PURPOSE: To provide a complete defrosting at the heat exchanger for heat source by a method wherein a coolant passage for the heat exchanger for the hat source is divided into a plurality of branch passages and one of them is used for a combined heating and defrosting circuit and further installed at a central part of the heat exchanger for the heat source or downstream of the heat source.

CONSTITUTION: Heat exchanger unit 3 for the heat source is constructed such that a heat exchanger part 10 and a heat exchanging pipe 12 are arranged in series through a pressure reduction unit 11 and a main coolant circuit including a series connected paralllel circuit containing a check valve 4 and a pressure reduction unit 9, and then the heat exchanger part 10 and the heat exchanger pipe 12 are placed downstream of the heat source. In this arrangement, the coolant is flowed as shown in a solid arrow line during a defrosting operation, a less volume of flow of the coolant is found for the heat exchanging pipe 12 due to a resistance in the capillary tube 11 and much of the flow rate of the coolant is flowed in the upstream heat exchanging pipe 3 containing much volume of frosting. Thereby, a high defrosting efficiency and a complete defrosting operation can be performed.