Return-side smoke detection is required for HVAC systems serving more than one story if their capacity is at least how many CFM?

• A. 5000 CFM
• B. 15000 CFM
• C. 10000 CFM
• D. 2000 CFM

Answer: (B)

Return-side smoke detection is used because it catches smoke as air returns to the air handler, which is a common path for smoke to travel between floors in a building. When an HVAC system serves more than one story and handles a large volume of air, specifically 15,000 cubic feet per minute or more, detecting smoke on the return side allows the system to respond quickly—shutting dampers and initiating smoke control sequences to contain the fire and protect occupants’ egress. The 15,000 CFM threshold reflects the point at which cross-floor smoke spread becomes a higher risk and centralized detection is warranted. For smaller systems, or those serving a single story, detectors in the occupied spaces or in the ducts may be sufficient because the potential for rapid multi-story spread is lower.