A duct smoke detector is a device or group of devices used to detect the presence of smoke in the airstreams of ductwork sections of the HVAC air-handling systems in industrial, commercial or large institutional facilities.
Duct smoke detection not only serves to assist in preventing the spread of toxic smoke and combustion gases, it can also be used to assist in protecting property and equipment.
Purpose of Duct Smoke Detectors
Primary Purpose: Prevent injury, panic and property damage by reducing the spread of smoke
NFPA 90A: Prevent the distribution of smoke through the supply air duct system and exhaust outside
Codes and Applications
NFPA 72 Installation guidelines : Duct smoke detectors shall be installed in all new and replacement air handling units, air conditioning units, package units and fans where required by NFPA 90A and NFPA 101.
Systems capable of 2,000 CFM (5 ton cooling) require supply-side detectors
On systems over 15,000 CFM, supply and return detectors are required
When activated duct smoke detectors shall:
Stop their respective fan(s)
Cause an audible and visible signal in an approved location to indicate a detector trouble condition
Initiate fire smoke dampers
Note: Duct smoke detectors are not intended to initiate a general evacuation
Principles of Operation
Duct smoke detectors work by creating a pressure differential between the sampling tube and exhaust tube on the housing to draw air from the duct.
When the duct smoke detector senses smoke, the:
Fan system will be turned off (N.C. contacts)
Trouble alarm signal will be initiated (N.O. contacts)