I have a 7200 BTU portable air conditioner. We bought it back in '96 I believe. This model has the intake on the casing and the hot air vent is sticking out the window. Because the intake is inside the house and your exhaust hose is outside, you are not replacing all the air the AC is sucking in from the room. This leads to the rest of the house sucking in outside air to compensate for the inequality of pressure.

In a nutshell, my portable cooled my bedroom enough for me to be able to sleep through the hottest part of the day as long as it was aiming right at me.

Lessons Learned:
Look for a portable that has an intake that can be placed outside the house.

Good Luck