How to Replace an Electric Water Heater Thermostat

Instructions on how to replace an electric water heater thermostat. Find out how it works, problems associated with the thermostat failure, and how to avoid them.

Highlights

How water heater thermostat works

Every electric water heater is equipped with one or two thermostats to control the flow of electrical current, sent to another thermostat or heating element. 

These are known as upper and lower and they control the upper and lower heating elements. If the electric water heater is not producing any hot water or the temperature is not as requested, some testing needs to be performed to determine which thermostat is bad.

Furthermore, these thermostats will allow one heater to work in a simultaneous mode so they heat water at the same time, or non-simultaneous mode. Water heaters thermostats are surface mounted and are firmly attached against the side of the tank.

These are designed to sense the heat of hot water and through the tank surface, set the temperature and by using the ECO feature or high limit switch (if applicable) protect the unit and the user from extremely high temperatures.

How to change the temperature on the thermostat

The hot water temperature inside the storage tank is factory set on 120 F as the safety precaution, so by using the dial you can easily change the output temperature. If you increase the temperature setting above the factory pre-set temperature there will be a danger of scalding injuries and heating might not be economical. The time needed for scalding burns or serious injury is as: if the temperature is set on 120 F it takes more than 5 minutes to produce injuries, for 140 F less than 5 seconds and for 160 F about half a second.

  • Before you change the temperature on the water heater thermostat, make sure that the electrical supply to the unit is turned OFF.
  • Once the power is OFF, remove the cover for the full access to the thermostat.
  • Use the screwdriver and rotate the adjustment dial clockwise to the desired temperature.
  • Bring the cover back and turn the power ON.

How to replace an electric water heater thermostat

Water heater thermostat replacement procedure applies to either the lower or upper element.

  • Before replacing make sure that the electrical power to the unit and breaker is turned OFF.
  • Remove the cover and insulation to have an access to the thermostat. Remove the plastic cover.
  • Make sure that there is no electrical power to the heater, by using the voltmeter.
  • Disconnect wires from the thermostat. Make sure they are marked so you don't have to guess when reconnecting.
  • Remove the thermostat from the mounted bracket.
  • Use a stiff brush and remove the scale and debris from the tank surface.
  • Now install a new and identical element so the back is flush with the tank surface. Keep in mind that the improper mounting will result to an incorrect water heating operation.
  • Refer to a wire diagram located on the cover to reconnect the wires.
  • Bring the cover and insulation back and resume the power.
  • Adjust the thermostat to the desired temperature and run the heater for test.

Electric water heaters are usually equipped with the two thermostats: lower and upper one. Upper one has the high limit which is non-adjustable and must be reset manually when it trips. If the hot water inside the heater increases to an extremely high temperature the high limit switch is triggered, it interrupts the heating so it requires manual reset.

Problems when the thermostat is not working properly

  • If the temperature is set too low, there will be not enough or no hot water inside the tank.
  • If the temperature is set too high, the outgoing water is too hot.
  • Overheated water and continuous operation are the result of an improperly installed thermostat - not in contact with tank.

When selecting the location for the electric water heater, it is important to provide an adequate clearance between the thermostat and surrounding elements so the thermostat can easily be used for temperature change, replaced if broken and cleaned as part of the regular maintenance.

Words of caution

Do not perform any work on the thermostat or water heater with the power ON. Always make sure that the breaker is in OFF position. It is recommended to call a professional for thermostat replacement.

Summary

As said before, this guide is for homeowners who are capable of doing DIY home projects. The guide provides some useful tips that will help you learn how to replace an electric water heater thermostat, identify and solve problems that affect the proper heating.

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