The Air Duct Council (ADC) emphatically disagrees with the proposed arbitrary five (5) feet length limit of flexible air ducts for residential applications in the next edition of the Uniform Mechanical Code (UMC). No technical justification has been provided to support this limitation and thus it is neither warranted nor justified.
All air ducts exhibit pressure loss due to friction regardless of the material the ducts are made from. Each duct type will exhibit different friction characteristics, some more, some less than others, that is unique to the duct type and material. The important aspect is to properly size all ducts to account for the pressure drop characteristic associated with whatever type of duct material is being used. In addition, air ducts should be installed per applicable industry standards and per the manufacturer’s requirements, regardless of which duct type is used.
Due to the helical-nature of the inner liner and the roughness factor of the materials used to manufacture flexible ducts, the pressure drop per foot is generally higher than that of the same diameter smooth galvanized pipe. When sizing ducts per ACCA’s Manual D, as required by the codes, the indicated diameter of a flexible duct run may well be larger than if sized using smooth galvanized duct. A properly sized duct system that uses flexible air duct will take the pressure drop characteristic and difference of the flexible duct material into account and thus the expected performance will be achieved.
Flexible Air Ducts have been installed throughout the country for more than 50 years without an arbitrary length limitation. The proposed code change to limit the length of flexible air ducts to five (5) feet in residential applications, suggested for inclusion in the 2018 UMC, is not substantiated nor warranted.