Above-ground duct systems shall conform to the following:
- Equipment connected to duct systems shall be designed to limit discharge air temperature to a maximum of 250°F (121°C).
- Factory-made air ducts shall be constructed of Class 0 or Class 1 materials as designated in Table M1601.1.1(1).
- Fibrous duct construction shall conform to the SMACNA Fibrous Glass Duct Construction Standards or NAIMA Fibrous Glass Duct Construction Standards.
- Minimum thickness of metal duct material shall be as listed in Table M1601.1.1(2). Galvanized steel shall conform to ASTM A 653. Metallic ducts shall be fabricated in accordance with SMACNA Duct Construction Standards Metal and Flexible.
- 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 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.
CLASSIFICATION OF FACTORY-MADE AIR DUCTS
|DUCT CLASS||MAXIMUM FLAME SPREAD INDEX|
GAGES OF METAL DUCTS AND PLENUMS USED FOR HEATING OR COOLING
|Round ducts and enclosed rectangular ducts|
|14 inches or less||0.0157||28||0.0145|
|16 and 18 inches||0.0187||26||0.018|
|20 inches and over||0.0236||24||0.023|
|Exposed rectangular ducts|
|14 inches or||0.0157||28||0.0145|
|Over 14a inches||0.0187||26||0.018|
For SI: 1 inch = 25.4 mm.
- For duct gages and reinforcement requirements at static pressures of 1/2 inch, 1 inch and 2 inches w.g., SMACNA Duct Construction Standard, Tables 2-1; 2-2 and 2-3 shall apply.
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 C 411 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.
- 3.1. For duct board, duct liner and factory-made rigid ducts not normally subjected to compression, the nominal insulation thickness shall be used.
- 3.2. For ductwrap, the installed thickness shall be assumed to be 75 percent (25-percent compression) of nominal thickness.
- 3.3. 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.
- 3.4. 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.
All longitudal 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. All 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 or tapes.
Closure systems used to seal 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 B-M” 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 (25.4 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 metal ductwork shall be installed in accordance with the manufacturer’s instructions. Round metallic ducts shall be mechanically fastened by means of at least three sheet metal screws or rivets spaced equally around the joint. Unlisted duct tape shall not be permitted as a sealant on any duct.
- 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.
- Continuously welded and locking-type longitudinal joints and seams in ducts operating at static pressures less than 2 inches of water column (500 Pa) pressure classification shall not require additional closure systems.
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 E 96, 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 not be 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 at least 1 inch (25 mm) high at the open sides.
Outdoor and return air for a forced-air heating or cooling system shall not be taken from the following locations:
- Closer than 10 feet (3048 mm) to an appliance vent outlet, a vent opening from a plumbing drainage system or the discharge outlet of an exhaust fan, unless the outlet is 3 feet (914 mm) above the outside air inlet.
- Where flammable vapors are present; or where located less than 10 feet (3048 mm) above the surface of any abutting public way or driveway; or where located at grade level by a sidewalk, street, alley or driveway.
- The fuel-burning appliance is a direct-vent appliance or an appliance not requiring a vent in accordance with Section M1801.1 or Chapter 24.
- 2.1. The return air shall be taken from a room or space having a volume exceeding 1 cubic foot for each 10 Btu/h (9.6 L/W) of combined input rating of all fuel-burning appliances therein.
- 2.2. The volume of supply air discharged back into the same space shall be approximately equal to the volume of return air taken from the space.
- 2.3. Return-air inlets shall not be located within 10 feet (3048 mm) of a draft hood in the same room or space or the combustion chamber of any atmospheric-burner appliance in the same room or space.
- Rooms or spaces containing solid-fuel burning appliances, if return-air inlets are located not less than 10 feet (3048 mm) from the firebox of those appliances.
- An unconditioned crawl space by means of direct connection to the return side of a forced air system. Transfer openings in the crawl space enclosure shall not be prohibited.