• CHAPTER15
    EXHAUST SYSTEMS 

    User notes:

    About this chapter: Chapter 15 is specific to exhaust systems related to clothes dryers, domestic cooking, toilet rooms, bathrooms and whole-house ventilation systems. Included are requirements for exhaust discharge locations, protection of exhaust ducts from damage, exhaust duct construction, duct length limits, and exhaust termination clearances. This chapter contains prohibitions for exhaust recirculation and discharge locations and addresses the design of whole-house ventilation systems required by Chapter 3.

    Code development reminder: Code change proposals to this chapter will be considered by the IRC—Plumbing/Mechanical Code Development Committee during the 2018 (Group A) Code Development Cycle. See explanation on page iv.

    SECTIONM1501
    GENERAL

    M1501.1Outdoor discharge.

    The air removed by every mechanical exhaust system shall be discharged to the outdoors in accordance with Section M1504.3. 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.

    SECTIONM1502
    CLOTHES DRYER EXHAUST

    M1502.1General.

    Clothes dryers shall be exhausted in accordance with the manufacturer’s instructions.

    M1502.2Independent 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.

    M1502.3Duct 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.

    M1502.3.1Exhaust termination outlet and passageway size.

    The passageway of dryer exhaust duct terminals shall be undiminished in size and shall provide an open area of not less than 12.5 square inches (8065 mm2).

    M1502.4Dryer exhaust ducts.

    Dryer exhaust ducts shall conform to the requirements of Sections M1502.4.1 through M1502.4.7.

    M1502.4.1Material and size.

    Exhaust ducts shall have a smooth interior finish and shall be constructed of metal not less than 0.0157 inch (0.3950 mm) in thickness (No. 28 gage). The duct shall be 4 inches (102 mm) nominal in diameter.

    M1502.4.2Duct 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. Where dryer exhaust ducts are enclosed in wall or ceiling cavities, such cavities shall allow the installation of the duct without deformation.

    M1502.4.3Transition 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 not greater than 8 feet (2438 mm) in length. Transition ducts shall not be concealed within construction.

    M1502.4.4Dryer exhaust duct power ventilators.

    Domestic dryer exhaust duct power ventilators shall conform to UL 705 for use in dryer exhaust duct systems. The dryer exhaust duct power ventilator shall be installed in accordance with the manufacturer’s instructions.

    M1502.4.5Duct length.

    The maximum allowable exhaust duct length shall be determined by one of the methods specified in Sections M1502.4.5.1 through M1502.4.5.3.

    M1502.4.5.1Specified 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 M1502.4.5.1. The maximum length of the exhaust duct does not include the transition duct.

    TABLE M1502.4.5.1

    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.

    M1502.4.5.2Manufacturer’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 M1502.4.5.1 shall be used.

    M1502.4.5.3Dryer exhaust duct power ventilator.

    The maximum length of the exhaust duct shall be determined in accordance with the manufacturer’s instructions for the dryer exhaust duct power ventilator.

    M1502.4.6Length identification.

    Where the exhaust duct equivalent length exceeds 35 feet (10 668 mm), 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.

    M1502.4.7Exhaust 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.

    M1502.5Protection 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 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 not less than 2 inches (51 mm) above sole plates and below top plates.

    SECTIONM1503
    DOMESTIC COOKING EXHAUST EQUIPMENT 

    M1503.1General.

    Domestic cooking exhaust equipment shall comply with the requirements of this section.

    M1503.2Domestic cooking exhaust.

    Where domestic cooking exhaust equipment is provided, it shall comply with one of the following:

    1. 1.The fan for overhead range hoods and downdraft exhaust equipment not integral with the cooking appliance shall be listed and labeled in accordance with UL 507.

    2. 2.Overhead range hoods and downdraft exhaust equipment with integral fans shall comply with UL 507.

    3. 3.Domestic cooking appliances with integral downdraft exhaust equipment shall be listed and labeled in accordance with ANSI Z21.1 or UL 858.

    4. 4.Microwave ovens with integral exhaust for installation over the cooking surface shall be listed and labeled in accordance with UL 923.

    M1503.2.1Open-top broiler exhaust.

    Domestic open-top broiler units shall be provided with a metal exhaust hood having a thickness of not less than 0.0157 inch (0.3950 mm) (No. 28 gage). Such hoods shall be installed with a clearance of not less than 1/4 inch (6.4 mm) between the hood and the underside of combustible material and cabinets. A clearance of not less than 24 inches (610 mm) shall be maintained between the cooking surface and combustible material and cabinets. The hood width shall be not less than the width of the broiler unit and shall extend over the entire unit.

    Exception: Broiler units that incorporate an integral exhaust system, and that are listed and labeled for use without an exhaust hood, shall not be required to have an exhaust hood.

    M1503.3Exhaust discharge.

    Domestic cooking exhaust equipment shall discharge to the outdoors through a duct. The duct shall have a smooth interior surface, shall be air tight, shall be equipped with a backdraft damper and shall be independent of all other exhaust systems. Ducts serving domestic cooking exhaust equipment shall not terminate in an attic or crawl space or areas inside the building.

    Exception: Where installed in accordance with the manufacturer’s 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.

    M1503.4Duct material.

    Ducts serving domestic cooking exhaust equipment 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. 1.The duct is installed under a concrete slab poured on grade.

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

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

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

    5. 5.The PVC ducts are solvent cemented.

    M1503.5Kitchen 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 M1505.4.4.

    M1503.6Makeup air required.

    Where one or more gas, liquid or solid fuel-burning appliance that is neither direct-vent nor uses a mechanical draft venting system is located within a dwelling unit’s air barrier, each exhaust system capable of exhausting in excess of 400 cubic feet per minute (0.19 m3/s) shall be mechanically or passively provided with makeup air at a rate approximately equal to the exhaust air rate. Such makeup air systems shall be equipped with not fewer than one damper complying with Section M1503.6.2.

    Exception: Makeup air is not required for exhaust systems installed for the exclusive purpose of space cooling and intended to be operated only when windows or other air inlets are open.

    M1503.6.1Location.

    Kitchen exhaust makeup air shall be discharged into the same room in which the exhaust system is located or into rooms or duct systems that communicate through one or more permanent openings with the room in which such exhaust system is located. Such permanent openings shall have a net cross-sectional area not less than the required area of the makeup air supply openings.

    M1503.6.2Makeup air dampers.

    Where makeup air is required by Section M1503.6, makeup air dampers shall comply with this section. Each damper shall be a gravity damper or an electrically operated damper that automatically opens when the exhaust system operates. Dampers shall be located to allow access for inspection, service, repair and replacement without removing permanent construction or any other ducts not connected to the damper being inspected, serviced, repaired or replaced. Gravity or barometric dampers shall not be used in passive makeup air systems except where the dampers are rated to provide the design makeup airflow at a pressure differential of 0.01 in. w.c. (3 Pa) or less.

    SECTIONM1504
    EXHAUST DUCTS AND EXHAUST OPENINGS

    M1504.1Duct construction.

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

    M1504.2Duct length.

    The length of exhaust and supply ducts used with ventilating equipment shall not exceed the lengths determined in accordance with Table M1504.2.

    Exception: Duct length shall not be limited where the duct system complies with the manufacturer’s design criteria or where the flow rate of the installed ventilating equipment is verified by the installer or approved third party using a flow hood, flow grid or other airflow measuring device.

    TABLE M1504.2

    DUCT LENGTH

    DUCT TYPEFLEX DUCTSMOOTH-WALL DUCT
    Fan airflow rating (CFM @ 0.25 inch wca)50801001251502002503005080100125150200250300
    Diameterb (inches)Maximum lengthc, d, e (feet)
    3XXXXXXXX5XXXXXXX
    4564XXXXXX1143110XXXXX
    5NL8142162XXXNL1529151284XX
    6NLNL1589155181XNLNLNL16811253259
    7NLNLNLNL161784019NLNLNLNLNL1488854
    8 and aboveNLNLNLNLNL18911169NLNLNLNLNLNL198133

    For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm.

    1. a.Fan airflow rating shall be in acordance with ANSI/AMCA 210-ANSI/ASHRAE 51.

    2. b.For noncircular ducts, calculate the diameter as four times the cross-sectional area divided by the perimeter.

    3. c.This table assumes that elbows are not used. Fifteen feet of allowable duct length shall be deducted for each elbow installed in the duct run.

    4. d.NL = no limit on duct length of this size.

    5. e.X = not allowed. Any length of duct of this size with assumed turns and fittings will exceed the rated pressure drop.

    M1504.3Exhaust openings.

    Air exhaust openings shall terminate as follows:

    1. 1.Not less than 3 feet (914 mm) from property lines.

    2. 2.Not less than 3 feet (914 mm) from gravity air intake openings, operable windows and doors.

    3. 3.Not less than 10 feet (3048 mm) from mechanical air intake openings except where the exhaust opening is located not less than 3 feet (914 mm) above the air intake opening. Openings shall comply with Sections R303.5.2 and R303.6.

    SECTIONM1505
    MECHANICAL VENTILATION

    M1505.1General.

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

    M1505.2Recirculation of air.

    Exhaust air from bathrooms and toilet rooms shall not be recirculated within a residence or circulated to another dwelling unit and shall be exhausted directly to the outdoors. Exhaust air from bathrooms, toilet rooms and kitchens shall not discharge into an attic, crawl space or other areas inside the building. This section shall not prohibit the installation of ductless range hoods in accordance with the exception to Section M1503.3.

    M1505.3Exhaust equipment.

    Exhaust equipment serving single dwelling units shall be listed and labeled as providing the minimum required airflow in accordance with ANSI/AMCA 210-ANSI/ASHRAE 51.

    M1505.4Whole-house mechanical ventilation system.

    Whole-house mechanical ventilation systems shall be designed in accordance with Sections M1505.4.1 through M1505.4.4.

    M1505.4.1System 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 as providing supply ventilation.

    M1505.4.2System controls.

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

    M1505.4.3Mechanical ventilation rate.

    The whole house mechanical ventilation system shall provide outdoor air at a continuous rate as determined in accordance with Table M1505.4.3(1) or Equation 15-1.

    Equation 15-1 (Equation 15-1)

    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 M1505.4.3(1) is multiplied by the factor determined in accordance with Table M1505.4.3(2).

    TABLE M1505.4.3(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 M1505.4.3(2)

    INTERMITTENT WHOLE-HOUSE MECHANICAL VENTILATION RATE FACTORSa, b

    RUN-TIME PERCENTAGE IN EACH 4-HOUR SEGMENT25%33%50%66%75%100%
    Factora4321.51.31.0
    1. a.For ventilation system run time values between those given, the factors are permitted to be determined by interpolation.

    2. b.Extrapolation beyond the table is prohibited.

    M1505.4.4Local exhaust rates.

    Local exhaust systems shall be designed to have the capacity to exhaust the minimum airflow rate determined in accordance with Table M1505.4.4.

    TABLE M1505.4.4

    MINIMUM REQUIRED LOCAL EXHAUST RATES FOR ONE- AND TWO-FAMILY DWELLINGS

    AREA TO BE EXHAUSTEDEXHAUST RATES
    Kitchens100 cfm intermittent or 25 cfm continuous
    Bathrooms-Toilet RoomsMechanical exhaust capacity of 50 cfm intermittent or 20 cfm continuous

    For SI: 1 cubic foot per minute = 0.0004719 m3/s.