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603.1 General
The pressure classification of ducts shall be not less than the design operating pressure of the air distribution in which the duct is utilized.
603.2 Under Floor or Crawl Space
Air ducts installed under a floor in a crawl space shall be installed in accordance with the following:
  1. Shall not prevent access to an area of the crawl space.
  2. Where it is required to move under ducts for access to areas of the crawl space, a vertical clearance of not less than 18 inches (457 mm) shall be provided.
603.3 Metal Ducts
Ducts shall be supported at each change of direction and in accordance with SMACNA HVAC Duct Construction Standards — Metal and Flexible. Riser ducts shall be held in place by means of metal straps or angles and channels to secure the riser to the structure.
Metal ducts shall be installed with not less than 4 inches (102 mm) separation from earth. Ducts shall be installed in a building with clearances that will retain the full thickness of fireproofing on structural members.
603.3.1 Rectangular Ducts
Supports for rectangular ducts shall be installed on two opposite sides of each duct and shall be riveted, bolted, or metal screwed to each side of the duct at intervals specified.
603.3.2 Horizontal Round Ducts
Horizontal round ducts not more than 40 inches (1016 mm) in diameter where suspended from above shall be supported with one hanger per interval and in accordance with Section 603.3.2.1 through Section 603.3.2.3.
603.3.2.1 Tight-Fitting Around the Perimeter
Ducts shall be equipped with tight-fitting circular bands extending around the entire perimeter of the duct at each specified support interval.
603.3.2.2 Size of Circular Bands
Circular bands shall be not less than 1 inch (25.4 mm) wide nor less than equivalent to the gauge of the duct material it supports.
Exception: Ducts not more than 10 inches (254 mm) in diameter shall be permitted to be supported by No. 18 gauge galvanized steel wire.
603.3.2.3 Connection
Each circular band shall be provided with means of connecting to the suspending support.
603.3.3 Earthquake Loads
Ducts located in structures that are installed in areas classified as seismic design category C, D, E, or F shall be in accordance with the building code.
603.4 Factory-Made Air Ducts
Factory-made air ducts shall be listed and labeled in accordance with UL 181 and installed in accordance with the terms of their listing, the manufacturer's installation instructions, and SMACNA HVAC Duct Construction Standards — Metal and Flexible.
Factory-made air ducts shall not be used for vertical risers in air-duct systems serving more than two stories and shall not penetrate a fire-resistance-rated assembly or construction.
Factory-made air ducts shall be installed with not less than 4 inches (102 mm) of separation from earth, except where installed as a liner inside of concrete, tile, or metal pipe and shall be protected from physical damage.
The temperature of the air to be conveyed in a duct shall not exceed 250°F (121°C). Flexible air connectors shall not be permitted.
603.4.1 Length Limitation
Factory-made flexible air ducts and connectors shall be not more than 5 feet (1524 mm) in length and shall not be used in lieu of rigid elbows or fittings. Flexible air ducts shall be permitted to be used as an elbow at a terminal device.
Exception: Residential occupancies.
603.5 Flexible Air Ducts
Flexible air ducts shall comply with UL 181, and shall be installed in accordance with the manufacturer's installation instructions and SMACNA HVAC Duct Construction Standards — Metal and Flexible.
Flexible air duct installations shall comply with the following:
  1. Ducts shall be installed using the minimum required length to make the connection.
  2. Horizontal duct runs shall be supported at not more than 4 feet (1219 mm) intervals.
  3. Vertical risers shall be supported at not more than 6 feet (1829 mm) intervals.
  4. Sag between support hangers shall not exceed 1/2 inch (12.7 mm) per foot (305 mm) of support spacing.
  5. Supports shall be rigid and shall be not less than 11/2 inches (38 mm) wide at point of contact with the duct surface.
  6. Duct bends shall be not less than one duct diameter bend radius.
  7. Screws shall not penetrate the inner liner of non-metallic flexible ducts unless permitted in accordance with the manufacturer's installation instructions.
  8. Fittings for attaching non-metallic ducts shall be beaded and have a collar length of not less than 2 inches (51 mm) for attaching the duct.
    Exception: A bead shall not be required where metal worm-gear clamps are used or where attaching metallic ducts using screws in accordance with the manufacturer's installation instructions.
  9. Duct inner liner shall be installed at not less than 1 inch (25.4 mm) on the collar and past the bead prior to the application of the tape and mechanical fastener. Where mastic is used instead of tape, the mastic shall be applied in accordance the mastic manufacturer's instructions.
  10. Duct outer vapor barriers shall be secured using two wraps of approved tape. A mechanical fastener shall be permitted to be used in place of, or in combination with, the tape.
  11. Flexible air ducts shall not penetrate a fire-resistance-rated assembly or construction.
  12. The temperature of the air to be conveyed in a flexible air duct shall not exceed 250°F (121°C).
  13. Flexible Air ducts shall be sealed in accordance with Section 603.10.
603.6 Plastic Ducts
Plastic air ducts and fittings shall be permitted where installed underground and listed for such use.
603.7 Protection of Ducts
Ducts installed in locations where they are exposed to mechanical damage by vehicles or from other causes shall be protected by approved barriers.
603.8 Support of Ducts
Installers shall provide the manufacturer's field fabrication and installation instructions.
Factory-made air ducts that are in accordance with UL 181 shall be supported in accordance with the manufacturer's installation instructions. Other ducts shall comply with SMACNA HVAC Duct Construction Standards — Metal and Flexible.
603.9 Protection Against Flood Damage
In flood hazard areas, ducts shall be located above the elevation required by the building code for utilities and attendant equipment or the elevation of the lowest floor, whichever is higher, or shall be designed and constructed to prevent water from entering or accumulating within the ducts during floods up to such elevation. Where the ducts are located below that elevation, the ducts shall be capable of resisting hydrostatic and hydrodynamic loads and stresses, including the effects of buoyancy, during the occurrence of flooding to such elevation.
603.10 Joints and Seams of Ducts
Joints and seams for duct systems shall comply with SMACNA HVAC Duct Construction Standards — Metal and Flexible. Joints of duct systems shall be made substantially airtight by means of tapes, mastics, gasketing, or other means. Crimp joints for round ducts shall have a contact lap of not less than 11/2 inches (38 mm) and shall be mechanically fastened by means of not less than three sheet-metal screws equally spaced around the joint, or an equivalent fastening method.
Joints and seams and reinforcements for factory-made air ducts and plenums shall comply with the conditions of prior approval in accordance with the installation instructions that shall accompany the product. Closure systems for sealing factory made air ducts and plenums shall be listed and labeled in accordance with UL 181A or UL 181B, and marked in accordance with Table 603.10.
TABLE 603.10 CLOSURE MARKINGS
TYPE OF DUCTWORK STANDARD TYPE OF CLOSURE SYSTEM MARKING
Rigid Metallic or Rigid Fiberglass UL 181A Pressure Sensitive Tape 181A-P
Rigid Metallic or Rigid Fiberglass UL 181A Mastic Tape 181A-M
Rigid Metallic or Rigid Fiberglass UL 181A Heat Sensitive Tape 181A—H
Flexible Air Ducts UL 181B Pressure Sensitive Tape* 181B-FX*
Flexible Air Ducts UL 181B Mastic* 181B-M*
* Mechanical fasteners shall be used in conjunction with a listed pressure sensitive tape or mastic in accordance with UL 181. Nonmetallic mechanical fasteners shall be listed and labeled in accordance with UL 181B and labeled "181B-C".
603.10.1 Duct Leakage Tests
Ductwork shall be leak-tested in accordance with the SMACNA HVAC Air Duct Leakage Test Manual. Representative sections totaling not less than 10 percent of the total installed duct area shall be tested. Where the tested 10 percent fail to comply with the requirements of this section, then 40 percent of the total installed duct area shall be tested. Where the tested 40 percent fail to comply with the requirements of this section, then 100 percent of the total installed duct area shall be tested. Sections shall be selected by the building owner or designated representative of the building owner. Positive pressure leakage testing shall be permitted for negative pressure ductwork. The permitted duct leakage shall be not more than the following:
(Equation 603.10.1)
Lmax = CLP0.65
Where:
Lmax = maximum permitted leakage, (ft3/min)/100 square feet [0.0001 (m3/s)/m2] duct surface area.
CL = six, duct leakage class, (ft3/min)/100 square feet [0.0001 (m3/s)/m2] duct surface area at 1 inch water column (0.2 kPa).
P = test pressure, which shall be equal to the design duct pressure class rating, inch water column (kPa).
603.11 Cross Contamination
Exhaust ducts and venting systems under positive pressure shall not extend into or pass through ducts or plenums.
603.12 Underground Installation
Ducts installed underground shall be approved for the installation and shall have a slope of not less than 1/8 inch per foot (10.4 mm/m) back to the main riser. Ducts, plenums, and fittings shall be permitted to be constructed of concrete, clay, or ceramics where installed in the ground or in a concrete slab, provided the joints are sealed and duct is secured in accordance with SMACNA HVAC Duct Construction Standards — Metal and Flexible. Metal ducts where installed in or under a concrete slab shall be encased in not less than 2 inches (51 mm) of concrete, secured in accordance with SMACNA HVAC Duct Construction Standards-Metal and Flexible.
603.13 Air Dispersion Systems
Where installed, air dispersion systems shall be completely in exposed locations in duct systems under positive pressure, and not pass through or penetrate fire-resistant-rated construction. Air dispersion systems shall be listed and labeled in accordance with UL 2518.

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