Wall/window-mounted air conditioners: These have a strong tendency to freeze over. There are way too many reasons that it happens: high humidity, high smoke/dust content in the air, pets, etc. When they freeze over, it drops the output significantly. Solution? Get a few towels, place them under the unit, take the front plastic cover/screen off the AC, and take a hair dryer to it. The ice will thin down so that you can pull larger chunks off of it, which you'll then dump in the sink/tub. If you do this every other day, you'll save yourself a lot of electricity, and the hassle of having to remove the ice once it's built up good.
