Before you head out to buy a brand new thermostat, you should find out if the one on your water heater is broken beyond repair or not. There is one sure way of doing that, which I have explained in detail below. If the test fails, the thermostat is broken and you will have to replace it.
Steps to Test a Thermostat
Follow the steps to ensure the functionality of your thermostat. To do this test, you will need a multimeter to check if there are any live points on the heater and to check the thermostat as well.
Turn Off Power
First up, shutting down the water heater. There should be a circuit breaker on the heater's electrical panel. Trip it to shut off the power to the heater. If there is a fuse instead of a circuit breaker, pull the fuse out. Do both if you have both on the heater (a rarity, but you do get these), just to be sure. Remove the fuse and keep them with you, so that no one else may put them back in when you're not around. To check if the power really is off, put the black probe of the multimeter on the black jack (common) and the red probe into the red jack (live). Set the multimeter to 'AC Voltage'. If the voltage shown is less than one-tenth of a volt, that means it's OFF.
Remove the Panel Cover and Insulation
Some heaters come with one panel and some have two. Remove both and use the voltmeter to check for any live points along the inside of the panel to make sure that the heater is grounded. Next comes the insulation on the thermostat and the heating element wiring. Remove both carefully. You will have colored wires surrounding the thermostat, separate them to reach it.
Test Terminal Resistance
You will have two terminals on the top and two on the bottom. Set your multimeter to 'Resistance' or 'Ohms'. Remove the wires that run through the thermostat and touch the probes to the contacts. If it is fine, you should get a reading or a beep. If the thermostat is broken, then you will not get any reading on your multimeter. This means that it is not closing the circuit when the power is on and completing the circuit, it is broken and you need to fit a new one. You can also test the water heating element for integrity at the same time.
Replacing the Thermostat
If you're like me, you will want to draw the wires that run through the thermostat before disconnecting them.
- Get a picture on a digital camera or write down which colored wire goes where.
- There will be two snap-in spring loaded metal ears that stick out on the lower side of the thermostat.
- Unscrew it and then pull the ears back in using the screwdriver. Gently slide out the thermostat.
- Place the new one in on the same position where the older one was. The ears of the new one should snap into place.
- Give it a light shake to make sure it's in place. Screw the new thermostat in and reconnect the wires back to the same contacts referring to the photo or the notes.
- Adjust the temperature of the thermostat to 120ºF. Push the red 'Reset' button to set it with the water heater.
- Fit the insulation covering back on and place back behind the panel. Screw the panel back in. Turn the power back on and place your ear to the drum of the heater. In some time, you should hear the water boiling inside, which means your repair was successful.