M-305.1 Flexible air ducts: Flexible air ducts, both metallic and nonmetallic, shall conform to the requirements of UL 181 for Class 0 or Class 1 flexible air ducts, and shall be so identified. Flexible air ducts shall not be limited in length.

M-305.2 Flexible [duct] air connectors: Flexible air connectors, both metallic and nonmetallic, shall conform to the requirements of UL 181 for Class 0 or Class 1 flexible air connectors, and shall be so identified.

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M-305.3 Penetrations prohibited: Flexible air ducts and flexible air connectors shall not pass through any wall, floor, ceiling or fireresistance rated assembly.

M-305.4 Air temperature: The temperature of air in flexible air ducts or flexible air connectors shall be less than 250 degrees F. (121 degrees C.). Flexible air ducts and air connectors shall be installed with a minimum clearance to an appliance as specified for the labeled equipment.

M-305.5 Low-, medium- and high-pressure systems: Flexible air ducts and flexible air connectors shall be rated for the pressure of the air distribution system in accordance with Section M-304.5.

M-305.6 Installation: Flexible air ducts and flexible air connectors shall be installed in accordance with the manufacturer's installation instructions.

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M-306.3.1 Labeling: Duct insulation shall be labeled by the manufacturer. The label shall indicate the [insulating] thermal resistance (R) value at the specified installation thickness, the flame spread rating and the smoke-developed rating.

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M-307.1 General: An air distribution system shall be designed and installed to supply the required distribution of air at the specified static pressure. The installation of an air distribution system shall not affect the fire protection requirements specified in the building code.

M-307.1.1 Cutting or notching: A structural member shall not be cut, notched or pierced [in excess of the limitations specified] except as provided for in the building code [listed in Chapter 21, unless proven safe by a structural analysis].

M-307.3 Supports: Ducts shall be supported with approved hangers at intervals not exceeding 10 feet (3048 mm) or by other approved duct support systems designed in accordance with the building code. Flexible and other premanufactured ducts shall be supported in accordance with the manufacturer's installation instructions.

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M-307.6 Registers, grilles and diffusers: Duct registers, grilles and diffusers shall be installed in accordance with the manufacturer's installation instructions. Supply air duct registers, grilles and diffusers shall [have] be provided with opposed blade damper assemblies or equivalent volume adjustment.

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M-307.7 [Visual] [d] Duct openings: Duct openings and branch take-offs in bathrooms, toilet rooms and changing rooms shall prevent visual observation and "cross-talk" from adjoining rooms.

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M-307.10 Private garage ducts: Where a warm-air space heating system serves both a dwelling unit and an attached or detached garage, supply duct outlets in the garage shall be located not less than 6 feet 6 inches above the garage floor and shall be provided with an approved backdraft damper. Return air shall not be taken from the garage. Outdoor makeup air shall be supplied to the return air system at a rate not less than the rate of supply air to the garage. Volume dampers shall not be installed in outdoor makeup air ducts or openings.

M-308.1.1 Intake openings: Mechanical and gravity outside air intake openings shall be located a minimum of 10 feet (3048 mm) from any hazard or noxious contaminant such as vents, chimneys, plumbing vents, streets, alleys, parking lots and loading docks except as otherwise specified in this code. [Where a source of contaminant is located within 10 feet (3048 mm) of an intake opening, such opening shall be located a minimum of 2 feet (610 mm) below the contaminant source.]

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M-309.2.2 Return air systems:

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Exception: Smoke detectors are not required in the return air system where the space served by the air distribution system is protected by a system of area smoke detectors in accordance with the building code. The area smoke detectors shall comply with Section M-309.4.

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M-309.2.3 Common supply and return air systems: Where multiple air handling systems share common supply or return air ducts or plenums with a combined capacity greater than 2,000 cfm, the supply air system shall be provided with smoke detectors in accordance with Section M-309.2.1. Where the combined capacity of such air handling units is greater than 15,000 cfm, additional smoke detectors shall be installed in the return air system in accordance with Section M-309.2.2.

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M-311.6.1 Alternative materials: Where the products being exhausted are detrimental to the duct material required in Section M-311.6, ducts shall be constructed of alternative materials that are compatible with the exhaust products in accordance with the manufacturer's recommendations.

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M-312.2 Vent penetrations: Flexible ducts that vent clothes dryers shall not penetrate or be located within any fireblocking, draftstopping or any wall, floor/ceiling or other assembly required by the building code to be fireresistance rated, unless such duct is constructed of galvanized steel or aluminum of the thickness specified in Table M-304.1 and the fireresistance rating is maintained in accordance with the building code.

M-312.[2] 3 Vent size: The minimum diameter of the vent shall be as specified by the manufacturer but not less than the diameter of the appliance outlet.

M-312.[3] 4 Cleanout: Each vertical riser shall be provided with a means for cleanout.

M-312.[4] 5 Vent materials: Dryer vent materials shall be of galvanized steel or aluminum of the thickness specified in Table M-304.1, or of flexible exhaust duct material.

M-312.[5] 6 Flexible ducts: Flexible ducts, where installed, shall be installed in accordance with the clothes dryer manufacturer's installation instructions. [Flexible ducts that vent clothes dryers shall not penetrate or be located within any firestopping, draftstopping or any wall, floor/ceiling or other assembly required by the building code listed in Chapter 21 to be fireresistance rated, unless such duct is constructed of galvanized steel or aluminum of the thickness specified in Table M-304.1 and the fireresistance rating is maintained in accordance with the building code listed in Chapter 21.]

M-312.[6] 7 Commercial or multiple installations: Vent systems for commercial or multiple-dryer installations shall conform to the requirements of Sections M-312.6.1 through M-312.6.3.

M-312.[6] 7.1 Makeup air: Installations exhausting more than 200 cubic feet per minute (0.09 m3/s) shall be provided with makeup air.

M-312.[6] 7.2 Exhaust fan motors: Exhaust fan motors installed in vent systems shall be located outside of the air stream.

M-312.[6] 7.3 Interlock: In multiple installations, the fan shall operate continuously or be interlocked to operate when any individual unit is operating.

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