M1601.1 Material and construction requirements for duct systems can be divided into two major categories: those for above-ground ducts and those for underground ducts. The demands placed on these categories are different; therefore, they are treated separately in the code and in the following commentary. Adequately sized supply air and return air ductwork is essential for efficient and proper circulation of conditioned air. Ducts must be large enough to allow sufficient air flow through appliances to satisfy refrigerant coil air flow demands and to avoid excessive temperature rise. As a practical matter, restriction of the supply duct system is an infrequently encountered problem. However, an inadequately sized return system is often found in the field, particularly on heat pump installations. The code does not provide specific requirements for sizing duct systems, but instead relies on ACCA Manual D or other sizing criteria, as approved by the code official.

 

 

 

 

 

 

 

 

M1601.1.1 Metallic ducts are usually constructed using galvanized sheet steel. Duct size is based on required airflow, system pressure, flow velocity and pressure losses caused by friction. Duct material thickness is determined by duct size, static pressure of the system, distance between supports and whether the duct is reinforced. Metallic ducts must be constructed with the minimum thicknesses specified in Table M1601.1.1(2), which bases the minimum required duct thickness on the geometry of the duct, the material used and the major dimension of the duct (the diameter for round ducts and the widest side for rectangular ducts). Nonmetallic ducts and duct materials must be tested and classified in accordance with the provisions of UL 181. Only Class 0 and Class 1 ducts can be used. Class 0 indicates flame spread and smoke-developed indexes of zero; Class 1 indicates a flame spread index not greater than 25 and a smoke-developed index not greater than 50, when tested to ASTM E 84. The exception to the 25 flame spread index for nonmetallic ducts is Item 6 of this section which allows installation of plastic ducts with a flame spread index of 200 or less above ground. However, it should be noted that there is an ICC committee interpretation that requires plastic ducts installed above ground to meet the Class I flame spread index requirement of 25. UL 181 requires that a nonmetallic duct be tested to determine its fire-performance characteristics, corrosion resistance, mold-growth resistance, humidity resistance, leakage resistance, temperature resistance, erosion resistance and structural performance. Air ducts that conform to the requirements of UL 181 are identified by the manufacturer’s or vendor’s name, rated velocity, negative and positive pressure classification and duct material class. Fibrous ducts are constructed of a composite material of rigid (high-density) fiberglass board and a factory- applied facing, typically reinforced aluminum. The surface of the fibrous duct that is exposed to the airflow is sealed with a fiber-bonding adhesive that prevents erosion of the fiberglass material. The factory-applied exterior duct-board facing contributes to the strength and rigidity of the composite material, acts as a heat reflector, serves as a vapor barrier and is an integral component of the joining method used to construct fibrous ducts. The material is available in board form for shop or field fabrication into rectangular sections or in ten-sided duct form, which approximates a circular cross section. Fibrous ducts take advantage of the inherent insulating qualities of the glass fiber material. The air friction factors for fibrous ducts are higher than those for sheet metal because of the relatively rough surface finish of the former. Construction of fibrous glass ducts must conform to the requirements of the SMACNA Fibrous Glass Duct Construction Standards, which provides details for the design and fabrication of air distribution systems using fibrous glass ducts. The SMACNA standards referenced in this section and the previous section are enforceable extensions of the code. The maximum discharge temperature permitted by industry standards for warm air heating systems is 250°F (121°C). This section prohibits nonmetallic ducts from being used in applications in which the air temperature would exceed this maximum because the material has not been tested to withstand higher temperatures, and high temperatures will cause accelerated aging of the duct material. Gypsum board is a composite material commonly used for the construction of air plenums and shafts. Gypsum board can reduce construction costs because it is a common component of building construction assemblies. By serving a dual purpose, gypsum board eliminates the need for independent duct construction. The use of gypsum board to form ducts and plenums is specifically regulated to prevent deterioration of the gypsum board material. Air temperatures that exceed 125°F (52°C) will, over time, dry both the paper facing and the gypsum of the gypsum board, leading to deterioration of the panel. Gypsum board can also deteriorate when exposed to moisture, which will happen if the surface temperature of the gypsum board is lower than the airstream dew-point temperature, causing water to condense on the surface of the gypsum board. For these reasons, gypsum board cannot be used for air distribution systems using evaporative cooling equipment. It is further restricted to return-air system applications only, a maximum airstream temperature of 125°F (52°C) and an airstream dew-point temperature continuously below the temperature of the gypsum board surface. Evaporative cooling equipment, such as “swamp coolers,” uses water as a refrigerant. The resulting addition of moisture to the airstream could cause deterioration of the gypsum board. Common in residential construction is the stud-space and joist-space being used as a plenum. These spaces are limited to use for return air only because the negative pressures within the return-air plenum with respect to surrounding spaces will decrease the likelihood of spreading smoke to other spaces via the plenum. Also, the temperature and moisture content of heated, cooled and conditioned supply air could cause a fire hazard or deterioration of the construction materials exposed in the spaces. The space must not be a part of a fire-resistance- rated assembly because the ASTM E 119 test does not consider the impact of air movement within the assembly on the fire-resistance rating. This restriction is a concession to the convenience and cost-saving potential of this method of moving air. The use of stud spaces inherently means the interconnection of different floor levels by the concealed space. Because of the hazard of such an interconnection, the use of this type of plenum is limited to return air from one floor level only for each independent stud cavity. All cavities not used for air movement must be isolated from the plenum by fireblocking constructed and installed in accordance with this code. Commentary Figure M1601.1.1(1) shows an example of an acceptable stud and joist space installation. The bottom plate of the wall is cut away for the plenum to function, and fireblocking is installed in the joist and stud space to limit communication of the plenum with other spaces. The stud cavity shown is being used to return air from one floor level only, conducting it to the space below, where the air-handling equipment is located. Commentary Figure M1601.1.1(2) shows an example of an unacceptable stud and joist space installation. Whether viewed as a shaft or as a duct, a stud-cavity plenum penetrates floor assemblies and is, therefore, subject to the floor penetration protection requirements of Section R321.3.1 which may require floor penetration protection. In as much as air is returned from more than one floor level through the same stud cavity, this section prohibits this type of installation because it creates a direct connection from one floor level to another by means of a concealed cavity. Such direct connections would act as a chase or chimney, allowing fire and smoke to spread quickly upward through the building. Thus, stud-cavity return-air plenums are subject to the same restrictions and requirements as floor openings and penetrations regulated by Section R321.3. Stud-space and joist-space plenums must be viewed as an exception to the fireblocking provisions of Section R602.8 because one or more fireblocks (wall plates) must be removed or relocated to construct the plenums. The code does not mention the type of materials allowed for “panning” the bottom of open joists to create joist-space plenums. Traditionally, sheet metal has been used; however, composite materials are also used. The building official must determine what materials are acceptable for joist panning.

TABLE 1601.1.1(1) This table, referred to in Section M1601.1.1, contains the maximum flame spread ratings for the two classes of factory-made duct construction.

 

 

 

 

 

 

 

 

 

 

 

M1601.1.2 Ducts installed underground must be able to resist the forces imposed on them by the materials that encase them, the forces created by floodwaters in and around them and corrosion. The ASHRAE Handbook of HVAC Systems and Equipment recommends that all underground ducts and fittings be round to provide optimum structural performance. Unlike round ducts, square or rectangular ducts offer little resistance to deformation or collapse caused by the structural loads associated with burial. Metal ducts must have either a protective coating to resist corrosion or be completely encased in concrete a minimum of 2 inches (52 mm) thick all around. Concrete- encased ducts may eventually corrode; however, the air passageway will be maintained because of the remaining concrete enclosure. All nonmetallic ducts and metallic ducts with factory-applied protective coatings must be approved, and all such duct installations must be in accordance with the manufacturer's installation instructions. Application of any field-applied protective coating must be approved. Great care must be taken to protect underground ducts from damage prior to placing the concrete or installing the permanent structure above them. Plastic duct and fitting systems designed for underground applications are available and allow corrosion-resistant and waterproof installations. Plastic ducts have the advantage of being corrosion resistant. The external coating properties of ASTM D 2412 assure that the pipe has the ability to resist deformation from the loads associated with direct burial. Plastic ducts rapidly lose strength as their temperature approaches their maximum service temperature. At temperatures above 150°F (65.5°C), PVC pipe is substantially weakened and deformation and/or collapse is possible. Underground ducts must be sloped to drain to an accessible point in the event that water enters the duct through duct openings or from the subsoil. Water can cause corrosion, deterioration of duct materials and duct blockages; therefore, sloping the duct to drain to a collection point will allow removal of water. The code does not require that ducts be water tight. This section requires that the duct be sealed before the concrete is poured to prevent concrete from entering the duct.

 

 

 

 

 

 

 

 

 

 

TABLE M1601.1.1(2) The metal thicknesses allowed by Table R603.3 have been used in dwelling unit construction without evidence of failure. Therefore, the lighter gages are justified in this limited application.

 

 

 

 

 

 

 

 

 

 

M1601.2 Several types of factory-made air ducts are available for a variety of applications; they are listed products that comply with UL 181. Air ducts are designed to convey air to or from a conditioned area. Labeling for compliance with UL 181 ensures that ducts have been fabricated from materials deemed satisfactory for the conditions of use.

 

 

 

 

 

 

 

 

 

 

 

M1601.2.1 Because duct systems connect most rooms in a building, they can provide a path for fire and smoke to travel throughout that building. Duct coverings and linings are exposed to the surrounding environment and to the airstream in the duct. To reduce the possible spread of fire and smoke, duct coverings and linings must be tested to ASTM E 84. These materials are limited to a maximum flame spread index of 25 and a smoke-developed index of 50, which correspond to Class 1 material. In addition, duct coverings and linings must be rated for the air temperatures expected; this avoids degradation of these materials. To verify that duct coverings and linings will not present a fire hazard, these materials must be tested in the form in which they will be installed at their rated temperatures but to not less than 250°F (121°C) in accordance with the procedures of ASTM C 411. This minimum temperature for testing represents the maximum temperature that industry standards will permit in the airstream of a warm-air heating appliance. It is important that the duct coverings and linings are tested in their composite form rather than having tests conducted on each component that comprises the product (i.e., the insulation, the facing and the adhesive). Each component could pass the ASTM E 84 tests individually, but could fail when combined into the final assembly to be installed in the field. ASTM E 84 dictates the test methods required for duct coverings and linings, including a requirement for testing of systems representative of the actual field installation. ASTM E 2231 was added to the code because it contains the specimen preparation and mounting procedures necessary to ensure that the specimen tested in the laboratory is as close as possible to the actual field installation. This will result in a safer field installation where the actual performance of the material can be more accurately predicted. To assist inspectors, the code requires that duct insulation have a label with the manufacturer’s name, thermal resistance (R), and the flame/smoke indexes. A third-party agency must provide quality control inspections at the manufacturer’s facility in accordance with the requirements for labeling. Testing performed by an independent agency must determine the insulating R-value, the flame spread index and the smoke-developed index. The 36-inch (914 mm) label intervals are intended to increase the likelihood that every cut piece of insulation and flexible duct will have a label. The thermal performance of duct insulation is dependent on its “installed” condition including the compressed condition, as is the case for duct wrap insulations. This section is intended to provide manufacturers, installers and inspectors with specific guidance for meeting the intent of the code. For example, installed duct wrap is assumed to have a thickness of 75 percent of the nominal uninstalled thickness. The R-value on the product will account for the decreased thermal resistance caused by compression of the product.

 

 

 

 

 

 

 

 

 

M1601.2.2 Isolators must be built from materials that will withstand the temperatures and pressures of typical conditioned air passing through the duct. The 10-inch (254 mm) length limitation prevents installation of long lengths of nonrigid duct material, which could adversely affect the integrity of the duct system.

 

 

 

 

 

 

 

 

 

 

M1601.3 As a result of increasing energy costs, ducts are being insulated more frequently to reduce losses. The following sections discuss the details of individual duct insulation installation requirements.

 

 

 

 

 

 

 

 

 

 

 

M1601.3.1 Air leaking from ducts in unconditioned spaces must be minimized to prevent energy losses. Joints must be made air tight by approved methods, which include tapes, mastics, gasketing and other approved closure systems. This section requires sealing of joints, seams and connections in accordance with the requirements of the UL standard applicable to the type of duct. UL 181A applies to rigid ductwork and UL 181B applies to flexible ductwork. The appropriate markings for tapes and mastics are also specified in this section to provide the installer and the inspector guidance for an acceptable installation. It is not the intent of this section to require sealing of the longitudinal “snap lock” seams on round duct or the spiral seams on round spiral duct. Note that Section 603.9 of the International Mechanical Code® (IRC®) prohibits the use of unlisted duct tape as a duct sealing method because of concern for the service life of the tape. Listed (metal foil) tapes are required for sealing. Tape alone cannot be substituted for mechanical fasteners because these are needed to ensure a structurally sound duct system.

 

 

 

 

 

 

 

 

 

 

M1601.3.2 These material requirements and the 10-foot (3048 mm) maximum spacing requirement prescribe structural support for metal ductwork to limit deflection and maintain alignment. Nonmetallic ducts must be supported in accordance with the manufacturer’s installation instructions because these ducts are produced in various configurations.

 

 

 

 

 

 

 

 

 

 

 

M1601.3.3 This section states that all openings around ducts at ceiling and floor levels must be fireblocked in accordance with Section R602.8. The fireblocks retard the spread of fire to other areas in the dwelling.

 

 

 

 

 

 

 

 

 

 

 

 

M1601.3.4 Where a duct is installed outdoors or in an unconditioned area (such as an attic), it can be exposed to humidity and temperature differentials that can create condensation on the outside of the duct. A vapor retarder must be installed on the exterior of the duct to protect the duct and/or insulation from damage caused by moisture. Duct insulation and coverings must not penetrate fireblocked assemblies. If the exterior insulation and vapor retarder burn in a fire, a pathway will open for the fire to spread through the duct penetration.

 

 

 

 

 

 

 

 

 

 

M1601.3.5 Factory-made air ducts must be listed and labeled for underground installations. Ducts not listed for inground installations may not have sufficient strength to withstand the loads applied by these types of installations. Therefore, they must not be installed in tile, metal pipe, concrete or masonry.

 

 

 

 

 

 

 

 

 

 

 

M1601.3.6 A physical separation from earth is the best method of protecting metal ducts from the effects of corrosion.

 

 

 

 

 

 

 

 

 

 

 

M1601.3.7 Ducts in garages and ducts penetrating separation walls or ceilings between garages and living spaces must be designed to prevent fire and smoke from easily entering the living spaces. See the commentary for Section R309.1.1.

 

 

 

 

 

 

 

 

 

 

 

M1601.3.8 In buildings and structures located in flood hazard areas, ducts must be installed above the design flood elevation or must be capable of preventing water from entering the ducts and capable of withstanding the forces of buoyancy and moving water. See the commentary for Section R324.1.5.

 

 

 

 

 

 

 

 

 

 

 

M1601.4 The code permits use of an under-floor crawl space as a supply plenum if certain conditions are fulfilled. A counterflow furnace is normally used in the space above to discharge the conditioned air to an under-floor plenum. Floor registers are installed in openings cut through the floor for delivery of the conditioned air to the space above. A noncombustible receptacle must be installed under each floor register to catch matches, embers or other sources of ignition that might fall through the floor registers. See Commentary Figure M1601.4 for an example of an under-floor plenum system.

 

 

 

 

 

 

 

 

 

 

 

M1601.4.1 Loose combustible scrap must be removed from an under- floor space used as a plenum. The space must be substantially airtight because the furnace will place the plenum under a slight positive pressure to force conditioned air out of the registers into the rooms above. If there are leaks in the plenum walls, the conditioned air will escape to the outside and result in energy loss.

 

 

 

 

 

 

 

 

 

 

M1601.4.2 The enclosing materials of an under-floor plenum, including the sidewall insulation, are limited to materials having a flame spread index less than 200.

 

 

 

 

 

 

 

 

 

 

M1601.4.3 A duct must be installed through the floor from the furnace outlet to at least 6 inches (152 mm) below combustible framing in the under-floor plenum. A noncombustible receptacle must be installed below the opening in the plenum to deflect the warm air from the furnace. The duct through the floor provides assurance that air is delivered to the plenum without leaking into the furnace enclosure. The receptacle deflects the supply air from the furnace to provide improved air distribution in the plenum. This receptacle must be suspended from the floor members and must not be more than 18 inches (457 mm) below the floor opening to ensure proper deflection of supply air. In addition, the receptacle must extend 3 inches (76 mm) beyond the opening on all sides and the perimeter and have a vertical lip at least 1 inch (25 mm) high on all sides to catch any sparks, embers or other sources of ignition.

 

 

 

 

 

 

 

 

 

 

 

M1601.4.4 An 18-inch by 24-inch (457mmby 610 mm) opening in the floor is required to provide access to the under- floor plenum to permit maintenance and repairs in the crawl space.

 

 

 

 

 

 

 

 

 

 

 

M1601.4.5 Because exposed wood joists often form under-floor plenums and flooring, the outlet air temperature from the furnace must be limited to 200°F (93°C). Furnaces must also be equipped with an automatic control that starts the air circulation fan when the air temperature in the furnace reaches 150°F (66°C). This requirement serves to inhibit the plenum air temperature from approaching 150°F (66°C) because the volume of the under-floor space cools the heated air delivered by the furnace through mixing with the cooler plenum air.