If the water coming from your faucet is too hot and turning down the setting on the water heater thermostat has no effect, the problem is very likely a malfunctioning thermostat on your water heater. The thermostat is allowing the heating element or burner to remain on too long, heating the water beyond what is safe or comfortable. Overheated water can be a serious scald hazard and can indicate another problem with your hot water heater—a failed temperature and pressure relief (TPR or T&P) valve. The TPR valve is there to release excess pressure from the water tank if the water gets too hot. If there's no evidence that the valve has opened recently, it may be time to replace the TPR valve as well as the thermostat.
Rule out High Mineral Content
Another cause of overheating is water that has high mineral content. The minerals can sometimes collect on the heating elements, causing them to corrode and become encrusted. When this happens, the heating elements begin to work harder and overheat. If this situation continues, eventually the elements will burn out. Heating elements can be replaced, and the offending sediments can be cleaned out by flushing the tank.
Testing the Thermostat
There is a simple test you can perform to test your water heater thermostat: Mark the current temperature setting with a marker, then turn down the temperature on the thermostat. Wait a few hours, then check the water temperature at a faucet. If the temperature has not cooled down, it means the thermostat has failed. Also, test the TPR valve as recommended by the manufacturer, and replace the valve if it fails the test.
Call a Pro for a Gas Water Heater Thermostat Problem
The steps for replacing a bad thermostat are very different for gas and electric water heaters. On a gas water heater, you have to replace the entire gas control valve because the thermostat is built into it. Because changing the gas valve involves several gas connections, this is a job best left to a plumber or heating contractor. Also, the control valve may be expensive enough to warrant replacing the entire water heater (especially if it's old), rather than just the valve.
Replacing Electric Water Heater Thermostats
Replacing a thermostat on an electric water heater is much simpler, and you can make the repair without having to drain the tank. The procedure is no more difficult than replacing a standard wall switch, but water heaters use 240-volt power, so be extra careful to shut off the power and test the wires for voltage before touching any electrical connections or wires.
Most electric water heaters use dual heating elements, and each heating element has its own thermostat. In situations where the water is too hot, it is best to replace both thermostats. While it is possible to test the thermostats individually with a multi-tester to determine which is bad, thermostats are relatively inexpensive and it usually best to replace them both. If one thermostat has failed, it may not be long before the other fails; replacing them both may save you from needing to do it again in the future.
Tools and Supplies Needed
The new thermostat must match the old thermostat exactly, or it must be an approved alternative replacement part from the water heater manufacturer.
- Non-contact voltage tester
- New thermostats
It is not necessary to drain the water tank or wait for the water to cool before replacing the thermostat on an electric water heater.
Turn off the Power
Turn off the breaker to the water heater circuit in your home's service panel (breaker box). Water heater breakers are double-pole type and typically are rated for 30 amps, but on some water heaters, the amperage rating may be higher.
Expose the Thermostat
Remove the metal access cover over the thermostat (and heating element) compartment on the water heater tank; you may need to remove a screw to free the panel. Remove the insulation just behind the cover, being careful not to touch any wiring.
Confirm the power is off by testing all of the wires and screw terminals inside the thermostat compartment, using a non-contact voltage tester. The tester should indicate that there is no voltage present.
Remove the Old Thermostat
Take a photo or make a drawing of the wire connections on the thermostat to make sure that you get them right when installing the new unit.
Loosen the two screw terminals on the thermostat, using a screwdriver. Pull each wire from its terminal. Remove the thermostat from its retaining clips, and take it out of the compartment. Be very careful not to bend or damage the clips, as they may not be repairable.
Install the New Thermostat
Fit the new thermostat into the retaining clips so the thermostat rests flush against the wall of the water tank. Connect each of the two circuit wires to the appropriate screw terminal on the thermostat, matching the wiring of the original thermostat. Tighten the screw terminals with a screwdriver, making sure the connections are tight.
Set the temperature to the desired setting on the thermostat, using a flat-blade screwdriver. For safety and energy efficiency, the recommended setting is 120 F.
Reinstall the insulation and compartment cover. Restore power to the circuit by switching on the circuit breaker. Allow the water heater to heat up for at least two hours, then test the water at a faucet to confirm the desired temperature.