The velocity at the outlet of a fan is measured in which unit?

• A. Feet per minute
• B. Miles per hour
• C. Inches per second
• D. Meters per second

Answer: (A)

In HVAC practice, the velocity at a fan outlet is expressed in feet per minute because this unit directly ties to calculating air flow. If you know how fast the air is moving (velocity) and the duct’s cross-sectional area, you can multiply them to get volumetric flow in cubic feet per minute (CFM). This makes feet per minute the natural companion to area in square feet, keeping calculations straightforward and aligned with common duct sizing and fan ratings.

Miles per hour isn’t convenient for ductwork because it scales awkwardly for flow calculations and often results in impractically large numbers. Inches per second could be used, but it would require extra conversions to relate to flow rates, making it less practical for routine design and measurement. Meters per second is a perfectly valid unit, but in US practice and RETA-style testing, customary units like feet per minute are the standard, so that’s the expected answer.