A single rooftop unit with 20,000 CFM supplies a three-story building. How many smoke detectors are needed?

• A. 2
• B. 3
• C. 4
• D. 5

Answer: (B)

Smoke detectors in HVAC systems are installed to quickly shut down the air handling unit when smoke is detected, preventing smoke from being spread through the building. When a single rooftop unit serves multiple floors, each floor ideally represents a separate zone that needs monitoring. This ensures that if smoke is detected on one floor, the system can respond appropriately for that zone without unnecessarily shutting down airflow to all floors.

In this setup, there are three floors served by the unit, so one detector per floor is required, totaling three detectors. The unit’s size or airflow (20,000 CFM) doesn’t determine the number of detectors; it’s the number of separate zones or floors that matters.