For that matter, air conditioning could include humidifying or dehumidifying, but it doesn't: only cooling. Why weren't air conditioners called air coolers?
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According to the Oxford English Dictionary, air conditioning has changed in meaning over time:
The term was first used in the original sense in the early 1900s:
By the later part of the century, it was used solely to refer to cooling warmer air:
So air conditioning was originally the cleansing of air, which already included temperature control. However, the meaning shifted over time to refer to any temperature control--but usually the cooling of warm air. It also shifted from industrial to more personal scope.
Probably the reason that air conditioning refers to "cooling" is because central heating was introduced first. Then the remaining problem to be solved became one of "too hot," rather than "too cold."
Also, air conditioners (and refrigerators) use a coolant called freon, that "conditions" the air. There are no similar chemicals that act on the air when it is heated.
I suspect that at the time they were introduced "conditioning" sounded like a comforting and effective thing to do - rather than merely cooling. In the same way that food mixers were called "food processors" when they were introduced because it sounded modern and technical.
A close ancestor to the modern air conditioner units was first made in 1902 by an American engineer by the name of Willis Carrier. The machine at that time was called "Apparatus for Treating Air".