<human>: Summarize the given text in a few sentences.

Converted part of a 2 car garage into a guest room. Unfortunately, we spent a butt ton of money creating a duct running from our home a/c unit into the room. It doesn't help! It's so hot here we have got to do something else. We didn't add any windows and the door is in the middle of the garage. We are looking at buying a portable a/c unit (Wall mounts are better but hella expensive). Even though the units are "portable", they all still recommend placing the unit near a window or door. Why is that? What would happen if we didn't do it? Would the room just not cool or is there some kind of drip line that I'm not aware of?
<bot>: want to buy a portable a/c unit and I'm wondering why it would need to be near a window/door.