Ariati UK 2 x Pack Connector with Backdraught Shutter Flap 100mm - 4-inch Non-Return Valve Damper Flap to Connect Duct Pipe or Flexible Ducting Hose Backdraft Stopper

Job done

Product is easy connect to flexible ducting or rigid pipework, but the one way value only partially opens with the air flow from a typical fan and thus impedes the air flow. I found the best result was to orientate the axis of the flap at about 10 degrees off the vertical (when mounting in a horizontal duct), so that gravity can still close it, but so that if offers less resistance to the air flow. If you install it with the flap axis horizontal, the air flow must lift the full weight of the flap (which it can't).Also, the flap can make a bit of noise when the wind outside disturbs it.

Had 2 seperate fans blowing into the same duct causing backdraft when only one in use so fitted these ending the problem. Also stops cold air coming in when the fans are off. They do need to be fitted so the flap is blown "up" to open so that it's own weight closes it.

Ok for price. Could be more robust. Works as intended

Great does the job it's intended for, just make sure you are putting it on the right way round.

Flimsy . Flap doesn't move freely

Do the job.

Just the job