I started another post thinking that there was a problem with my water heater thermostat, which I ended up replacing. But my water heater works fine, except when I run the Watts recirculating pump I have installed in the system and then it trips the thermostat after working and doing what it should do for a short time. This pump has been working fine for the last three years with no problems. It is still recirculating water as it is supposed to do, but now all of a sudden it is tripping the thermostat on the water heater after working for a little length of time. What could be wrong? I really need this in my system as it takes a ridiculously long time to get hot water at my kitchen sink without it.
John Young's description is on the right track.
You'll need a clamp meter to check the current for each of the two elements. With a cold tank the top element is turned on first. Once the set temperature is satisfied the upper thermostat switches to the lower element.The elements are 4500 watts each at 240 volts and it'll draw approximately 18.75 amps. With wires removed from the thermostat, each element will be ~12-13 ohms each.
What is happening as the water circulates with a hot tank, the lower element will turn on as the temperature goes below its set temper (120º), if the top thermostat water get below the set temperature, it switched power to the upper heating element, then it trips the thermostat because the upper heating element is bad. Corrosion on will occur on a heating element and burn a hole in it and that is a short or over current. I seen when doing an ohm meter check, the element will check OK on resistance and other times it an open. But I do not think this is the scenario.
The magic going on is when after the thermostat has it's breaker tripped, assuming its the upper element will turn on first, it does not make sence its the upper element because you do get hot water eventually, therefore,
I'm assuming the lower heating element is bad. When you reset the thermostat, water gets hot from the upper element then switched to the lower element and, but with out the circulator running there is no trip. If this happen in a few minutes the lower element is turning on and it trips the upper thermostat.
I would do a current or ohm test and replace both elements since the short may be intermittent. They are not that expensive, $12-$15 each at home depot.