Wall unit air conditioners, also known as through-the-wall air conditioners, take in warm air and exhaust it out the back of the unit while sending cool air back into the room. Wall unit air conditioners require more planning and installation than window units because they are mounted permanently and cannot be removed.
To mount one, a hole is cut in an exterior wall, and a sleeve installed. A sleeve is necessary to support the weight of the air conditioner, as the wall itself is not strong enough to. The advantages of wall unit air conditioners versus window air conditioners are that the use of the window is not lost and they are more airtight, making them more energy-efficient.