A method of operating a heat exchange assembly, the heat exchange assembly comprising a first circuit (3; 5) having a first (4a; 6a) and a second temperature sensor (4b; 6b), and a first flow meter (7; 9), the heat exchange assembly comprising a second circuit (5; 3) having a third temperature sensor (6a; 4a), the heat exchange assembly comprising a heat pump having a compressor and an expansion valve and coupling the first circuit (3; 5) to the second circuit (5; 3) for heat exchange between the first circuit (3; 5) and the second circuit (5; 3), the heat exchange assembly further comprising a first heat exchanger (16; 17) coupling the first circuit (3; 5) to the heat pump and a second heat exchanger (17; 16) coupling the second circuit (5; 3) to the heat pump, the heat exchange assembly further comprising a power meter selected from a compressor meter connected to the compressor or from a second circuit meter comprising the third temperature sensor (6a; 4a), a fourth temperature sensor (6b; 4b) mounted to the second circuit (5; 3), and a second flow meter (9; 7) mounted to the second circuit (5; 3), the method comprising: reading a first temperature signal from a sensor selected from the first temperature sensor (4a; 6a) or the second temperature sensor (4b; 6b), and reading a second temperature signal from the third temperature sensor (6a; 4a); determining a first expected coefficient of performance from the first and second temperature signals; starting the compressor; characterised in that the method comprises the steps of: after starting the compressor, reading a third signal from the first temperature sensor (4a; 6a), a fourth signal from the second temperature sensor (4b; 6b), a fifth signal from the first flow meter (7; 9), and a sixth signal from the power meter; determining an actual coefficient of performance as a function of the third to sixth signals; comparing the actual coefficient of performance to the first expected coefficient of performance; and if the actual coefficient of performance is less than the first expected coefficient of performance: modifying the first expected coefficient of performance as a function of the actual coefficient of performance. reading a first temperature signal from a sensor selected from the first temperature sensor (4a; 6a) or the second temperature sensor (4b; 6b), and reading a second temperature signal from the third temperature sensor (6a; 4a); determining a first expected coefficient of performance from the first and second temperature signals; starting the compressor; characterised in that the method comprises the steps of: after starting the compressor, reading a third signal from the first temperature sensor (4a; 6a), a fourth signal from the second temperature sensor (4b; 6b), a fifth signal from the first flow meter (7; 9), and a sixth signal from the power meter; determining an actual coefficient of performance as a function of the third to sixth signals; comparing the actual coefficient of performance to the first expected coefficient of performance; and if the actual coefficient of performance is less than the first expected coefficient of performance: modifying the first expected coefficient of performance as a function of the actual coefficient of performance. after starting the compressor, reading a third signal from the first temperature sensor (4a; 6a), a fourth signal from the second temperature sensor (4b; 6b), a fifth signal from the first flow meter (7; 9), and a sixth signal from the power meter; determining an actual coefficient of performance as a function of the third to sixth signals; comparing the actual coefficient of performance to the first expected coefficient of performance; and if the actual coefficient of performance is less than the first expected coefficient of performance: modifying the first expected coefficient of performance as a function of the actual coefficient of performance.