Should a fan blow in or out?

  It's very hot today, and I have a fan.  This fan has the ability for both exhaust (blowing air out the back of the fan) or intake (blowing air into the room).

  The question: what is best for cooling a room, blowing the hot air out or blowing the cool air in?

  The answer may depend on the circumstances.  If you want to cool a certain POINT in the room, then blowing the cool air at the point is best -- air is funneled to the point at high speed which produces both an influx of cold air and a wind chill.  

  But what happens if you want to cool off an entire room?  Or you want to keep a room cool while you are not there?  An air conditioner will work, but it's expensive to operate and it may be bad for the environment.

  Consider the following example.  You get home from work and it's 75 degrees outside after a hot day of 90.  The air in your room is still at 90, but the air outside is still at 75.  If there has been little circulation in the room over the course of the day, you may assume that floor is cooler than the ceiling -- after all, heat rises.  You may also assume that the room is relatively airtight and there are several other rooms of roughly the same temperature -- 90 degrees -- adjacent to the room with the fan.

  If we leave the fan on intake for the whole day, we may be blowing 90 degree air into the apartment during the course of the day at a faster rate than it would have entered during the course of the day if the fan weren't on to begin with.  Wind chill would have no effect on the air itself -- that's a human physiological reaction.  The only place which would be cool would be one area of the room where you are not (since you are at work).

  If we leave the fan on exhaust for the whole day, on the other hand, it will be difficult for the
hotter air outside to make it into the room.  It will help clear the entire room and ensure that air doesn't stay in the room long enough to get really hot.  However, how much good would that do?  If most of the air is blown out at window level (maybe a couple of feet above the ground), that air would have to be replaced by something -- and that something would likely just be the hotter air closer to the ceiling.

  If there is no fan at ALL (and the fan is out of the window), it's easier for air to leave or enter the room than it would be if the fan were in the window (and off).  After all, the fan is blocking the exit.

  So think about it.  There are lots of cases.