From the Ask-the Builder website, an email:
"I encourage you to do some research ( including some independant of roof vent manufacturers) but you will find these vents(the aluminum spinning vents) have generally less than one square foot of roof opening and do not induce air movement while spinning. Larger, less conspicuous static vents are more efficient at moving air.
*I am not suggesting the manufacturers are misleading everyone but simply allowing us to carry on with the wrong idea of how they work. *
*If the vent is powered by a motor then it would be different. *
As far as static operation, the most efficient is simply a hole in the roof. (But that would not be practicle). The vent is merely a cover to protect the hole while allowing the air to pass… and this comes at a cost of some restriction and therefor lost of dynamic energy in the gas.
*The only purpose of the rotation is mainly to help keep the open vent area clear of debris. Also there may be a benefit that these rotary vents tend to be higher. *
*But dont take my word for it …check with a mechanical engineer specialised in gas dynamics. *
*Bottom line… you can’t expect those fins to both turn the vent and to draw air out imultaneously… it defies the laws of physics. (I know it looks like it is doing that, but it cannot) *
I have Bachelor’s degree in Mechanical Engineer, and although not a practising engineer, I studed gas fluidynamics gas dynamica and this is really simple first year stuff. Basically look up how a turbine blade functions."