The NFPA-72 states that “Fire alarm duct detectors that are installed in the air duct system shall not be used as a substitute for open air protection“. When smoke or a fire is detected, the entire (HVAC) air conditioning system is disabled to prevent the spread of the fire to other area’s of the building.
The primary goal of a duct detector on a fire alarm system is to detect smoke during a fire and prevent panic, injury and property damage by reducing the spread and recirculation of smoke in a commercial building. An example would be a heating, ventilating or air conditioning (HVAC) fan motor overheats causing smoke to be sensed by the duct detector in the air supply duct.
In this case, the fire alarm duct detector is equipped with a relay that will immediately cut the power to the fan motor before significant amounts of smoke can be distributed throughout the occupied building. It can also serve to protect the air conditioning system from itself in the event of smoke and fire damage, and works in conjunction with other life safety systems to prevent the spread of fire and oxygen flow.