Got your attention didn't I, Harley? A space 24'x40'x8' is 7,680 cubic feet; to change that 10 times in 24 hours you have to move 76,800 cubic feet of air. That works out to 53 cubic feet per minute; does that sound a little better? The 3" pipe out means a 3" pipe in to me. If the pipe is run into your frunace retun; the air will be pumped upstairs by the furnace and then come back down to be removed. You could use sheet metal pipe if it is any cheaper but either kind should be insulated. Drawing winter air into the heated space will cause a lot of condensation.
Sorry I upset you.