I believe it needs the fresh air for cooling purposes...you'd risk overheating, if you've ever been on the other side of the unit (outside) while it's running you would know the amount of heat the unit puts out...probably more than it could cool efficiently, not to mention the nasty/stale/re-circulated air it'd be blowing around...
this, of course, is based on no facts whatsoever & my own common sense...er, good luck.
