Heaters may be used as the primary source of heating, or they can be used for personal or supplemental warmth. Electric heaters run on electricity and are suitable for indoor spaces since they don't release fumes. Electric process heaters are used in manufacturing processes to heat materials. Gas, oil, and kerosene heaters are operated with fuel, so they're ideal for open areas that don't have access to electricity. Hydronic boilers and heaters use hot water or steam to provide warmth.