How to Replace a Water Heater Thermostat on the Electric Models: DIY Tips

Learn how to replace a water heater thermostat on the electric models fast and easy. See how does a thermostat work, explore the types, common problems, and how to test and troubleshoot it.

How to replace a water heater thermostat: Things to consider

What is a water heater thermostat?

Water heater thermostats are important parts of both electric and gas heating devices. These adjustable thermostats are designed to set and control the temperature of the outgoing hot water.

In electric models, thermostats are surface mounted, installed in a bracket to hold the element against the side of the tank.

The back surface of the thermostat is in full contact with the tank, and therefore it responds to tank surface temperature change. On the electric tank-type heaters, thermostats are located behind the access panels.

What is the purpose of a water heater thermostat?

Every electric water heater has at least one heating element for water heating, one thermostat for the temperature control, and one high limit switch to protect the unit from the excessive temperature.

Different hot water temperature is needed for various applications. Hot water is needed for the shower, dish-washing, clothes washing, and this is why a hot water heater thermostat is used to set or adjust the temperature.

The purpose of the thermostat is to control the electrical current that is sent either to another thermostat or a heating element when there is a request for a temperature change.

Electric water heaters with a storage tank of 30 and more gallons are mainly equipped with two heating elements and two adjustable thermostats, one on each heating element. The upper thermostat is the main one and has a high-limit switch attached. The lower thermostat is the first one that senses a change in water temperature.

Upper thermostats are not the same as the lower ones. They also do not operate simultaneously.

The high limit switch is located in the same area as the upper thermostat, and it has a manually reset (ECO) button that trips when the extremely high water temperature is present (above 170 F). By pressing the ECO button (reset), the unit is ready for normal operation, but if it often trips, more investigation should be done.

When adjusting the thermostat on a hot water heater, the recommendation is to set both thermostats to almost the same temperature or set the top element to a slightly lower temperature, so the bottom element activates first and heats the water from the bottom up.

Water heaters that are built with the lower tank capacity and for point of use service utilize only one heating element and one thermostat - usually wired for 120 V. They also have a high limit switch.

How do water heater thermostats work

If the water inside the heater's tank is cold, the upper thermostat activates and sends the power of 240 V to the upper heating element to heat the water. The electric element heats the water as long as there is a need or until the upper thermostat is satisfied.

Water is now hot, but only in the upper part of the tank. This is good if there is high demand for hot water. However, what is happening with the bottom part of the tank, where water is still cold?

As the upper thermostat is satisfied, the power gets redirected to the lower thermostat. Once the bottom heating element gets 240 V, the heating process starts. Now, when the bottom thermostat becomes satisfied, and the set temperature is reached, the power gets interrupted, stopping the process.

Now, all the water in the tank is hot.

How to set the temperature on a thermostat

The temperature range on different brands and water heater models is very similar. It usually goes from around 100 F to 140 F. When shipped to the customer, units have a factory pre-set temperature of 120-125 F. This is a safe and energy efficient setting that protects users, especially kids and seniors, from scalding water.

It is important to know that if exposed to hot water of 120 F, it takes more than 5 min. to produce burns, for 130 F less than 30 sec and for temperatures of 160 F less than a second.

To change the temperature setting on electric water heaters, follow the steps below.

Adjusting the thermostat on a hot water heater is easy, and you should do it only when the power is off.

• Disconnect all the power to the unit on the main switch of the fuse box or the main circuit breaker.
• Remove the access panel, cover, and fold the insulation away from the controls to clear the way to the controls.
• Using the screwdriver, rotate the adjustment knob to the desired position (temperature).
• Bring the cover, access panel, and insulation back and turn on the electrical supply and the water heater.

Troubleshooting thermostat problems

No hot water

• Check and replace the upper thermostat if it is defective.
• If the high limit switch has tripped, manually reset an ECO button.
• Check and replace the thermostat if it is grounded.
• Thermostat out of calibration - calibrate if possible; if not, replace it.

Not enough hot water

• The thermostat is set too low, increase the temperature.
• Check and replace the malfunctioning lower thermostat, or if improperly wired.
• Check if the thermostat is flush with the tank. If not, make sure it is fully and tightly attaching the water heater.

Slow hot water recovery

The lower thermostat is defective - replace it.

Water is too hot

• If the thermostat setting is too high, lower the temperature.
• Make sure the thermostat is flush with the tank.
• Check if the thermostat is out of calibration and replace it if needed.

Protection against high hot water temperatures

There are few ways to protect you and others from accidental burns.

One solution is to install the temperature limiting valve that attaches to faucets to limit hot water flow or install a mixing valve to reduce the hot water temperature by mixing cold and hot water.

How to test a thermostat

Testing heating elements and thermostats usually go hand in hand, as some tests are not accurate when the heating element is open or grounded.

To perform the test, you need tools such as the screwdriver and multimeter.

The guide below is for checking the electric water heater from the Rheem/Ruud manufacturer, and it can be used for other brands as well.

• Check the power to the water heater using the thermostat terminals. If there is power, the meter should read 240 V (terminal 1 and 3). If there is no power, check the power supply.
• Check the power to the high limit switch (terminals 2 and 4).
• Next, turn the dial on the lower thermostat to its lowest setting, so it doesn't activate, and increase the temperature on the upper thermostat, so it kicks in (but only if the water is cold or temperature is below the set one). This procedure is to check the upper thermostat.
• Check if there is power between the upper thermostat and a heating element by placing the meter prongs on terminal 1 and the blue wire of the heating element. If the meter shows 240 V, that means there is power between them. Repeat the procedure, but instead, terminal 1 uses terminal 2.
• If there is no power, the thermostat is broken and should be replaced.

Next is to check the lower thermostat.

• Turn the temperature dial on the upper thermostat to its lowest setting and to its highest on the lower thermostat, so it calls for heat.
• Put one probe on terminal 1 and red wire of the lower heating element. The power is being supplied if the reading shows 240 V.
• Now put the probes on terminal 2 and red wire of the lower heating element. If the power is not supplied, the thermostat is defective and should be replaced.

How to replace a water heater thermostat on electric models (video included)

Note: Fixing a thermostat on a water heater is not recommended – it is better to replace it. Get a new one which is of the same type and has the same specs.

Changing a thermostat is a DIY project and here you will find an easy-to-use replacement guide both upper and lower thermostats:

• Turn OFF the electric power to the unit.
• Remove the front panel door and fold up the insulation pad.
• Remove the plastic cover over the thermostat.
• Using the multimeter, check once again if the power is OFF.
• Unclip the thermostat from the bracket.
• Disconnect wires from the thermostat. Label wires for proper re-connection to the new element.
• Remove the water heater thermostat.
• Install the new thermostat into the bracket.
• Attach wires to the thermostat.
• Set the thermostat to the original temperature setting.
• Cycle test the heater to check that everything works properly.
• Bring the removed elements back.
• Restore the power to the unit.
• Run the unit.

It is recommended to check, clean, or replace the heating elements as well.

When bringing the thermostat back, make sure it is firmly attached to the surface of the tank (flush). Behind the access cover, there is a wiring diagram; use it to properly re-connect wires to the thermostat.

If your electric device is equipped with two heating elements and two thermostats, the lower ones will operate more frequently, have exposure to the lime build-up and fail more often than the upper ones.

When looking for water heater thermostats, it is important to know the voltage of the heating element. Some technical data can be found on the back of the element or the main energy guide label.