Air treatment device for heating of air, comprising an air handling unit , which air handling unit comprises a heat exchanger , arranged to, in a heating mode, recover heat energy from a first air stream and transfer it to a second air stream , for heating of the second air stream , and that the air treatment device further comprises a heat pump , arranged to, in a heating mode, recover heat energy from the first air stream and transfer it to a second air stream , for heating of the second air stream , and which heat pump comprises a circulation circuit for refrigerant , an evaporator arranged in the first air stream , in the airflow direction after the heat exchanger , and a condenser arranged in the second air stream , in the airflow direction after the heat exchanger , and that the heat pump further comprises a compressor with a suction side and a pressure side connected to the circulation circuit and arranged, in the flow direction of the refrigerant in the heating mode, after the evaporator and before the condenser , and further the circulation circuit comprises an expansion valve and a first reservoir , which are arranged, in the flow direction of the refrigerant in the heating mode, after the condenser and before the evaporator on a liquid pipe , and a second reservoir arranged between the evaporator and the compressor , characterised in that the circulation circuit further comprises an injection circuit having a first end connected between the condenser and the expansion valve at the liquid pipe , and a second end arranged at the suction side of the compressor , and which injection circuit comprises an injection valve arranged to direct at least a subset of the refrigerant from the high pressure side to the low pressure side, such that the subset of the refrigerant thus not pass through the expansion valve and the evaporator , for the reduction of the heat pump discharge super heat, and that the circulation circuit comprises a pressure sensor arranged to measure a pressure (P) on the high pressure side of the circulation circuit and a temperature sensor arranged to measure a hot gas temperature (T2) on the high pressure side after the compressor , the air treatment device comprises means for calculating a condensation temperature (T1) based on the measured pressure (P), and further means for calculating a so-called discharge super heat (DSH), which is the difference between the condensation temperature (T1) and the hot gas temperature (T2), and the injection valve is arranged to control the amount of refrigerant through the injection circuit based on the value of the discharge super heat (DSH), to control the same, wherein a heat exchanger is arranged with a first side connected to the circulation circuit , with a first input from the second reservoir and with a first output connected to the circulation circuit before the connection of the second end of the injection circuit , and the heat exchanger is further provided with a second side connected to the injection circuit with a second input from the injection valve and a second output to the compressor suction side , wherein the heat exchanger is arranged to exchange energy between the high pressure side and low pressure side of the circulation circuit , for evaporation of liquid refrigerant in the injection circuit . <EOS>