Self-closing window fan cover

Self-closing gravity cover for window fan.
The design is for placing on the outside of the glass or on the wall where the fan tube comes out. The goal is to keep cold air or water out during the winter or rain when the fan is off.

Outer diameter 113 mm. Inner diameter 105 mm.

The valve closes on its own due to gravity and opens on its own when the fan is running.
When it is cold outside and hot inside, it is possible to get a slight shaking of the valve and to hear a slight clatter of open-closed every 1-2 seconds. However, it works very well and does not allow cold air to pass through a stopped fan.

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