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How to Test a Water Heater Thermostat and Replace it if Needed

 
 

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Your hot water isn't hot, and the manual says to; check the thermostat. But do you know how to test a water heater thermostat? And, if it proves to be faulty, do you know how to replace your water heater thermostat? All good questions that this article is going to answer!

If your electric water heater isn't heating water, there are two likely reasons. It could be a faulty heating element, or a faulty thermostat. This article will give you step-by-step instructions on how to test a water heater thermostat, and replace it if necessary. It's actually not as hard as it sounds!


Water Heater Thermostat Basics

The electric water heater thermostat is located under an access panel on the front of the water heater tank. 

The thermostat is responsible for keeping the water in the tank hot. It does this by opening and closing contacts in order to turn on and off the flow of electricity to the heating elements which are submerged in the tank and actually heat the water.

There is a bi-metal switch that sits flat against the wall of the water heater's tank. As the water inside the tank is heated, the steel tank gets warmer and heats the bi-metal switch within the thermostat.

If your water heater is set to the recommended 120°F, then once the temperature of the thermostat reaches 120°F the bi-metal switch opens and the flow of electricity is shut down to the heating element.

The opposite happens when the heat drops below the 120°F set point. The bi-metal switch closes, and the electricity is allowed to flow to the heating element.

This video will show you how your water heater thermostat works.

Watch the Video


Identifying Which Thermostat is Faulty

The majority of water heaters with a capacity of 30-gallons or more use two heating elements, each with their own thermostat.

The thermostats are wired so that only one heating element is ON at a time.

The upper thermostat brings the top 1/3 of the water in the tank to the temperature set point. It then shuts down and switches the electricity to the lower thermostat.

The two thermostats are not the same and when one fails, the water heater will react differently.

How to Identify Which Thermostat Failed

  • Upper Thermostat Failure - If your upper thermostat (or heating element) is defective, then the water heater will stop heating water entirely. Since the upper thermostat controls the lower thermostat, when the upper fails, the lower never turns on.
  • Lower Thermostat Failure - If your lower thermostat (or heating element) is defective, you'll either notice the hot water is lukewarm or the hot water quickly runs out. This is because the upper thermostat will signal the heating element to heat the top 1/3 of the water in the tank, but the lower thermostat is faulty and unable to turn the lower heating element on. 

How to Test a Water Heater Thermostat

First, you need to check for continuity. To do this you'll need a Phillips and flat head screwdriver and a multimeter. Before working on your electric water heater, always turn off the power. Then verify that the power is off. Here's how:

Shut Off the Power to Your Water Heater

  • Flip the breaker to OFF at the main electrical panel.
  • Remove the access cover to the thermostat.
  • Remove the insulation and plastic protective cover.
  • Touch one probe of the multimeter to grounded metal (such as the unpainted metal tank housing).
  • Touch the other probe to each thermostat terminal, and both heating element terminals.
  • The multimeter should indicate no voltage (Zero) at all times.
  • Watch the video to visually see how to turn off the power to your water heater.

How to Test a Water Heater Thermostat for Continuity

With your power turned off at the main electrical panel, it's time to test your water heater's thermostat.

Step One

  • Check if the reset button has tripped. If it has, go ahead and reset it. 
  • Remove the wires from the thermostat. Pay attention to how they are connected.

Step Two

  • Touch one of the multimeter leads to the left side terminal by the reset button.
  • Touch the other multimeter lead to the other terminal on the left side of the reset button.
  • The multimeter should read close to zero if the thermostat has continuity. If it does not read close to zero, the thermostat is faulty and needs to be replaced.
  • Repeat the test on the right side terminals by the reset button.

Step Three

  • Test the lower portion of the thermostat, by touching one lead to the common terminal (next to the temperature setting) and the second lead to the left heating element terminal.
  • If the water in the tank is below the thermostat's set temperature, the multimeter should read close to zero ohms of resistance.
  • Move the lower lead from the left heating element terminal to the right heating element terminal. The multimeter should read NO continuity.
  • If the water in the tank is above the thermostat's set temperature, you should see the exact opposite results.
  • If the thermostat is working as it should, you'll need to test the lower thermostat. 

Step Four

  • Disconnect the power wires to the lower thermostat.
  • Touch a lead to one of the terminals, and the second lead to the other terminal.
  • If the water temperature is below the thermostat's set temperature, the multimeter should display close to zero ohms of resistance.
  • If the water temperature is above the thermostat's set temperature setting, the multimeter should display no continuity.
  • An opposite reading indicates a faulty thermostat.

Watch the Video


How to Buy New Water Heater Thermostats

If you've determined that one of your water heater's thermostats is faulty and needs to be replaced, it's a best practice to replace them both. Thermostats can be purchased individually or within a kit.

Buying a Tune-up Kit

You may want to consider purchasing a tune-up kit that includes both the upper and lower thermostats as well as 2 new heating elements. It may cost a few more dollars, but in most cases you'll notice a difference in how your water heater performs.

This is especially a good idea if your water heater has at least a few more years of service.

ZERO EWH-01 Electric Water Heater Tune-Up Kit, Universal type for TOD and APCOM type, Fit for Most Water Heaters

Universal Tune-up Kit

The ZERO EWH-01 Tune-up Kit fits most electric water heaters. It includes 1 upper and lower non-simultaneous water heater thermostat, and 2 screw-in HWD heater elements.


Rheem also offers an OEM tune-up kit for electric water heaters.

If you do decide to replace your heating elements at the same time, make sure that the heating elements are the correct size and type for your water heater. This article will help you find the right heating element.

Buying Upper or Lower Thermostats

Although it's always a good idea to replace both thermostats at the same time, sometimes it simply makes sense to replace the faulty one.

This is especially true if a new water heater is in your future, and you're simply trying to get your current water heater up and running until you can purchase a new one.

Camco 08163 Upper Thermostat, Black

Camco Upper Thermostat

The Camco 08163 Upper Thermostat has a build-in reset switch, adjustable temperature settings, and comes with a 1-year warranty.


If you prefer, Rheem also manufactures an OEM upper thermostat.

If you need the lower thermostat, the Camco 8123 is a good inexpensive option, and of course, Rheem offers one as well

Single Element Water Heaters

We've focused on double element water heaters, but some, especially tanks 20 gallons or under, will heat the water with a single heating element. These water heaters are wired differently and require a single element thermostat.

They may look very similar to an upper dual element thermostat, however, they have fewer wire terminals. 

Camco 07843 Single Element Water Heater Thermostat with HLC

Camco Single Element Thermostat

The Camco 07843 Single Element Thermostat has a build-in safety switch, adjustable temperature settings, and includes a protective cover.


How to Replace a Water Heater Thermostat

Replacing a water heater thermostat is very easy, and you should be able to do it yourself without any problem. However, many homeowners feel more comfortable hiring a professional plumber to handle the job.

You'll need a Phillips and flatbed screwdriver, a multimeter, as well as your new thermostat. Be sure you've purchased the correct thermostat for your water heater.

Shut Off the Power to Your Water Heater

  • Flip the breaker to OFF at the main electrical panel.
  • Remove the access cover to the thermostat.
  • Remove the insulation and plastic protective cover.
  • Touch one probe of the multimeter to grounded metal (such as the unpainted metal tank housing).
  • Touch the other probe to each thermostat terminal, and both heating element terminals.
  • The multimeter should indicate no voltage (Zero) at all times.

Step One

  • Disconnect the thermostat wires. Pay attention to how they'll be reconnected.
  • Carefully lift the thermostat out of the retaining bracket.

Step Two

  • Replace the old thermostat with a new one. Verify it is the correct thermostat for your heater.
  • Check that the back of the thermostat is positioned tightly against the tank.
  • Reconnect the wires to the thermostat.
  • Replace the protective cover.

Step Three

  • Check and adjust the temperature setting to 120°F.
  • Replace the insulation and access cover.
  • Turn the water heater power ON.

Watch the Video


Red questions on a white background

Common Questions

Here are some common questions homeowners have asked regarding water heater thermostats:

Are Electric Water Heater Thermostats Universal?

No. Some water heaters use two heating elements and others heat the water with a single element. These are not the same, so you will want to make sure you purchase the correct type for your water heater.

Also, in dual element water heaters, the lower and upper thermostats are not the same and can not be used interchangeably. The upper thermostat is larger and has more terminals than the lower.

It's always a best practice to check the information tag on your water heater to make sure you purchase the correct thermostat. There are dozens of residential electric water heater manufacturers and most can use OEM parts, but taking a few extra minutes upfront can save you time, money and frustration later.

How to Reset a Water Heater Thermostat?

If your thermostat tripped, you can open the access cover on the front of the tank and remove the insulation. There should be a red button on the thermostat. Simply push it in to reset. If it trips again you have a more serious problem (see below). 

What Causes a Thermostat to Trip?

There are several reasons a thermostat can trip. The most common are a faulty thermostat or heating element, but it can also be caused from a loose electrical connection, or even a faulty reset button. If you are having difficulty identifying the cause, you should contact a professional plumber.

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