Okay, I think i've managed to get an okay-looking air-duct system going, although to do so I had to futz with various things a teeny bit, but nothing major...

There'll have to be stairs on both sides of Level 2 due to a curving duct there, and new air-ducts and vents will have to be extended to certain sections when they themselves are added. And the fans... well... I did my best with those...
Now, if that's alright, someone else'll have to add in a Carl-Shivan lurking around in there, as I don't even want to attempt it with such accomplished MSPaint Artistes around here... *Looks at Carl*