How to Test Your Water Heater Thermostat

Your first step is to fully power down the water heater to avoid any risk of electrocution. Water and electrical currents do not exactly mix.

Turn off the water heater using the breaker on your main electrical panel. In most cases, this involves flipping the breaker to the “off” position. Your panel should list which breaker goes to which part of the house or device, so scan the breakers for the one that provides juice to the water heater. Flip this breaker off, and you’re good to go.

Even though you just shut the power off, your safety should be your number one priority. That's why you should double-check the electricity in your home to ensure no voltage is present. You can use a voltage tester or multimeter to check for voltage before proceeding to the next step.

Now that there is no risk of electrocution, start testing the water heater thermostats by making them accessible. Head to the heater's access cover. If you don’t know where to find it, check the instructions for the heating system, which should explain all the parts of your water heater.

Use a flathead screwdriver to remove the access panels for both the upper and lower thermostats. If a layer of insulation covers the thermostats, fold them out of the way and secure them with tape for now.

Using a digital multimeter can be slightly confusing until you get the hang of it. For this process, power the multimeter and use the dial or knob to set it to the absolute lowest setting for ohms of resistance. If needed, check the multimeter’s instructions for how to adjust the ohms setting, as some models require unique adjustment processes.

The upper thermostat has a reset button that goes off when the water in the tank overheats. Give this button a quick inspection to make sure it doesn’t require a reset. Press the reset button if necessary but still go ahead with the remaining steps, as you already opened up the access panel and prepared the multimeter.

Even though you turned off the power at the main breaker, disconnecting the power wires will isolate the thermostat from the rest of the system and give you an accurate reading. Disconnect these wires, but take note of their orientation for when you reconnect them later. Use a screwdriver, if necessary, to unscrew the terminals and release the wires. Perform this step for both the upper and lower thermostats.

Set the upper thermostat temperature to its highest setting or maximum temperature. With your multimeter turned on and set to ohms, place one of the leads on the reset terminal, which is the one next to the reset button. Place the second lead on the left-side terminal where the power wires are. Check the multimeter reading. If the reading is zero, the thermostat is functioning as it should. If the reading is 1, the thermostat lacks continuity and will require replacement.

Set the upper thermostat temperature to its lowest setting or minimum temperature. Repeat the continuity test from the previous step. However, this time, place the second multimeter lead on the right-side terminal. Check the multimeter reading. If the reading is zero, the thermostat is functioning as it should. If the reading is anything other than zero, the thermostat needs replacing.

To test the lower thermostat continuity, turn the thermostat temperature up to its maximum. The lower thermostat only has two terminals. Place one multimeter lead on one terminal and the other on the second terminal. If you get an ohm reading of 0, the lower thermostat is fine. If you get no reading, a 1, or any other reading, the lower thermostat requires replacement.

Once you've noted all the readings, you'll want to compare the data with the temperature information from the upper thermostat. If the upper and lower thermostats show opposite readings, the lower thermostat requires replacement.

Finish things up by returning everything you disassembled to its proper location. This includes reconnecting the wires to the thermostats, placing protective covers on each thermostat, closing up the access panel, and restoring power via the breaker.