An air-cooled chiller (100) includes a compressor (12); a cooler (14); a heat recovery heat exchanger (16), wherein the heat recovery heat exchanger is connected between an output of (12b) the compressor and an input header (20) of an air heat exchanger (60). A solenoid valve (30) is located in an input header (20) of the air heat exchanger to divide the input header into a first portion (20a) and a second portion (20b). A controller (32) is configured to control the solenoid valve (30). A second valve (34) is located in the output header (36) to divide the output header into a first portion (36a) and a second portion (36b). There is also provided a method of operating the air-cooled chiller and a method of retrofitting an existing serial-concept air cooled chiller, to provide the present air-cooled chiller.