Originally Posted by Minus08
It flaps because of the low pressure area created on the outside of the damper by the high winds. The same way a plane's wing creates lift.
Exacty! Winds coming across the roof peak, especially, create a low pressure area.. Exactly why you must run chimneys up high enough to get outside the venturi/aerodynamic zone.
Another good approach, if you have a peaked roof and attic with gable, is put in a vent (dryer type) in that wall up near/at the peak and under the eave. Multiple vents? Run them together into a single aluminum vent pipe and out through that one exit point. (Even saves the heat - less moisture - for your attic!)