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An electric boiler is a device that uses electrical energy to boil water.
Small electric water boilers are portable household appliances, used to prepare hot beverages or other food items. They may provide a reservoir of hot water to be kept at a fixed temperature or to allow tea to be steeped at a controlled temperature. They are manually filled with water every time hot water is desired.
An electric steam boiler is intended to produce steam for process industry or other uses, such as drying, heat treating, and others. These are generally permanently installed systems with connections to a water supply and piping to transport product steam to the point of use. In places where electric power is relatively low cost compared to fossil fuels, it may be economically practical to use an electric boiler for steam central heating. For example, in Winnipeg, Canada, during the Second World War, large central electric steam boilers were used for a district heating system, using surplus hydroelectric power. The intent was to conserve coal fuel for more critical wartime needs.
Control and safety
Even the tabletop appliance will have a control switch or at least an over temperature cut out to prevent fire if the appliance is started with no water in it. Larger installations have more elaborate controls to ensure that sufficient water supply is available to prevent burn out of the heating elements. These may include interlocks, pressure switches or other measures. 
A boiler is a pressure vessel and industrial boilers are constructed, operated and inspected in accordance with local regulations to reduce the risk of explosion. For example, permanently installed pressure relief valves will operate if a control malfunction results in excessive pressure in the boiler.
Normally resistance heating elements are insulated from the water going through the boiler; sensitive ground fault leakage current detection may be installed to alarm or shut off power if an insulation failure is detected. Electrode boilers put the water supply in direct contact with the electrical supply; current collectors or other features may be provided in piping to prevent dangerous electrical hazards on connected piping.
Any dissolved minerals in the water will be concentrated by the evaporation and will tend to form a scale coating heating surfaces. Even potable water may contain enough minerals to eventually clog or scale a boiler. Household appliances can be cleaned readily, but permanent boiler installations require control of make-up water supply chemistry to reduce scaling.
Electric heating is generally highly efficient; since there is no stream of waste combustion gases emitting from the boiler, nearly all the purchased energy appears in the product hot water or steam in useful form. This high efficiency even in small sizes partly offsets the generally higher cost of electric energy compared to fossil fuels. An electric boiler can be placed in operation quickly, as there is no furnace to reach high temperatures.  Environmental impact is displaced to the source of the grid electricity; however, if fossil fuel is being consumed to make electricity, overall efficiency will be lower than direct use of the fuel.
Operationally, an electric boiler is a convenient process unit that is easy to control and that requires no space for fuel storage nor for an exhaust gas stack. There is no blower for combustion air and only minimal pumps required for operation, so the boiler is quiet. 
- Boiler (power generation) boilers fired by fuel used to operate electric generators