Water heaters or cylinders have two thermostats: one that is associated with the upper heating element and another to control the lower heating element. This improves the water heater's efficiency by only turning on the heating element in the part of the water heater with the coolest water. If large quantities of hot water are quickly used, both thermostats will engage. When your hot water is consistently too hot or too cool, adjust your thermostat to change the temperature. If the water temperature does not improve within 24 hours, you will need to change the thermostat.
Turn off the power to the water heater at the main circuit breaker panel. Open either the upper or lower access panel on the hot water heater, depending on which thermostat you are changing. These are held in place by two screws or by one screw and a tab that is inserted into a slot on the water heater. Move any insulation that is covering the thermostat to allow you to access it.
Use a screwdriver to loosen the screws that connect the wires to the thermostat. Pull the wires that are wrapped around the screws loose to remove the wires from the thermostat. Note which wire is connected to each of the terminals. Remove the screws that fasten the thermostat onto the water heater. Compare the new thermostat to the old one to make sure they match.
Hold the new thermostat in position on the water heater and screw it in place. Wrap the wires around the screws and use the screwdriver to attach the wires to the thermostat by tightening the screws. Adjust the temperature setting on the new thermostat to the desired level by turning the screw in the centre of the temperature dial on the thermostat with a screwdriver in the desired direction.
Cover the thermostat and heating element exteriors with the insulation that you removed in Step 1. Replace the screws in the access panel to secure it to the water heater. Turn the electricity back on to the water heater and monitor it for about an hour to make sure that the water is heating to the desired temperature. If it is too hot or cool, adjust the thermostat temperature again.
Make sure that the electricity has been turned off to the water heater before starting to replace electrical components.