UL (Underwriters Laboratories), in their 181 Standard for Factory-Made Air Ducts and Air Connectors, defines two categories of flexible “ducts”.

The UL Listed Air Duct must pass all of the tests in the UL 181 Standard. Air Ducts are labeled with a square or rectangular shaped label showing their respective listing. There is no limitation on the length of runs when using UL Listed Air Ducts.

The UL Listed Air Connector must pass only a limited number of the UL 181 tests, and is labeled with a round shaped label, which states “for installation in lengths not over 14 feet.”