You will find that the element (probably lower) is burned out.

Stop resetting the safety and fix the problem.

Generally if there is a defective element the water heater would not produce sufficient quantities of hot water, but the post states that it works fine for a few days, then mysteriously pops the thermostat limit switch.

While that could be attributed to a defective element I would suspect one of two other possibilities first.

HOW TO TEST

Begin by switching the circuit breaker off that supplies the water heater or pull the service disconnect if one is present to kill all power to the water heater.

Disconnect one of the two wires to the bottom element, then test the resistance of the element with an ohm meter. Typically if the element is okay you should get about 10 - 15 ohms of resistance. If the resistance is zero the element is burned out and if the ohm meter needle moves to infinity the element is shorted out. In either case you will have to change the element.

If the elements test okay you can then replace the wires and check the new thermostat to make sure all the electrical connections are tight. If you have a loose connection that would introduce resistance, which in turn raises the amp draw and would trip the overload.

Next check the upper thermostat mounts. Most electric water heater thermostats are mounted by means of steel spring clips. Quite often when changing a thermostat people tend to bend the clips slightly and the thermostat is a bit loose. A thermostat MUST BE held tightly against the wall of the tank so that the thermal sensor on the back of the thermostat is in tight register with the tank wall. If the clips are bent and the thermostat is even a bit loose there is a slight air gap between the thermal sensor and the tank wall. In this case the thermostat is not properly sensing the tank temperature and the temperature will rise until the overload trips out on thermal overload (too hot).

The solution is to first remove the thermostat, then use a piece of sandpaper to lightly buff the tank wall to remove any rust or scale, then bend the spring tabs back a bit and re-insert the thermostat making sure it is tight against the tank wall.

While you have the power off, open the top service cover where the main electrical cable is connected to the water heater and check all the electrical connections to make sure they are tight and not corroded.

You should now be able to close all the service covers, and restore the service disconnect or main power breaker.

Lazypup: A few corrections. If the resistance is 0 ohms it is shorted if it is infinity it is open. Also increased resistance will reduce amp draw. The extra resistance at the loose connection will cause excess heat at that connection.

I know that we've had this debate before, but in my experience, an older heater that is tripping the heat limit is likely to have a burned-out lower element that is shorted and thus on all of the time at a reduced wattage.

I'm with Jake, once again as I have discovered this before also.