In Spanish, the word for heater is calefactor. This term is commonly used in many Spanish-speaking countries to refer to various types of heating devices, whether they are electric heaters, gas heaters, or central heating systems.
In some contexts, you may also hear the word estufa, which generally refers to a stove or heater, particularly in the context of cooking or home heating. However, if you want to specifically refer to a heating appliance, calefactor is the most appropriate term.