Give your water heater’s thermostat the once-over
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.
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.
Your 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.
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.
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.
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.
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.
The 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.
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.
Now, the fun begins, as it’s time to test the upper thermostat. Place your multimeter lead on the reset terminal of the upper thermostat, which is usually next to the reset button. If not, look for the terminal that did not have power wires to remove.
Keep your first lead on the reset terminal and move the second lead to the left-side terminal (the one with the power wires). Look at the multimeter’s reading at this point. If it reads 1, your thermostat has no continuity and likely requires replacement. If it reads 0, you’re in the clear. Repeat this process with the right-side terminal.
Before moving on to the lower thermostat, check on how the upper thermostat regulates the temperature, which can also contribute to water heater issues. This simple process requires the multimeter, so place the lead on the left common terminal on the lower half of the thermostat. While staying in place, move the second lead to the upper heating element terminal. If your water temperature is lower than it should be, the first reading will be 0, and the second reading will be 1.
Move the second lead to the lower heating element terminal, but keep the first lead on the left common terminal. If the temperature is higher than it should be, the first reading will be 1, followed by the second reading of 0. Make a note of these readings for later.
This process is similar to checking the continuity of the upper thermostat. Place your leads on the two terminals. If the reading is 0 ohms or close to it, the water temperature is lower than it should be. If the reading is 1 ohm, there is no continuity and, therefore, no heat regulation.
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.
If you find that the upper and lower thermostats have opposing readings, be sure to replace the old thermostat with a new one. Ensure the replacement water heater thermostat is compatible with your water heater. Then, follow the steps in the instruction manual to have your new thermostat up and running in no time.
If your thermostat readings come back normal, then your thermostat might not be the problem after all. In this case, you may want to look into how to replace a water heater element, as this is another faulty mechanism in water heaters that can lead to some of the same symptoms as faulty thermostats.
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.
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.
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 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.
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.
Yes, you can replace a water heater thermostat. Replacing the thermostat might be easier than fixing it. This job will take an hour or two of work for a pro, plus the cost of the replacement part. Most water heater thermostats are universally compatible and interchangeable.
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.
If you think your water heater is over 10 years old, you should consider replacing it before an emergency makes the decision for you. To determine the age of your water heater, take a look at the brand name and serial number on your unit—sometimes, the date of manufacture is printed on the same tank label with the serial number. If not, you can consult with an online water heater industry source, such as the Building Intelligence Center, to determine the manufacturing date.
