What type of ducts must be a single length and comply with UL 2158A?

Transition ducts are specifically designed to connect different types of ventilation systems or components. According to UL 2158A, which is a standard for clothes dryer transition ducts, these ducts must be constructed in a manner that allows for effective exhaust of air and lint while minimizing the risk of fire hazards. The requirement for these ducts to be a single length helps to eliminate obstructions and joints where lint could accumulate, potentially leading to blockage and increased fire risk.

This standard ensures that transition ducts maintain a consistent cross-section and do not have any seams or connections that could compromise their performance or safety over time. This focus on safety and performance makes the single-length requirement an essential aspect of the code that governs the use of transition ducts in residential applications.

In contrast, other duct types may have different construction and performance requirements based on their application, and they might not be mandated to be a single length under UL 2158A.