Patent Publication Number: US-8983283-B2

Title: Method and apparatus for operating an electric water heater using adjustable temperature setpoints

Description:
FIELD OF THE INVENTION 
     The present invention relates to electric water heaters and more particularly to a control system for controlling the capacity of an electric water heater for energy efficiency. 
     BACKGROUND OF THE INVENTION 
     Electric water heaters are conventionally used in residential and commercial buildings to supply the occupants of the building with a reservoir of hot water. The water heater typically includes a tank that is fluidly coupled to a water supply of the building at an inlet and is fluidly coupled to building fixtures such as faucets, showers, and dishwashers at an outlet. The water heater tank receives cold water from the building water supply at the inlet and heats the water to a set point temperature using lower and upper heating elements. The lower and upper heating elements raise the temperature of the water disposed within the water heater tank to the set point temperature by converting current from a building power supply into radiant heat. The heated water is stored within the tank and is held at the set point temperature by the heating elements so that a supply of hot water is constantly and consistently provided at a desired temperature. 
     Conventional electric water heaters typically include a control system that monitors a temperature of water disposed within the water tank to ensure that the water contained therein is maintained at a predetermined set point temperature. The set point temperature is typically a consumer-selected setting that allows the consumer to determine a temperature of the hot water to be produced by the water heater. The control system continuously monitors the temperature of the water within the tank via a temperature sensor and compares the sensed temperature to the set point temperature. The control system generally includes an upper temperature sensor associated with the upper heating element and a lower temperature sensor associated with the lower heating element. The upper temperature sensor and lower temperature sensor each provide information regarding the water temperature near the respective elements. The respective sensors, in combination with the upper and lower heating elements, allow the control system to selectively heat the water disposed within the tank when the sensed temperature falls below the set point temperature. 
     In operation, the upper heating element of a conventional electric water heater is energized by the control system to heat a volume of water generally between the upper heating element and a top of the tank (i.e., an upper zone of the tank). Once the water in the upper zone of the tank is at the set point temperature, the control system de-energizes the upper heating element and energizes the lower heating element. The lower heating element heats a volume of water generally above the lower heating element and below the upper heating element (i.e., a lower zone of the tank). The lower heating element remains energized until the water within the lower zone of the tank is at the set point temperature. 
     Water, when heated, rises due to the physical properties (i.e., density) of heated water relative to the cooler water within the tank. Therefore, as the lower heating element heats water, the heated water rises within the tank and cold water descends toward the lower heating element. The descending cold water mixes with the passing hot water and is heated by the lower heating element. This process continues until the entire volume of water disposed within the lower zone of the tank reaches the set point temperature. 
     When a consumer draws hot water from the tank, the initial hot water drawn from the tank outlet is disposed within the top zone of the tank, near the upper heating element and upper temperature sensor. When the hot water exits the tank, a fresh supply of cold water is introduced into the tank at an inlet. The inlet is generally disposed at a bottom of the tank, below the lower heating element. The incoming cold water eventually contacts the lower heating element as the hot water is displaced (i.e., drawn from the tank at the outlet). At this point, the lower temperature sensor detects the influx of cold water and relays the information to the control system. The control system processes the information from the lower temperature sensor and energizes the lower heating element to heat the incoming cold water until the set point temperature is achieved. 
     If the consumer does not use all of the hot water available in the tank, the lower heating element remains energized and continues to heat the water (as described above) until the set point temperature is reached. However, there are instances when the consumer draws a sufficient volume of hot water from the tank such that the volume of cold water entering the tank reaches the upper heating element. Such an occurrence is known as a “deep draw” event. A deep draw event is identified when the upper temperature sensor detects a significant drop in temperature due to the incoming cold water. Upon detection of the incoming cold water, the control system de-energizes the lower heating element and energizes the upper heating element in an effort to quickly heat the smaller volume of cold water above the upper element to the set point temperature before the water exits the tank. 
     When the consumer stops using hot water, the influx of cold water is similarly stopped. At this point, the upper heating element continues to heat water disposed in the upper zone of the tank until the upper temperature sensor detects that the water disposed in the upper zone is at the set point temperature. The control system then de-energizes the upper heating element and energizes the lower heating element to heat the water disposed within the lower zone of the tank. The lower heating element remains energized until the lower temperature sensor detects that the temperature of the water disposed within the lower zone is at the set point temperature. In this manner, conventional hot water heaters include a control system that responds to a draw of hot water from the tank by continually heating the entire volume of water disposed within the tank to the set point temperature. 
     The capacity of an electric water heater is conventionally understood as the volume of water that the water heater is able to heat and maintain at a set point temperature. For example, an eighty-gallon water heater can heat and store eighty gallons of water. In this regard, then, the capacity of the eighty-gallon water heater is eighty gallons. 
     The effective capacity of the water heater that is realized by a consumer, however, is greater than the simple volume capacity of the water heater that was just described. This is so because a consumer does not typically use water at the set point temperature when a call for “hot water” at a household fixture is made. While the set point temperature for a water heater can vary, it is not uncommon that the set point is at 120° F. or higher. A consumer demand for “hot water” at a fixture, however, generally is for water at a comfortable temperature that is well below the set point temperature. Consequently, in order to produce the “hot water” that is used by the consumer, water drawn from the water heater is mixed with cold water from the building water supply. Thus, for example, for every gallon of “hot water” that is used by the consumer, only a half-gallon of water is drawn from the water heater. This effectively increases the amount of “hot water” that the electric water heater can provide to a consumer. 
     As a general proposition, the higher the set point temperature of the water heater, the lower the volume of water that needs to be drawn from the water heater in order to produce “hot water” for the consumer. Similarly, the lower the set point temperature of the water heater, the higher the volume of water that needs to be drawn from the water heater in order to produce “hot water” for the consumer. Thus, the effective capacity of the water heater can be adjusted by raising or lowering the set point temperature of the water heater. For example, a lower set point temperature would require more water from the water heater to produce the desired “hot water.” Thus, hot water from the water heater is used faster and the effective capacity of the system is reduced. Conversely, raising the set point temperature would require less water from the water heater to provide the same “hot water.” Increasing the set point temperature, therefore, increases the capacity of the water heater. 
     A conventional control system for an electric water heater generally operates to maintain the entire volume of water in the tank at the set point temperature, as described above. These control systems operate independent of the actual demands for hot water made by the consumer. Therefore, even if the consumer&#39;s requirements for “hot water” were regularly smaller than the effective capacity of the water heater, the water heater would nonetheless repeatedly heat all of the water to the set point temperature all of the time. 
     Therefore, it is desirable to provide a control system that can continuously monitor and adjust the effective capacity of an electric water heater based on consumer demands in order to save energy associated with operation of the electric water heater. Furthermore, it is also desirable to provide a control system that enables the electric water heater to satisfy government energy standards, while simultaneously providing a consumer with an adequate “hot water” capacity. 
     SUMMARY OF THE INVENTION 
     Accordingly, a method and apparatus for operating an electric water heater is provided. The invention comprises a control system including a control module and a consumer interface module. The control module controls operation of the electric water heater by selectively energizing and/or de-energizing one or more heating elements. The consumer interface module enables a user to input a set point temperature for the electric water heater and select an energy savings mode. The control module continuously monitors and adjusts the user-selected set point temperature until a capacity of the water heater matches consumer usage. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a schematic representation of an electric water heater that is operated in accordance with the principles of the present invention; 
         FIG. 2  is a schematic representation of a consumer interface module of the electric water heater of  FIG. 1 ; 
         FIG. 3A  is a schematic representation of a control module incorporating an electronic upper limit sensor for an electric water heater in accordance with the principles of the present invention; 
         FIG. 3B  is a schematic representation of a control module incorporating a bimetal upper limit switch and electronic upper limit sensor for an electric water heat in accordance with the principles of the present invention; 
         FIG. 4  is a flowchart that describes the operation of an energy saver module for an electric water heater in accordance with the principles of the present invention; 
         FIG. 5  is a flowchart that describes the operation of an electric water heater in accordance with the principles of the present invention; 
         FIG. 6  is a flowchart that illustrates operation of a consumer interface module for an electric water heater controller in accordance with the principles of the present invention; 
         FIG. 7  is a flowchart that describes the operation of a water temperature differential module in accordance with the principles of the invention; and 
         FIG. 8  is a schematic representation of a control system for a hot water heater according to the invention and incorporating a sensor module, a control algorithm, and a control module. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     With reference to the figures, an electric water heater  10  is provided and includes a control module  12 . The control module  12  adjusts an effective capacity of the electric water heater  10  by continuously monitoring and adjusting a set point temperature of the water heater  10  until an optimum effective capacity of the electric water heater  10  is achieved. As used herein, the term module refers to an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group), and memory that execute one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality. 
     The set point temperature is a consumer-selected input and is generally defined as the maximum temperature that the consumer selects for the heated water that exits the water heater  10 . The effective capacity of the water heater  10  is generally defined as the ability of the water heater  10  to provide a volume of water at a “delivered temperature.” The delivered temperature is the temperature of the water as used by the consumer at a fixture. The delivered temperature of the water is generally lower than the set point temperature because the delivered temperature is usually achieved by mixing water from the water heater  10  at the set point temperature with cold water from the building water supply. 
     The effective capacity of the water heater  10  is directly related to the set point temperature as follows: the higher the set point temperature, the lower the volume of hot water that is necessary to be mixed with the cold water to produce the water at the fixture at the delivered temperature. Conversely, the lower the set point temperature, the higher the volume of hot water that is necessary to be mixed with the cold water to produce the water at the fixture at the delivered temperature. Therefore, there is a direct correlation between the set point temperature and the effective capacity of the water heater  10 . 
     The control module  12  monitors and controls the effective capacity of the water heater  10  by selectively adjusting the consumer-selected set point temperature. In so doing, the control module varies the effective capacity of the water heater  10  to meet the specific needs of the consumer. By adjusting the effective capacity of the water heater  10  to meet the demand of the consumer, the control module  12  is able to minimize energy consumption of the water heater  10  while maintaining the ability to produce a satisfactory volume of hot water for the consumer. 
     With reference to  FIG. 1 , the electric water heater  10  is shown to include a tank  14 , an upper heating element  16 , and a lower heating element  18 . The tank  14  defines an interior  11  having a volume and includes an inlet  20  and an outlet  22 , both fluidly coupled to the interior  11 . The inlet  20  is also fluidly coupled to a water supply  24 , while the outlet  22  is also fluidly connected to the hot water pipes leading to the building fixtures, such as faucets, showers, dishwashers, and clothes washers, etc., which are schematically represented at  26 . The inlet  20  receives a constant supply of cold water under pressure from the building water supply  24  such that the interior  11  of the tank  14  is always full of water. Hot water only exits the tank  14  through the outlet  22  when a demand for hot water is made at one of the fixtures  26  throughout the building. Cold water, therefore, only enters the tank  14  when hot water exits the tank  14  through the outlet  22 . 
     The upper heating element  16  and the lower heating element  18  each extend through a side wall  25  of the tank  14  and generally into the interior  11 . The upper heating element  16  is disposed near an upper wall  32  of the tank  14 . The lower heating element  16  is disposed near a lower wall  34  of the tank  14 . The lower heating element  18  is generally closer to the lower wall  34  of the tank  14  than the upper heating element  16  is to the upper wall  32 . 
     The upper and lower heating elements  16 ,  18  receive current from a power supply  30  via the control module  12 . The control module  12  regulates each of the upper and lower heating elements  16 ,  18  between an ON state and an OFF state. 
     The electric water heater  10  also includes a sensor module  35  (see,  FIG. 8 ) in communication with the control module  12 . The sensor module  35  comprises an upper temperature sensor  36  and a lower temperature sensor  38 , each in communication with the control module  12 . Outputs from the upper and lower temperature sensors  36 ,  38  which correspond to their respective temperature readings are monitored by the control module  12 . 
     The upper temperature sensor  36  is disposed adjacent to the upper heating element  16  to monitor a temperature of water within the tank  14  in an upper zone (i.e., generally between the upper heating element  16  and the upper wall  32 ). The lower temperature sensor  38  is disposed adjacent to the lower heating element  18  to monitor a temperature of water within the tank  14  in a middle zone (i.e., generally between the lower heating element  18  and the upper heating element  16 ). The temperature sensors  36 ,  38  are preferably thermistors, such as NTC thermistors, but could be any suitable temperature sensor that can accurately and reliably provide an output which is indicative of the temperature of the water residing within the tank  14  near the sensor. 
     In addition to the foregoing, the sensor module  35  could also comprise two or more upper temperature sensors  36  disposed near the upper heating element  16 . Such an arrangement would provide redundant temperature readings at the upper heating element  16 . In a device having such an arrangement, the control module  12  would monitor the output from the plurality of sensors  36  and the sensor output indicative of the highest measured temperature would be used to control the operation of the upper heating element  16 . In addition, the control module  12  can compare the respective outputs from the sensors  36  for a self-diagnostic procedure. For example, if the difference between the output of any two sensors  36  is above a predetermined threshold value, the control module  12  could detect a sensor fault and require that the water heater  10  be shut down for maintenance or repair. 
     Further, the sensor module  35  could also include a flow sensor  37  disposed at the inlet  20  or the outlet  22  of the tank  14 . The flow sensor  37  could monitor a flow of water entering or exiting the tank  14 . Therefore, output from the flow sensor  37  could be used by the control module  12  to control the operation of the upper and lower heating elements  16 ,  18 . The flow sensor  37  could also be used to determine the volume of water that has been drawn from the water heater  10  over a period of time. 
     Referring now to  FIG. 2 , the control module  12  includes a consumer interface module  45  having a liquid crystal display (LCD)  40 , a series of light-emitting devices (LEDs)  42 , and a speaker  44 , all contained within a control module housing  46 . The LCD  40  displays the operating parameters of the electric water heater  10  such as the set point temperature (see bar graph  41  of  FIG. 2 ), an energy savings level (e.g., 0, 1 or 2), and other useful information such as the date and time. In addition, the LCD  40  may be backlit to allow use of the control module  12  in a dark or dimly-lit basement. The LEDs  42  are positioned adjacent to the LCD  40 , but may also be incorporated into the LCD  40  to visually indicate operating parameters of the electric water heater  10 . The speaker  44  allows the control module  12  to audibly alert a consumer of a particular condition of the water heater  10 . In addition to the foregoing, the control module  12  also includes at least user-input device  48  (e.g., a button) to enable the consumer to communicate with the consumer interface  45 . 
     Turning to  FIG. 3A , the control module  12  also comprises a microcontroller  50  in communication with the sensor module  35  and the consumer interface module  45 . The microcontroller  50  is powered by a power supply  52  disposed generally within the control module housing  46 . The power supply  52  receives power from line voltages L 1 , L 2 . 
     A limit control module  51  controls power to the heating elements  16 ,  18  based on readings from the upper and lower temperature sensors  36 ,  38 . The limit control module  51  of  FIG. 3A  is shown as an electronic limit control module  53  and essentially acts as a backup device to the microcontroller  50 . For example, if the microcontroller  50  fails to cut power to the upper and lower heating elements  16 ,  18 , the electronic limit control module  53  shuts down the heating elements  16 ,  18  based on readings from the upper and lower temperature sensors  36 ,  38 . The limit control module  51  could also include a bimetal snap disc thermostat  55 , as shown in  FIG. 3B . The bimetal snap disc thermostat  55  receives line voltages L 1 , L 2  and selectively prevents power from reaching the upper and lower heating elements  16 ,  18 . 
     In either of the foregoing configurations, the limit control module  51  is a separate circuit from the microcontroller  50  and selectively cuts power to the upper and lower heating elements  16 ,  18  based on readings from the upper and lower temperature sensors  36 ,  38 . The limit control module  51  only cuts power to the upper and lower heating elements  16 ,  18  when the microcontroller  50  fails to do so based on readings from the upper and lower temperature sensors  36 ,  38 . 
     The microcontroller  50  is also in communication with a sensor conditioning module  54  and a relay and driver module  56 . The sensor conditioning module  54  receives the output from the respective temperature sensors  36 ,  38  and directs the output to the microcontroller  50  and electronic limit control module  51 . The relay and driver module  56  receives event messages from the microcontroller  50  based on input from the upper and lower temperature sensors  36 ,  38  to toggle the upper and lower heating elements  16 ,  18  between the ON state and the OFF state by selectively allowing line voltage L 1 , L 2  to supply current to the respective heating elements  16 ,  18 . 
     Operation of the electric water heater  10  and associated control module  12  is best understood with reference to  FIGS. 4-7 . Generally speaking, the control module  12  monitors the consumer&#39;s hot water usage over time and provides an effective capacity for only the amount of hot water that is actually needed. The control module  12  can reduce the effective capacity by reducing a consumer-selected selected set point temperature by a setback value and recommend a reduction in the consumer-selected set point temperature if further reductions to the set point temperature are not possible. The control module  12  can increase the effective capacity by recommending an increase in set point temperature. In this manner, the control module  12  is able to tailor the effective capacity of the water heater  10  to the actual hot water consumption of the consumer. 
     When the water heater  10  is initially installed, the tank  14  is completely filled with cold water from the building water supply  24  via the inlet  20 . At this point, all of the water within the tank  14  is substantially at the same temperature (i.e., cold). The consumer selects a set point temperature setting at the consumer interface  45  by depressing one of the buttons  48 . The set point temperature represents the temperature of the water that the control module  12  seeks to achieve in the tank  14  within a tolerance. The tolerance recognizes that the actual water temperature within the tank may be different from the measured temperature provided by sensors  36 ,  38 . The set point temperature can be set, for example, to one of twenty temperature settings. The twenty settings are exemplified by the bar graph of  FIG. 2 , though more or fewer temperature settings could be used. The respective temperature settings provide the control module  12  adjusts the effective capacity of the water heater  10 . 
     In addition to selecting the desired set point temperature, the consumer is also able to select a desired energy savings setting, for example 0—No Energy Savings, 1— Moderate Energy Savings, or 2— Aggressive Energy Savings. Selecting an energy level provides the control module  12  with the ability to adjust the consumer set point temperature to tailor effective capacity. The energy savings levels are exemplified by levels 0, 1, 2 ( FIG. 2 ) but could include additional energy savings levels. The consumer selects the respective energy savings setting at the consumer interface  45  by depressing one of the buttons  48 . 
     The first energy savings setting, 0—No Energy Savings, does not allow the control module  12  to lower the consumer-selected set point temperature. The second energy savings level, 1—Moderate, allows the control module  12  to lower the consumer-selected set point temperature by an initial setback value. Thus, the temperature to which the water in the water heater  10  will be heated is the control set point temperature, i.e., the consumer-selected set point temperature minus the initial setback value. As already described, a lower water temperature in the tank  14  reduces the effective capacity of the electric water heater  10 . At the reduced set point temperature, the consumer draws more hot water from the tank  14  in order to obtain water at a desired temperature. Energy savings, though, is realized because the entire volume of water in the tank  14  is heated to a lower temperature. 
     The third energy savings setting, 2—Aggressive, similarly allows the consumer-selected set point temperature to be lowered by the initial setback value. In addition, the second energy savings setting allows the control module  12  to lower the set point temperature still further, by up to a maximum setback value. With the maximum setback value, the control module  12  can further reduce the effective capacity of the water heater  10  in an effort to optimize the energy efficiency of the water heater  10  based on consumer demand for hot water. 
     Once the consumer selects a set point temperature and energy savings setting, the control module  12  initially controls the water heater  10  based on the respective consumer inputs (i.e., set point temperature and energy savings setting). 
     In operation, the control module  12  first determines the control set point temperature based on the initial setback value. Note that regardless of which energy savings level is selected (i.e., 1 or 2), the control module  12  initially sets the control set point temperature to a value equal to the consumer-selected set point temperature minus the initial setback value, unless the energy savings level chosen is 0—No Energy Savings. In so doing, the control module  12  generates a control set point temperature that is lower than the consumer-selected set point temperature, reducing the effective capacity of the water heater  10 . With the control set point temperature determined, the control module  12  then controls the function and operation of the electric water heater  10  as previously described. 
     Once the water heater  10  is at the control set point temperature the control module  12  monitors hot water consumption by the consumer. By monitoring the upper heating element  16 , the control module  12  is able to react to hot water usage and adjust effective capacity. As previously discussed, the upper heating element  16  is only energized during a deep draw event when the incoming cold water contacts the upper temperature sensor  36 . Therefore, the control module  12  is able to determine that the water heater  10  has excess effective capacity when the upper heating element  16  has not been energized for a predetermined period. In addition, the control module  12  is able to determine that there is a need for additional effective capacity if the upper heating element  16  has been energized for a predetermined period. 
     It should be noted that the predetermined amount of time is generally referred to as a “cycle” and is usually at least one week in duration to allow for a week&#39;s worth of household events that may give rise to a deep draw event such as, for example, laundry day. The control module  12  may also collect usage data to generate historical usage data (i.e., water usage over time). The control module  12  may then utilize the collected historical data to develop usage patterns. The usage patterns may be used by the control module  12  in anticipating setback temperatures for different times of day or days of the week. In this manner, the control module  12  may control the capacity of the water heater  10  based on historical information to prepare for certain household events. 
     For example, if laundry day falls on Thursday for three consecutive weeks, the control module  12  may increase the effective capacity of the water heater  10  on Wednesday night in anticipation of laundry day. Conversely, if a consumer is routinely away from home on Saturdays and Sundays, the water heater  12  may reduce the effective capacity on Friday night. Therefore, the control module  12  may be used to tailor energy consumption based on consumer water usage and may collect data to anticipate future water usage. 
     If the control module  12  determines that there is excess effective capacity in the water heater  10 , the control module  12  will take one of two actions. First, if the energy savings setting is set to level 1, the control module  12  must continue to control the water heater at the consumer-selected set point temperature minus the initial setback value. If conditions warrant a further decrease in effective capacity, however, the control module  12  alerts the consumer via consumer interface module  45  to change the energy savings setting from level 1 to level 2. Second, if the energy savings setting is set to level 2, the control module  12  lowers set point temperature by the maximum set back value to further reduce the effective capacity of the water heater  10 . However, the control module  12  is only permitted to reduce the set point temperature by the maximum setback value. 
     Conversely, if the control module  12  determines that there is not enough effective capacity in the water heater  10 , the control module  12  increases the effective capacity by raising the control set point temperature, but is limited in doing so by the consumer-selected set point temperature. 
       FIG. 4  details an exemplary savings module  58  for use by the control module  12  for determining when an increase or a decrease in effective capacity is warranted. The energy savings module  58  utilizes the control module  12  and associated sensor module  35  to tailor the effective capacity of the water heater  10  to the specific needs of the individual consumer by continuously monitoring the consumer&#39;s hot water usage. Initially, the control module  12  compares the consumer-selected set point temperature to a threshold cutoff temperature, which is too low to allow operation of the energy savings module  58  (i.e., a setback from the consumer-selected set point temperature would result in a cold water condition). In one exemplary embodiment, the cutoff temperature is between 115 degrees Fahrenheit and 120 degrees Fahrenheit. Therefore when the consumer-selected set point temperature is lower than the cutoff temperature (i.e., 115-120 degrees Fahrenheit), the energy savings module  58  sets the control set point temperature at the consumer-selected set point temperature at  62  as the control module  12  cannot setback the temperature lower than 115 degrees Fahrenheit. At this point, the control module  12  maintains the water disposed within the tank  14  at the consumer-selected set point temperature by selectively toggling the upper and lower heating elements  16 ,  18  between the ON and OFF states. 
     If the consumer-selected set point temperature is above the cutoff temperature, the control module  12  reduces the consumer-selected set point temperature by the initial setback amount to the control set point temperature at  64 . Once the control set point temperature is determined, the control module  12  maintains the water within the tank  14  at the control set point temperature by selectively toggling the upper and lower heating elements  16 ,  18  between the ON and OFF states. 
     The control module  12  controls the water heater  10  at the control set point temperature for one cycle (i.e., at least one week). The control module  12  monitors the sensor module  35  to determine if the upper heating element  16  has been energized during the cycle at  66 . If the upper heating element  16  has been energized during the cycle, the control module  12  concludes that the water heater  10  has experienced a deep draw event and requires additional effective capacity at  68 . However, if the upper element  16  has not been energized during the cycle, the control module  12  references a timer to determine whether the cycle has expired at  70 . If the timer has expired (indicating that the cycle has ended), the control module  12  concludes that the water heater  10  has not experienced a deep draw event within the last cycle at  72 . At this point, the control module  12  concludes that the set point temperature should be further reduced to decrease the effective capacity of the water heater  10 . 
     The control module  12  determines a float range for the setback value based on whether the upper heating element  16  has been energized during the last cycle at  74 . The float range defines an amount the control module  12  is allowed to either increase or decrease the set point temperature to effectuate a change in effective capacity. The control module  12  is limited in implementing the float range by the maximum setback value as the control module  12  is not permitted to reduce the consumer-selected set point temperature more than the maximum setback value at  76 . In addition, the control module  12  is limited by the cutoff temperature (i.e., 115-120 degrees Fahrenheit). 
     If the control module  12  determines that additional energy savings are possible because the upper heating element  16  has not cycled for a predetermined time, or that the water heater  10  is not producing enough hot water to keep up with demand (i.e., the upper heating element  16  is regularly cycled ON), the control module  12  alerts the consumer. The control module  12  notifies the consumer that at least one of the set point temperature setting or the energy savings level should be adjusted to allow the control module  12  the flexibility to optimize performance of the water heater  10 . The control module  12  recommends such action through use of a performance monitoring module  78  to rectify an over capacity or an under capacity situation. 
     With particular reference to  FIG. 5 , operation of the performance monitoring module  78  is described. The performance monitoring module  78  generates a recommendation to the consumer to save energy by selecting a lower set point temperature or generates a recommendation to the consumer to increase the set point temperature based on hot water demand history. For example, if the setback value is equal to the maximum setback value, the control module  12  cannot further reduce the consumer-selected set point temperature even if there is excess effective capacity in the water heater  10 . Therefore, the only way for the control module  12  to reduce the effective capacity of the water heater  10  is to start at a lower consumer-selected set point temperature. Therefore, the control module  12  must alert the consumer that the consumer-selected set point temperature should be adjusted. 
     The control module  12  first determines if the setback value equals the maximum setback value at  80 . If the setback value equals the maximum setback value, and the upper heating element  16  has not cycled ON for a predetermined period of time, the control module  12  recommends to the consumer via the LCD  40 , LED  42 , and/or speaker  44  that the consumer-selected set point temperature should be reduced to realize further energy savings at  82 . If the consumer reduces the set point temperature, the control module  12  is able to further reduce the effective capacity of the water heater  10  by calculating the control set point temperature from a lower consumer-selected set point temperature. Such a reduction in effective capacity ultimately saves the consumer energy as excess water is not needlessly heated. In this manner, even though the control module is restricted from reducing the consumer-selected set point temperature by the maximum setback value, the control module  12  can still further reduce the effective capacity of the water heater  10 . 
     If the setback amount is zero, and the upper heating element  16  has been cycled ON during a previous period, the control module  12  determines that an increase in effective capacity is necessary at  84 . At this point, the control module  12  alerts the consumer of the need for additional effective capacity at  86  and recommends increasing the consumer-selected set point temperature via the LCD  40 , LED  42 , and/or speaker  44 . If the control module  12  is able to properly control the effective capacity of the water heater  10  based on hot water demand and consumer-selected input, the control module  12  displays that the system is functioning within its limits and is able to sufficiently optimize the effective capacity of the water heater  10  at  88 . 
     In each of the foregoing situations, the control module  12  must alert the consumer to either raise or lower the consumer-selected set point if the maximum setback is achieved. The control module  12  makes such recommendations through a consumer interface display module  90 . 
     The consumer interface display module  90  for use with the above-described performance monitoring module  78  is shown in  FIG. 6 . The consumer interface display module  90  determines whether the LCD  40  recommends an increase in the consumer-selected set point temperature and whether the consumer has acted on the recommendation  92 . If the consumer has acted on the recommendation, the recommendation is removed and the display  40  notes that the system is functioning within limits. At this point, the control module  12  sets the setback value to be generally equal to the initial setback value plus the amount that the consumer-selected set point temperature was increased  96 . 
     Similarly, the consumer interface display module  90  determines whether the LCD  40  recommends a decrease in the consumer-selected set point temperature and whether the consumer has acted on the recommendation  98 . If the consumer has acted on the recommendation, the recommendation is removed and the display  40  that the system is functioning within limits and is able to sufficiently optimize the effective capacity of the water heater  10  at  100 . At this point, the control module  12  sets the setback value to be generally equal to the initial setback value minus the amount that the consumer-selected set point temperature was decreased  102 . 
     It should be noted that for the consumer interface display module  90 , the consumer&#39;s acting on the recommendation (i.e., to raise or lower the set point temperature range) does not immediately change the temperature of the water disposed within the tank  14 . Following the recommendation simply shifts the control module&#39;s  12  operational limits so that the control module  12  has greater flexibility to further adjust the effective capacity of the water heater  10  when necessary in view of hot water demand history, thereby realizing greater energy efficiency. 
     The control module  12 , by optimizing effective capacity of the water heater  10 , allows more hot water to be available at lower set point temperatures, as demonstrated by the differential module  104  of  FIG. 7 . 
     During periods of non-use, the temperature of water within the tank  14  will fall due to heat escaping through tank walls. Therefore, maintaining the tank  14  at a lower temperature reduces energy loss. At lower set point temperatures, the water within the tank  14  is only allowed to vary from the set point temperature a small amount to increase the average temperature of the tank  14 . Reducing the operating range of the tank  14  at lower set point temperatures ensures that there is enough hot water within the tank  14  to deliver water at a comfortable temperature (i.e., the delivered temperature). 
     For higher set point temperatures, the differential module  104  allows a wider temperature differential (i.e., 12° F.) between the set point temperature and the temperature of the water at which the heating elements  16 ,  18  are energized. For lower temperatures, the differential module  104  allows a narrower temperature differential (i.e., 7° F.). This relationship allows more hot water to be available at lower set point temperatures. For example, a set point temperature of 145° F. requires a differential of 12° F., thereby allowing the water to range between 133° F. and 157° F. A set point temperature of 105° F. requires a differential of 7° F., thereby allowing the water to range between 98° F. and 112° F. 
     Each degree lost by the water heater  10  during non-use has a greater impact in reducing effective capacity at lower set point temperatures than at higher set point temperatures. Maintaining the temperature of the water close to the set point temperature allows more hot water to be available. 
     Therefore, by controlling the effective capacity of the water heater  10  to a state that minimizes the set point temperature (i.e., by reducing the consumer-selected set point temperature by the setback value), more hot water is available at lower set point temperatures and energy is saved. 
       FIG. 8  schematically represents the relationship between the control module  12 , sensor module  35 , energy savings module  58 , performance monitoring module  78 , user interface module  90 , and differential module  104 . Each of the modules  35 ,  58 ,  78 ,  90 ,  104  communicate with the control module  12  to aid the control module  12  in continuously adjusting the set point temperature of the water heater  10  until the effective capacity and energy use are optimized. 
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.