Common Exhaust Duct Not Shared by Fans in Separate Dwellings

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Common exhaust duct not shared by fans in separate dwellings
Common exhaust duct not shared by fans in separate dwellings
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Local Mechanical Exhaust

Common exhaust duct not shared by fans in separate dwellings.

  1. Install separate exhaust ducts for separate units.

If fans from separate dwellings do share a common exhaust duct, one of the following must apply:

  1. The fans must run continuously, OR
  2. Each outlet must have a back-draft damper to prevent cross-contamination when the fan is not running.

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 Kitchen Exhaust Fans and Bathroom Exhaust Fans). Ideally, each exhaust fan should have its own individual duct to the outside and each unit should have its own ducting to prevent cross contamination. However, in multi-unit dwellings, such as condominiums or townhouses, 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 (See Back-Draft Dampers at Shared Common Exhaust Duct). ENERGY STAR permits multiple units to share a common exhaust duct if each fan has a back-draft damper to prevent cross-contamination when the fan is not running or if all fans connected to the common exhaust duct are set to run continuously.

Exhaust fans in separate dwelling units should not share a common exhaust
Figure 1 - Exhaust fans in separate dwelling units should not share a common exhaust. 

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 unit. The dampers should open when the fan is actively exhausting and should shut when the fan is off. When the exhaust fan is operating, the back-draft damper is pushed open by airflow to allow air to exit through the exhaust duct.
    Exhaust fans in separate dwelling units can share a common exhaust if each exhaust fan is equipped with a back-draft damper to prevent cross contamination

    Figure 2 - Exhaust fans in separate dwelling units can share a common exhaust if each exhaust fan is equipped with a back-draft damper to prevent cross contamination. 

  2. Remember to remove any packing tape used to hold the dampers closed during shipping.
    The back-draft damper is open when the fan is actively exhausting and closes when the fan is off
    Figure 3 - The back-draft damper is open when the fan is actively exhausting and closes when the fan is off. 
Ensuring Success

In multi-unit dwellings, such as condominiums or townhouses, 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, all of the fans are set to run continuously, or 3) each fan outlet has a back-draft damper to prevent cross-contamination when the fan is not running.

Climate

No climate specific information applies.

CAD

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.

2009, 2012, 2015, and 2018 International Energy Conservation Code (IECC)

This topic is not specifically addressed in the IECC.

2009, 2012, and 2015 International Residential Code (IRC)

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

2018 IRC

Section M1505.2 Recirculation of air. Exhaust air from bathrooms and toilet rooms to not be recirculated within a residence or to another dwelling unit and must be exhausted directly to the outdoors. Exhaust air from these rooms cannot discharge into an attic, crawl space or other area inside the building. This section shall not prohibit the installation of ductless range hoods in accordance with the exception to Section M1503.3. 

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.

More Info.

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