Why can't duct detectors be used to replace area detection?

Duct detectors are designed to monitor the air flow within an HVAC system, specifically detecting smoke or heat from the air being drawn into the ducts. If the air handling unit (AHU) is not operating, the duct detector may not function as intended because it relies on the movement of air to transport potential smoke or heat to the detector. In instances where the AHU is off, there would be no air flow through the ducts for the detector to sense, significantly diminishing its effectiveness in identifying hazards.

On the other hand, area detection systems, such as smoke detectors located throughout a building, can consistently monitor the environment regardless of whether an HVAC system is actively running. This characteristic makes them more reliable for detecting conditions that might indicate a fire, especially in spaces that may not be connected to a duct system.

In conclusion, the dependence on the AHU's operation is a critical limitation of duct detectors that prevents them from effectively replacing area detection systems.