Set for assembling a heat source unit of an air conditioner at the site of the air conditioner, the set comprising a heat source heat exchanger module (2) having a first casing (4), a heat source heat exchanger (3) and a compressor module port(6) fluidly communicated with the heat source heat exchanger, a compressor module (1) having a second casing (8) separate from the first casing, a compressor (9) and a heat source heat exchanger module port(10) fluidly communicated with the compressor, wherein the heat source heat exchanger module and the compressor module are fluidly communicatable via the compressor module port and the heat source heat exchanger port, a main board (12) comprising a control logic of the air conditioner and a first electric connector, wherein each of the modules has a circuit board (5) comprising a second electric connector, the circuit boards being configured for data communication with the main board upon electrical connection of the circuit boards and the main board via the first and second connectors. Further, a method for assembling a heat source unit of an air conditioner at the site of the air conditioner using the aforesaid set is suggested.