Informational Guide

How To Test Water Heater Thermostat

by Andrew

If your hot water isn't working, it can cause quite a stir. Most manuals will tell you to 'check the thermostat,' but to the average Joe, this can be quite tricky. An electric water heater comes with two thermostats; both control different heating elements. How exactly do you check to see if they’re working?  

If you need to check your thermostat, you’re in the right place. Follow this guide for a step-by-step on how to test a water heater thermostat, and get your hot water flowing again.  

Common Reasons For Testing Thermostats In Hot Water Heaters 

The thermostats are located on the side of the water heater. They’re responsible for controlling the temperature of the water, making sure it reaches 120F. There’s also a high limit switch, which will be triggered if the temperature goes over 170F.  

The thermostats control the heating elements inside of the tank; the upper thermostat controls the lower. The switch is shut when the water needs to heat, and vice versa. If you’re not getting hot water, there could be several reasons: 

water heater
  • Loose/Bad Wiring 
    If one of your wires becomes loose, then it poses a serious hazard. The trip switch (or reset button) prevents any problems before they occur, and this could be a reason it stops working.  
  • Bad Thermostat 
    The two thermostats in your tank communicate to one another to determine which is on and off, so they don’t work at the same time. If one is bad, it could stay permanently on, causing the trip switch to activate once the temperature is too high.  
  • Burnt Heating Element 
    The heating elements have a metal coating that heats the water up. If they burn out, the metal cracks and exposes wires to water. This means the water will continue heating up even after the thermostat is off.  You can check your thermostat for any of these problems with a simple test, which we will explain how to do below.  

How To Test A Water Heater Thermostat With A Multimeter 

This is the guide on how to test electric water heater thermostat with multimeter. To test your water heaters thermostats, you’ll need two tools: a flathead screwdriver and multimeter. 

The multimeter will read your thermostats with ohms (Ω) of resistance; this is how much current is flowing through the opposition, such as insulators. All materials will resist a current somewhat.  

It’s also very important to turn off the power supply to the water heater. Failing to do so can cause a serious hazard to you.  

  • Step 1: Locate the breaker and thermostats on your water heater. 
    This is also called the trip switch, the reset button, etc. The thermostats are on two panels, one above and one below, that cover them.
  • Step 2: Remove access covers. 
    You’ll need to unscrew the panels from the side of the water heater. Use the flathead screwdriver to unscrew the access covers, and place them aside. You’ll also need to remove the cutout flap, which covers the thermostats. Finally, unscrew the plastic covers on the thermostats and place them somewhere safe. 
  • Step 3: Set your multimeter up. 
    Make sure it’s on the lowest setting for ohms of resistance. You’re checking to see how much resistance there is from the wiring. 
  • Step 4: Inspect trip switch. 
    This will be on the top thermostat. If the water heater has tripped, you'll need to reset it by pressing the trip switch. Before moving on, you need to remove the wires from the thermostat. Make sure you note how they're connected so that you can reconnect them when you are finished. 
  • Step 5: Check the readings. 
    Once the wires are removed, place one prong on the reset terminal. Then place the second prong on the other side, the one that had the wire running into it. If the ohms of resistance reading is above 1, your thermostat is faulty and needs replacing. You want the resistance reading to be as close to zero as possible. Do this again to the other side, which had a wire through it.
  • Step 6: Checking the elements. 
    Place one prong on the common terminal, whichever side the wire came from. The common terminal is usually found in the lower half of the water heater. Place the other prong to the upper heating element terminal. If the water temperature is lower than it should be, the first reading will be 0 ohms, followed by 1 ohm.  Repeat this on the lower heating element terminal, and if the water is higher than it should be, readings will be 1ohm followed by 0 ohms.  
  • Step 7: Check the lower thermostat. 
    Repeat the readings check on the upper thermostat, connecting one prong to the reset terminal, and one on the side you removed the wire from. If the water temperature is lower, then it'll read 0 ohms; higher will read 1 ohm.
  • Step 8: Check your readings are accurate. 
    If the upper and lower thermostats show opposing readings, then your thermostats are faulty, or your heating elements have burned out, and you’ll need to replace them. 
ohmmeter

Can You Test A Water Heater Thermostat Without A Multimeter? 

It is not recommended to test the thermostat of a water heater without a multimeter. You should not test the electric currents with anything other than a multimeter, as this can be a serious hazard. There is a simple test you can do without checking any of the electrics.  

Run one of your hot water faucets, and get a feel of the temperature. You can do this with your hands or with a thermometer if you so choose.  

  • The upper thermostat is damaged or burnt out if the water comes out cold and doesn’t heat up.  
  • The lower thermostat is damaged or burnt out if the water starts out hot but quickly becomes cold again. 

In both of these circumstances, if you don’t own a multimeter, it’s best to call in a professional to help determine the cause of the problem. If you’re ever unsure,  you’re always better off having someone take a look at it.   

Testing the Thermostat Of Popular Water Heater Models 

A.O. Smith Water Heaters 

A.O. Smith recommends turning off all electricity supply to the heaters and removing the power wires. The steps on testing remain the same, but you will find the terminals on the bottom of the thermostat rather than on the sides. The terminals will need to be unscrewed a little to access the wires.  

Rheem Water Heaters 

You should follow the instructions above as directed, and follow any necessary precautions to avoid injury or death. Before checking the terminals on the thermostat, Rheem recommends checking there isn’t a malfunctioning reset button that could be causing your heater to trip. If you’re satisfied that it’s not that, you should follow the instructions above.  

rheem water heater installing

Atwood Water Heaters 

There are two different thermostats attached to an Atwood model. One is labeled 'T’Stat,’ which is the propane gas control. The second is E.C.O., which stands for ‘Emergency Cut Off,’ which turns off the heater when the water is too hot. To test both, you will find the heating element terminals and power lines underneath each label, and you can easily test both with the multimeter.  

Suburban RV Water Heaters 

The thermostats on a Suburban model are next to each other, enclosed in a plastic cover. While removing the wires, you should make sure you know where each one goes, as cross wiring can be dangerous. The terminals are easily accessible under the casing and can be tested with a multimeter using the instructions above.  

People Also Ask (FAQs)

How long do water heater thermostats last? 

You can expect approximately 8 - 12 years of use from your thermostats before they will need replacing. If you notice any issues with its use during this time, it’s best to test it or call a professional.  

What happens if the water heater thermostat goes bad? 

The lower heater thermostat is controlled by the upper, so if the upper one is malfunctioning, the lower will never switch on. You’ll be able to tell this if there’s no hot water. If the lower thermostat is not working correctly, the hot water will go cold quickly.  

What are some reliable thermostat brands to buy? 

A.O. Smith is one of the first companies to be mentioned in regards to water heaters. Their company is reliable and will provide solid working water heaters. Rheem is also reliable, producing high-quality and innovative water heater thermostats. Bosch is the forefront in technology and produces tankless water heaters that are efficient.  

Conclusion

Losing your hot water is infuriating, and now with the tools to test your thermostats yourself, you can determine the issues with your tank and whether it’s simply the case of a power surge or something more serious. You should always take necessary precautions when testing, and if you're ever unsure, or it doesn't look safe, there is no shame in calling a professional. Good luck! 

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