Today I saw a ventilator that goes in the window or wherever, it was just a free running fan blade (go in either direction) It got me thinking, how would it work? may be smoke and mirrors. Surely friction losses would make it less efficient than a hole. External roof fan vents work by external wind driving a sort of turbine which then extracts air. May be wind passing window at right angles to the fan may cause a vacuum which might cause fan to spin and extract, but I would assume it would do the same with just a hole. Any one prove it works?
You can get one here.
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