The tank heater is a device used to heat the water in the tank. It is quite similar to the heating elements found in common household appliances, such as water heaters, ovens, and clothes dryers to name a few. Its construction is a high heat-resistive element encased in a heat-treated stainless steel tube. Magnesium oxide powder is used as filler and insulation to keep the resistive wire in place and away from the casing. Elements are built for specific applications and are not necessarily interchangeable. They are rated for applied voltage and a specific power output (wattage), which will determine the resistance (ohms value).