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When you want the air to flow only one way through a duct, you need to use a backdraft damper. The Fantech-RSK6 has light aluminum, butterfly blades that are spring loaded to close and seal.

12" motorized damper for zoning round ducting.  The YITA-MDA300 is a well tested, 24V motorized damper that seals well, is simple to install, and has been tested for tens of thousands of cycles.  It will accept 300mm or 12" diameter ducting that it will capture with small, wedge shaped stops.  It works extremely well with the Heyoka Damper control box: HEY-DM120CB2.

When you want the air to flow only one way through a duct, you need to use a backdraft damper.  The Fantech-RSK12 has light aluminum, butterfly blades that are spring loaded to close and seal.