A faulty thermostat can cause problems for your home's hot water supply. You may have no hot water at all, or it may allow the water to heat beyond safe temperatures. It is best to be doubly safe when working on high-voltage appliances, including a 240-volt hot water heater. After you turn off the power supply, test the heater connections with a voltage meter to be sure the power is off. Changing the thermostat should take only about 30 minutes for the average handyman.
Turn off the power at the main panel. Unscrew the holding panel screws and remove the panel from the front of the heater. Test the wires on the exposed thermostat with the voltage meter before proceeding.
Number the wires before removing them from the thermostat. You can use bits of masking tape and a black marker for this, and it will help you remember the order of connections when you hook up the new thermostat.
Loosen the old thermostat from the mounting clip and lift it out. Put the new one into the mounting clip and reconnect the wires.
Set the heater settings to the manufacturer's recommended temperature. Make sure the heater is full of water, then restore the power at the main circuit panel. Locate the red reset button on the front of the thermostat and push it.
Replace the access panel.
Do not restore the power to a water heater that is not full of water. It burns out the element.