1. The Second Law of Thermodynamics

Heat can only spontaneously flow from a hotter object to a colder one, and cannot spontaneously flow from a colder object to a hotter one.

2. The Principle of Air Conditioning

In winter, an air conditioner extracts heat from the low-temperature outdoor environment, raises it to a high temperature level through compressor work, allowing heat to flow spontaneously from the high-temperature side to the low-temperature indoor space, achieving heating. In summer, the process is reversed: indoor heat is compressed, heated, and released outdoors.

This process fully complies with the Second Law of Thermodynamics: heat cannot spontaneously transfer from a colder object to a hotter one, but reverse heat transfer can be forced by inputting external mechanical work.