Local Mechanical Exhaust
If fans share common exhaust duct, back-draft dampers installed.
- Install back-draft dampers on all fans sharing a common exhaust duct.
- Remove all packing tape from back-draft dampers.
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. 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 grille or entering the room.
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. 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 1 - The back-draft damper on this exhaust fan is closed to prevent cross contamination.
Figure 2 - The back-draft damper on this exhaust fan is open when the exhaust fan is operating.
2. Remember to remove any packing tape used to hold the dampers closed during shipping.
Figure 3 - The back-draft damper opens when the fan is actively exhausting and closes when the fan is off.
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.
Figure 1 - Separate exhaust fans can share a common exhaust if each exhaust fan is equipped with a back-draft damper to prevent cross contamination.
No climate specific information applies.
ENERGY STAR Version 3, (Rev. 6)
HVAC System Quality Checklist, Local Mechanical Exhaust. If fans share common exhaust duct, back-draft dampers installed.
DOE Challenge Home
Exhibit 1: Mandatory Requirements. Certified under ENERGY STAR Qualified Homes Version 3.
This topic is not specifically addressed in the 2009 IECC.
This topic is not specifically addressed in the 2009 IRC.
This topic is not specifically addressed in the 2012 IECC.
This topic is not specifically addressed in the 2012 IRC.