Certain embodiments provide a heating system configured to heat at least a part of a subsurface formation. The system includes an electrical power supply. A heater section includes one or more electrical conductors electrically coupled to the electrical power supply. The heater section is configured to be placed in an opening in the formation. At least one of the electrical conductors includes ferromagnetic material. The heater section provides a first heat output when time-varying electrical current is applied to the heater section below a selected temperature, and provides a second heat output approximately at and above the selected temperature during use. The second heat output is reduced compared to the first heat output. The system is configured to allow heat to transfer from the heater section to a part of the formation. The system is configured to allow a frequency of the applied time-varying electrical current to be varied.