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The following round fan is installed in a residential roof in a house in Pennsylvania. The roof also has soffit and ridge vents. The round part of the fan spins, but it is unpowered (by electricity). So what is this vent or fan called, and what function does it serve that is not served by ridge vents?

Mystery roof fan

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They're typically called air turbines or roof vent turbines. They can turn if the outside wind is blowing or if hot air is simply rising through them. They're a type of unpowered exhaust, just with moving parts. Typically you'll have ridge vents OR powered attic fans OR these. The advantage ridge vents have is they have no moving parts.

  • Yes, that is correct and they “suck” air (exhaust) out of the attic when there isn’t “cross ventilation” available. They can be noisy if rusty. – Lee Sam Jun 8 '18 at 22:24
  • @LeeSam and Machavity: So what is the function (and advantage, if any) of having the moving part on these, over other passive vents (be they ridge vents or other cut-in vents)? I imagine that if there's wind to turn these that same wind would draw roughly as much air through any other passive vent system. – feetwet Jun 8 '18 at 22:34
  • @feetwet The advantage with turbine vents is that with very little wind on the OUTSIDE of the house, it will move air in the attic. Whereas, “gravity vents” (whether ridge vents or individual vents) they allow “dead” areas in the attic, because it takes more wind to create air movement INSIDE the attic. The disadvantage is they are unsightly and can be noisy. – Lee Sam Jun 8 '18 at 23:19
  • @feetwet I had a couple of these I had to replace with powered fans. The main downside is they just didn't move enough air by themselves for the size attic we have. But as long as there's hot air flowing, they passively increase the air pull, especially without any other exhaust mechanisms. – Machavity Jun 9 '18 at 0:41

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