The present invention is concerned with an electric downhole heating system (10) for formation heat treatment in the field of oil and gas production. The heater (10) contains separate wiring chamber (12), heating chamber (16), and cooling chamber (14), the latter being inserted between the wiring chamber (12) and the heating chamber (16). The heat treatment is carried out by inserting the heater (10) in a borehole to be treated. A gas, preferably nitrogen or air, is brought to the heater with a hose or tube (24). The gas goes through the wiring chamber (12) and cooling chamber (14), and is heated by following a tortuous path in the heating chamber (16) before it is expelled from the heater (10).