A set for assembling a heat source unit of an air conditioner at the site of the air conditioner, includes a heat source heat exchanger module having a first casing, a heat source heat exchanger and a compressor module port fluidly communicated with the heat source heat exchanger, a compressor module having a second casing separate from the first casing, a compressor and a heat source heat exchanger module port 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 comprising a control logic of the air conditioner and a first electric connector, wherein at least one of heat source heat exchanger and compressor modules has a circuit board comprising a second electric connector, the circuit board of such a module being configured for data communication with the main board upon electrical connection of the circuit board 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 uses such a set.