How They Work
Continuous power is provided to the upper thermostat, which supplies power to the lower thermostat. When the water heater is first powered on, the upper heating element turns on first. Once the upper thermostat detects that the desired temperature has been reached, it shuts off power supply and alternates to turn on the lower element. Because hot water rises, the upper 1/3 portion of the water heater will always get hot first. Once the lower thermostat detects that the bottom of the tank is up to temperature, the lower heating element will shut off. The heating elements run non-simultaneously (one heating element on at a time).
As hot water is used, the lower heating element will turn on and off as needed to refill the bottom 2/3 of the tank with hot water. It is only when the top (or final) 1/3 of the water is used that the upper heating element will cycle back on. This is the reason why the lower heating element does most of the work.
A typical 30-, 40-, 50-, or 80-gallon water heater will have two 4500-5500 watt heating elements. Smaller, special-purpose and utility water heaters up to 30 gallons may have a single heating element ranging from 1500-4500 watts.
The two most common materials for heating elements are stainless steel and copper. While copper is a better conductor and therefore transfers heat more efficiently, it tends to corrode much faster than stainless steel. This will vary depending on the amount and type of chemicals and minerals in the water.
Heating Elements
To test resistance you will use a multimeter. Turn the power off to the water heat at the breaker, then after testing to make sure the power is off, you will remove the two wires from the heating element. With the multimeter set to resistance setting, place a test probe touching each of the screws on the heating element.
A reading of 5-15 ohms indicates the element is testing good, though a typical reading for 4500-watt element is 12.8 ohms. The resistance will depend on material and wattage of the element. Open Lead (O.L.) indicates the element is bad and must be replaced.
To test for a short-circuit you will ensure power is off with wires removed from the element. Set your multimeter to resistance. Place one probe touching the water heater tank and the other probe touching one screw on the heating element. Repeat this test one more time touching the other screw. If there is low resistance, there is a short-circuit and the element must be replaced. If the multimeter reads open lead, there is no short-circuit and the element is testing good.
Testing Heating Elements
Testing resistance (Ohms)
Short-circuit test
Amperage test
To test amperage you will need the element to be turned on and running. To accomplish this, you can turn up the thermostat temporarily to cause the water heater to turn on, if needed. If you have a clamp meter, place a single wire to the element inside the clamp. If you are using a multimeter without a clamp, skip this test, as you will not be able to accurately measure amp draw. Most multimeters are either designed for DC voltage only, or have a limit of 10 Amps which is fuse-protected. A 4500-watt element will draw around 18.7 amps.
There are three tests you can perform on a heating element to ensure it is working properly. First is the resistance test, often referred to as "ohm out" the element. The resistance test will tell you if the inner heating element has burnt-out and has broken (like a light-bulb filament). The second test is for a short-circuit, also known as testing to see if the element is "grounded out". The third test is an amperage test to find out if the element is drawing the correct amount of amperage. Checking amperage usually only needs to be done if the water heater is otherwise testing good, but is not working properly.
Element Replacement
Turn off power.
Disconnect both wires from the element
Flush and drain the water heater
Unscrew the element with an element wrench (1.5" socket)
Screw in new element and reinstall wires (order doesn't matter)
Fill tank with water and bleed all air out of tank
Turn power back on and test for proper operation
Replacement of the heating elements is a simple process.
There are two temperature dials, one one each thermostat. They should be set to the same temperature so the water heater works as intended.
When you turn the temperature dial with a flat-head screwdriver your are loosening or tightening a spring clip that controls the distance of a metal disc that acts as a temperature switch. This metal disc contains dissimilar metals. When the disc is heated, it expands and makes contact across the terminals on the thermostat and the circuit is complete. When the disc cools it contracts and the circuit is broken.
The upper thermostat also holds the high-limit switch. This switch activates when the tank overheats or a short causes the contact to fail. This switch breaks the circuit to stop the flow of electricity to the terminals on the thermostats and the elements. This is the red reset button, also called an ECO (Emergency Cut Off) switch. Pressing the reset button should restore the flow of electricity to the thermostats if it was activated due to overheating.
Wiring diagram
Thermostats
Once you have determined that a thermostat has failed, you can do additional troubleshooting to determine which thermostat is not working properly. To do this, you can check for continuity between the contacts using your multimeter by shutting off the power for the water heater, and disconnecting all wires from the thermostats. Use the diagrams provided as a guide, and place the multimeter test probes at each side of the contacts.
Press the reset button then check for continuity across the terminals labeled #1, then do the same for #2. If there is not continuity this indicates the high-limit switch has failed and the upper thermostat must be replaced. Next, test the continuity across the terminals labeled as #3; if they have continuity the upper thermostat is working properly.
If the lower element is running, there is no continuity across the #3 terminals. Instead, there will be continuity across the upper #3 terminal to the #4 terminal. There should be continuity across the #5 terminals. If there is not continuity across #5 terminals, the lower thermostat is bad.
Finally, if there is continuity across terminals that there should not be, this indicates the thermostat has failed and the contact has welded itself shut.
While testing the thermostats you can turn the temperature dial on the upper thermostat to the highest setting and the lower thermostat to the lowest setting to cause the thermostat to try to turn on the upper element. You can do the reverse to cause the thermostat to to turn on the lower element. It is important to know which element is supposed to be running while you are testing for continuity between contacts. Although there will be no voltage because the power is off and the wires are disconnected, you will be able to test for continuity to complete these troubleshooting procedures.
Step 1: Test the two leads for AC voltage coming into the water heater.
Step 2: Press the reset button. If it clicks, it has been reset. Go on to step 3.
Step 3 (reset button clicks when pressed): Perform test on elements to check if either element is grounded to the tank (short-circuit test). If both elements test good this most likely indicates the thermostats contacts are stuck/welded shut and the thermostat is sending power continuously to both elements. Replace the bottom thermostat, and then the upper thermostat if needed.
Testing Thermostats
Quick Guide
Additional Testing
Diagram of flow of electricity while upper element is on.
4
Diagram of flow of electricity while lower element is on.
Thermostat Replacement
Replacement of the upper and lower thermostat is the same, simple process. The key is taking a picture before you begin so that you know where all the wires go.
Turn off the power at the breaker.
Remove every wire from the thermostat.
Pull back the clips holding the thermostat in place.
With clips held back, slide thermostat up and out.
Slide new thermostat into place. Ensure clips are in place.
Hook up all wires exactly as they were.
Set thermostats to desired temperature, and turn power on.
Test for proper operation.
Tools
The qualities you want in an element wrench is enough strength that it will not bend on a stubborn element, and for it to have a face flat enough to be able to grip the element.
In addition to voltage, for a multimeter to be all-purpose for a plumber it should be able to test:
Milli-Amps, DC for testing thermocouples and thermopiles on gas water heaters,
AC/DC amps for testing draw on water heaters, pump and other appliances. Testing larger amp draw generally requires a clamp meter.
Continuity and resistance (Ohms)
You should also have alligator clips for clamping your test leads onto wires to free up your hands.
Troubleshooting
All of the detailed steps you need to properly test an element and thermostat are already listed above, so this section will serve as a guide to help diagnose the problem.
No hot water
Test for voltage coming in to upper thermostat. If there is not 240v coming in to two upper terminals (120v each), check fused disconnect or breaker.
Check reset switch by pressing the red button. If it clicks, it was tripped because the switch overheated.
If reset button clicks, check for a shorted element. If elements are not shorted to the tank, then the thermostat likely has failed due to a faulty contact.
If reset button did not click, check the upper element for proper operation. If the upper element is bad neither element will turn on.
Not enough hot water
Test the lower element. Not enough hot water is usually a bad lower element.
If all tests indicate the lower element is good, test the lower thermostat.
Water is warm, but not hot
Test thermostat settings. If set properly and the water will not get hot, but never runs out of warm water, this indicated the dip tube is broken and the incoming cold water is mixing with the hot water. Replace the dip tube.
Odor problems
If the hot water comes out smelling like rotten eggs, or hydrogen sulfide, this is caused by bacteria growing on the anode rod. Flushing will not fix the problem. Replace the anode rod.