in-line 4-inch pressure damper for air vent?

I am fixing a poor installation of a bath vent fan. The vent cover outside has the flappers that close (somewhat) when the fan is not under pressure, but when the wind blows in the winter, it still pops open sometimes and cold air gets in. Not pleasant to walk into the bath after a windy winter night.
It is 4 inch round vent pipe, and I'm wondering if there is anything like an in-line damper that works the same way- backflow of air is blocked by a flapper in the pipe, but when the fan is on, pressure going out pushes the flap open.
I'm open to any and all ideas, even if it means changing the 4" in the middle of the run to something else. I just want to reduce/eliminate the backflow of cold air.
Thanks, Keith
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Most bath fans have a damper on them at the exhaust. Remove the motor on your fan and stick your hand into the exhaust opening to feel it. Maybe it stopped functioning.
John Grabowski http://www.mrelectrician.tv

cold
night.
an
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