My building's central hair has been broken since before last summer and I caved. 

The advertised minimum window height varied between 21.5" and 22.5". GIven that the unit itself is only 20.25", I decided to chance it. Turns out a 21.25" opening is, with the support installed, exactly the minimum height for the opening. You could probably get away with 20.75" without the support.

Keeps my bedroom very cool. It's not silent, I wouldn't even call it "quiet", definitely not as quiet as the advertising leads you to believe. If you've slept in a hotel room in summer with the A/C running it's comparable to that. The noise doesn't bother me but it might bother light sleepers.