UPDATE:

*  There is a few more options - please check below and check fit test models for a right fit for your window !

This is a 100mm vent for UPVC windows that can be locked when slightly open. It vents all the way out , so no need to open the window wide to put a hose out, at the same time the window can still be locked making sure it's secure .

When printed in white, it's hardly noticeable from outside (thus my photo in black filament).

It's made in a way that the rain won't get inside and any water drains out the front.

I printed mine in PETG.

Outside diameter of an inlet is 100mm

EDIT: 

• Just added a vent that fits (at least some) windows that open inwards. There's a fit test file as well, so you can check if it fits your window (Look for Inward Opening files)
• added 150mm version too and few others

• added version for Single Window - it's a modified shape to fit a window that takes up a whole opening and opens outward. Please check photo below (and in print profiles):

  

• added version for Inward Opening Window - it's a modified shape to fit a window that opens inward. Please check photo below (and in print profiles):

  

• added version for 10mm Thicker Windows  - it's a modified shape to fit a window that's a bit thicker (mostly in europe, I think ).  Please check photo below (and in print profiles):

  

• added version for Windows that have a smaller gap when closed with a gap - Thinner ducting. Please check photo below (and in print profiles):

  

IT'S NOT FOR AC!  THE AIRFLOW IS NOT BIG ENOUGH FOR AC! DO NOT USE WITH AC UNIT!

As it's a large thing to print, I included a test model (a slice) just so you can make sure everything  fit nicely in the gap that your window provides.

Installation video 

When printing it needs painted supports on the round inlet bit. 

Don't use automatic supports as it'll put supports inside and you won't take them out.