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Water Heater Thermostats – Problems, Testing, Replacing

Note: This post may contain affiliate links. This means that at no cost to you, we may receive a small commission for made purchases.

Testing, replacing, and repairing a water heater thermostat can be difficult if you do not know what you are doing and going in blind.

But just with some simple instructions, the process can be made quick and easy.

This guide will give you the information you need to recognize problems, run tests and replace your water heater thermostat!

What Is A Water Heater Thermostat?

A water heater thermostat is an adjustable device that is attached to your water heater to control, change, and regulate the temperature of your hot water. 

They are usually mounted to the surface of your water heater and placed on a bracket that holds the device to your hot water tank.

This makes it, so the backside of your thermostat is fully connected to your hot water tank and is designed to react and respond to the temperature of this surface.

Water heater thermostats that are placed on the tank are usually located near or behind the access panels of the device.

What Are Water Heater Thermostats Used For?

Most electric-powered water heaters will have at least one heating element that is, of course, used to heat the water, as well as at least one thermostat for temperature control, and at least one high limit switch which is utilized to protect the heater from reaching an excessively high temperature.

Water heaters will need to help water reach varying temperatures for the varying uses hot water will have in the building or section of a building it is responsible for.

For example, hot water is needed for showers, dishwashing, and clothes washing, and even more, depending on what appliances you have installed.

This is why we need water heater thermostats, to control the separate temperatures needed for all these different functions to make sure it is appropriate for each. 

The specific use of the water heater thermostat is in how it adjusts the electrical current that will be sent to either another thermostat or a heating element that needs to change its temperature to what the thermostat dictates. 

If the electric heaters that a water heater thermostat is attached to have a storage tank size of about 30 or more gallons you will need two heating elements to fully control the temperature of the water, each of which will need its own thermostat. 

These are placed in different parts of the tank, the lower thermostat is used to detect and sense changes in the water temperature, while the upper thermostat will be the main one that includes the high-limit switch that is responsible for shutting down the device if it manages to reach a dangerous temperature.

These two separate thermostats are not identical due to having different functions and are not designed to work simultaneously.

As previously mentioned, the high-limit switch is attached to the upper thermostat and has a manual reset button that will activate when the water reaches dangerously high temperatures (usually above 170 Fahrenheit or 77 Celsius).

If you manually activate this button the heater will be ready for further operation, but if it has automatically tripped you should investigate the source of the issue before further use.

If you are using a water heater with two thermostats for best use, it is recommended to set both the upper and lower thermostat to almost identical temperatures.

As an alternative, you can set the top element to a just slightly lower temperature than the lower thermostat to ensure that the bottom thermostat will activate first to make sure the heat from the water is rising efficiently

If your water heater has a lower tank capacity so that they only need a single water heater thermostat per the single water heater, will also utilize the high-limit switch system.

How Do Water Heater Thermostats Work?

When the water located inside a water heater is at a lower temperature than required, the upper thermostat (if there are multiple thermostats, if not this is referring to the single thermostat) will send a power of 240 V to the higher heating element, so it can heat the water in that part of the tank.

This heating will carry on until the thermostat detects that it has reached the set temperature (see also ‘Thermostat Not Reaching Set Temperature’). 

Of course, now the water in this section of the tank is heated, but not the rest of the tank.

Usually, water is drawn from the higher portion of the tank first, so this is fine if there is a quick need for hot water, but what if there is another section of the tank that is still cool?

If the upper thermostat has had its dictated temperature reached then power will be redirected onto the lower thermostat and a similar power of 240 V will be sent to the lower heating element.

Once it reaches the set temperature and satisfies the lower thermostat the power will stop and now all the water in the tank will be sufficiently heated.

How Do You Set The Temperature On A Water Heater Thermostat?

The range of temperatures that can be set by a water heater thermostat varies depending on the brand you buy as well as being wholly reliant on the water heater it is attached to.

The range of temperatures will usually vary between 100 and 140 Fahrenheit (37 and 60 Celsius) and usually manufacturers will set the default temperature to somewhere between 120 and 125 as a default once installed (48 and 52 degrees Celsius).

Water Heater Thermostats - Problems, Testing, Replacing

This is seen as the safest setting to leave a thermostat at as well as being energy efficient and not wasteful. 

Some important information to know when working with hot water is that if you are exposed to water of 120 degrees Fahrenheit (48 degrees Celsius) you will need to be exposed for over 5 minutes to start having burns.

But for just 10 degrees higher at 130 Fahrenheit (54.5 degrees Celsius) you only need less than 30 seconds for burns.

By the time water reaches 160 Fahrenheit (71 Celsius), you will burn in under a second of exposure.  

This information is to spread awareness of the importance of safety around hot water as ignorance can cause irreversible damage.

Luckily changing the temperature of a water heater is quite simple and does not require much effort if the heater is functional.

However, changing the temperature setting of a water heater (see also ‘Adjusting Your Water Heater Settings: The Best Ways To Maximize Efficiency‘)  should only be adjusted once the power of the water heater is fully shut down.

  • Firstly double check that there is absolutely no power running to the heater, you can do this most efficiently by turning off the heater at your fuse box or with your main circuit breaker.
  • After you are absolutely sure there is no power you can remove the access panel, then the cover, and then fold the insulation away from the controls of the thermostat.
  • Once this has been done use a screwdriver to rotate and adjust the knob that will control the temperature of the heater. 
  • After this reinstall the cover, and then the access panel with the insulation. Once everything is back in place you can turn the power back on and your thermostat will be set to the set temperature if it is fully functional.

Before you start this process make sure you know all the steps, so you have all the tools with you and are prepared for the task at hand.

How To Solve Different Thermostat Problems?

There are a few problems with thermostats that people tend to run into quite often, these are some of the most common and the best most efficient ways to solve them as soon as possible.

When There Is A Complete Lack Of Hot Water

  • Check to see if the upper thermostat is faulty or functioning, if this is the case it will need replacing.
  • The high-limit switch may have been tripped by excessive temperatures, if this is the case it is safest to get a professional to check for issues, if this is cleared you can reset the system with the same high-limit switch.
  • The thermostat may have been grounded which means it will probably need fixing or replacing.
  • The thermostat may also need to be calibrated, the manual for your thermostat should have a guide on how to do this, but if you can not get this process to work you will need to replace your system.

When There Is An Insufficient Amount Of Hot Water

  • The thermostat may be set at an insufficient temperature, so you will need to change it to be higher.
  • There is a good chance that there is an issue with the lower thermostat which could be sourced in improper wiring, which can be fixed.
  • Check to see if the thermostat is flush with the tank, if this is not the case you need to ensure that it is properly attached to the water heater.

When The Water Is Not Reheating Quickly Enough

When The Water Is Being Heated Too Much

  • The simplest solution to this is that the thermostat may have its heat set too high, so it will need to be decreased.
  • Ensure that the thermostat is fully flush with the tank.
  • Make sure that the thermostat is fully calibrated and if this does not fix the issue it will likely need to be replaced.

How To Protect Against Excessively High Water Temperatures?

If you are severely anxious about water heating too high due to a repeating problem or if the water is gradually getting out of hand, you can purchase a temperature limiting valve that can be attached to water outlets like faucets that will work to limit heated water flow.

Water Heater Thermostats - Problems, Testing, Replacing

You can also get a mixing valve (see also ‘Hot Water Mixing Valve Buying & Installation Tips‘) that reduces the temperature of hot water by simply combining it with cold water.

How To Test The Functionality Of A Water Heater Thermostat?

These tests work to not just test the water heater thermostat but also the heating element that is included in the heater.

This is to avoid inaccuracies that can come from when a heating element is either open or grounded.

The only tools you will need to test a water heater thermostat with this method are a screwdriver and a multimeter.

  • First check the power being provided to the water heater from the thermostat terminals. If power is being provided the meter will display 240 V, but if there is no power make sure to check the power supply.
  • Check if the power is being supplied to the high-limit switch.
  • After this set the knob on the lower thermostat to the lowest setting to make sure it will not activate and after this raise the temperature for the upper thermostat and if it starts.
  • Then check if power is between the upper thermostat and the heating element by using the meter prongs on the heating element. If it displays 240V there is power, if not there is no power. Repeat this process with every power terminal present.
  • If you do not have any power the terminal is broken.
  • For checking the lower thermostat make sure the upper thermostat is on its lowest setting and the lower thermostat is on its highest to summon heat from the heating element. 
  • Use the same method of probing power from each terminal and heating element, if there is power it will read 240 V but if it does not, there is no power.

Do not attempt any of this process if you are not confident in your skills, contacting a professional can save many injuries or just causing problems worse than what you started with.

How To Replace The Water Heater Thermostat On A Water Heater?

It is much easier to replace a thermostat than fixing it, so this step by step will go over how to do this process.

  • Make sure there is no power to whichever thermostat you are replacing
  • First remove the front panel door and then fold the insulation padding
  • Then remove the plastic cover that is over the thermostat
  • Use your multimeter to double-check the power is off
  • Unclip your old thermostat from the bracket
  • Disconnect wiring from the thermostat and label them for reinstallation
  • Remove the thermostat
  • Install the new thermostat to the bracket
  • Reattach the labelled wires
  • Reset the thermostat to the correct temperature
  • Test a cycle on the heater
  • Reinstall the covers from the start
  • Reset the power and then run the thermostat

Make sure when reinstalling a thermostat to keep a few things in mind:

  • Clean all elements that are being reinstalled
  • Make sure everything is firmly attached to the tank and all properly rewired
  • Lower thermostats tend to fail more often than upper thermostats
  • Make sure you know the voltage required for your heater


This is all you need to know for home care and fixing of water heater thermostat (see also ‘How To Wire Water Heater Thermostats’) issues.

While everything can be one by yourself, if you doubt your confidence, do not hesitate to contact professionals to aid in the process!

author avatar
Charlie Hardcastle


On Key

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