Beginner’s Guide: How to Test a Water Heater Thermostat

Josh Miller

Josh is a contributing writer to the site and former HVAC specialist.

Last Updated on June 3, 2021

When a water heater isn’t working correctly, it’s helpful to know how to fix it. There are two main reasons why water heaters malfunction. Usually, there’s either a problem with the heating elements or the thermostats.

Tank water heaters have one or two thermostats. Knowing how to test a thermostat can help you find the component that’s not working. Once you identify the part, you can replace it with one that’s brand new.

If you have never tested a water heater thermostat, the task can seem challenging. However, once you learn a bit about thermostats, testing them isn’t difficult, and you can do it in a few simple steps.

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Table of Contents

Where are the Thermostats

Most modern and electric water heaters have two thermostats, one at the top and one at the bottom. When you look at an electric water heater, you’ll notice two panels on the side, one above and one below. Behind each panel, there is a thermostat and a heating element.

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The Different Parts of a Thermostat

The upper thermostat is slightly larger because it’s the central thermostat on the water heater. It monitors the tank’s temperature, connects to the upper heating element, and it also controls the flow of electricity to the lower thermostat. The lower thermostat only monitors temperature and is connected to the lower heating element.

The top thermostat has two modules. The module on top is called the thermo reset module. You’ll notice the thermo reset module has a red button in the middle. You can press this button to reset the thermostat back to factory settings. The part right below the reset module is the temperature controller, and it also has a terminal that connects to the lower thermostat.

The thermo reset module is also an important safety feature. It makes sure the temperature is always in a safe zone. If the temperature starts getting too high and exceeds 180°F, the high-limit switch trips and stops the water heater from working. This causes the red button to pop out, and you have to reset the settings.

How Do Thermostats Work on a Water Heater

When learning how to test a water heater thermostat, it’s crucial to understand how thermostats work.

Thermostats work by opening and closing terminals that allow voltage to flow to the heating elements. Many people don’t know that the two heating elements on an electric water heater never work simultaneously.

The electric wires come in through the top of the water heater and send electricity to the reset module on the top thermostat. As long as the temperature is inside a safe zone, the electricity goes onto the temperature controller on the top thermostat and powers the upper heating element.

Heated water is less dense than cool water and rises to the top of the tank. When the water around the upper heating element has become hot, the thermostat notices, and it then starts sending electricity to the lower thermostat. When the bottom water gets hot and reaches a specific temperature, both thermostats stop working

How to Test a Water Heater Thermostat

When learning how to test a water heater thermostat, it’s essential to know how to use a continuity meter and perform a continuity test. This allows you to know if there’s any electricity flowing through specific parts of the thermostat.

Test the Thermostats in 6 Simple Steps

In addition to the continuity meter, you’ll also need a screwdriver. Testing the thermostats won’t take longer than 20 minutes, and you only need to follow these simple steps. 

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Step 1: Turn off the Water Heater

The first thing you should do is shut off the power supply to the unit. You can do this by turning off the breaker switch or by unplugging the water heater from the wall.

Step 2: Remove the Upper Access Panel

Using a screwdriver, remove the upper access panel. On most water heaters, there’s a piece of spray foam insulation. You’ll need to remove it to access the thermostat. Below the insulation, there may also be a plastic protective cover.

Step 3: Disconnect the Wires

You’ll notice there’s a total of 7 screws on the thermostat. These are the thermostat’s terminals, and they are the points through which electricity flows. Notice the wires connected to the terminals.

You should disconnect all wires from the thermostat. You can do this by slightly unscrewing the terminals and pulling on the wires. Disconnecting the wires isolates the thermostat from the electric circuit.

Step 4: Test the Thermo Reset Module on the Upper Thermostat

Start by testing the thermo reset portion of the thermostat. Touch the two terminals on the left with the two leads from the continuity meter. If there’s continuity, the meter should read close to zero ohms of resistance. If the analog needle doesn’t move at all, or the display on the digital display doesn’t change, there’s no continuity. This means the thermostat is defective.

Repeat this test on the right-side terminals of the reset module. Once you have tested the four terminals on the reset module, test the lower portion of the thermostat. You’ll find three terminals there.

Step 5: Test the Lower Part of the Upper Thermostat

There should be two terminals on the left and one terminal on the right. Place both meter leads on the two right terminals. If the water is below the indicated setting, the meter should indicate close to zero ohms of resistance. Move one lead to the terminal on the right for the lower heating element, and the meter should show no continuity.

If the water is above the designated setting, the reverse should happen. When placing the leads on the upper left terminal and the right terminal, the meter should show continuity. There shouldn’t be any continuity when placing both leads on the left-side terminals.

Step 6: Test the Bottom Thermostat

On the bottom thermostat, you’ll only find two terminals. If the water is below the designated settings, there should be almost zero ohms of resistance. If the water is above the designated setting, then the meter should display no continuity.

Replace a Broken Water Heater Thermostat

Once you have identified the problem, you can replace the damaged thermostat with a new one. For extremely hot water, it’s usually recommended to change both thermostats, even if you only notice a problem in one. 

When installing the new thermostats, make sure to shut off the power completely.

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