• M1503.4
    Makeup Air for
    Range Hoods

    CHANGE TYPE: Modification

    CHANGE SUMMARY: Automatic operation of a mechanical damper is no longer required for supplying makeup air for kitchen exhaust systems exceeding a rating of 400 cubic feet per minute (cfm). Transfer openings are permitted to obtain makeup air from rooms other than the kitchen.

    2015 CODE: 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 mechanically or naturally 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 not less than one damper. Each damper shall be a gravity damper or an electrically operated damper that automatically opens when the exhaust system operates. Dampers shall be accessible 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.

    M1503.4.1 Location. 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.

    CHANGE SIGNIFICANCE: The 2009 IRC introduced provisions for makeup air for high-velocity kitchen exhaust hoods that were capable of an airflow rate exceeding 400 cfm. Although this size of kitchen hood equipment is unusual in residential construction, the concern is that kitchens in modern homes are getting larger and oversized equipment is becoming more popular. With tighter building thermal envelopes for energy conservation, the high rate of exhaust requires outside makeup air to prevent negative pressure and the adverse effects on other appliances and systems. The code previously required an automatic damper that was interlocked with the exhaust hood so that outdoor makeup air was provided any time the hood fan was in operation.

    The 2015 IRC provides more flexibility in achieving adequate makeup air for high-velocity kitchen exhaust fans. The outdoor makeup air can be obtained either mechanically or naturally. In either case, a damper is required to provide a means of closure, reduce air leakage, and conserve energy. Electrically operated dampers must still be interlocked to automatically open when the exhaust system operates. The other option is to provide a gravity damper that opens in response to pressure differentials created when the exhaust fan operates. The gravity damper is balanced to close when exhaust fan operation ceases. Proponents of this change offered that allowing a gravity damper is compatible with other similar applications in the IRC and that the residential code does not require automatic motorized dampers elsewhere. Proponents also stated that a gravity damper has the added benefit of equalizing depressurization in the house during the operation of other equipment such as bath fans and clothes dryers. Both types of damper—gravity and motorized—require maintenance and may need to be replaced at some time. Therefore, the code requires the dampers to be accessible.

    The code also clarifies that the source of makeup air may be from a room or space other than the kitchen where the range hood is located. When outdoor air is introduced into another room through a mechanical damper or gravity damper, permanent openings of adequate size are required between the rooms for the makeup air to pass though. This provision recognizes that homeowners have valid reasons for not wanting the opening in the kitchen. Locating the opening in another room or bringing the makeup air in through the duct system allows the unconditioned air to mix and temper. This is beneficial in both the heating and cooling seasons in various climate zones. Proponents of this change reasoned that requiring the outside air opening in the kitchen created the possibility that it would be covered or otherwise disabled due to the discomfort of introducing unconditioned air to the kitchen.

    Required makeup air for kitchen exhaust hoods exceeding 400 cfm

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