Control and method for operating an electric water heater

1. A control for an electric water heater, the electric water heater including at least one sensor that determines a temperature of water in the water heater and generates a temperature signal indicative of the water temperature, the control comprising:

means for receiving the temperature signal;

means for setting a desired temperature:

means for generating a first control signal indicative of a first relationship between the temperature signal and a first temperature threshold, wherein the first temperature threshold is fixed and the first control signal turns off a heating element based on the first relationship; and

means for generating a second control signal indicative of a second relationship between the temperature signal and a second temperature threshold, wherein the second temperature threshold is variably based on the desired temperature and the second control signal turns off the heating element based on the second relationship,

wherein each of the first temperature threshold and the second temperature threshold is greater than the desired temperature.

2. The control of claim 1 further comprising a temperature limit means for receiving the temperature signal and generating a third control signal indicative of a third relationship between the temperature signal and a reference voltage signal, wherein the third control signal causes a heating element to at least one of turn on and turn off according to the third relationship.

3. The control of claim 2 wherein the temperature limit means includes at least one comparator for receiving the temperature signal and the reference voltage signal.

4. The control of claim 3 wherein the at least one comparator generates the third control signal.

5. The electric water heater control of claim 2 wherein the reference voltage signal is indicative of a temperature limit.

6. The electric water heater control of claim 2 further comprising means for receiving the second control signal and the third control signal, and for turning off the heating element according to at least one of the second control signal and/or and the third control signal.

7. The electric water heater of claim 2 wherein the voltage reference signal is indicative of the first temperature threshold.

8. The electric water heater control of claim 1 wherein the first control signal turns off the heating element if the temperature signal is greater than the first temperature threshold and the second control signal turns off the heating element if the temperature signal is greater than the second temperature threshold.

9. The electric water heater control of claim 1 wherein the first temperature threshold is greater than the second temperature threshold.

10. The electric water heater control of claim 1 wherein the second temperature threshold is less than the first temperature threshold.

11. An electric water heater comprising:

at least one sensor that determines a temperature of water in the water heater and generates a temperature signal indicative of the temperature;

at least one heating element that heats the water;

a control module that receives the temperature signal and de-energizes the at least one heating element based on a first relationship between the temperature signal and a first temperature threshold and a second relationship between the temperature signal and a second temperature threshold, wherein the first temperature threshold is fixed, the second temperature threshold is variably based on a desired temperature, and the first temperature threshold and the second temperature threshold are greater than the desired temperature; and

a temperature limit circuit that receives the temperature signal and de-energizes the at least one heating element according to the first relationship.

12. The electric water heater of claim 11 wherein the control module opens a first switch in order to de-energize the heating element if the temperature signal is greater than the first temperature threshold and opens a second switch in order to turn off the heating element if the temperature signal is greater than the second temperature threshold.

13. The electric water heater of claim 12 wherein the temperature limit circuit opens the second switch if the temperature signal is greater than the first temperature threshold.

14. The electric water heater of claim 11 wherein the first temperature threshold is greater than the second temperature threshold.

15. The electric water heater of claim 11 wherein the second temperature threshold is less than the first temperature threshold.

16. A method for operating an electric water heater comprising:

sensing a temperature of water in a water heater tank;

executing a first algorithm that turns off at least one heating element if the temperature exceeds a first temperature threshold, wherein the first temperature threshold is fix; and

executing a second algorithm that turns off the at least one heating element if the temperature exceeds a second temperature threshold that is variably based on a desired temperature,

wherein the first temperature threshold and the second temperature threshold are not equal and are greater than the desired temperature.

17. The method of claim 16 wherein the step of executing the first algorithm includes generating a first signal at a first control module if the temperature is greater than the first temperature threshold.

18. The method of claim 17 wherein the step of executing the second algorithm includes generating a second signal at a second control module if the temperature is greater than the second temperature threshold.

19. The method of claim 18 further comprising receiving the first signal and the second signal at a switching module that turns off one or more the at least one heating element in response to one of the first signal and/or the second signal.

20. The method of claim 18 wherein the second control module is a software module.

21. The method of claim 18 wherein the step of generating the first signal includes:

generating a temperature signal indicative of the temperature;

receiving the temperature signal at the first module;

generating a threshold signal indicative of the first temperature threshold;

receiving the threshold signal at the first module;

comparing the temperature signal to the threshold signal at the first module; and

generating the first signal if the temperature signal is greater than the threshold signal.

22. The method of claim 16 wherein the second temperature threshold is less than the first temperature threshold.

23. The method of claim 16 further comprising selecting the desired temperature.

24. The control of claim 1 wherein the first control signal controls at least one first switch in communication with the heating element and the second control signal controls at least one second switch in communication with the heating element.