Why does "air conditioning" always mean "cooling" and never "heating"?

For that matter, air conditioning could include humidifying or dehumidifying, but it doesn't: only cooling. Why weren't air conditioners called air coolers?

Solution 1:

According to the Oxford English Dictionary, air conditioning has changed in meaning over time:

Originally: the process of cleaning air and controlling its temperature and humidity in connection with certain manufacturing processes.

Now usually: the process of controlling the humidity, ventilation, and air temperature in a building, vehicle, etc., esp. to maintain a comfortably cool atmosphere in warm conditions; (also) an installation or appliance which performs this process, an air conditioner.

The term was first used in the original sense in the early 1900s:

1909 S. W. Cramer Useful Information Cotton Manufacturers (ed. 2) IV. v. 1395, I finally hit upon the compound word, ‘Air Conditioning’‥. Suggested by the use of the term ‘Conditioning’ in the treatment of yarn and cloth.

1930 Engineering 11 July 34/2 Air-conditioning is dealt with fully, with the methods for washing, cleaning, humidifying, cooling and drying the air.

By the later part of the century, it was used solely to refer to cooling warmer air:

1987 C. Phillips European Tribe i. 11 Only the odd car rubbering by in the boulevard below and the hum of the air-conditioning disturbed my sleep.

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.

Solution 2:

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.