Which types of detectors are commonly installed in air return ducts of large buildings?

• A. Smoke detectors
• B. Water-flow detectors
• C. Infrared detectors
• D. Duct detectors

Answer: (D)

Detectors installed in air return ducts are designed specifically for the moving-air environment of HVAC systems. In large buildings, smoke can quickly travel through the return air and spread wide if not detected early, so placing sensors directly in the ducts provides rapid detection where the air circulates. Duct detectors are built to handle airflow, dust, and temperature variations and are integrated with the building’s fire alarm and HVAC controls. When smoke is sensed, they can trigger alarms and automatically shut down or modify the HVAC system, close dampers, and help contain the fire and limit smoke spread. While other detectors serve different purposes (for example, room smoke detectors or security infrared devices), duct detectors are the appropriate choice for monitoring air return paths in large structures.