UL 181:2021 PDF Download

The UL 181:2021 standard sets out requirements for materials used in the fabrication of air duct and air connector systems. These requirements apply to systems used in accordance with various codes, including the International Mechanical Code (IMC), International Residential Code (IRC), Uniform Mechanical Code (UMC), and the Standards of the National Fire Protection Association for the Installation of Air-Conditioning and Ventilating Systems (NFPA No. 90A) and the Installation of Warm Air Heating and Air-Conditioning Systems (NFPA No. 90B).

The standard covers a range of air ducts and air connectors, including preformed lengths of flexible or rigid ducts, materials in the form of boards for field fabrication of rigid ducts, and preformed flexible air connectors.

Within the standard, air ducts and air connectors are classified into two categories:

1. Class 0 - Air ducts and air connectors with surface burning characteristics of zero.

2. Class 1 - Air ducts and air connectors with a flame-spread index of not over 25 without evidence of continued progressive combustion and a smoke-developed index of not over 50.

These classification criteria ensure that the materials used in the fabrication of air duct and air connector systems meet specific safety standards, particularly in terms of fire resistance and smoke development.