Testing Thermostats on Electric Hot Water Heaters
Problems with hot water temperature can usually be traced back to the thermostat or heating element. Symptoms can be either very hot water or lukewarm water, or rapid temperature swings. With a little instruction most homeowners can diagnose a faulty thermostat without having to hire a plumber. Larger hot water heaters have two heating elements, called dual-element hot water heaters. These heating elements are controlled by two thermostats. The lower thermostat heats your water as it fills the tank while the upper thermostat works as a backup to make sure the water stays at a desired temperature.
Duel Element Hot Water Heaters
Contrary to popular belief, both elements do not heat the water at the same time. The upper element will heat the cold water as it enters the tank. When the upper thermostat is satisfied the
upper element will shut down and the lower element will begin heating. If both thermostats were on at the same time, the current draw would be about 40 amps and the circuit breaker would trip.
When you use hot water the water at the lower portion of the bottom of the tank cools and the thermostat energizes the lower heating element. If the hot water is used faster than the bottom element can heat the water, the top thermostat will drop below its setpoint and it will turn off the bottom element and energize the top element to heat the water leaving the tank.
Dual Element Hot Water Heater
240V Single Element Thermostat
240V Dual Element Wiring Diagram
This testing will not be able to determine the status of the thermostats if the water is overheated. Overheating is typically caused by a grounded heating element.
Phillips and Standard Screwdrivers
It is suggested that the entire guide be read before testing the thermostats. Use caution when working with energized equipment. Never touch the terminal screws with anything other than the test probes on the multimeter.
Testing the Upper Thermostat
1. Turn the water heater power off. Remove both access panels, insulation and plastic safety covers.
Thermostat and Heating Element located below access panel.
2. Check to ensure that the reset button (red) has not tripped. This is a high-limit switch that is located on the thermostat. If the red button has popped out, you can usually push it back in to reset the thermostat. If it is tripped, or it trips repeatedly there may be a problem with the heating element.
Use a small screwdriver to set the upper thermostat temperature to the highest setting.
3. Set the lower thermostat temperature to the lowest setting.
4. Turn the water heater power on. Check the two wires above the reset button to be sure there is voltage coming into the water heater.
Larger water heaters, 40 gallons and up, are supplied with 240 volts. Voltage at the elements on this water heater should be around 240 volts.
5. Place the multimeter probes on the upper element terminal screws. If you do not have power at the element the thermostat is bad.
If you do have power, check the lower thermostat.
Testing the Lower Thermostat
1. Set the upper thermostat temperature to the lowest setting.
You should hear the thermostat click off as you turn the dial. If you don't hear it, give the water
time to heat up and then continue.
2. Set the lower thermostat temperature to the highest setting.
3. Check the voltage on the lower element. If you have power at the element allow the water to heat up. Lower the temperature on the thermostat. If you hear an audible click, the thermostat is good.
If you do not have power at the element go to the next step.
4. Test the lower thermostat for power. Place one multimeter probe on the top contact screw and the other probe on the metal shell of the water heater tank. It should read around 120 volts. If you do not get a reading you will need to replace the upper thermostat as the upper thermostat is delivers voltage to the lower thermostat.
If you have power go to the next step.
5. Place one meter probe on the lower contact screw and one probe on the shell of the water heater. You should have 120 volts, if not, replace the lower thermostat.