2,000 cfm (944L/sec).

(2) At each story prior to the connection to a common return and prior to any recirculation or fresh air inlet connection in air return systems having a capacity greater than 15,000 cfm (7080 L/sec), and serving more than one story.”

Exception Number One:

“Return system smoke detectors are not required when the entire space served by the air distribution system is protected by a system of area smoke detectors.” (NFPA 90A, 2002, 6.4.2.2)

Exception Number Two:

“Fan units whose sole function is to remove air from inside the building to outside the building.” (NFPA 90A, 2002, 6.4.2.3)

Application Documents

There are several important documents that provide guidance concerning the performance, application and installation of System Sensor duct smoke detectors:

• U.L. Standard 268A, Standard for Smoke Detectors for Duct Applications

• NFPA Standard 90A, Installation of Air Conditioning and Ventilating Systems

• NFPA 92A, Standard for Smoke-Control Systems Utilizing Barriers and Pressure Differences Nfpa 92A: Recommended Practice for Smoke-Control Systems (Gl-92a-88)

• NFPA 92B Standard for Smoke Management Systems in Malls, Atria, and Large Spaces NFPA 92B: Standard for Smoke Management Systems in Malls, Atria and Large Spaces (2009)

• NFPA Standard 72, National Fire Alarm Code

• NFPA Standard 101, Life Safety Code

• ASHRAE Handbook and Product Directory, “Fire and Smoke Control”