Connector with Backflow Flap, Check Valve, Connector, Ventilation Pipe, Round Duct, Ventilation System, Pipe Connector, Diameter 100 mm, White

works great

The media could not be loaded. No gaskets or o-rings to help seal, but should stop most back draft. Here is a much better one: AC Infinity 4" Backdraft Damper, One-Way Airflow Ducting Insert with Spring-Loaded Folding Blades for 4” Ducting in Range Hoods and Bathrooms Fans https://a.co/d/hodpLQlI used this one in a corner just above the room and the better one at the outside wall. Fits nicely into a 4" 90.

Nice damper for 4 inch metal ducting, used it to isolate the existing dryer exhaust duct, from an ad hoc fan driven exhaust to vent my cat’s sealed litter box cabinet. Must install vertically. The existing dryer flow creates outward suction which seals the damper so there is no backdraft towards the newly installed fan exhaust ( very necessary to minimize lint to build up). When the fan is in operation ( with or without the counter dryer suction) the damper opens up and allows the stink to vacate my basement.. just what I needed...( the damper is installed between the t-shaped duct and the flexible piping in the picture)

It worked, it fit, and should work good long term for my purposes.

I have been searching for some kind of 'automatic one way' check valve to install on a 4" air ventilation line. It needs to open with the relatively low air pressure provided by inline duct fans and shut completely when the fan is off. In my case this just did not work. It will open under the pressure of an inline fan but even when the fan is off it opens under the natural draft as colder air wants to move in through the fan as well. That might work for some applications but not what I needed. The flapper does seem to swing open and shut whether mounted horizontally or vertically. While I did not expect the flapper to be of heavy material I was surprised by how thin and light the body of the unit was. The housing on the flapper valve does appear to be pretty flimsy.

Lightweight. It was easy to install and works as supposed to

Fragile

This part has not spring and can be opened by pushing on either side of it, in other words if the wind blows into the ventilation fan the damper does not stop it, air will flow both directions so what is the point?