Product review (4 stars): Tried others, buy this one
What makes this on the best is a little 2 part door inside.  You can set the doors to closed and all the air goes outside in the summer.  In the winter you can turn the door 90 degrees so the warm air stays in the house.  The second little door is in the center of the bigger door.  If the screen gets clogged the air sill open the small door and allow the vent to go outside.