San Francisco Rotating Rooftop Vents: What are they? Why do they rotate?

July 3, 2010 5:10 PM

I've noticed these rotating vents since I've moved to San Francisco almost 4 years ago, but never found an answer. They are on a lot of the roofs out here, and seem to rotate with the wind, but sometimes not. Do they rotate with the wind? If so, why? Is it a smell issue? I am sure there is a basic, logical answer, but my google-fu is failing me today!
posted by sir_rubixalot to Home & Garden (10 answers total)

Are you talking about rotating chimney cowls? They keep the exhaust from being blown back down the chimney by the wind.
posted by Laen at 5:14 PM on July 3, 2010

Attic ventilation fans?
posted by busboy789 at 5:16 PM on July 3, 2010

If you're referring to these things, then they exist to cool the house or building down. Hot air in the roof rises and escapes through the vent. It is designed in such a way as to spin when the air passes through it. I had some on my home in Australia and they definitely help to cool the house. They do the same in winter, however, which is a bit of a pain.
posted by twirlypen at 5:21 PM on July 3, 2010

We call them turbines.
posted by girlbowler at 5:23 PM on July 3, 2010

Best answer: I think you mean turbine vents. They rotate in the wind, but they also rotate when the air in the attic (or whatever air space you have) is significantly warmer than the air outside (i.e. there is a pressure difference). They help exhaust air from the attic, cooling the building down and also removing humid air (which can condense and cause problems). They are common here in Canada (where they also help keep the attic colder in the winter, preventing the snow from melting on the roof and forming ice dams).
posted by ssg at 5:36 PM on July 3, 2010 [3 favorites]

Best answer: Also, since we live in the Land That Central Heating Forgot, many houses have gas heaters that vent through the roof. My house has several such vents, and each has a rotating vent cap. They direct the venting smoke/gas away and keep rain from getting in the vent pipe.
posted by gingerbeer at 6:09 PM on July 3, 2010 [1 favorite]

Response by poster: I thought they were turbine vents originally, but I couldn't remember what they were called and then I kept seeing both on the same roof; which could be explained by separate installs/fixes.

Thank you ssg and gingerbeer! I have a feeling it is due to both of these things.
posted by sir_rubixalot at 6:14 PM on July 3, 2010

Turbine ventilators are neat. Keeping rain out of the pipe isn't that hard with a little cap or whatever, but you don't want gusts of wind blowing back down your pipes and blowing out your pilot light or even just causing drafts. In heavy or gusty wind, the turbine spins faster, which does draw more air through, but not enough to extinguish a flame.
posted by aubilenon at 6:28 PM on July 3, 2010 [1 favorite]

Here in the south, people seem to generally call them 'whirlybirds.'
posted by box at 7:34 AM on July 4, 2010

seem to rotate with the wind, but sometimes not

Those on top of my building in Mountain View were rusted maybe, seized up so they didn't spin, the apartment very hot on certain summer days because of, while those on the buiding across the street would be spinning easily.
posted by Rash at 7:59 PM on July 5, 2010