Has anybody else noticed that air will blow through the vents when the sunroof is open, even if the A/C is off? It only happens if the lever is set to fresh air; it will stop if you switch to recirculate. It will also stop if you shut the sunroof.

Is this by design, or just a natural occurrence due to pressure differences inside and outside the car? Or, is it an actual fan that is activated automatically when the sunroof is open?

I sort of like that it does it because when I tilt the sunroof, I can leave the A/C off and still get a nice breeze through the vents. I've gotten 41 mpg on my commute this way

 

Somebody didn't pay attention in high school physics!

Bernoulli's principle - Wikipedia, the free encyclopedia

 

To more directly answer the question, that's the way its designed. You always have some measure of fresh air flowing through the car, fans on or off and windows open or closed. Opening the sunroof creates a low pressure in the cabin (thus the Bernoulli link above) so you notice more fresh air coming through the vents (since they are fed from the cowl area, which is a high-pressure area).

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