What is likely to cause a significant reduction in air flow in a flexible air duct?

A 90-degree bend in a flexible air duct is likely to cause a significant reduction in airflow due to the sharp turn it creates in the duct's path. When air travels through a duct, it moves in a straight line as easily as possible. A bend introduces turbulence and resistance, which can impede airflow and reduce efficiency. This reduction in airflow can lead to uneven heating or cooling in different areas of a space, as well as increased energy consumption as the HVAC system works harder to push air through the turn.

While excessive length of duct run can also contribute to reduced airflow, it is generally the bends and turns that create the most notable immediate impact by increasing friction loss. A dirty air filter can restrict airflow, but its effect is different from that of a duct's structural issues. Incorrect duct size can lead to airflow problems, but it does so in a more indirect manner rather than the immediate, pronounced effect seen with sharp bends. Therefore, the specific nature of a 90-degree bend makes it a more critical factor in significantly reducing airflow in a flexible air duct.