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CHAPTER 3: DUCT DESIGN
3.1 Installation Restrictions and Use Limitations
3.1.1 Flexible Duct Installation
Flexible ducts shall be installed in accordance with the conditions of their listing and manufacturers installation instructions.
3.1.1.1 Shall not be installed in concrete, buried below grade or in contact with the ground.
3.1.1.2 Shall be interrupted at the immediate area of operation of electric, fossil fuel or solar energy collection heat sources to meet listed equipment clearances specified.
3.1.2 Flexible Air Connectors Installation
3.1.2.1 Shall not be installed in lengths greater than 14 ft. [4.3 m] for any given run.
3.1.2.2 Shall not be spliced together with short collars to circumnavigate the 14 ft. [4.3 m] limitation.
3.1.2.3 Shall not pass through any wall, partition or enclosure of a vertical shaft with a 1 hour or more fire resistive rating and shall not pass through floors.
3.1.3 Vertical Risers
Shall not be used for vertical risers serving more than two stories in height.
3.1.4 Temperature Limitations
Shall not be used in systems with enter air temperature higher than 250°F [121°C].
3.1.5 Duct Penetrations
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Shall not penetrate a wall where fire dampers are required.
3.1.6 Additional Uses
Shall not be used to vent appliances for cooking, heating, fireplaces, and clothes drying unless approved and recommended by the appliance manufacturer.
3.2 Benefits
Flexible air ducts provide multiple benefits and advantages.
Foremost, except for those intentionally perforated for acoustical purposes, flexible air ducts have impervious inner liners—meaning they have zero leakage when properly connected and sealed. This attribute is instrumental in helping systems achieve the extremely low leakage levels desired today by most building owners.
In addition flexible air ducts come pre - insulated with multiple R - value offerings. This allows for lower duct installation costs as the ducts do not have to be wrapped and sealed after - the - fact.
Flexible air ducts also have inherently better sound attenuation performance due to the non - rigid inner liner. Net insertion loss values are higher as compared to standard sheet metal pipe.
Flexible duct are densely packaged allowing for easier transport to the job site and can be more easily routed around building obstructions.
When properly sized and installed per the procedures in this standard, flexible air ducts are a cost effective and efficient option for HVAC duct systems.
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