Central air conditioning systems were developed. These can cool an entire office or apartment building from a central unit, just as a building is heated from a large stove in the basement instead of a small stove in each room. In the same period, a small unit that can air-condition a single room was developed. After World War II, a large number of small units began to be used in private housing. Later developments were increasingly used in public buildings and private residences, combining heating and cooling devices in one system.

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