2009 IRC, International Residential Code for One and Two Family Dwellings. 2009. Fifth Printing. International Code Council, Washington, D.C.
Common Exhaust Duct Not Shared by Fans in Separate Dwellings
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Scope
Local Mechanical Exhaust
Common exhaust duct not shared by fans in separate dwellings.
- Install separate exhaust ducts for separate units.
If fans from separate dwellings do share a common exhaust duct, one of the following must apply:
- The fans must run continuously, OR
- Each outlet must have a back-draft damper to prevent cross-contamination when the fan is not running.
Notes:
Exhaust outlets from more than one dwelling unit may be served by a single exhaust fan if the fan runs continuously or if each outlet has a back-draft damper to prevent cross-contamination when the fan is not running.
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Description
Exhaust fans should always be ducted to a location outside the home (See Kitchen Exhaust and Bathroom Exhaust). 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.
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.
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.
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.
Right and Wrong Images
Presentations
None Available
Videos
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CAD Images
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Compliance
ENERGY STAR Version 3, (Rev. 6)
HVAC System Quality Checklist, Local Mechanical Exhaust. Common exhaust duct not shared by fans in separate dwellings. Exhaust outlets from more than one dwelling unit may be served by a single exhaust fan if the fan runs continuously or if each outlet has a back-draft damper to prevent cross-contamination when the fan is not running.
Exhibit 1: Mandatory Requirements. Certified under ENERGY STAR Qualified Homes Version 3.
This topic is not specifically addressed in the 2009 IECC.
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.
This topic is not specifically addressed in the 2012 IECC.
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.
Case Studies
None Available
References
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2009 IRC—International Residential Code for One and Two Family Dwellings
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2012 IRC—International Residential Code for One and Two Family Dwellings
2012 IRC, International Residential Code for One and Two Family Dwellings. 2012. Second Printing. International Code Council, Washington, D.C.
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DOE Challenge Home National Program Requirements
DOE. 2012. DOE Challenge Home National Program Requirements (Rev. 02). Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy, Washington, D.C.
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ENERGY STAR Qualified Homes, Version 3 (Rev. 06) Inspection Checklist for National Program Requirements
EPA. 2012. ENERGY STAR Qualified Homes, Version 3 (Rev. 06) Inspection Checklist for National Program Requirements. U.S. Environmental Protection Agency, Washington, D.C.
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HVAC System Quality Installation Rater Checklist
EPA. 2011. HVAC System Quality Installation Rater Checklist. U.S. Environmental Protection Agency, Washington, D.C.
