How to Test Water Heater Thermostat: DIY Test, Troubleshoot & Repair Guide

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In America, around 1.5 million water heaters are replaced every year.

That's a whole lot. However, not all water heater problems are worth replacing the heater. You can DIY troubleshoot some electric water heater thermostat defects at home. And this is what this guide is all about.

Now let's learn how to test and repair an electric water heater thermostat for water heater wiring, before you think of replacing electric water heater.

But first, what's the purpose of an electric water heater thermostat, and how does it work?

What Is The Purpose of an Electric Water Heater Thermostat?

We wash clothes, do the dishes, and shower with hot water. But we don't use the same level of hot water for these activities. And this is where the thermostat is important as it controls the temperature in the electric heater.

An electric water heater, on its most basic level, is a piece of electrical equipment made of 3 parts: a heating element, a thermostat, and a switch.

So the water heater thermostat acts like a switch that's activated by changes in the water temperature. Its primary purpose is to control the electric current entering the water heater and send it to the other thermostat (if there is one) or the heating element.

When it detects a low water temperature, water heater thermostats trigger the elements to produce heat. Mind you, the thermostat has no contact with the water in the tank.

So how does it know when the water is cold or hot?

The back of the water heater thermostat is held tightly against the tank with a clip. And the part where the tank meets the thermostat has no insulator. So when the temperature is low, the temperature of the tank is also low; and this is what the thermostat detects.

That said, when electricity enters the unit, the heating element becomes very hot and converts the power to heat. This then converts your cold water into hot water; reaching around 120°F.

And finally, the high limit switch prevents the hot water from getting scalding hot.

How Does An Electric Water Heater Thermostat Work?

There are two types of electric water heaters;

  • Single element water heater
  • And the dual element water heaters

The single-element type has a single-element thermostat and heating element. The tanks are usually smaller as a single thermostat is enough to control the temperature.

The dual-element water heater on the other has two thermostats and two heating elements. And most water heaters are dual-element water heaters, and this is what we'll focus on in this article. Although, a single-element water heater can be tested and replaced the same way.

They basically have two panels that house two heating elements and two thermostats.

The electric water heater thermostats produce heat in the tank by allowing the flow of electricity into the elements. However, both thermostats don't function the same way and at the same time.

The top thermostat controls the heating element in the top half, has a high limit switch, and is the primary thermostat. While the lower one controls the lower heating element and detects changes in the water temperature.

Keep in mind that the high limit switch which is located in the same area as the upper thermostat has a reset button that trips when the water gets extremely hot (over 170F).

So the upper thermostat gets activated when the water in the tank gets cold or lower than its set point. It then heats up the water by transmitting 240 V of power to the upper heating element. The two heating elements continue heating the water till it's satisfied with the temperature.

The thing is, only the water in the upper half gets hot while the one at the bottom is either cold or lukewarm. So after the upper thermostat heats up the water, the power goes to the lower thermostat which then transmits the power to the bottom heating element.

As the bottom heating element receives 240 V, it heats up the water at the bottom area to the set temperature before ending the process. Now the whole water in the tank is hot.

Problems with Electric Water Heaters

Sometimes, when your heating elements or water heater thermostats fail, you'll notice changes in the unit.

Your water heater may stop producing hot water if the upper element or thermostat fails. And if it's the lower element or thermostat that's faulty, you'll notice you quickly run out of hot water. However, these could be a result of the cold weather or pipes without proper insulation.

You may also notice that your unit is producing hot water at an extremely high temperature. Although, this could be a result of setting the temperature too high or changes in seasons.

If your water heater also takes too long to reheat, this may be indicative of a faulty electric water heater. Other problems include poor tank maintenance or high water pressure at home.

How to Test an Electric Hot Water Heater Thermostat and Fix it: Step by Step Guide

To be sure of what the problem really is, we're going to test both the thermostats and heating elements in our unit.

Note that when your elements are open and grounded, it could return a false result, and hence we're also testing them.

To carry out this test in the following steps, we'll need a Flathead and Philips screwdriver, and a digital multimeter. here's how to test a faulty water heater thermostat:

Step 1: Turn the power source off

On your circuit breaker, locate the water heater breaker panel and turn the water heater or hot water off.

Step 2: Remove the outer access panels

Now go to the side of the unit and remove the upper and lower thermostat access panels with a flathead screwdriver or 1/4-inch nut driver.

Step 3: Remove the insulation

You can either completely remove the insulation or fold it over the thermostat. Also, remove the plastic safety guard covering the thermostat and heating element. Also secure the insulator in place with tape, and try not to pull out the wiring as you work on this step.

Step 4: Check the high limit switch button

Check if the red high limit reset button has tripped. If it has, press it.

Sometimes, the red switch button may trip if the heating elements fail or the contacts on the thermostat have fused closed, or if the water heater thermostat is out of calibration.

Step 5: Disconnect the wires

Use your Philips screwdriver to disconnect the wire entering each terminal.

Step 6: Turn the temperature setting to the highest

Set the temperature on the upper thermostat to the highest, and set the scale on your multimeter to the RX1 setting.

Step 7: Check the thermostat and heating element with a multimeter

Set your analog or digital meter to its lowest resistance which should be 200 ohms. You should hear a click sound.

Then place the black probe on the left side screw terminal. Also, place the other red probe on the other terminal that's still on the left side.

Then take your reading to check the thermostat for continuity.

If the meter reads zero or a value very close to that, your thermostat is working properly. But if it reads wide, you may have a defective thermostat.


Step 8: Repeat the process for the right side

Also, set the upper thermostat on the right side to its lowest setting and place the probes on the screw terminals. This should also return a zero value.

Step 9: Take the meter reading on the lower thermostat

After testing the upper thermostat and it is working fine, check the lower thermostat by repeating the same process.

Have in mind that there are only two terminals in the lower thermostat. Do check the reading to see if it's zero.

If you're convinced the thermostats are in optimal condition, you can test for your heating elements. But if one or more of the thermostats need to be changed, read on.

How to Replace A Faulty Thermostat on an Electric Water Heater

Replacing a faulty thermostat is fairly easy. Plus it's cheap to get a new one.

So we're going to change the two electric water heater thermostats even if the problem is with a single-element thermostat. But before doing that, you'll need to get your thermostats from the same manufacturer. If you can't get that, another product from a suitable brand will suffice.

Tools you'll need.

  1. Flathead Screwdriver
  2. Philips Screwdriver
  3. Digital multimeter or voltmeter
  4. Replacement thermostat

Now here we go.

Step 1: Turn off the power supply to the heater

You don't want to work with power on. So head to the circuit breaker and switch off the power supply connected to the water heater.

Step 2: Remove the outer access panel and insulation

Electric water heaters have outer access panels that protect their thermostats and elements. Unscrew it and remove the insulating pad and plastic covering, but be careful not to touch the wires.

Step 3: Take out the Thermostat

Before you pull out the faulty old thermostat, take a picture of the wiring so you can know which wire goes into the terminal when you're connecting the new thermostat. Or you can label the wire. Also, confirm if it is switched off by using your multimeter.

Then unscrew the screw terminals with a Philips screwdriver and disconnect the wiring. Then, you can pry out the thermostat from its attachment clips and bracket. But do this gently so you don't damage the clips.

Step 4: Install the new thermostat

When you've successfully taken out the faulty thermostat, you're going to repair the new thermostat in the water heater.

Position it correctly so it sits well on the surface of the storage tank, and connect the right clips using the picture you took from the previous step.

Also, attach the circuit wires to their appropriate screw terminal and tighten the screws.

And it may be good thinking to check, clean, plus change the heating elements.

In case you want to get a new water heater, do your research before purchasing anything.

Step 5: Set the temperature of your new thermostat

When you're sure the wires are properly connected and done setting up your thermostat, use your flathead screwdriver to set the temperature to your desired temperature.

The recommended temperature is 120°F.

Step 6: Replace every other thing you took out

Now, you're all set up, it's time to replace your insulators and the chamber access panel.

After doing that, reconnect the power supply by switching on the water heater breaker on the circuit breaker.

Step 7: Cycle test your electric water heater

You can now switch on your water heater for two hours before running the hot water faucet to see if the heater heated up your water very well.

Other Problems with Water Heaters and How to Fix Them

Water leaks: Most times, leaking valves and plumbing connections are the culprits for water leaks. However, the problem could be caused by tank corrosion or loose elements in the water heater tank. While a corroded tank can't be repaired, you'll have to replace it. But if the heating element comes loose, tighten it back with an element wrench.

Noises from tank: If your tank makes noise like rumbling, popping, or high-pitched sounds, it may be as a result of boiling water. Excessive buildup at the tank's bottom can cause it to overheat and boil the water.

Fixing it is quite easy. First, drain the tank to flush out the sediment. If it doesn't work, replace the tank.

Rust-colored water or bad odor: If the water flowing out of your faucet is reddish, yellowish, or brownish, it signifies that some parts of your water heater tank are corroded and rusty. It could also mean the corrosion is in your pipes. Either way, you'll need a plumber to look into it.

And if the water flowing in your home smells like rotten eggs, it's likely there are bacteria in your hot water tank. To fix this, you may need to change the anode rod. However, you can't do this on your own. Also, call in the services of a professional plumber.

Wrapping up

Now you know the steps to take to easily test and replace your defective water heater thermostat. However, working with electricity can be very dangerous, so take precautions to ensure you always switch off the power supply to your water heater.

Another thing worth knowing is that sediment that forms at the bottom of water heater tanks is the major culprit in reducing the performance of water heaters. It can also cause your heater to overheat.

To make sure your heater lasts long without repairing or replacing it regularly, make it a point of duty to periodically flush the sediment.

Another option is to install a sediment filter and/or water softener.

Jim Spencer

Jim Spencer

Jim has been in the construction business for over 12 years with plenty of experience working on client projects, from start to finish. From kitchens to bathrooms and backyard spaces, he writes on a wide variety of topics surrounding home improvement.

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