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 ZeroEnergy Ready Home program, ENERGY STAR Certified Homes, and Indoor airPLUS.
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
- 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).
- Remember to remove any packing tape used to hold the dampers closed during shipping.
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.
Mechanically controlled dampers limit ventilation to prevent over-ventilation in humid or extreme temperature conditions.
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.
This topic is not specifically addressed.
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.
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.
Section 403.5 Mechanical ventilation (Mandatory). Automatic or gravity dampers are installed on all outdoor air intakes and exhausts.
Section R403.5 (R403.6 in 2015, 2018, and 2021 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.
Section R101.4.3 (in 2009 and 2012). 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.)
Chapter 5 (in 2015, 2018, 2021). The provisions of this chapter shall control the alteration, repair, addition, and change of occupancy of existing buildings and structures.
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 (M1507.3 in 2009 IRC) 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.
Duct length and diameters are specified in Table M1506.2 (Table 1504.2 in 2018 and 2021 IRC) and are based on the fan flow rate, which should be in accordance with ANSIE/AMCA 210-ANSIE/ASHRAE 51.
Section R102.7.1 Additions, alterations, or repairs. 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 the requirements of this code, unless otherwise stated. (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.
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.
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.
See Compliance tab.
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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.