A method of controlling a fan of a vapour compression system, the vapour compression system comprising a compressor, a heat rejecting heat exchanger, an expansion device and an evaporator arranged in a refrigerant circuit, said fan being arranged to provide a secondary fluid flow across the heat rejecting heat exchanger, the method comprising the steps of: - establishing an outlet temperature, T 1, of refrigerant leaving the heat rejecting heat exchanger, - establishing a temperature, T 2, of ambient air of the heat rejecting heat exchanger, - deriving a temperature difference, ΔT=T 1 -T 2, between the temperature (T 1) of refrigerant leaving the heat rejecting heat exchanger and the temperature (T 2) of ambient air of the heat rejecting heat exchanger, - comparing the temperature difference, ΔT, to a first threshold value and to a second threshold value, the second threshold value being smaller than or equal to the first threshold value, and - controlling the rotational speed of the fan on the basis of the comparing step wherein the step of controlling the rotational speed of the fan comprises the steps of: if the temperature difference, ΔT, is larger than the first threshold value, increasing the rotational speed of the fan, and if the temperature difference, ΔT, is smaller than the second threshold value, decreasing the rotational speed of the fan, and wherein the step of decreasing the rotational speed of the fan is performed by ramping down the rotational speed, characterized in that the fan is controlled with an asymmetric control aspect, in the sense that the rotational speed of the fan is increased rapidly and significantly by jumping up the rotational speed if it is established that the outlet temperature is above a dead zone delimited by a first outlet temperature and a second outlet temperature respectively corresponding to the first threshold value and the second threshold value, and the rotational speed of the fan is decreased carefully and gradually by ramping down the rotational speed if it is established that the outlet temperature is below the dead zone. - establishing an outlet temperature, T 1, of refrigerant leaving the heat rejecting heat exchanger, - establishing a temperature, T 2, of ambient air of the heat rejecting heat exchanger, - deriving a temperature difference, ΔT=T 1 -T 2, between the temperature (T 1) of refrigerant leaving the heat rejecting heat exchanger and the temperature (T 2) of ambient air of the heat rejecting heat exchanger, - comparing the temperature difference, ΔT, to a first threshold value and to a second threshold value, the second threshold value being smaller than or equal to the first threshold value, and - controlling the rotational speed of the fan on the basis of the comparing step wherein the step of controlling the rotational speed of the fan comprises the steps of: if the temperature difference, ΔT, is larger than the first threshold value, increasing the rotational speed of the fan, and if the temperature difference, ΔT, is smaller than the second threshold value, decreasing the rotational speed of the fan, and wherein the step of decreasing the rotational speed of the fan is performed by ramping down the rotational speed, characterized in that the fan is controlled with an asymmetric control aspect, in the sense that the rotational speed of the fan is increased rapidly and significantly by jumping up the rotational speed if it is established that the outlet temperature is above a dead zone delimited by a first outlet temperature and a second outlet temperature respectively corresponding to the first threshold value and the second threshold value, and the rotational speed of the fan is decreased carefully and gradually by ramping down the rotational speed if it is established that the outlet temperature is below the dead zone. if the temperature difference, ΔT, is larger than the first threshold value, increasing the rotational speed of the fan, and if the temperature difference, ΔT, is smaller than the second threshold value, decreasing the rotational speed of the fan, and wherein the step of decreasing the rotational speed of the fan is performed by ramping down the rotational speed, characterized in that the fan is controlled with an asymmetric control aspect, in the sense that the rotational speed of the fan is increased rapidly and significantly by jumping up the rotational speed if it is established that the outlet temperature is above a dead zone delimited by a first outlet temperature and a second outlet temperature respectively corresponding to the first threshold value and the second threshold value, and the rotational speed of the fan is decreased carefully and gradually by ramping down the rotational speed if it is established that the outlet temperature is below the dead zone.