Temperature limited heaters with thermally conductive fluid used to heat subsurface formations

A system, comprising:

• a heater comprising one or more electrical conductors, the heater configured to generate a heat output during application of electrical current to the heater, wherein the heater comprises a ferromagnetic material, and wherein the ferromagnetic material at least partially surrounds a non-ferromagnetic material;
• a conduit at least partially surrounding the heater;
• a fluid located in a space between the heater and the conduit, wherein the fluid has a higher thermal conductivity than air at standard temperature and pressure (STP) (0° C. and 101.325 kPa);
• and
• wherein the system is configured to provide (a) a first heat output below a selected temperature when time-varying electrical current is applied to the heater, and (b) a second heat output near or above the selected temperature when time-varying electrical current is applied to the heater;
• and
• wherein the system is configured to allow heat to transfer from the heater to a part of a subsurface formation.