Back-Draft Dampers at Shared Common Exhaust Duct

Scope Images
If fans share common exhaust duct, back-draft dampers installed
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 related codes and standards requirements, and criteria to meet national programs such as DOE’s Zero Energy Ready Home program, ENERGY STAR Certified Homes, and Indoor airPLUS.

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).
    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. (Image courtesy of Calcs Plus).

     

    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. (Image courtesy of Calcs Plus).

     

  2. Remember to remove any packing tape used to hold the dampers closed during shipping.
    The back-draft damper opens 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. (Image courtesy of Calcs Plus).

     

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

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

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Compliance

The Compliance tab contains both program and code information. Code language is excerpted and summarized below. For exact code language, refer to the applicable code, which may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

ENERGY STAR Certified Homes, Version 3/3.1 (Rev. 09)

This topic is not specifically addressed.

DOE Zero Energy Ready Home (Revision 07)

Exhibit 1 Mandatory Requirements.
Exhibit 1, Item 1) Certified under the ENERGY STAR Qualified Homes Program or the ENERGY STAR Multifamily New Construction Program.
Exhibit 1, Item 6) Certified under EPA Indoor airPLUS.

American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) 62.2-2010, ASHRAE 62.2-2013, and ASHRAE 62.2-2016

ASHRAE Standard 62.2-2010 Ventilation for Acceptable Indoor Air Quality in Low-Rise Residential Buildings (available for purchase at link above). The standard provides minimum requirements for mechanical and natural ventilation systems intended to provide acceptable indoor air quality in low-rise residential buildings.

2009 International Energy Conservation Code (IECC)

Section 403.5 Mechanical ventilation (Mandatory). Automatic or gravity dampers are installed on all outdoor air intakes and exhausts.

2012, 2015, and 2018 IECC

Section R403.5 (R403.6 in 2015 and 2018 IECC) Mechanical ventilation shall meet the requirements of the International Residential Code or the International Mechanical Code. Automatic or gravity dampers are installed on all outdoor air intakes and exhausts. 

Retrofit: 

2009, 2012, 2015, 2018, and 2021 IECC

Section R101.4.3 (Section R501.1.1 in 2015, 2018, and 2021 IECC). Additions, alterations, renovations, or repairs shall conform to the provisions of this code, without requiring the unaltered portions of the existing building to comply with this code. (See code for additional requirements and exceptions.)

2009 International Residential Code (IRC)

Section M1507.3 Ventilation rate. Kitchen fans should have an exhaust rate of 100 cfm intermittent or 25 cfm continuous.

2012 IRC

M1503 Range hoods should discharge to the outdoors through a metal duct made of galvanized steel, stainless steel, or copper; with a smooth interior surface that is air tight and equipped with a back draft damper. The duct should be independent of other exhaust systems and should not terminate in an attic, crawl space, or other inside space. Exhaust fans capable of exhausting ≥ 400 cfm should be mechanically or naturally provided with makeup air at a rate equal to the exhaust air rate. The makeup air system should have a gravity or electrically operated damper that automatically opens when the system operates and that is accessible for inspection.

M1505 Open-top broiler units should have a min. 28-gauge metal exhaust hood with ¼-inch clearance between the hood and the underside of combustible material or cabinets. The hood should be at least as wide as the broiler, and extend over the entire unit, discharge to the outdoors, and be equipped with a back draft damper.

M1506  Exhaust opening should terminate at least 3 feet from property lines, 3 feet from operable and non-operable openings into the building, and 10 feet from mechanical air intakes, except where the opening is located 3 feet above the air intake.

Section M1507.4 Minimum Required Local Exhaust Rates for One- and Two-Family Dwellings. Kitchen fans should have an exhaust rate of 100 cfm intermittent or 25 cfm continuous.

2015 IRC

Same as 2012. Duct length and diameters are specified in Table M1506.2 and are based on the fan flow rate, which should be in accordance with ANSIE/AMCA 210-ANSIE/ASHRAE 51.

Retrofit: 

2009, 2012, 20152018, and 2021 IRC

Section N1101.3 (Section N1107.1.1 in 2015 and 2018, N1109.1 in 2021 IRC). Additions, alterations, renovations, or repairs shall conform to the provisions of this code, without requiring the unaltered portions of the existing building to comply with this code. (See code for additional requirements and exceptions.)

Appendix J regulates the repair, renovation, alteration, and reconstruction of existing buildings and is intended to encourage their continued safe use.

This Retrofit tab provides information that helps installers apply this “new home” guide to improvement projects for existing homes. This tab is organized with headings that mirror the new home tabs, such as “Scope,” “Description,” “Success,” etc. If there is no retrofit-specific information for a section, that heading is not included.

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. 

More Info.

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References and Resources*
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Building Science Measures
Building Science-to-Sales Translator

Quality Installed Exhaust Fans = Professionally Installed Exhaust Fans

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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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