How to Test Your Water Heater Thermostat
Give your water heater’s thermostat the once-over
- Digital multimeter
- Flathead screwdriver
- Insulation (optional)
- Tape (optional)
- Replacement thermostat (optional)
If you’re not getting any hot water while running a bath or washing the dishes, you might have a malfunctioning water heater thermostat. Luckily, you can learn how to test a water heater thermostat with some simple household tools. This guide will walk you through the steps for inspecting the upper and lower thermostat so you can get insight into what’s causing a lack of hot water.
Prepping to Test a Home’s Water Heater Thermostat
Read the manual and research the location and type of thermostats included in the system to familiarize yourself with your water heater's inner workings. Many types of water heaters include more than one thermostat.
As a general rule of thumb, if your system handles more than 30 gallons simultaneously, it likely features two heating elements and two associated thermostats. A heater’s upper thermostat is the primary sensor for the system, with the lower thermostat acting as a backup tied to the remaining heating element.
The type of issue you’re experiencing can dictate which of your two thermostats is failing. The following list covers general guidelines. You should check both thermostats regardless of the underlying problem.
No hot water: If your system isn’t producing any hot water, an upper thermostat issue is a likely culprit.
Some hot water, but not enough: Again, this indicates a problem with the upper thermostat.
Slow hot water recovery: If it takes a while for your faucets to “wake up” and funnel hot water, it’s likely a problem with the lower thermostat.
Excessive heat: If your water is too hot, the thermostat is set or calibrated incorrectly.
High-limit switch issues: If your heater’s red reset button keeps popping out, an upper thermostat malfunction is likely the cause, as the upper thermostat regulates heat.
Turn Off the Power to Your Water Heater
Photo: brizmaker / Adobe StockYour first step is to fully power down the water heater to avoid any risk of electrocution. Water and electrical currents do not exactly mix.
Turn off the water heater using the breaker on your main electrical panel. In most cases, this involves flipping the breaker to the “off” position. Your panel should list which breaker goes to which part of the house or device, so scan the breakers for the one that provides juice to the water heater. Flip this breaker off, and you’re good to go.
Check for Electricity
Even though you just shut the power off, your safety should be your number one priority. That's why you should double-check the electricity in your home to ensure no voltage is present. You can use a voltage tester or multimeter to check for voltage before proceeding to the next step.
Remove the Access Panels
Now that there is no risk of electrocution, start testing the water heater thermostats by making them accessible. Head to the heater's access cover. If you don’t know where to find it, check the instructions for the heating system, which should explain all the parts of your water heater.
Use a flathead screwdriver to remove the access panels for both the upper and lower thermostats. If a layer of insulation covers the thermostats, fold them out of the way and secure them with tape for now.
Set the Multimeter
Using a digital multimeter can be slightly confusing until you get the hang of it. For this process, power the multimeter and use the dial or knob to set it to the absolute lowest setting for ohms of resistance. If needed, check the multimeter’s instructions for how to adjust the ohms setting, as some models require unique adjustment processes.
Check the Reset Button in the Upper Thermostat
Photo: BillionPhotos.com / Adobe StockThe upper thermostat has a reset button that goes off when the water in the tank overheats. Give this button a quick inspection to make sure it doesn’t require a reset. Press the reset button if necessary but still go ahead with the remaining steps, as you already opened up the access panel and prepared the multimeter.
Disconnect the Wires
Even though you turned off the power at the main breaker, disconnecting the power wires will isolate the thermostat from the rest of the system and give you an accurate reading. Disconnect these wires, but take note of their orientation for when you reconnect them later. Use a screwdriver, if necessary, to unscrew the terminals and release the wires. Perform this step for both the upper and lower thermostats.
Upper Thermostat Test: Part 1
Set the upper thermostat temperature to its highest setting or maximum temperature. With your multimeter turned on and set to ohms, place one of the leads on the reset terminal, which is the one next to the reset button. Place the second lead on the left-side terminal where the power wires are. Check the multimeter reading. If the reading is zero, the thermostat is functioning as it should. If the reading is 1, the thermostat lacks continuity and will require replacement.
Upper Thermostat Test: Part 2
Set the upper thermostat temperature to its lowest setting or minimum temperature. Repeat the continuity test from the previous step. However, this time, place the second multimeter lead on the right-side terminal. Check the multimeter reading. If the reading is zero, the thermostat is functioning as it should. If the reading is anything other than zero, the thermostat needs replacing.
Lower Thermostat Test
Photo: Kostiantyn / Adobe StockTo test the lower thermostat continuity, turn the thermostat temperature up to its maximum. The lower thermostat only has two terminals. Place one multimeter lead on one terminal and the other on the second terminal. If you get an ohm reading of 0, the lower thermostat is fine. If you get no reading, a 1, or any other reading, the lower thermostat requires replacement.
Compare the Data
Once you've noted all the readings, you'll want to compare the data with the temperature information from the upper thermostat. If the upper and lower thermostats show opposite readings, the lower thermostat requires replacement.
Reassemble and Test the Thermostat
Finish things up by returning everything you disassembled to its proper location. This includes reconnecting the wires to the thermostats, placing protective covers on each thermostat, closing up the access panel, and restoring power via the breaker.
Symptoms of a Bad Thermostat on Your Water Heater
A faulty water heater thermostat can cause several inconveniences when using hot water in your home. Fortunately, replacing a faulty thermostat costs far less than replacing your entire water heater, so it's important to identify the root cause of the problem.
With this in mind, here are the signs that your water heater’s thermostat needs replacing:
You have no hot water.
You're running out of hot water quickly.
The water is too hot.
It takes a long time for your water heater to heat the water.
You keep having to reset the high-temperature limit switch.
If you’re experiencing other problems with your water heater, you might want to consider how long water heaters last to see if it’s time to replace the unit. For example, a water heater leaking from the top likely isn’t caused by a faulty thermostat, but it could be a sign that your water heater has old and deteriorating parts.
So, if your water heater’s approaching the 10-year mark or has other problems beyond the thermostat, consider upgrading to a water heater that won’t disappoint you every time you hop in the bathroom for a warm shower.
DIY vs. Hiring a Pro to Test a Water Heater Thermostat
This is a budget-friendly and efficient project for budding amateurs, but DIYing this test only lets you know if one or both of your thermostats require replacement. Your best bet is to contact a water heater repair contractor near you if you need a replacement. The average cost to repair a water heater is around $590, while water heater replacement costs range from $875 to $1,750 on average.
A pro will also double-check your inspection work and root out any other issues contributing to inefficiency within your water heating system. Don’t replace the thermostat on your own unless you have experience with electrical systems.
Frequently Asked Questions
The best water heater thermostat setting for safety is 120 degrees Fahrenheit. If it’s below 120 degrees, you risk bacteria growth in the water tank. You can turn up the water heater temperature if necessary. Most modern thermostats ship with the default temperature set to 140 degrees, but they are easily changed to 120 when necessary.
Thermostats for a water heater cost $150 to $200. In other words, if your system needs new upper and lower thermostats, the parts cost $300 to $400.These are not the most expensive gadgets in the world, but they do include some sensitive components that drive the cost up.
In most cases, replacing the thermostat does not require completely draining the water heater. Your pro will need to power down the entire system at the primary breaker box.
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