HVAC Contractor Practice Exam 2025 - Free HVAC Practice Questions and Study Guide

The airflow rate is the primary factor that determines the size of a duct required in an HVAC system. This is because the airflow rate, typically measured in cubic feet per minute (CFM), directly correlates with how much air needs to be transported to achieve desired temperature control and comfort levels within a conditioned space.

When calculating duct size, engineers utilize the airflow requirements of the HVAC system to ensure that sufficient air can be delivered to each area without excessive velocity, which could lead to noise issues and reduce comfort. If the duct is too small for the required airflow, it can create higher resistance and lead to increased friction loss, which can reduce the efficiency of the system.

In contrast, while temperature difference, friction loss, and material of the duct are also important considerations, they support the overall design rather than dictate duct size directly. Temperature difference impacts system performance and load calculation, while friction loss must be minimized for efficiency, but it is the airflow rate that is pivotal in determining the duct dimensions to ensure adequate circulation and balance in the HVAC system.