What factor primarily determines the CFM delivered by a duct?

The factor that primarily determines the CFM (cubic feet per minute) delivered by a duct is the duct cross-sectional area. This is because the cross-sectional area directly influences the volume of air that can flow through the duct at a given velocity. A larger cross-sectional area allows for more air to pass through in a set amount of time, thus increasing the CFM.

When designing duct systems, it's crucial to consider the size of the duct because it affects both the airflow capacity and the overall efficiency of the HVAC system. While factors like duct shape, length, and air pressure can also impact airflow, the maximum airflow rate achievable through a duct is most significantly governed by its cross-sectional dimensions. These dimensions establish the pathway through which air can circulate, making them a pivotal element in airflow calculations and system performance.