Air source heat pump heat transfer medium is a special substance, its boiling point under normal pressure is minus 40℃, the freezing point is below minus 100℃, the substance is a liquid when it is cold, but it is easy to be evaporated into a gas, and vice versa. In actual operation, the evaporation temperature of the heat transfer medium in the air source heat pump is about minus 20 ° C, so the ambient temperature of 5 ° C is also "hot" for such a low temperature, and even the temperature of snow, such as 0 ° C, is also hot in comparison, therefore, some heat energy can still be exchanged.