Which capacity triggers supply-side smoke detection according to NFPA 90A?

• A. 1000 CFM
• B. 2000 CFM
• C. 8000 CFM
• D. 12000 CFM

Answer: (B)

In NFPA 90A, the need for supply-side smoke detection ramps up with larger HVAC systems to catch smoke before it spreads through the ductwork. The standard specifies a threshold at 2,000 CFM of supply air. When the system delivers 2,000 CFM or more, a smoke detector in the supply duct is required. Detecting smoke there allows the HVAC to respond quickly—typically by shutting down the air handler or reconfiguring the system—so smoke isn’t pushed into occupied spaces.

This threshold is chosen to balance cost and risk: smaller systems don’t pose the same rapid-smoke-spread concern, while larger systems could distribute smoke much more quickly if not detected early.