What is the minimum length required if a duct must offset 5" using a field fabricated mitred offset?

In HVAC systems, when creating a mitred offset in a duct, it's important to ensure that the duct's transition accommodates the necessary change in direction while minimizing airflow restrictions. The minimum length required for an offset typically considers not only the physical change in direction but also the effect of that angle on airflow.

For a 5-inch offset, the general practice is to use a length that is sufficient to allow the ducts to connect without creating excessive turbulence. In the case of a mitred offset, a longer length helps to smooth out the airflow as it changes direction. The requirement of 20 inches demonstrates a balance between ensuring adequate airflow and the dimensions of the ducts involved. A length of 20 inches allows for a gentler transition and reduces the potential for pressure drop or noise associated with sharper bends, which could be more prevalent if a shorter length were used.

Utilizing this minimum length standard helps HVAC professionals maintain system efficiency and airflow characteristics, which is crucial for proper functioning and comfort. This length is guided by industry standards and best practices to optimize duct design.