What is the maximum angle allowed for flexible ducts?

The maximum angle allowed for flexible ducts, as indicated by the correct choice, refers to a limitation based on the design and practical function of ductwork. Flexible ducts are designed to easily navigate around obstacles within a system, but introducing sharp bends can greatly affect airflow, efficiency, and system performance.

The stipulation that the maximum bend should not exceed double the diameter of the duct ensures that the bend remains gradual enough to minimize restrictions in airflow. A bend exceeding this guideline could create significant pressure drops, turbulence, and increased resistance, which would degrade the effectiveness of the HVAC system. Additionally, excessive bending can put stress on the material of the duct, potentially leading to tears or failure over time.

In contrast, guidelines around angles like 45 degrees or 90 degrees are general rules of thumb for rigid ducts, where turns must be planned to prevent creating sharp corners that can obstruct flow. Such angles are not recommended for flexible ducts precisely because they can lead to the aforementioned issues. Thus, specifying an angle based on the duct's physical dimensions, like double its diameter, provides a more flexible yet practical rule for maintaining efficiency and protecting the integrity of the ducts.