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INSPECTION & INSTALLATION
4.8 Fittings, Fasteners & Sealants
4.8.1 Fittings
Sheet metal fittings to which flexible ducts with plain ends are attached shall be beaded and have a minimum of 2 inches [50 mm] collar length. Beads are optional for fitting when using metal worm - gear clamps or when attaching metallic flexible ducts using sheet metal screws. Sheet metal sleeves used for joining two sections of flexible duct with plain ends shall be a minimum of 4 inches [100 mm] in length and beaded on each end. Beads are optional for sleeves when using metal worm gear clamps or when joining metallic flexible ducts using sheet metal screws.
4.8.2 Mastic
Mastic shall be listed and marked “ UL181B - M ”.
4.8.3 Tapes
Tape shall be listed and marked “ UL181B - FX ”.
4.8.4 Nonmetallic Fastener (Plastic ties)
Plastic ties often called zip ties shall be listed and marked “ UL 181 B - C ”.
Flexible duct secured with nonmetallic fasteners shall be limited to 6 inches W.G. [1500 Pa] positive pressure.
4.8.5 Metallic Fasteners
Metallic fasteners are not required to be listed.
4.8.6 Screws
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ADC does not recommend the use of metal screws for making connections and splices with non - metallic flexible air ducts. Procedures and materials (tapes, mastic, fasteners) for connecting and splicing non - metallic flexible ducts are evaluated using UL181B Standard which does not address the use of metal screws. Potentially, metal screws can damage the components in some non - metallic flexible ducts.
ADC allows the use of sheet metal screws for making connections and splices with metallic flexible air ducts.
4.9 Typical Installation Locations
4.9.1 Ducts in Attics
When installing flexible ducting in attics, the ducts can be suspended from the rafters using an appropriate strap material and proper spacing. Flexible ducts can also rest on ceiling joists or truss supports provided the support centerline spacing does not exceed four (4) feet and the support surface is at least 1 - 1/2” in width. (Refer to Section 4.5 Supporting Flexible Ducts) Avoid installing ducts in areas where exposure to direct sunlight will occur (e.g. turbine vents, sky lights canopy windows, etc.). As most flexible ducts are intended for Indoor Use Only, this exposure to UV radiation can lead to eventual deterioration of the outer vapor barrier. (Refer to 3.7 UV Light) Avoid running ducts unnecessarily high up against the roof line. Attic spaces are generally significantly hotter closer to the roof line and this elevated temperature can significantly affect the energy efficiency of the duct system and potentially increase the occurrence of condensation.
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