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My landlord claims that roof turbine ventilators move more air when they do not spin—e.g. by stalling it using a piece of wire tied through the vent and the brace. His reasoning, roughly, is that when the turbines are spinning, all it is doing is equalizing the pressure, and claims to observe that the air in the attic is still; whereas when the turbines are stalled, he claims to have observed more air, dust, etc. moving around and leaving the attic.

I fail to see how this could be the case. My understanding of pressure equalizing is that it means that air must be flowing from high pressure (in the attic) to low pressure (outside). By allowing the turbine to spin freely, there would be less resistance/reactive force that keeps the air from moving out of the attic; whereas when the turbine is stopped, the air moving out of the attic wants to move the turbine but it can't (i.e. more resistance). And when something (e.g. wind) spins the turbine, then the turbines will pull air out of the attic; immobilizing the turbine means no air will be pulled out.

I will not be able to correct him if he is wrong (he made up his mind 30+ years ago). But in case I run into anyone else actually interested in a response to his argument, I'd like to be able to explain this better. What would be a more intuitive/straightforward/easy-to-understand/etc. way to explain this?

  • I don't suppose 30 years ago he worked for McDonnell-Douglas. – Harper Feb 23 at 13:51
  • Nope, but he was an air force brat, so he seems to know quite a bit about planes, and can usually tell more about a plane by hearing it than I can by looking at it. – chrstphrchvz Feb 23 at 14:02
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Wow, landlords can really be dummies sometime. The simplest answer is that the turbine company paid an engineer to design/improve their turbine to be most effective relative to the cost of production. If stopping the blades were more effective then there would be no bearings for them to spin on and the turbines would cost less to build. The landlords idea could be improved by taking the turbine off the base and leaving the attic open to the sky. This, however would allow rain into the attic just as what would happen with a driving rain with stopped blades. Empirevent.com has a nice website with photos. You can easily see into the interior when the vent is stopped and that is the path for rain to enter.

  • There is a certain type of person who is way overconfident in his ability to analyze certain situations. They are sure they know more than anyone else, including trained professionals. – Jim Stewart Feb 23 at 16:00

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