For each element, the unit requires a matching thermostat, so dual element units have two thermostats, and single element models only one.
When replacing one thermostat, it is a good idea to go ahead and replace both the upper and lower thermostats.
Testing the Thermostats on a Dual Element Water Heater
Since most electric water heaters use both an upper and lower element, the process outline below is for them. To test, you will need a multimeter and screwdriver. You can pick up a good inexpensive multimeter online or any local hardware store.
Testing the UPPER thermostat:
- Turn off the power to the water heater.
- Remove the access panels to the elements along with the insulation underneath.
- Use a screwdriver to set the upper thermostat to its highest setting.
- Set the lower thermostat to its lowest setting.
- Turn the power to the water heater back on.
- Make sure there is voltage coming into the water heater by checking the 2 wires above the reset button. It should read 240 volts.
- Use your multimeter to check for power on the upper element terminal screws. If no power, this thermostat is bad and needs to be replaced. If there is power, test the lower thermostat.
Testing the LOWER thermostat:
- Set the upper thermostat to its lowest setting.
- Set the lower thermostat to its highest setting.
- Make sure there is voltage on the lower element. If the multimeter shows power at the element, allow a few minutes for the water to heat up.
- Lower the temperature on this thermostat and wait for an audible click which indicates the thermostat is functioning correctly.
- If there is no voltage at the element, test the lower thermostat for power.
- Place one of the multimeter probes on the top contact screw and the other probe on the metal casing of the water tank. It should show a reading of ~120 volts.
- If there is no reading, the upper thermostat will need replacing since it provides voltage to the lower thermostat.
- If there is a reading, place one probe on the lower contact screw and the other probe on the metal casing of the water tank. It should show approx. 120 volts. If not, the lower thermostat will need to be replaced.
Electric Water Heater Thermostat Replacement
Always disconnect the power or shut off the breaker before doing any work on a water heater. To change thermostats, you will need to remove the access panel and safety cover (do this for both upper and lower access panels on dual element units). For the sake of safety, use a voltage meter to make certain the power is off. Make a simple diagram of how the wires are connected to the diagram. Remove the wires from the thermostat.
Thermostats are held in place by a special bracket that holds it firmly against the tank, allowing the thermostat to sense the internal water temperature. Gently pry out one side of the bracket while twisting that side of the thermostat upwards to prevent the bracket from locking in place again. Repeat for the other side. Repeat for the lower thermostat on dual element models.
Take the old thermostat(s) with you to purchase a replacement. While most thermostats are interchangeable, the best results come from accurately matching the components.
If the bracket was damaged during thermostat removal, you can get a replacement bracket as well. The old bracket can be pried upward to remove it and the new one can be placed flat against the tank and slid downward until it lock into place.
Replace each thermostat by sliding it evenly downward into the holding bracket until it locks in place. Replace the wiring exactly as it was removed. Replace the safety cover(s) and access panel(s). Restore power and allow the water heater to work for one hour before testing the hot water.
If you suspect a heating element has also gone bad, see How To Replace A Water Heater Element.
Gas Water Heater Thermostat Replacement
Gas water heaters use a different type of thermostat system. On most gas water heaters, the gas control valve, which includes both a thermostat and a heat limiting device, are located on the outside of the unit, near the bottom. It can be identified as the component with the controls for temperature and the pilot light.
If the thermostat on a gas water heater goes bad, you will have to replace the gas control valve component.