A method for controlling a vapour compression system , the vapour compression system comprising a compressor unit comprising at least two compressors , a heat rejecting heat exchanger , a receiver , an expansion device and an evaporator being arranged in a refrigerant path, the expansion device being arranged to control a supply of refrigerant to the evaporator , at least one of the compressors being a main compressor being fluidly connected to an outlet of the evaporator and at least one of the compressors being a receiver compressor being fluidly connected to a gaseous outlet of the receiver , the vapour compression system further comprising a bypass valve fluidly interconnecting the gaseous outlet of the receiver and the at least one main compressor , wherein in the receiver the refrigerant is separated into a liquid part and a gaseous part, the liquid part of the refrigerant leaves the receiver via a liquid outlet , and is supplied to the expansion valve , wherein the method comprises the steps of: - measuring or deriving a pressure difference across the bypass valve , - deriving a mass flow rate of refrigerant through the bypass valve , based at least on the pressure difference across the bypass valve , and using a fluid model, - deriving a minimum mass flow rate of refrigerant required to operate the receiver compressor , based on a minimum displacement volume of the receiver compressor and using a fluid model taking prevailing operating conditions into account, - comparing the derived mass flow rate of refrigerant through the bypass valve and the derived minimum mass flow rate of refrigerant required to operate the receiver compressor , - starting the receiver compressor and closing the bypass valve in the case that the derived mass flow rate of refrigerant through the bypass valve exceeds the derived minimum mass flow rate of refrigerant required to operate the receiver compressor , and - controlling a pressure prevailing in the receiver by operating the receiver compressor in the case that the derived mass flow rate of refrigerant through the bypass valve exceeds the derived minimum mass flow rate of refrigerant required to operate the receiver compressor , and controlling the pressure prevailing in the receiver by operating an opening degree of the bypass valve in the case that the derived mass flow rate of refrigerant through the bypass valve is lower than the derived minimum mass flow rate of refrigerant required to operate the receiver compressor . - measuring or deriving a pressure difference across the bypass valve , - deriving a mass flow rate of refrigerant through the bypass valve , based at least on the pressure difference across the bypass valve , and using a fluid model, - deriving a minimum mass flow rate of refrigerant required to operate the receiver compressor , based on a minimum displacement volume of the receiver compressor and using a fluid model taking prevailing operating conditions into account, - comparing the derived mass flow rate of refrigerant through the bypass valve and the derived minimum mass flow rate of refrigerant required to operate the receiver compressor , - starting the receiver compressor and closing the bypass valve in the case that the derived mass flow rate of refrigerant through the bypass valve exceeds the derived minimum mass flow rate of refrigerant required to operate the receiver compressor , and - controlling a pressure prevailing in the receiver by operating the receiver compressor in the case that the derived mass flow rate of refrigerant through the bypass valve exceeds the derived minimum mass flow rate of refrigerant required to operate the receiver compressor , and controlling the pressure prevailing in the receiver by operating an opening degree of the bypass valve in the case that the derived mass flow rate of refrigerant through the bypass valve is lower than the derived minimum mass flow rate of refrigerant required to operate the receiver compressor . <EOS>