A complementary heating system uses components of an A/C loop. Hot refrigerant vapour is injected in the evaporator where it heats up the airflow penetrating the passenger cabin. The energy is provided by the compressor compressing the refrigerant against a gas expansion device. A 3 -way diversion valve makes it possible to switch between the classical A/C operation and the new additional heating function. The Additional Heating System (AHS) has been installed on cars and tested in a climatic windtunnel and during field tests.