A CU is the essential element in the AC set. It allows the hot, stale air and all excess warmth from the premise to be let out in the surrounding environment, while the normalized breathing air is transported inside. These devices come at a variety of shapes and sizes, and are suited for numerous variations of systems. There are several system alternatives. The one, which is most widespread and simple is a window unit, which is installed in a window, and comprises two fans. One of them transports the cool air into the premise, while the other one takes the excess warmth outside. Another type is known as a split system, a more intricate type, where the cooling element and heat outlet are separated. These systems may provide AC for a single room or for a whole apartment or house. There are also portable units, that allow the relocation of the inside element. They connect the cooling element and the heat outlet with special flexible hoses.