Heat pump system comprising a refrigerant circuit, the refrigerant circuit comprising an evaporator and a heat exchanger which is adapted to exchange heat between the refrigerant circuit and an entity to be heated, wherein the refrigerant circuit further comprises a bypass line bypassing the heat exchanger and the refrigerant circuit in addition to the heat exchanger and the evaporator comprises a heater which is adapted to heat refrigerant which is routed through the bypass line, characterised in one of the following: - the refrigerant circuit comprising a main expansion valve located between a first port of the heat exchanger and the evaporator and the refrigerant circuit further comprising a main compressor located between the evaporator and a second port, different from the first port, of the heat exchanger, the main expansion valve is located between the bypass line and the heat exchanger and/or the main compressor is located between the heat exchanger and the bypass line, and the bypass line comprising in series the heater as well as an auxiliary expansion valve, being adapted to expand refrigerant flowing in the bypass line before entering the heater, and/or an auxiliary compressor, being adapted to compress refrigerant flowing through the bypass line after leaving the heater and before entering the evaporator; - the refrigerant circuit comprising a main expansion valve located between a first port of the heat exchanger and the evaporator and the refrigerant circuit further comprising a main compressor located between the evaporator and a second port, different from the first port, of the heat exchanger, the main expansion valve is located between the bypass line and the heat exchanger and/or the main compressor is located between the heat exchanger and the bypass line, and the refrigerant circuit comprising a liquid pump, being adapted to pump refrigerant flowing in the bypass line, the liquid pump being preferably connected in series with the heater between a point where the bypass line branches off the line connecting the evaporator with the main expansion valve and the heater. a refrigerant circuit, the refrigerant circuit comprising an evaporator and a heat exchanger which is adapted to exchange heat between the refrigerant circuit and an entity to be heated, wherein the refrigerant circuit further comprises a bypass line bypassing the heat exchanger and the refrigerant circuit in addition to the heat exchanger and the evaporator comprises a heater which is adapted to heat refrigerant which is routed through the bypass line, characterised in one of the following: - the refrigerant circuit comprising a main expansion valve located between a first port of the heat exchanger and the evaporator and the refrigerant circuit further comprising a main compressor located between the evaporator and a second port, different from the first port, of the heat exchanger, the main expansion valve is located between the bypass line and the heat exchanger and/or the main compressor is located between the heat exchanger and the bypass line, and the bypass line comprising in series the heater as well as an auxiliary expansion valve, being adapted to expand refrigerant flowing in the bypass line before entering the heater, and/or an auxiliary compressor, being adapted to compress refrigerant flowing through the bypass line after leaving the heater and before entering the evaporator; - the refrigerant circuit comprising a main expansion valve located between a first port of the heat exchanger and the evaporator and the refrigerant circuit further comprising a main compressor located between the evaporator and a second port, different from the first port, of the heat exchanger, the main expansion valve is located between the bypass line and the heat exchanger and/or - the refrigerant circuit comprising a main expansion valve located between a first port of the heat exchanger and the evaporator and the refrigerant circuit further comprising a main compressor located between the evaporator and a second port, different from the first port, of the heat exchanger, the main expansion valve is located between the bypass line and the heat exchanger and/or the main compressor is located between the heat exchanger and the bypass line, and the bypass line comprising in series the heater as well as an auxiliary expansion valve, being adapted to expand refrigerant flowing in the bypass line before entering the heater, and/or an auxiliary compressor, being adapted to compress refrigerant flowing through the bypass line after leaving the heater and before entering the evaporator; - the refrigerant circuit comprising a main expansion valve located between a first port of the heat exchanger and the evaporator and the refrigerant circuit further comprising a main compressor located between the evaporator and a second port, different from the first port, of the heat exchanger, the main expansion valve is located between the bypass line and the heat exchanger and/or the main compressor is located between the heat exchanger and the bypass line, and the refrigerant circuit comprising a liquid pump, being adapted to pump refrigerant flowing in the bypass line, the liquid pump being preferably connected in series with the heater between a point where the bypass line branches off the line connecting the evaporator with the main expansion valve and the heater.