Above-ground duct systems shall conform to the following:
- Equipment connected to duct systems shall be designed to limit discharge air temperature to not greater than 250°F (121°C).
- Factory-made ducts shall be listed and labeled in accordance with UL 181 and installed in accordance with the manufacturer’s instructions.
- Fibrous glass duct construction shall conform to the SMACNA Fibrous Glass Duct Construction Standards or NAIMA Fibrous Glass Duct Construction Standards.
- Field-fabricated and shop-fabricated metal and flexible duct constructions shall conform to the SMACNA HVAC Duct Construction Standards—Metal and Flexible except as allowed by Table M1601.1.1. Galvanized steel shall conform to ASTM A653.
- The use of gypsum products to construct return air ducts or plenums is permitted, provided that the air temperature does not exceed 125°F (52°C) and exposed surfaces are not subject to condensation.
- Duct systems shall be constructed of materials having a flame spread index of not greater than 200.
- 7.1. These cavities or spaces shall not be used as a plenum for supply air.
- 7.2. These cavities or spaces shall not be part of a required fire-resistance-rated assembly.
- 7.3. Stud wall cavities shall not convey air from more than one floor level.
- 7.4. Stud wall cavities and joist-space plenums shall be isolated from adjacent concealed spaces by tight-fitting fireblocking in accordance with Section R602.8.
- 7.5. Stud wall cavities in the outside walls of building envelope assemblies shall not be utilized as air plenums.
DUCT CONSTRUCTION MINIMUM SHEET METAL THICKNESS FOR SINGLE DWELLING UNITSa
|ROUND DUCT DIAMETER|
|1/2 inch water gage||1 inch water gage|
|Thickness (inches)||Thickness (inches)|
|12 to 14||0.013||0.018||0.016||0.023|
|15 to 17||0.016||0.023||0.019||0.027|
|19 to 20||0.019||0.027||0.024||0.034|
|RECTANGULAR DUCT DIMENSION|
|1/2 inch water gage||1 inch water gage|
|Thickness (inches)||Thickness (inches)|
|9 to 10||0.013||0.018||0.016||0.023|
|11 to 12||0.016||0.023||0.019||0.027|
|13 to 16||0.019||0.027||0.019||0.027|
|17 to 18||0.019||0.027||0.024||0.034|
|19 to 20||0.024||0.034||0.024||0.034|
For SI: 1 inch = 25.4 mm, 1 inch water gage = 249 Pa.
- Ductwork that exceeds 20 inches by dimension or exceeds a pressure of 1 inch water gage (250 Pa) shall be constructed in accordance with SMACNA HVAC Duct Construction Standards—Metal and Flexible.
Duct insulation materials shall conform to the following requirements:
- The flame spread index is not greater than 25 and the smoke-developed index is not greater than 450 at the specified installed thickness.
- The foam plastic is protected in accordance with the ignition barrier requirements of Sections R316.5.3 and R316.5.4.
- The foam plastic complies with the requirements of Section R316.
- Duct coverings and linings shall not flame, glow, smolder or smoke when tested in accordance with ASTM C411 at the temperature to which they are exposed in service. The test temperature shall not fall below 250°F (121°C). Coverings and linings shall be listed and labeled.
- External reflective duct insulation shall be legibly printed or identified at intervals not greater than 36 inches (914 mm) with the name of the manufacturer, the product R-value at the specified installed thickness and the flame spread and smoke-developed indices. The installed thickness of the external duct insulation shall include the enclosed air space(s). The product R-value for external reflective duct insulation shall be determined in accordance with ASTM C1668.
- 4.1. For duct board, duct liner and factory-made rigid ducts not normally subjected to compression, the nominal insulation thickness shall be used.
- 4.2. For ductwrap, the installed thickness shall be assumed to be 75 percent (25-percent compression) of nominal thickness.
- 4.3. For factory-made flexible air ducts, The installed thickness shall be determined by dividing the difference between the actual outside diameter and nominal inside diameter by two.
- 4.4. For spray polyurethane foam, the aged R-value per inch measured in accordance with recognized industry standards shall be provided to the customer in writing at the time of foam application. In addition, the total R-value for the nominal application thickness shall be provided.
Longitudinal and transverse joints, seams and connections in metallic and nonmetallic ducts shall be constructed as specified in SMACNA HVAC Duct Construction Standards—Metal and Flexible and NAIMA Fibrous Glass Duct Construction Standards. Joints, longitudinal and transverse seams, and connections in ductwork shall be securely fastened and sealed with welds, gaskets, mastics (adhesives), mastic-plus-embedded-fabric systems, liquid sealants or tapes. Tapes and mastics used to seal fibrous glass ductwork shall be listed and labeled in accordance with UL 181A and shall be marked “181A-P” for pressure-sensitive tape, “181 A-M” for mastic or “181 A-H” for heat-sensitive tape.
Tapes and mastics used to seal metallic and flexible air ducts and flexible air connectors shall comply with UL 181B and shall be marked “181 B-FX” for pressure-sensitive tape or “181 BM” for mastic. Duct connections to flanges of air distribution system equipment shall be sealed and mechanically fastened. Mechanical fasteners for use with flexible nonmetallic air ducts shall comply with UL 181B and shall be marked 181B-C. Crimp joints for round metallic ducts shall have a contact lap of not less than 1 inch (25 mm) and shall be mechanically fastened by means of not less than three sheet-metal screws or rivets equally spaced around the joint.
Closure systems used to seal all ductwork shall be installed in accordance with the manufacturers’ instructions.
- Spray polyurethane foam shall be permitted to be applied without additional joint seals.
- Where a duct connection is made that is partially inaccessible, three screws or rivets shall be equally spaced on the exposed portion of the joint so as to prevent a hinge effect.
- For ducts having a static pressure classification of less than 2 inches of water column (500 Pa), additional closure systems shall not be required for continuously welded joints and seams and locking-type joints and seams of other than the snap-lock and button-lock types.
Duct insulation shall be installed in accordance with the following requirements:
- A vapor retarder having a maximum permeance of 0.05 perm [2.87 ng/(s • m2 • Pa)] in accordance with ASTM E96, or aluminum foil with a minimum thickness of 2 mils (0.05 mm), shall be installed on the exterior of insulation on cooling supply ducts that pass through unconditioned spaces conducive to condensation except where the insulation is spray polyurethane foam with a maximum water vapor permeance of 3 perm per inch [1722 ng/(s • m2 • Pa)] at the installed thickness.
- Exterior duct systems shall be protected against the elements.
- Duct coverings shall not penetrate a fireblocked wall or floor.
The space shall be cleaned of loose combustible materials and scrap, and shall be tightly enclosed. The ground surface of the space shall be covered with a moisture barrier having a minimum thickness of 4 mils (0.1 mm). Plumbing waste cleanouts shall not be located within the space.
A duct shall extend from the furnace supply outlet to not less than 6 inches (152 mm) below the combustible framing. This duct shall comply with the provisions of Section M1601.1. A noncombustible receptacle shall be installed below any floor opening into the plenum in accordance with the following requirements:
- The receptacle shall be securely suspended from the floor members and shall be not more than 18 inches (457 mm) below the floor opening.
- The area of the receptacle shall extend 3 inches (76 mm) beyond the opening on all sides.
- The perimeter of the receptacle shall have a vertical lip not less than 1 inch (25 mm) in height at the open sides.
- Openings shall not be located less than 10 feet (3048 mm) measured in any direction from an open combustion chamber or draft hood of another appliance located in the same room or space.
- The amount of return air taken from any room or space shall be not greater than the flow rate of supply air delivered to such room or space.
- Return and transfer openings shall be sized in accordance with the appliance or equipment manufacturers’ installation instructions, Manual D or the design of the registered design professional.
- Taking return air from a kitchen is not prohibited where such return air openings serve the kitchen only, and are located not less than 10 feet (3048 mm) from the cooking appliances.
- Dedicated forced-air systems serving only the garage shall not be prohibited from obtaining return air from the garage.
- Taking return air from an unconditioned crawl space shall not be accomplished through a direct connection to the return side of a forced-air furnace. Transfer openings in the crawl space enclosure shall not be prohibited.
- Return air from one dwelling unit shall not be discharged into another dwelling unit.
Restricted return air occurs in buildings when returns are located in central zones and closed interior doors impede air flow to the return grill or when ceiling spaces are used as return plenums and fire walls restrict air movement from one portion of the return plenum to another. Provisions shall be made in both residential and commercial buildings to avoid unbalanced air flows and pressure differentials caused by restricted return air. Pressure differentials across closed doors where returns are centrally located shall be limited to 0.01 inch WC (2.5 pascals) or less. Pressure differentials across fire walls in ceiling space plenums shall be limited to 0.01 inch WC (2.5 pascals) by providing air duct pathways or air transfer pathways from the high pressure zone to the low zone.
- Transfer ducts may achieve this by increasing the return transfer 11/2 times the cross sectional area (square inches) of the supply duct entering the room or space it is serving and the door having at least an unrestricted 1-inch undercut to achieve proper return air balance.
- Transfer grilles shall use 50 square inches (of grille area) to 100 cfm (of supply air) for sizing throughthe-wall transfer grilles and using an unrestricted 1- inch undercutting of doors to achieve proper return air balance.
- Habitable rooms only shall be required to meet these requirements for proper balanced return air excluding bathrooms, closets, storage rooms and laundry rooms, except that all supply air into the master suite shall be included.