Any reason why there's that rotating slitted vent thing atop some urban air pipes?

robboy

If by air pipes you're referring to roof vents, yes. They rotate when the heat of the enclosed building attic or above ceiling space rises to the vanes of the vent, thus pulling more of the unwanted heat away from the building. Some are electrically controlled so that when the temperature in the attic reaches so many degrees it activates automatically and then shuts down when the space is sufficiently cooled. It's a terrific energy saving device. Apr 06 04, 2:51 PM

satguru

There are also cowls on chimneys to stop birds falling down, but as robboy says, the moving vent part has that function. The ones without vents are basically to let the gas out but not objects in, and I wake up to one every day from my window, as I sleep in the loft room. Apr 06 04, 4:07 PM