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I live on a hill top. Brutal wind. Not so in the valley. Anyhow, all night long I could hear the flapper plastic door/valves swing open sucking out heat from my bathroom vent fans. (When the fans were off).
So I put in 24vac power dampers on relays activated by the bathroom fan on 120v. Problem solved. Prior to that I had 2 four inch holes in the house venting heat into the outside. It has probably been covered before. If so, I apologize.
Now, when the bathroom fan runs, unimpeded exhaust to outside. Fan off = sealed drum tight.
On my 3, or 4 th year. Maybe more. Works great in a tight house. ICF construction with radiant floor heat.