4 inch Backdraft Damper

What a 4 inch Backdraft Damper Is Stopping

A duct with nothing in it is a two way street. When the fan is off, outside air walks back down the run, bringing cold, smell and whatever is living in the roof space with it. On a sealed room it also breaks the pressure relationship you built the extraction around. A backdraft damper closes that street in one direction. Two blades sit inside the body, each on its own independent hinge. That independence is the design detail worth paying for, because a single shared shaft means one sticky blade holds the other open. The body is galvanized steel and the whole assembly pushes into the duct, so there is no flange to fit and no transition to build. It installs in any orientation. The blades close onto a neoprene perimeter seal rather than metal onto metal, which is what makes the closure quiet as well as tight.

Why Static Pressure Decides Whether It Opens at All

One figure matters more on a small duct than anywhere else. The blades need between four and eight hundredths of an inch of static pressure before they lift. That is not very much pressure at all. A small fan at the end of a long flexible run, with a loaded carbon filter in front of it, does not always make it. The symptom is a fan that runs while nothing moves, and the cause looks like a dead fan rather than a shut damper. Two habits between them avoid it entirely. Keep the run short and use rigid duct where the layout allows, since flexible duct costs several times the resistance for the same length. Second, check the damper by hand at commissioning with the fan at its lowest intended speed rather than at full. A unit that will be run at thirty percent overnight needs to open at thirty percent. The damper is sized for a four inch duct and suits the smaller Vortex in-line fans.

Key Features

• Duct size: Four inches, matching a four inch inline fan or run
• Function: Stops air travelling back down the duct when the fan is off
• Blades: Two, each on an independent hinge so neither can jam the other
• Seal: A neoprene perimeter gasket rather than metal onto metal
• Noise: That gasket is what makes the closure quiet as well as tight
• Body: Galvanized steel throughout the housing and both blades
• Mounting: The whole assembly inserts into the duct, with no flange to fit
• Orientation: Installs in any position
• Opening pressure: Between four and eight hundredths of an inch of static
• Commissioning: Check it opens at the lowest speed the fan will actually run

Technical Specifications