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Maintenance Thermostat

How to test electric water heater thermostat with digital multimeter

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A faulty water heater thermostat is an inconvenience to you, your family, or your business. Although there are different types of failures, a malfunctioning electrical system is a common problem. The point of failure may vary and could be faulty wiring or loose connections, a bad heating element, or a water heater thermostat.

A failure in the thermostat, heating element, or both could lead to completely cold water. Or partly warm water that does not reach the set temperature.

If your water heater stops providing you with hot water according to your settings and requirements, you can test the element and thermostat to ensure they are functioning properly. 

Thermostat role in water heaters

The main work of a thermostat in a heater is to regulate and maintain the water temperature, hence ensure safety and optimize energy use. When it starts malfunctioning, it cannot control the water temperature to suit your requirements. 

In practice, the thermostat controls the heating elements to maintain the water at the set temperature. When the water is cold, the thermostat automatically powers the elements to heat the water to the desired temperature. Upon reaching the set temperature, the thermostat shuts off the element, until the water gets colder again. As such, its work is to ensure the water temperature remains constant at a certain set level. Consequently, it reduces the risks of heating the water to very high and unnecessary temperatures, hence reducing the energy use without compromising comfort.

water heater thermostat

Types of water heater thermostat failures and impact

A faulty thermostat may not provide any control, or may only function partially and could result in your water not reaching the usual hotness levels.

Potential faults include;

  • A completely open thermostat that does not power the element at all.
  • A partial open thermostat – heats the water partially such that the temperature does not reach the desired level.
  • A short circuit- In this case, the thermostat does not provide any control and will continuously heat the water. Most often, a shorted thermostat results in the tripping of the heater reset button or high limit switch. 

Most standard water heaters have two thermostats, an upper one and a lower one. If one of these fails, it impacts the water heating process differently. For example, when the upper thermostat goes bad, you will notice that cold water will run out of the heater when you turn on the hot water tap. On the other hand, with a faulty lower thermostat, the water is initially hot, but it becomes cold after some time.

How to test the thermostat with a digital multimeter

Once you suspect that a thermostat is faulty, it is best practice to test and confirm that it requires a replacement.

Before testing the thermostat, confirm that the heating element is in working condition. Otherwise, an open-circuited heating element or one with a short circuit to the ground will result in some misleading measurements.

Please check here on how to test a heating element with a multimeter.

Unless physically broken, the common problems with the thermostats are due to failure in electrical continuity, which usually gets faulty due to wear and tear, unstable or poor quality power, etc. To identify the problem, you can use an analog or digital multimeter to test for continuity. 

Tools for testing water heater thermostat

Although the tools may vary according to the model, below is the list of the items you require.

  • A flat screwdriver 
  • Philips screwdriver
  • Nut driver 
  • Digital multimeter
  • Non-contact voltage tester
  • Basic technical skills

Let us now look at the steps you need to test the thermostat with a multimeter.

Step 1: Disconnect electrical supply to the heater

For safety reasons and accurate measurements, disconnect the power from the electrical panel. You can do this by turning off the breaker. To confirm that there is no power; use a meter or non-contact voltage tester at the terminals

Step 2: Remove the water heater’s access panels

Use a flat screwdriver or nut driver to remove the lower and upper thermostat access covers. Roll the insulation that covers the thermostat back and also remove the internal plastic covers. You can tape the insulating material in a place and put it out of the way as you work on the thermostat.

Step 3: Confirm that there is no power to the water heater

To avoid short circuits or electric shocks, ensure that the electrical supply is off. You can use a digital meter to measure the voltage across the thermostat terminals, which should give zero volts if off. Alternatively, use a non-contact voltage tester to confirm. 

Step 4: Confirm Reset button has not tripped

The reset button may trip when there is a problem with eth thermostat. For example, it may trip the reset switch if the thermostat has short-circuited contacts or out of calibration. Also, the reset button may trip due to a faulty heating element. 

Step 5: Disconnect the thermostat wiring

Before removing the cables, draw the wiring diagram to avoid making mistakes when reconnecting. Alternatively, take a picture using a camera or even a mobile phone. 

To physically remove the wires, use the Phillips #2 screwdriver to loosen the screw and pull each cable out. Repeat this for each of the terminals.

Step 6: Check the thermostat continuity with a digital meter

Set the digital multimeter to the lowest resistance range. Most meters have a typical low range of 200ohms and may include a tone.

Place one of the meter probes on the screw terminal on the left side of the thermostat. Place the second probe on the other terminal, still on the left side. The digital meter should display a low resistance reading, hence confirming continuity. If the meter reading is very high or open, the thermostat is faulty and will require a replacement. 

Testing water heater thermostat

Repeat the test on the terminals on the right side and also for the other thermostat.

If the thermostats are functional and the water heater still does not function properly, test the wiring and heating elements. 

Please see our guide on how to test a heating element with a digital meter.

Conclusion

An electric water heater uses a thermostat to maintain the water temperature at the desired level. A failure in the thermostat makes it hard to control the heating elements and the water temperature. 

Most often, the water does not get hot as per the set temperature. Fortunately, you can test a thermostat with only a few tools and basic skills. However, you need to be careful and ensure that you have disconnected the electrical power before checking the thermostat for continuity. Once you identify a faulty device, remove and replace it with a similar model.