My range hood has a horizontal flap in it that is shut when the fan is not going, and opens upwards when the fan starts. The flap is of course to stop the wind coming in when the fan is not in use.
I've just made another range hood for a very confined space. Nobody makes o ne small enough, and I don't even want to discuss that. But I can't make a flap valve that opens and shuts properly. I have a 100mm ID pipe that goes from the range hood to outside, and I have cut a long rectangular hole in it and have a hinged curved flap inside. Bu t a 40 watt fan won't open an aluminium flap. I've just made a very lightwe ight plastic flap, and that gets hingebound. There's just too much friction . Perhaps I could have a circular flap inside the pipe, but unless it's much smaller than 100mm it will foul on the pipe when it opens. Any other ideas?