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How to Test a Water Heater Thermostat With a Multimeter

how to test a water heater thermostat

Is your water heater not delivering hot water anymore, or does water turn cold after a few minutes? It could be a problem with the heating elements or the thermostat. Determining the culprit is possible if you know how to test a water heater thermostat.

The thermostat controls the water heater’s preset water temperature, giving you hot water with a consistent temperature. Unfortunately, it can also go bad in the long run. Performing an electric water heater thermostat troubleshooting can help evaluate the current thermostat status.

This guide shows you how.

Things You Need for This Tutorial

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Testing water heater thermostat requires a few things.

  • Owner’s Manual

It would be best to learn the thermostat’s location in your water heater before testing it with a digital multimeter. You can check the owner’s manual for an illustration of the water heater’s different components to determine where is thermostat on hot water heater.

Most electric water heaters have two thermostats, which control the water temperature in the upper and lower tank sections. The thermostats will have an access panel cover protecting them against the elements.

  • Digital Multimeter

You can use an analog multimeter if you want because it is inexpensive. However, I prefer a digital device because I can read the results through its large LCD screen.

Analog devices have a needle that points to the value range. If you have visual problems, you might not be able to read the results accurately.

  • Other Tools

Although not mandatory, I recommend getting a non-contact voltage detector (people call it a voltage stick). You need this to double-check the water heater’s electrical power status before proceeding with the thermostat testing.

Another indispensable tool is the screwdriver. You might want to check the manual for the fastener type securing the wires to the thermostat. Most units have slotted screws, requiring a flat-tipped screwdriver.

Steps for Testing a Water Heater with a Multimeter

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Step 1. Cut off the water heater’s power supply.

Most electric water heaters do not have a manual power on/off switch because they have their dedicated electrical connection to a circuit breaker.

Switch off the breaker that connects to the water heater at the circuit breaker panel. You should see a label alongside the breaker, depending on who installed the system.

If there are no labels, you might not have any choice but to turn off the entire circuit breaker panel. You can also try switching off the breakers one at a time until you know the water heater is turned off.

Step 2. Access the water heater thermostats.

Use a screwdriver to loosen and remove the screws fastening the thermostat access panel covers. Open the upper panel first before detaching the lower thermostat compartment.

Each thermostat will have a plastic insulating cover and a cutout flap on top. Flip the cutout flap down or up and secure it with tape. Use a flat-tip screwdriver to peel off the plastic insulating cover. Remove the insulation material and keep it safe.

Pro Tip: Use a non-contact voltage detector to check the electric current in the thermostats after removing the plastic insulating cover. If the device detects voltage, double-check the circuit breaker panel and retest.

Do not proceed with testing hot water heater thermostat with the water heater powered on.

Step 3. Prepare your multimeter.

Plug the black male plug into the multimeter’s COM port, and insert the red plug into the Voltage port (marked VΩmA). Turn the multimeter dial to the Resistance Mode (marked ΩnS), and set the range according to the water heater thermostat’s Ohms rating.

If you are unsure about the water heater’s resistance rating, you can set the multimeter’s range value higher. You can always reset the multimeter to a lower range later. If the device reads OL, the range value is too low.

You can also use a multimeter with an Auto-Range feature.

Step 4. Prepare the thermostats.

Thermostats have a built-in safety feature (the reset button), allowing them to cut off electrical power (tripping the power) in an emergency. Push this button to reset it and ensure there is no electricity running through the system.

Isolate the thermostats by unscrewing the fasteners connecting electrical power wires to the thermostat terminals. Note the wires’ orientation to help you during replacement.

Step 5. Test the thermostats with the multimeter.

You are now ready to check hot water heater thermostat with your multimeter. This step can be challenging for first-timers, but we will go through each process one at a time.

First, touch the reset terminal with a multimeter probe. Do not worry about which electrode to use because electrical resistance is not directional. You can find the reset terminal next to the thermostat’s reset button.

Next, place the other multimeter electrode on the thermostat’s left side terminal, ensuring to maintain a distance from the first probe.

Third, read the value in the multimeter. You will want the electric water heater thermostat test to return a zero (0.00) or close-to-zero (0.001) value, indicating your thermostat is still functional. However, if the multimeter reads “1.00,” you might have a busted thermostat that requires replacement.

Lastly, reposition the second lead to the thermostat’s right-side terminal, while keeping the first probe on the reset terminal. Read the results and interpret accordingly (remember, 0 is good, 1 is bad).

Step 6. Test the water heater’s water temperature settings.

Sometimes a working thermostat does not regulate the preset water temperature. For example, you might dial 120 degrees Fahrenheit, but the thermostat sets it to 110 degrees or 130 degrees.

Position a multimeter lead on the thermostat’s left common terminal. You can find this in the thermostat’s lower half.

Next, touch the second probe on the thermostat’s upper heating element post, and read the result. If it shows 0.00, your water heater heats the water at a lower temperature than the preset temperature.

Third, reposition the second electrode to the lower heating element post. If the reading is 1.00, the water heater heats the water higher than the preset temperature.

Repeat the process for the lower thermostat, noting the results.

Remember, 1.00 indicates a water temperature higher than the preset value, while a 0.00 result shows the opposite (the water temperature is lower than the preset temperature).

Step 7. Compare the readings for the upper and lower thermostats.

Analyze the results for the upper and lower thermostats. Ideally, they should be similar. If not (i.e., lower thermostat value = 0.00, upper thermostat value = 1.00), the lower thermostat is almost always defective.

You might want to replace the lower thermostat before wrapping it up.

Step 8. Adjust the thermostats.

I recommend adjusting the thermostats before completing the process of checking thermostat on water heater.

You should see a dial next to the thermostat. Turn the dial with a flat-tip screwdriver to compensate for the readings. If the result is 1.00, you must turn the dial to the left to decrease the water temperature. Do the opposite if the value is 0.00.

Step 9. Put back the water heater components.

Re-secure the power wires to the thermostat terminals, and reinstall the plastic insulating material. Put on the cutout flap and reposition the access panel over the thermostat assembly. Tighten the panel screws.

Conclusion

The process of how to test a water heater thermostat is pretty straightforward, requiring only the opening of the access panel and removing the power wires before testing. You can execute this procedure in thirty minutes or less.

If you find this guide advantageous in maintaining your water heater, would you share it with other people? I will be happy to entertain any questions or comments you might have related to this article.

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