NFPA 90B-2018 Standard for the Installation of Warm Air Heating and Air-Conditioning Systems.
4.1.1.1.4 A Class 0 or Class 1 rigid or fexible air duct shall not be used as a vertical air duct that is more than two stories in height. 4.1.1.1.5 A Class 0 or Class 1 rigid or fexible air duct shall not be used in an air duct containing air at a temperature in excess of 121°C (250°F). 4.1.1.2* Supply ducts shall be installed in conformance with the following: (1) The conditions of their listing (2) SMACNA Fibrous Glass Duct Construction Standards (3) SMACNA HVAC Duct Construction Standards — Metal and Flexible (4) SMACNA Residential Comfort System Installation Standards Manual 4.1.2 Air Connectors. Air connectors shall be permitted to be used as limited-use, fexible air ducts that shall not be required to conform to the provisions for air ducts where they meet the requirements in 4.1.2.1 through 4.1.2.7. N 4.1.2.1 Air connectors shall conform to the requirements for Class 0 or Class 1 air connectors when tested in accordance with ANSI/UL 181. N 4.1.2.2 Class 0 or Class 1 air connectors shall not be used in ducts containing air at temperatures in excess of 121°C (250°F). N 4.1.2.3 Air connector runs shall not exceed 4.27 m (14 ft) in length. N 4.1.2.4 Air connectors shall not pass through any wall, parti‐ tion, or enclosure of a vertical shaft that is required to have a fre-resistance rating of 1 hour or more. N 4.1.2.5 Air connectors shall not pass through foors. N 4.1.2.6 An air connector shall not be interrupted by a short collar or any other ftting on one side and then connected to another air connector on the other side when penetrating a foor or a wall, partition, or enclosure of a vertical shaft that is required to have a fre-resistance rating of 1 hour. N 4.1.2.7 Multiple air connector runs shall not be spliced together in an attempt to exceed the length limitation in 4.1.2.3.NFPA 90B pdf download.