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Back-Draft Dampers at Shared Common Exhaust Duct

    Scope
    Scope Images
    Image
    If fans share common exhaust duct, back-draft dampers installed
    Scope

    If multiple exhaust fans in the home share a common exhaust duct, back-draft dampers are installed for each fan.

    • Inspect each fan to see if it has an integral back-draft damper.
    • If no back-draft damper is included, install dampers immediately above each fan to ensure that each fan has its own damper.
    • Remove packing tape from the back-draft dampers before installing.

    See the Compliance Tab for links to related codes and standards and voluntary federal energy-efficiency program requirements.

    Description
    Description

    Exhaust fans should always be ducted to a location outside the home (see the guides Kitchen Exhaust and Bathroom Exhaust). Ideally, each exhaust fan should have its own individual duct to the outside. However, builders sometimes prefer to connect the exhaust fans to a common exhaust duct, for reasons of layout or a wish to minimize penetrations through the roof. A common exhaust duct can be used if each fan has a back-draft damper to prevent cross-contamination when the fan is not running. In very humid climates, HVAC installers will sometimes also install a motorized damper in the duct closer to the roof outlet to prevent moist air from drifting in when the fan is off and condensing on fan parts or entering the home. Some exhaust fan models come with an integral motorized damper to prevent airflow when the fan is not operating. Inspect the fans and add a damper if one is not already included with the fans.

    How to Install Back-Draft Dampers in Exhaust Fans to Make a Shared Duct Possible

    1. Install back-draft dampers where the exhaust duct meets the exhaust fan in each fan unit.  The dampers should open when the fan is actively exhausting and should shut when the fan is off (Figures 1 and 2). When the exhaust fan is operating, the back-draft damper is pushed open by airflow to allow air to exit through the exhaust duct (Figure 3).
    Image
    The back-draft damper on this exhaust fan is closed to prevent cross contamination
    The back-draft damper on this exhaust fan is closed to prevent cross contamination
    The back-draft damper on this exhaust fan is closed to prevent cross contamination
    Figure 1. The back-draft damper on this exhaust fan is closed to prevent cross contamination (Source: Courtesy of Calcs Plus). 
    Image
    The back-draft damper on this exhaust fan is open when the exhaust fan is operating
    The back-draft damper on this exhaust fan is open when the exhaust fan is operating
    The back-draft damper on this exhaust fan is open when the exhaust fan is operating
    Figure 2. The back-draft damper on this exhaust fan is open when the exhaust fan is operating (Source: Courtesy of Calcs Plus).

       2. Remember to remove any packing tape used to hold the dampers closed during shipping.
     

    Image
    The back-draft damper open when the fan is actively exhausting and closes when the fan is off
    The back-draft damper open when the fan is actively exhausting and closes when the fan is off
    The back-draft damper open when the fan is actively exhausting and closes when the fan is off
    Figure 3. The back-draft damper opens when the fan is actively exhausting and closes when the fan is off (Source: Courtesy of Calcs Plus).
    Success
    Ensuring Success

    In homes with multiple exhaust fans, the HERS rater will inspect the ventilation system exhaust ducts to ensure that one of the following is true: 1) each unit has its own exhaust duct that is individually ducted to the outside, or 2) if the units share a common exhaust duct, each fan has a back-draft damper to prevent cross-contamination when the fan is not running.

    Climate
    Climate

    Mechanically controlled dampers limit ventilation to prevent over-ventilation in humid or extreme temperature conditions.

    Training
    Right and Wrong Images
    Image
    Fan shares exhaust and does not have dack-draft damper installed
    Fan shares exhaust and does not have dack-draft damper installed
    Image
    Fan shares exhaust and has a back-draft damper installed
    Fan shares exhaust and has a back-draft damper installed
    Image
    Back-draft damper still has a piece of tape that prevented it from rattling during shipping
    Back-draft damper still has a piece of tape that prevented it from rattling during shipping
    Image
    Packing tape has been removed and damper will be able to function properly once fan is installed
    Packing tape has been removed and damper will be able to function properly once fan is installed
    Videos
    Compliance
    Retrofit
    Existing Homes

    SCOPE

    If exhaust fans share a common duct, verify that each fan has its own damper; install if missing.

    See the Scope tab for additional job specifications.

    If the project will involve working in the attic, see the assessment guide, Pre-Retrofit Assessment of Attics, Ceilings, and Roofs.

    For more on supply ventilation, see the U.S. Department of Energy’s Standard Work Specifications on exhaust ventilation.

    DESCRIPTION

    If exhaust fans share a common duct, verify that each fan has its own damper; if dampers are missing or nonfunctioning, install new dampers, as described in the Description tab.

    COMPLIANCE

    See Compliance tab. 

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    Building Science Measures
    Building Science-to-Sales Translator

    Quality Installed Exhaust Fans = Professionally Installed Exhaust Fans

    Image(s)
    Technical Description

    Exhaust fans should be professionally installed for best performance, especially if the fans are used as part of the home’s indoor air quality ventilation system. A trained technician will ensure that the fan has an adequate flow rate for the application, the damper is functional, the exhaust vent is properly installed to vent to the outside, air sealing is done around the fan box and vent duct, noise levels meet sone requirements, and if timers are to be used, that they are properly installed and set.

    Professionally Installed Exhaust Fans
    Sales Message

    Professionally installed exhaust fans follow best practices for effective operation. What this means to you is assured removal of odors and contaminants. Wouldn’t you agree it’s important to get full value from your investment in advanced technology components?

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