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Newbern Hardware and Furniture

230 East Main Street  Newbern, TN  38059  (731) 627-3219  Store Hours are 7:30am - 5pm Mon -Sat
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How Does an Electricial Water Heater Work?

This is one of the most often asked questions in our store along with questions regarding why the water heater does not perform.  This page is designed to allow the homeower understand the principles regarding these units.

A Few Words of Caution

This page is merely to be used as a guide and is not intended to be all inclusive.  It is your responsibility to consult with a licensed and trained professional when working on equipment such as a water heater.  Many areas require licensed professionals to perform this operation.  If in doubt, consult your electrical inspector.

Not understanding electricity can be dangerous and without proper safety techniques, electricity can KILL you.  Electrical safety can be achieved by turning off the breaker or removing the fuse at the power panel supplying the water heater. 

Absolutely never engergize a water heater without the tank COMPLETELY full of water.  Energizing a dry tank will certainly damage the element(s).  You must remove the trapped air in a tank for it to be completely filled.  This can be accomplished by opening the hot water at a faucet and opening the supply water to the tank.  Purge water through the unit until all the air is removed from the tank

Fundamentals of the Water Heater Circuit

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 (resistance heating) the water in the upper third of the tank.  This is the typical wiring becuase this will deliver the quickest supply of warm water tbecause water is removed from the top of the tank first.  Once the upper thermostat is satisfied, the lower element will begin to heat.

As water is heated, the density of the liquid is reduced.  This creates a natural circulation of hot and cold water within the tank.   Eventually the temperature in the upper section of the tank will satistfy the upper thermostat and then the bottom thermostat/element is energized. At this time the upper element will no longer heat.   If enough water is drawn from the tank to cool the upper third of the tank, the upper thermostat will send power to the upper element first thereby shutting off the lower element. When the upper third of the tank is reheated, power will again be switched to the lower element.  This wiring scheme allows for only one element to heat at a time.

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. The majority of the time this is the cause of the problem.  The symptoms include warm water and/or very limited supply of hot water. 

Note:  If water temperature in the tank reaches 180°F, the manual reset high limit switch will be tripped and the unit will not heat. This switch (located on the upper thermostat) can be reset by firmly pushing the red button.
Control Circuit

The standard single-phase control circuit consists of an upper thermostat with integrated high temperature switch, a lower thermostat, two heating elements and wires. (Diagram 3).  Voltage between L1 and L2 should be 220 V.  Voltage between any lead and ground will be 110 V.

Provided the high temperature reset switch is not tripped, 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. This is bridge 1 to 2 on the thermostat.  Once the water temperature  reaches the desired the set point,  Bridge 1 to 2 is broken and Bridge 1 to 4 is made.  Once this is achieved, the lower element is fully energized until the remaining water in the tank reaches the lower thermostat setting.  If at any time the water temperature exceeds 180°F the high limit control switch will trip, shutting off all 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.

Troublshooting the thermostats, elements, and reset switch with your multimeter will be made easier provided you understand the flow of the electricity and the gate logic.  You can determine if the elements are faulty by removing the wiring and testing across the elements measuring the resistance of the element.  You cannot properly perform a continuity test on the element(s) while it is connected to the thermostat.    A new element in air will typically have 10-15 ohms of resistance.