Air ducts must pass 15 tests of the UL181 Standard for Safety for Factory Made Air Ducts and Air Connectors. However, air connectors are required to be pass only 12 of the 15 tests. Therefore, most models codes and the National Fire Protection Association (NFPA) require that air connectors shall not be installed in lengths greater than 14 feet. An installer may NOT increase this length by using a splice between 14' sections of air connectors. The installed length of Air Duct is not restricted.