If the water coming from your faucet is too hot, it is almost always due to a thermostat malfunction. The thermostat is allowing the heating element to remain on too long, which heats the water beyond what is safe or comfortable. Overheated water can be a hazard as it can scald you in just a few seconds. The pressure relief valve could also be malfunctioning. The pressure relief valve is in place so that, in the event that the water gets too hot, the water pressure relief valve will open to allow the excess pressure to bleed off.
Testing the Thermostat
There are some simple means of testing your water heater for the aforementioned issues. They include testing the thermostat by marking the current temperature setting with a marker and then turning the temperature down on the thermostat of the water heater. Check the water after waiting for a few hours. If the temperature is cooler, the problem is solved. If the water is still too hot, the temperature pressure relief valve might need to be replaced.
Another cause of overheating could be due to water that has a high mineral content. The minerals can sometimes collect on the heating elements, causing them to corrode and become encrusted. When this happens, the heating element begins to work harder and overheat. If this situation continues, eventually the element will fail and burn out. Elements can be replaced, and the offending sediments can be cleaned out.
Repairing a bad thermostat is different depending on the type of water heater that you have: gas or electric. To repair a thermostat on a gas water heater, you need to replace the gas control valve, as the thermostat is built into it. Changing the gas valve is a pretty involved process, and must be done safely and carefully as you are dealing with natural gas.
The part is also fairly expensive ($75 - $300), so make sure you have tested the water heater completely before you assume that the thermostat is the problem. Once you have determined the thermostat is the problem:
- Start by shutting the gas supply off to the water heater.
- Drain the water heater, and disconnect the burner gas supply tube, the pilot light gas supply tube, and the thermocouple tube from the gas valve.
- Disconnect the gas supply pipe from the gas valve itself, leaving the gas valve completely disconnected from the water heater.
- At this point you can begin to install the new gas valve. Assuming that you have purchased the proper replacement, everything should go back together pretty easily. After reconnecting the gas valve to the water heater, reconnect the gas supply pipe to the valve. Then reconnect the thermocouple, burner gas supply tube, and the pilot light gas supply tube.
- When everything is reconnected, you can turn the gas supply shut off valve back on. You should make certain that you check all of your gas line connections with soap bubbles to check for gas leaks. If you notice any bubbles at a joint, continue to tighten the joint until the bubbles stop.
- If no leaks are found, it is safe to relight the pilot and refill the water heater tank.
- When the tank is filled again, you can turn the gas valve to the on position and set the thermostat to the desired temperature. The most common setting is around 120 degrees Fahrenheit. At this point the thermostat replacement on your gas water heater is finished.
To replace the thermostat on an electric water heater:
- Begin by turning off the breaker in the panel that supplies power to the water heater. Before proceeding with the repair, it is wise to check the wire connections with a voltmeter to make certain that the power is indeed turned off. Since most electric water heaters have two thermostats and two heating elements, we will assume that you have already done the troubleshooting to determine the proper one to replace.
- It is not necessary to drain the water tank or wait for the water to cool to replace the thermostat on an electric water heater. Simply remove the cover to the thermostat and the insulation to expose the wire connections to the unit.
- Take a photo or draw out the wire connections to make sure that you get them right when installing the new unit.
- After you have made note of the wiring of the thermostat, you may disconnect all of the wires to the unit.
- The thermostat is usually held in place by a metal spring. Remove the spring and slide out the old thermostat.
- Proceed to install the new thermostat to the side of the tank. The thermostat must sit flush to the water tank.
- Put the springs that hold the thermostat in place back on and reconnect the wires.
- Use a screwdriver to set the temperature to the desired level, although 120 degrees Fahrenheit is a common setting for a residential water heater.
- Reinstall the insulation, covers, and turn the power back on to the water heater.
If you have done this replacement in a short time, the heating element may not come on right away. If the water has cooled, you can place your ear against the water heater to hear a slight fizzing sound. That's the sound of the water being heated. The cost of parts is about $30 - $50.