Which of the following is NOT an acceptable use for flexible air duct connections?

Connecting flexible air ducts directly to a furnace is not considered an acceptable use because it can lead to a number of issues. Flexible ducts are designed to be more adaptable and are often used to make transitions between fixed, rigid components in an HVAC system. When connecting directly to a furnace, using rigid ductwork is preferred because it provides a more stable and secure connection. Rigid ducts minimize the risks of air leaks and provide better airflow, which is essential for efficient furnace operation.

In contrast, the other options involve acceptable uses of flexible duct connections. Attaching to a rigid duct allows for adjustments in alignment or orientation without compromising the integrity of the rigid ductwork. Passing through a ceiling is also acceptable, as flexible ducts can navigate around obstacles while maintaining airflow. Similarly, passing through walls is permissible, enabling routing flexibility in an HVAC system layout. Each of these applications leverages the versatility of flexible air ducts while still adhering to best practices in HVAC installation and performance.