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
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
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