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 the electric heater comprises a liquid inlet and a liquid outlet for the primary liquid; a control head adapted to determine a temperature 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 fluid in said main heating circulation, said second liquid passage being in thermal contact with said first liquid passage to heat said secondary heating fluid 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; the electric heater comprises a ground electrode arranged at the liquid inlet and a ground electrode arranged at the liquid outlet, wherein the ground electrodes are arranged perpendicular to the liquid flow, such that the primary liquid passes the ground electrodes, and the ground electrodes having different openings for the flow of the primary liquid. characterized in that said control head is adapted to determine a pressure of said primary heating liquid; said secondary heating fluid is a secondary heating liquid; said electric heater is connectable to a power supply comprising at least two power lines and provides a ground potential; said heating system further comprising 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 the ground potential. an electric heater adapted to heat a primary heating liquid by applying an electric current directly to said primary heating liquid, wherein the electric heater comprises a liquid inlet and a liquid outlet for the primary liquid; a control head adapted to determine a temperature 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 fluid in said main heating circulation, said second liquid passage being in thermal contact with said first liquid passage to heat said secondary heating fluid 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; the electric heater comprises a ground electrode arranged at the liquid inlet and a ground electrode arranged at the liquid outlet, wherein the ground electrodes are arranged perpendicular to the liquid flow, such that the primary liquid passes the ground electrodes, and the ground electrodes having different openings for the flow of the primary liquid. characterized in that said control head is adapted to determine a pressure of said primary heating liquid; said secondary heating fluid is a secondary heating liquid; said electric heater is connectable to a power supply comprising at least two power lines and provides a ground potential; said heating system further comprising 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 the ground potential.