The invention relates to a refrigerant circuit and a process for operation of the refrigerant circuit of an HVAC system of a vehicle, particularly an electric or hybrid vehicle. The refrigerant circuit includes a primary circuit with a compressor, a heat exchanger for heat transmission between the refrigerant and the environment, a collector, a first expansion member, a heat exchanger for supply of heat from the fresh air to be conditioned of the vehicle interior to the refrigerant and a heat exchanger arranged switched in parallel to the heat exchanger. Further, the refrigerant circuit is provided with a secondary train that extends starting from a tapping point positioned between the compressor and the heat exchanger up to a connection point and is provided with a heat exchanger for heat transmission from the refrigerant to the fresh air to be conditioned for the vehicle interior, and a subsequent control valve.