Water Heater Thermostat Replacement, Do It Yourself & Save... 

The water heater thermostat is the device that turns the source of heat on and off to produce and maintain the home's supply of hot water. The device consists of a bimetal switch that opens and closes based on the expansion and contraction of the metal. The device can be adjusted to maintain the desired hot water temperature. It is a simple matter of turning the pointer on the unit with a small standard screwdriver until it is on the desired water temperature.

When the temperature of the water in the storage tank is below the thermostat's setpoint, the switch is closed. This supplies electrical power to the element. 

As current flows through the element, heat is produced and transferred to the water in the tank.

When the water temperature reaches the setpoint, the bimetal in the switch deforms and opens the electrical contacts. This interrupts power to the element and stops current flow and the production of heat.


The typical home's water heater will have two thermostats and they are referred to as the upper and lower thermostats. They are not identical. The upper unit typically has three wire connections while the lower unit only has two.

Smaller units such as for apartments will only have one thermostat.

Testing the water heater thermostat... 

It is a simple process to test one of the thermostats if you think it is malfunctioning. The unit can fail by sticking closed which will result in the water temperature being too high and the limit control tripping.

The water heater thermostat can also fail by not closing when required. To check for this condition, start by shutting off the power to the unit. Verify that there is no power supplied to the unit. Then you can check the resistance across the connections on the thermostat. You can place one of the leads from your meter on each of the two wire connections on the left side of the unit on the upper thermostat or the only two connections on the lower unit. Then, you can turn the setpoint adjustment. The resistance value should go to zero as you raise the setpoint and should go to infinity as you lower the setpoint.

 Replacement procedure... 

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If you find that one of the units must be replaced, ensure that you get the proper replacement. Then, you should mark the wires and their connection points and then disconnect them. The thermostat can then be slid out of the clasp that holds it to the tank. Slide the replacement in and reconnect the wires and ensure that they are tight.

You should complete your water heater thermostat replacement by turning the power to the unit back on. Observe the proper operation of the water heater and ensure that the thermostat is set at the desired temperature. (The temperature should normally be set at 120 degrees to save energy and avoid burns.)