Many people even plumbing and electrical professionals have trouble figuring why the red reset button on the upper thermostat of an electric water heater trips. The red reset button or, high limit known as the Energy Cut Off or, ECO is designed to trip when the water in the tank becomes too hot cutting off power to the elements. The ECO is only a manually reset thermostatic switch that shuts off the power to the water heater when the water in the heater reaches 180 Degrees F. The ECO has no other function.
There are a few different things that can cause the ECO to trip all of which involve the ECO getting too hot.
A bad thermostat that does not shut off allowing continuous heating of the water even after the set temperature has been reached is a possible cause of ECO tripping. Your water heater has 2 thermostats, an upper and lower under the two access panels, as well as two elements. Either thermostat sticking in the on position could cause the element it serves to continue heating the water until the water in the tank is heated to 180 Degrees F when the ECO opens cutting power to the water heater.
A loose electrical connection either where the wires connect to the upper thermostat or an internal loose connection can generate heat caused by high resistance inside the thermostat causing the ECO to sense the heat of the bad connection rather than the temperature of the water in the tank causing the ECO to trip.
A bad element that is shorted to ground near the center of the element may also cause the water in the tank causing the ECO to trip. Because the element is shorted to ground near the center of the element the short does not have a high enough current draw to trip the circuit breaker and it continues to heat. However, the way the water heater electrical power is switched the power still is applied to the element even when the thermostat shuts off. The water heater is supplied with 240-volts which means both of the power leads are hot leads. Only 1 side is switched and in normal operation that is fine. But, when the element is shorted to ground there is still the power on the unswitched side available, which will flow through half of the element to ground as 120-volts continuing to heat the water until the ECO trips.
Your residential electric water heater is wired for 240-volt non-simultaneous operation where the power is initially applied to the upper element, then when the upper thermostat is satisfied it switches power to the lower thermostat for heating the lower section. Once the lower thermostat is satisfied the power to the element is shut off until either thermostat calls for heating again.
Troubleshooting is relatively easy with basic electrical knowledge and skills with a screwdriver and a basic multimeter. Caution is needed as the electrical voltage is 240-volts AC and severe injury or even death can result from electrical shock. The ECO is also a critical safety item of your electric water heater. If the water in the water heater was allowed to continue heating uncontrolled the one remaining safety device before a water heater explosion is a T&P valve functioning properly. If you do not have the skills required to safely and properly test your water heater please call a Licensed Plumber. 411Plumb assumes no liability for this advice or your ability to accomplish it safely.
Testing The ECO
Under the upper side access panel measure for voltage passing through the ECO switch to the thermostat section of the upper thermostat. If the voltage is present and being supplied to the thermostat section then move on to testing the thermostat and elements. If voltage is not present then the ECO switch requires resetting. If the water in the tank is still to hot it must be cooled down to allow resetting the ECO switch. Run hot water from a faucet to cool the tank. If the ECO switch will not reset replace the upper thermostat. If the Eco switch resets then move on to testing the thermostat and elements.
Testing the Elements
To test the elements turn off the power to the water heater and remove both the upper and lower side access panels. Disconnect both wires off the screw terminals on each heating element. Measure across the two screws with a multimeter set top resistance and the resistance should read about 12.8 Ohms for a 240-Volt/4500-Watt element. Then measure resistance from each screw to the water heater tank. This measurement should be an open circuit or infinite reading. If there is any connection from either screw to the tank the element is shorted and needs replacement.
Testing the Thermostat
With the tank cold turn on power to the water heater and measure for voltage from the top thermostat to the upper element, it should read 240-Volts. Check for power being fed to the lower thermostat and element, in this case it should read 0-Volts. Once the upper thermostat is satisfied the power supplied to the upper element should be 0-Volts and the voltage supplied to the lower thermostat and element should read 240-Volts. Once the lower thermostat is satisfied the lower thermostat should shut off power to the lower element.
Note: If testing reveals a fault of the internal wiring of the water on a water heater with foam insulation the wiring is generally not repairable and the water heater should be replaced.