Probably was left open when someone was working up there and wanted some cool air. It should be connected to something, either a splitter box (one big duct feeds a box that then has smaller ducts running from it to the rooms) or directly to a register.
Do you know if you have dampers? And btw, ducts should never be laying on the attic floor but should be strapped up. That's a part of your problem right there. Also check to make sure all ducts have no kinks in their runs.
If you do not have dampers on the ducts to control air flow do you have the aforementioned splitter boxes or "Y" configurations up there where again, a Y would consist of one bigger duct breaking off into two smaller ducts at a Y connection? It is always best to have dedicated runs and not split or Y'ed runs, especially without dampers.
Last edited by Doc Holliday; 09-05-2012 at 01:20 PM.