A serious problem confronting theoperation of air source heat pumps is the degradation in performance caused by frosting ofoutdoor units. In practical applications

the probability of frosting in local climatic conditions determines the applicability of most defrosting technologies. The degree of frosting of air source heat pumps is not only affected by climate

but also affected by system performance and even the user's usage habits. Consequently

it has been difficult to make a quantitative judgment of how to overcome these problems. Therefore

the concept of freezing degree hours is proposed to characterize the effects of climatic conditions on frost formation. In order to make the calculation of freezing degree hours closer to reality

this study collected operational data of air source heat pumps in many cities. The characteristic temperature difference between evaporation temperature and outdoor dry bulb temperature

with different power types

was obtained by fitting the operational data. The characteristic temperature difference can be used to correct the specific fixed value that is assumed in traditional designs. Finally

according to the analysis of freezing degree hours under different characteristic temperature differences

combined with the frost characteristics of different power types in 275 cities

the frost climate zones of these cities were classified

thereby providing a reference for the design of air source heat pump systems across different frost climate zones and the development of control strategies.