The typical flow pattern is the high vents allow the heated air out. The lower ones let fresh air in, they are all on one side this helps generate a gradient the coolest place is on the floor in front of the low vents. Plexi is very clear and neutral colour, glass has a green tint to it. I have loads of plexi (acrylic I don't always buy 'plexiglas brand') none of mine is yellow or even yellowing...