Electric Water Heaters
Most electric water heaters used in residences are the automatic storage type. Although some instantaneous heaters are in use, the high electric power input required makes them uneconomical to operate when compared with fuel-fired types.
An electric water heater generally consists of a vertical tank with a primary heating element or resistor inserted near the bottom of the tank. Some water heaters have a secondary heating element located in the upper one-fourth of the tank (see Figure 4-62). The number of heating elements used in the heater will depend on the size of the storage tank. Large-capacity storage tanks will require two heating elements.
The manual and thermostatic (automatic) controls are located inside the storage tank. A water heater thermostat is designed to automatically open or close the electrical circuit to the heating element(s) whenever the hot-water temperatures exceed or fall below
the temperature range of the water heater. Depending on the size of the storage tank, the water heater will be equipped with either one or two thermostats.
The rated voltage for electric water heaters is 240 volts. The thermostats controlling the primary and secondary heating elements are generally set at 150°F.
Several types of heating units are available for electric water heaters, but the two most popular are probably the immersion element and the strap-on unit. The immersion element is inserted
through an opening in the side of the tank. The strap-on unit is externally mounted on the surface of the tank.
A larger storage tank capacity is required for electric water heaters than for fuel-fired types in order to compensate for the limited recovery rate. As a result, initial equipment costs are higher. Care should be taken not to oversize or undersize the storage tank. Incorrect sizing will result in an inefficient water heater.
Electric water heaters of the automatic storage type are generally available in storage tank capacities ranging from 30 to 140 gallons. The electric power input requirement will range from 1600 to 7000 watts.