Which side of an HVAC system does NFPA 90A require smoke detection for systems with capacity of at least 2000 CFM?

• A. Return-side
• B. Supply-side
• C. Both sides
• D. Neither side

Answer: (B)

NFPA 90A focuses on preventing smoke from being carried into occupied spaces by the HVAC system. For large air-handling units, specifically those at 2,000 CFM or more, the standard requires smoke detectors on the supply side. Detecting smoke in the supply air is crucial because it allows the system to respond quickly—shutting down the supply fan or dampering the intake—to stop distributing smoke into rooms. Detecting on the return side would mean smoke has already circulated back from spaces, which is less effective for preventing spread. So, for systems at or above 2,000 CFM, the requirement is to place smoke detection on the supply side.