What is the equivalent length added for a 90-degree elbow in ductwork?

The equivalent length added for a 90-degree elbow in ductwork is an important consideration in HVAC design, as it affects the overall resistance to airflow in the system. The correct answer of 5 feet reflects a standard for calculating how much extra resistance is introduced by the elbow in comparison to straight duct runs.

This value recognizes that when air flows through a duct, changes in direction, such as those created by elbows, introduce turbulence and additional friction, which can significantly impact the efficiency of air movement. HVAC engineers and building inspectors use this equivalent length for accurate sizing of ductwork, ensuring that heating and cooling systems operate effectively and that they maintain proper airflow throughout a building.

In practice, while values for equivalent lengths can vary based on duct material and diameter, 5 feet is often used as a general rule of thumb for a standard 90-degree elbow, making it a reliable reference point for inspectors and professionals in the field. Understanding these concepts is essential when evaluating and designing efficient duct systems in residential buildings.