The frost on the heat exchanger of air-source heat pump has a great influence on the operating performance. The frosting experiment based on a cascade type air-source heat pump water heater is carried out in the enthalpy-difference chamber in this paper

and the outdoor evaporator of the unit is a finned-tube heat exchanger. The experiment is conducted under the conditions that the outdoor temperature is from -18℃ to 6℃ and outdoor relative humidity is from 70% to 90%. Experimental results show that: First of all

the amount of frosting almost changes linearly with the frosting time which is of maximum in the outdoor temperature range of -3~3℃

and the lower outdoor temperature

the more frosting amount; Secondly

the outdoor relative humidity has a great influence on the quantity of frost

and the more humidity

the more frosting quantity; Finally

the frosting factor is proposed based on the relationships of the area and form of evaporator coil

the time of frosting

the difference of the dew point temperature and the temperature on the evaporator coil and the frosting quantity

and is from 6.51 to 10.31kg/(m2?℃?min) with the average value 8.58×10-4 kg/(m2?℃?min).