Standard Electric Operation

Standard Electric Water Heater

This section provides an overview of standard electric water heaters. The first part of this section has illustrations, photographs, and a brief description of each part. The second part of this section describes the operation of standard electric water heaters. When this section is completed, the reader should have a basic understanding of how a standard electric water heater transfers heat into water.

Element – The element consists of an inner wire surrounded by filler material enclosed in a sheath of copper or stainless steel. The thermostat allows electrical current to flow through the inner wire, and from the wire’s resistance, creates heat, which is transferred through the filler material to the outer sheath and is then absorbed by the water. Elements may be available in different wattages, and materials, to meet specific heating requirements.

When installing a new electric water heater, or following draining the tank for maintenance purposes, the tank should be completely re-filled before applying power to the elements. Energizing a heating element that is not fully submerged in water is referred to as “dry firing,” and will cause the element to immediately burn out.

Control Circuit – The standard single-phase control circuit consists of a high limit control switch with a reset button, upper thermostat, lower thermostat, two heating elements, and wires. The upper thermostat first sends electrical energy to the upper element until the water temperature in the upper third of the tank reaches the thermostat setting. Power is then transferred to the lower element until the remaining water reaches the lower thermostat setting. If the water temperature exceeds 170°F, the high limit control switch will trip, shutting off power to the elements. Single element water heaters have one element mounted at the bottom of the tank, controlled by a single thermostat and high limit switch.

Standard Electric Water Heater Operation

Electric water heaters operate by use of either one or two direct immersion heating elements, controlled by either thermostats or a microprocessor control module, which heat the water in the tank to the desired temperature. Many options exist, including the wattage/voltage of the elements, the type of material of which the elements are constructed, the amount of insulating foam surrounding the tank, in addition to the storage capacity of the tank.

The standard residential electric water heater control circuit consists of a manual reset high limit switch, an upper thermostat, lower thermostat, two heating elements, and wires.

When power is initially turned on to the unit, the upper element is energized and heats the water in the upper third of the tank (see figure 1).

When the upper third of the tank is heated to the temperature set on the upper thermostat, power is switched to the lower heating element. The lower element continues to heat until the water temperature in the lower portion of the tank is heated to the lower thermostat setting (see figure 2).

As hot water is drawn from the top of the tank, the dip tube delivers cold water to the bottom of the tank (see figure 3).

Eventually the cold water mixes with the hot, lowering the temperature to below the lower thermostat setting and the bottom element is energized. If enough water is drawn to cool the upper third of the tank, the upper thermostat will send power to the upper element first. When the upper third of the tank is heated, power will again be switched to the lower element (see figure 4). 

If the upper element burns out, the water heater will cease to function because the upper thermostat will never be satisfied and power will never be switched to the lower element.

If the water temperature in the tank reaches 170°F, the manual reset high limit switch will be tripped. This switch can be reset by firmly pushing the red button above the upper thermostat. (See the image above with reset button.)

Single element water heaters have one element mounted at the bottom of the tank controlled by a single thermostat and high limit switch.