The invention includes a refrigerant circuit for a cooling operation and a heat pump operation. The refrigerant circuit has a high pressure area and a low pressure area, including at least one heat source/heat sink, a compressor, an expansion device, at least one thermal interior space module, and an internal heat exchanger. The internal heat exchanger has a high pressure side part and a low pressure side part, wherein the high pressure side part is disposed between the expansion device and the heat source/heat sink. The invention also includes at least one metering device through which the high pressure side part of the internal heat exchanger is operable during the heat pump operation at a medium pressure level intermediate a pressure level in the high pressure area and a pressure level in the low pressure area of the refrigerant circuit.