Heat pump system, comprising a first and a second hermetic compressor (C1, C2), a condenser , a first electronically controlled expansion valve , and an evaporator , wherein the first compressor and the second compressor (C1, C2) are connected in series via a first conduit segment , such that the compression is a two-stage compression when the first and second compressor (C1, C2) are both operated, a first oil separator arranged behind the first hermetic compressor (C1) and being connected with the first hermetic compressor (C1) via a first oil conduit for feeding back oil from the first oil separator to the first hermetic compressor (C1), a second oil separator arranged behind the second hermetic compressor (C2) and being connected with the second hermetic compressor (C2) via an second oil conduit for feeding back oil from the second oil separator to the second hermetic compressor (C2), wherein the first expansion valve is arranged in flow direction in front of the evaporator; a control unit coupled to the first expansion valve for electrically controlling the first expansion valve , a second expansion valve being arranged within a second conduit segment which is connected to the first conduit segment ; wherein the first and/or second hermetic compressor (C1, C2) is a variable speed compressor, wherein at least one of the first and second compressors (C1, C2) is operable independently; and wherein the compressor of the first and second compresssor (C1, C2) which is not operated is by-passed via a bypass conduit , the heat pump further comprising a third conduit segment coupled to the condenser and comprising a valve , the heat pump being characterised by a fourth conduit segment connected with its first end to the third conduit segment and with its second end to the first expansion valve , a fifth conduit segment coupled to the condenser and comprising a valve , wherein the valve of the third conduit segment is coupled optionally to a liquid receiver and a filter drier in a series arrangement as well as to a valve in the fourth conduit segment , wherein the valve of the fifth conduit segment is coupled to the output of the filter drier and to a valve such that the third conduit segment and the fifth conduit segment are coupled, said valve being coupled to the second expansion valve via second conduit segment and to a heating loop of a condensate pan via a sixth conduit segment , wherein the other end of the condensate pan is coupled to one end of an economizer , and the other end of the economizer is coupled in turn to the valve of the fourth conduit segment , wherein the first and second compressor (C1, C2) are arranged such that nominal oil levels of the first and second compressor (C1, C2) are on same geodetical height at least when the heat pump system is not operated, wherein the first and second oil separators are connected to each other via a conduit segment , which comprises a valve , wherein the valve is closed when the heat pump system is operated, and wherein the valve is opened when the system is not operated for an oil equalization, the economizer being arranged directly between the second expansion valve and the first conduit segment , wherein a steam injection is performed via the economizer into the first conduit segment between the first and second compressor (C1, C2), wherein the refrigerant of the steam injection is diverted from the sub-cooled refrigerant in front of the evaporator in flow direction and is evaporated in counterflow with respect to a major part of the refrigerant within the economizer , wherein the first and/or second compressor (C1, C2) is connected with the control unit being adapted to control the speed of the first and/or second compressor (C1, C2) in dependence on a performance demand. <EOS>