A heating system to heat a main heating circulation, said heating system comprising: an electric heater adapted to heat a primary heating liquid by applying an electric current directly to said primary heating liquid, wherein said electric heater is connectable to a power supply comprising at least two power lines and said heating system is configured such that said primary heating liquid flows through said electric heater along a flow path, said electric heater further comprising: a central electrode connected to a central electrode terminal; and a cylindrical outer electrode connected to an outer electrode terminal, wherein said central electrode and said cylindrical outer electrode are separated by said flow path in a coaxial arrangement such that an electric current flows between the central electrode and said cylindrical outer electrode when said at least two power lines are connected to said central electrode terminal and to said outer electrode terminal; a control head adapted to determine a temperature and a pressure of said primary heating liquid; a heat exchanger with a first liquid passage for said primary heating liquid and a second liquid passage for a secondary heating liquid in said main heating circulation, said second liquid passage being in thermal contact with said first liquid passage to heat said secondary heating liquid while cooling said primary heating liquid; a pump; and a plurality of tubes connecting said electric heater, said control head, said heat exchanger and said pump defining a circulation for said primary heating liquid, wherein said pump is adapted to pump said primary heating liquid such that heat is transferred from said heater via said heat exchanger into said main heating circulation, and wherein said power supply further comprises a ground line, characterized in that said heating system further comprises metal fittings being arranged at ends of at least some of said plurality of tubes, wherein said metal fittings are adapted to provide an electric connection between said primary heating liquid and said ground potential; and said electric heater further comprises a ground electrode connectable to said ground line and provided perpendicular to said flow path at the liquid outlet of the electric heater, wherein said ground electrode comprises an opening to provide a passage for said primary liquid. an electric heater adapted to heat a primary heating liquid by applying an electric current directly to said primary heating liquid, wherein said electric heater is connectable to a power supply comprising at least two power lines and said heating system is configured such that said primary heating liquid flows through said electric heater along a flow path, said electric heater further comprising: a central electrode connected to a central electrode terminal; and a cylindrical outer electrode connected to an outer electrode terminal, wherein said central electrode and said cylindrical outer electrode are separated by said flow path in a coaxial arrangement such that an electric current flows between the central electrode and said cylindrical outer electrode when said at least two power lines are connected to said central electrode terminal and to said outer electrode terminal; a central electrode connected to a central electrode terminal; and a cylindrical outer electrode connected to an outer electrode terminal, wherein said central electrode and said cylindrical outer electrode are separated by said flow path in a coaxial arrangement such that an electric current flows between the central electrode and said cylindrical outer electrode when said at least two power lines are connected to said central electrode terminal and to said outer electrode terminal; a control head adapted to determine a temperature and a pressure of said primary heating liquid; a heat exchanger with a first liquid passage for said primary heating liquid and a second liquid passage for a secondary heating liquid in said main heating circulation, said second liquid passage being in thermal contact with said first liquid passage to heat said secondary heating liquid while cooling said primary heating liquid; a pump; and a plurality of tubes connecting said electric heater, said control head, said heat exchanger and said pump defining a circulation for said primary heating liquid, wherein said pump is adapted to pump said primary heating liquid such that heat is transferred from said heater via said heat exchanger into said main heating circulation, and wherein said power supply further comprises a ground line, characterized in that said heating system further comprises metal fittings being arranged at ends of at least some of said plurality of tubes, wherein said metal fittings are adapted to provide an electric connection between said primary heating liquid and said ground potential; and said electric heater further comprises a ground electrode connectable to said ground line and provided perpendicular to said flow path at the liquid outlet of the electric heater, wherein said ground electrode comprises an opening to provide a passage for said primary liquid.