CHAPTER 15
EXHAUST SYSTEMS


  • SECTION M1501
    GENERAL

    M1501.1 Outdoor discharge.
    The air removed by every mechanical exhaust system shall be discharged to the outdoors in accordance with Section M1506.2. Air shall not be exhausted into an attic, soffit, ridge vent or crawl space.

    Exception: Whole-house ventilation-type attic fans that discharge into the attic space of dwelling units having private attics shall be permitted.

    SECTION M1502
    CLOTHES DRYER EXHAUST

    M1502.1 General.
    Clothes dryers shall be exhausted in accordance with the manufacturer's instructions.

    M1502.2 Independent exhaust systems.
    Dryer exhaust systems shall be independent of all other systems and shall convey the moisture to the outdoors.

    Exception: This section shall not apply to listed and labeled condensing (ductless) clothes dryers.

    M1502.3 Duct termination.
    Exhaust ducts shall terminate on the outside of the building. Exhaust duct terminations shall be in accordance with the dryer manufacturer's installation instructions. If the manufacturer's instructions do not specify a termination location, the exhaust duct shall terminate not less than 3 feet (914 mm) in any direction from openings into buildings. Exhaust duct terminations shall be equipped with a backdraft damper. Screens shall not be installed at the duct termination.

    M1502.4 Dryer exhaust ducts.
    Dryer exhaust ducts shall conform to the requirements of Sections M1502.4.1 through M1502.4.6.

    M1502.4.1 Material and size.
    Exhaust ducts shall have a smooth interior finish and be constructed of metal having a minimum thickness of 0.0157 inches (0.3950 mm) (No. 28 gage). The duct shall be 4 inches (102 mm) nominal in diameter.

    M1502.4.2 Duct installation.
    Exhaust ducts shall be supported at intervals not to exceed 12 feet (3658 mm) and shall be secured in place. The insert end of the duct shall extend into the adjoining duct or fitting in the direction of airflow. Exhaust duct joints shall be sealed in accordance with Section M1601.4.1 and shall be mechanically fastened. Ducts shall not be joined with screws or similar fasteners that protrude more than 1/8 inch (3.2 mm) into the inside of the duct.

    M1502.4.3 Transition duct.
    Transition ducts used to connect the dryer to the exhaust duct system shall be a single length that is listed and labeled in accordance with UL 2158A. Transition ducts shall be a maximum of 8 feet (2438 mm) in length. Transition ducts shall not be concealed within construction.

    M1502.4.4 Duct length.
    The maximum allowable exhaust duct length shall be determined by one of the methods specified in Section M1502.4.4.1 or M1502.4.4.2.

    M1502.4.4.1 Specified length.
    The maximum length of the exhaust duct shall be 35 feet (10 668 mm) from the connection to the transition duct from the dryer to the outlet terminal. Where fittings are used, the maximum length of the exhaust duct shall be reduced in accordance with Table M1502.4.4.1. The maximum length of the exhaust duct does not include the transition duct.

    TABLE M1502.4.4.1
    DRYER EXHAUST DUCT FITTING EQUIVALENT LENGTH

    DRYER EXHAUST DUCT FITTING TYPEEQUIVALENT LENGTH
    4 inch radius mitered 45 degree elbow2 feet 6 inches
    4 inch radius mitered 90 degree elbow5 feet
    6 inch radius smooth 45 degree elbow1 foot
    6 inch radius smooth 90 degree elbow1 foot 9 inches
    8 inch radius smooth 45 degree elbow1 foot
    8 inch radius smooth 90 degree elbow1 foot 7 inches
    10 inch radius smooth 45 degree elbow9 inches
    10 inch radius smooth 90 degree elbow1 foot 6 inches
    For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 degree = 0.0175 rad.

    M1502.4.4.2 Manufacturer's instructions.
    The size and maximum length of the exhaust duct shall be determined by the dryer manufacturer's installation instructions. The code official shall be provided with a copy of the installation instructions for the make and model of the dryer at the concealment inspection. In the absence of fitting equivalent length calculations from the clothes dryer manufacturer, Table M1502.4.4.1 shall be used.

    M1502.4.5 Length identification.
    Where the exhaust duct is concealed within the building construction, the equivalent length of the exhaust duct shall be identified on a permanent label or tag. The label or tag shall be located within 6 feet (1829 mm) of the exhaust duct connection.

    M1502.4.6 Exhaust duct required.
    Where space for a clothes dryer is provided, an exhaust duct system shall be installed. Where the clothes dryer is not installed at the time of occupancy the exhaust duct shall be capped or plugged in the space in which it originates and identified and marked “future use.”

    Exception: Where a listed condensing clothes dryer is installed prior to occupancy of the structure.

    M1502.5 Protection required.
    Protective shield plates shall be placed where nails or screws from finish or other work are likely to penetrate the clothes dryer exhaust duct. Shield plates shall be placed on the finished face of all framing members where there is less than 11/4 inches (32 mm) between the duct and the finished face of the framing member. Protective shield plates shall be constructed of steel, shall have a minimum thickness of 0.062-inch (1.6 mm) and shall extend a minimum of 2 inches (51 mm) above sole plates and below top plates.

    SECTION M1503
    RANGE HOODS

    M1503.1 General.
    Range hoods shall discharge to the outdoors through a single-wall duct. The duct serving the hood shall have a smooth interior surface, shall be air tight, shall be equipped with a back-draft damper, and shall be independent of all other exhaust systems. Ducts serving range hoods shall not terminate in an attic or crawl space or areas inside the building.

    Exception: Where installed in accordance with the manufacturer's installation instructions, and where mechanical or natural ventilation is otherwise provided, listed and labeled ductless range hoods shall not be required to discharge to the outdoors.

    M1503.2 Duct material.
    Single-wall ducts serving range hoods shall be constructed of galvanized steel, stainless steel or copper.

    Exception: Ducts for domestic kitchen cooking appliances equipped with down-draft exhaust systems shall be permitted to be constructed of schedule 40 PVC pipe and fittings provided that the installation complies with all of the following:

    1. The duct is installed under a concrete slab poured on grade;

    2. The underfloor trench in which the duct is installed is completely backfilled with sand or gravel;

    3. The PVC duct extends not more than 1 inch (25 mm) above the indoor concrete floor surface;

    4. The PVC duct extends not more than 1 inch (25 mm) above grade outside of the building; and

    5. The PVC ducts are solvent cemented.

    M1503.3 Kitchen exhaust rates.
    Where domestic kitchen cooking appliances are equipped with ducted range hoods or down-draft exhaust systems, the fans shall be sized in accordance with Section M1507.4.

    M1503.4 Makeup air required.
    Exhaust hood systems capable of exhausting in excess of 400 cubic feet per minute (0.19 m3/s) shall be provided with makeup air at a rate approximately equal to the exhaust air rate. Such makeup air systems shall be equipped with a means of closure and shall be automatically controlled to start and operate simultaneously with the exhaust system.

    SECTION M1504
    INSTALLATION OF MICROWAVE OVENS

    M1504.1 Installation of a microwave oven over a cooking appliance.
    The installation of a listed and labeled cooking appliance or microwave oven over a listed and labeled cooking appliance shall conform to the terms of the upper appliance's listing and label and the manufacturer's installation instructions. The microwave oven shall conform to UL 923.

    SECTION M1505
    OVERHEAD EXHAUST HOODS

    M1505.1 General.
    Domestic open-top broiler units shall have a metal exhaust hood, having a minimum thickness of 0.0157-inch (0.3950 mm) (No. 28 gage) with 1/4 inch (6.4 mm) clearance between the hood and the underside of combustible material or cabinets. A clearance of at least 24 inches (610 mm) shall be maintained between the cooking surface and the combustible material or cabinet. The hood shall be at least as wide as the broiler unit, extend over the entire unit, discharge to the outdoors and be equipped with a backdraft damper or other means to control infiltration/exfiltration when not in operation. Broiler units incorporating an integral exhaust system, and listed and labeled for use without an exhaust hood, need not have an exhaust hood.

    SECTION M1506
    EXHAUST DUCTS AND EXHAUST OPENINGS

    M1506.1 Ducts.
    Where exhaust duct construction is not specified in this chapter, construction shall comply with Chapter 16.

    M1506.2 Exhaust openings.
    Air exhaust openings shall terminate not less than 3 feet (914 mm) from property lines; 3 feet (914 mm) from operable and nonoperable openings into the building and 10 feet (3048 mm) from mechanical air intakes except where the opening is located 3 feet (914 mm) above the air intake. Openings shall comply with Sections R303.5.2 and R303.6.

    SECTION M1507
    MECHANICAL VENTILATION

    M1507.1 General.
    Where local exhaust or whole-house mechanical ventilation is provided, the equipment shall be designed in accordance with this section.

    M1507.2 Recirculation of air.
    Exhaust air from bathrooms and toilet rooms shall not be recirculated within a residence or to another dwelling unit and shall be exhausted directly to the outdoors. Exhaust air from bathrooms and toilet rooms shall not discharge into an attic, crawl space or other areas inside the building.

    M1507.3 Whole-house mechanical ventilation system.
    Whole-house mechanical ventilation systems shall be designed in accordance with Sections M1507.3.1 through M1507.3.3.

    M1507.3.1 System design.
    The whole-house ventilation system shall consist of one or more supply or exhaust fans, or a combination of such, and associated ducts and controls. Local exhaust or supply fans are permitted to serve as such a system. Outdoor air ducts connected to the return side of an air handler shall be considered to provide supply ventilation.

    M1507.3.2 System controls.
    The whole-house mechanical ventilation system shall be provided with controls that enable manual override.

    M1507.3.3 Mechanical ventilation rate.
    The whole-house mechanical ventilation system shall provide outdoor air at a continuous rate of not less than that determined in accordance with Table M1507.3.3(1).

    Exception: The whole-house mechanical ventilation system is permitted to operate intermittently where the system has controls that enable operation for not less than 25-percent of each 4-hour segment and the ventilation rate prescribed in Table M1507.3.3(1) is multiplied by the factor determined in accordance with Table M1507.3.3(2).

    TABLE M1507.3.3(1)
    CONTINUOUS WHOLE-HOUSE MECHANICAL VENTILATION SYSTEM AIRFLOW RATE REQUIREMENTS

    DWELLING UNIT
    FLOOR AREA
    (square feet)
    NUMBER OF BEDROOMS
    0 – 12 – 34 – 56 – 7> 7
    Airflow in CFM
    < 1,5003045607590
    1,501 – 3,00045607590105
    3,001 – 4,500607590105120
    4,501 – 6,0007590105120135
    6,001 – 7,50090105120135150
    > 7,500105120135150165
    For SI: 1 square foot = 0.0929 m2, 1 cubic foot per minute = 0.0004719 m3/s.

    TABLE M1507.3.3(2)
    INTERMITTENT WHOLE-HOUSE MECHANICAL VENTILATION RATE FACTORSa, b

    RUN-TIME PERCENTAGE IN EACH 4-HOUR
    SEGMENT
    25%33%50%66%75%100%
    Factora4321.51.31.0
    a. For ventilation system run time values between those given, the factors are permitted to be determined by interpolation.
    b. Extrapolation beyond the table is prohibited.

    M1507.4 Local exhaust rates.
    Local exhaust systems shall be designed to have the capacity to exhaust the minimum air flow rate determined in accordance with Table M1507.4.

    TABLE M1507.4
    MINIMUM REQUIRED LOCAL EXHAUST RATES FOR
    ONE- AND TWO-FAMILY DWELLINGS

    AREA TO BE EXHAUSTED EXHAUST RATES
    Kitchens 100 cfm intermittent or 25 cfm continuous
    Bathrooms-Toilet Rooms Mechanical exhaust capacity of 50 cfm intermittent or 20 cfm continuous
    For SI: 1 cubic foot per minute = 0.0004719 m3/s.