FIELD OF THE INVENTION 
   1. Technical Field 
   This disclosure relates to water heaters. In particular, this disclosure relates to water heaters having an electronic control system. 
   2. Background 
   In one aspect, a water heater includes a water container/tank and a burner to heat the water in the tank. A water heater may also include a controller for regulating the burner. By regulating the burner, the controller at least partially determines the hot water output of the water heater. For example, when the controller is associated with a temperature monitoring probe within the container and a thermostat, it may relate a sensed parameter such as water temperature with a control parameter such as a predetermined temperature range to determine whether operation of the burner is needed to achieve a desired hot water output. 
   For a gas water heater, the controller may comprise a gas valve. For example, the gas valve may act as a switch for turning the burner on and off. The gas valve may also be controlled electronically to operate according to a control algorithm. For example, a control algorithm may be designed for regulating operation of the heating element to meet various environmental and/or efficiency targets. 
   During the heating cycle in a typical storage-type water heater, hot water tends to rise to the top of the tank and cold water tends to settle at the bottom. The amount of difference in temperature between the top of the tank and the bottom is affected by many parameters including the placement of the temperature monitoring probe, output and size of the burner, the material composition of the tank and/or combustion compartment, the rate and frequency of water usage and the like. This difference in temperature between the top of the tank and the bottom is commonly referred to as “stacking.” 
   Stacking is prominent in conditions where the hot water supply is cycled on and off frequently, that is, where the hot water is drawn to point where the burner is activated by the controller, and then the water is turned off shortly thereafter. In this situation, a substantial amount of standby hot water already exists in the tank. The application of further heat magnifies the stacking problem by further raising the temperature of the water at the top portion of the tank. As such, continuous cycling over a prolonged period can create further unwanted stacking. 
   As one might infer, placement of the temperature monitoring probe, cold water intake and hot water exit within the tank are factors that influence stacking. Currently, a significant amount of development time is spent in identifying locations to place these elements within the tank that will trade off hot water capacity against a maximum desirable water temperature under worst case stacking conditions. 
   One method for controlling stacking is disclosed in U.S. Pat. No. 6,560,409 to Troost. Troost describes a method wherein the frequency of removal of water from a water heater is monitored and related to water temperature to control the operation of a heating element. For example, the temperature control set point may be depressed or reset in response to a water temperature condition in the tank and the frequency of water removal over a time period. While suitable for its intended purpose, it would be advantageous to extend the methods to control stacking in Troost to applications that will increase the hot water output and energy efficiency of a typical water heater. 
   For example, while it is desirable to control stacking, it is also desirable to add cold water to the lowest portion of tank as possible to maximize hot water output. The endurance of a water heater with respect to its ability to sustain hot water output is measured as its “first hour” rating. Minimizing the mixing of the intake cold water with the heated water at the top of the tank will maximize the first hour rating. However, to increase the hot water provided while avoiding adverse conditions, additional advances in controlling stacking should be achieved. 
   SUMMARY  
   We provide a water heater including a water container having a cold water inlet and, a hot water outlet, a combustion chamber adjacent the water container, a burner associated with the container, a temperature sensor associated with the container that detects water temperature in an upper portion of the container, and a controller that activates the burner, the controller establishing an initial water temperature set point, comparing detected water temperature with the set point when the burner is activated, reducing the set point by a first selected amount when the water temperature decreases by less than a selected amount while the burner is activated or increasing the set point by a second selected amount when the water decreases by a selected amount or more while the burner is activated. 
   We also provide a method of controlling a water heater including detecting water temperature in an upper portion of a water container in the water heater, establishing an initial water temperature set point, comparing detected water temperature with the set point when a burner of the water heater is activated, reducing the set point by a first selected amount when the water temperature decreases by less than a selected amount while the burner is activated or increasing the set point by a second selected amount when the water decreases by a selected amount or more while the burner is activated. 
   We further provide a water heater including a water container having a cold water inlet, a hot water outlet, and a combustion chamber adjacent the water container, a dip tube connected to the cold water inlet and extending downwardly from a top portion of the water container and having a length that is between about 85% and about 90% of the height of the water container, a burner associated with the combustion chamber, an upper sensor associated with the water container that detects water temperature in an upper portion of the water container, a lower temperature sensor associated with the water container that detects water temperature in a lower portion of the water container; and a controller that activates the burner in response to temperature sensed by the upper and lower sensors. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1   a  is a schematic partial sectional view of a water heater which emphasizes certain features of the water heater and omits others for ease of understanding. 
       FIG. 1   b  is a schematic partial sectional view of the water heater of  FIG. 1   a  having an additional temperature sensor. 
       FIG. 2   a  is a schematic frontal view of a controller associated with the water heater shown in  FIG. 1  and connected to a tank internal temperature sensor. 
       FIG. 2   b  is a schematic frontal view of a controller associated with the water heater shown in  FIG. 1  and connected to a tank external temperature sensor. 
       FIG. 3  is a flow diagram which illustrates the operation of various aspects of the invention. 
   


   DETAILED DESCRIPTION 
   It will be appreciated that the following description is intended to refer to specific aspects of our water heaters selected for illustration in the drawings and is not intended to define or limit those water heaters, other than in the appended claims. 
   Turning now to the drawings generally and  FIGS. 1 and 2 , in particular, an environment for the implementation of aspects is shown which includes a water heater  10 . One skilled in the art will appreciate that the water heater  10  comprises several components, some of which are shown and some of which are not. These additional water heater  10  components, while relevant to operation of the water heater  10 , are not particularly pertinent to the description herein and, as such, are not described herein. 
   Water heater  10  includes an outer jacket  12 , which surrounds foam insulation  14 . The foam insulation  14  surrounds a water tank  16 . A top pan  18  caps the jacket  12  on its upper end and a bottom pan  20  caps the jacket  12  on its lower end. An inlet  22  in the upper portion of the tank  16  provides for cold water to enter the tank  16  through, for example, an inlet tube  23 . Similarly, an outlet  24  allows for hot water to exit through the upper portion of the tank  16 . 
   The water heater  10  further includes a burner  26 . The burner  26  may comprise any commercially available burner. The burner  26  is positioned to receive fuel from a fuel line  28 , which connects to a gas valve  30 , which connects to a fuel supply line  32  connected to a fuel supply that is not shown. Burner  26  may be further positioned within a combustion chamber  34  and above an air intake opening  36  in the bottom pan  20  to receive combustion air. 
   A temperature monitoring sensor  38 , shown in  FIGS. 1 and 2 , is associated with the tank  16  for monitoring the temperature of water in the tank  16 . The temperature monitoring sensor  38  may be positioned to monitor the temperature of the water in the upper portion of the tank  16  as shown in  FIG. 1   a,  as an example. Sensor  38  may be internal to tank  16  as shown in  FIG. 2   a  or external. For example, a thermistor can be used on the outside of tank  16  as shown in  FIG. 2   b . Any type of sensor may be used such as thermocouples, RTD&#39;s, bimetals and the like. The temperature monitoring sensor  38  further provides information related to the frequency of hot water removal from the tank  16 . For example, a decrease in water temperature at the upper portion of the tank  16  may be correlated with the frequency of hot water removal from the tank  16 . 
   A plurality of temperature monitoring sensors  38  may be associated with the tank  16  as shown in  FIG. 1   b,  as an example. For example, the plurality of temperature monitoring sensors  38  may be electrically linked together to monitor the water temperatures in upper and lower portions of the tank  16  relative to each other. A plurality of temperature monitoring sensors  38  may be utilized to average the water temperatures in various portions of the tank  16 . 
   The upper sensor helps monitor and control stacking and facilitates lengthening the dip tube for more hot water delivery. Tables 1 and 2 below shows the effect of lengthened dip tubes. Thus, it is advantageous to have the length of the dip tube be within about 85%—about 90% of the tank height. 
   
     
       
             
           
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               Water Inlet Dip tube Length 
             
           
        
         
             
                 
                 
                 
               Invention Dip 
                 
                 
               Invention 
             
             
                 
                 
               Std Dip tube 
               tube Length 
               Invention Dip 
               Std Dip tube 
               Dip tube % 
             
             
               Model 
               Tank Ht (in) 
               Length (ind) 
               (in) 
               tube Increase % 
               % Tank Ht 
               Tank Ht 
             
             
                 
             
           
        
         
             
               40 Gallon 
               47.5 
               34 
               42 
               24 
               72 
               88 
             
             
               50 Gallon 
               46.25 
               34 
               42 
               24 
               74 
               91 
             
             
                 
             
           
        
       
     
   
   
     
       
             
             
             
             
           
             
             
             
             
           
         
             
               TABLE 2 
             
             
                 
             
             
                 
               First Hour 
               First Hour 
                 
             
             
               Model 
               Recovery Std 
               Recovery Invention 
               % Improvement 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               40 Gallon 
               69 
               79 
               14.5 
             
             
               50 Gallon 
               80 
               91 
               13.75 
             
             
                 
             
           
        
       
     
   
   A controller  40 , shown in  FIG. 2 , is associated with a fuel control element such as gas valve  30  and is utilized to receive signals indicative of water temperature and the frequency of water removal. Controller  40  subsequently sends signals to control the operation of the heating element  26 . For example, the frequency of water removal may be signaled by monitoring decreases in water temperature at the upper portion of the tank  16 . Such temperature monitoring is achieved by temperature monitoring sensor  38 . Alternatively, the frequency of water removal may be monitored directly or by other means well known to those skilled in the art such as, for example, water flow or water pressure monitoring. 
   The controller  40  may include a microprocessor  42 . One example of microprocessor  42  may be of the type disclosed in U.S. Pat. No. 6,560,409, the subject matter of which is incorporated herein by reference. Other microprocessors may be employed. 
   The microprocessor  42  may be operable to receive selected inputted information such as, for example, water temperature information from the temperature monitoring sensor  38 . The microprocessor  42  may be pre-programmed and/or programmable to set and/or adjust the temperature control set point based on the inputted information. For example, the microprocessor  42  may be pre-programmed and/or programmable to continuously vary the temperature control set point when the water temperature is within a predetermined range. The microprocessor  42  may be further pre-programmed and/or programmable to adjust the temperature control set point on a selected basis depending on various local conditions and parameters for a particular water heater  10 . The microprocessor  42  may carry out thermostat functions for the controller  40  by providing signals to activate and/or deactivate the burner  26  according to a pre-programmed and/or programmable control algorithm. 
   An example of the system operation is now described. The microprocessor  42  may be pre-programmed and/or programmable to adjust the temperature control set point as a function of water usage. The microprocessor may include two operating modes. In a first standard operating mode, the microprocessor  42  is operable to set or maintain a user-selectable temperature control set point. In a second energy saver operating mode, the microprocessor is operable to regulate the temperature control set point according to a control algorithm. For example, the second operating mode may be activated during periods when the burner is activated. In the second operating mode, the microprocessor may receive signals indicating the water temperature from the temperature monitoring sensor  38 . If the water temperature drops by less than 10° F. while the burner  26  is activated, the microprocessor  42  will reduce the temperature control set point by 2° F. If the water temperature drops by more than 10° F. while the burner  26  is on, the microprocessor  42  will increase the temperature control set point by 3° F. The microprocessor may be further pre-programmed and/or programmable to prevent the new set point from being permitted to exceed the initial set point or fall below a predetermined minimum set point. For example, the minimum temperature control set point may be 115° F. The initial set point, minimum set point and the predetermined magnitudes for adjusting the set point may be user-selectable. 
   The temperature control set point may be continuously variable and thus, lower on average to minimize the conditions in which stacking may occur in the tank  16 . This second energy saving operating mode may reduce heat loss by up to about  30  percent when hot water demand is low for a gas water heater. The microprocessor  42  may also be pre-programmed and/or programmable to continuously vary the temperature control set point more or less, or for different water temperature ranges than those described above. 
     FIG. 3  is a flow diagram which illustrates one example of the operation. While the microprocessor  42  may perform operations herein, another element or a combination of elements may also be operable to perform some or all of the operations described. As such, the flow diagram should be understood as only one example for implementing aspects this disclosure. 
   The plot  300  begins at step  302  wherein the microprocessor  42  determines whether a first or second operating mode is selected and whether the temperature control set point is above a minimum set point. For example, the minimum set point may be 115° F. If the microprocessor  42  is set to a first operating mode or the temperature control set point is below the minimum set point, the microprocessor  42  will operate in a first operating mode wherein the microprocessor is operable to set and maintain a temperature control set point for the burner  26  at step  304 . 
   If the second operating mode of the microprocessor  42  is selected and the temperature control set point is above a minimum set point, the microprocessor  42  will be operable to continuously vary the temperature control set point within a predetermined/selected water temperature range at step  306  by first recording the initial set point at step  308  and monitoring the water temperature versus the set point during a period when the burner is on at step  310 . For example, the temperature monitoring sensor  38  may be associated with the microprocessor  42  for monitoring the water temperature in the tank  16 . If the burner  26  is not on, the microprocessor  42  will maintain the temperature control set point until a burner  26  is on period and stand by in the selected mode. 
   When the burner  26  is on at step  312 , the microprocessor  42  determines whether the water temperature has decreased less than a predetermined magnitude. For example, the predetermined magnitude may be 10° F. or any magnitude determined to represent a selected flow rate of hot water out of the tank  16  and for which an adjustment in the set point is determined to be desirable. If the water temperature decreases by less than a predetermined magnitude, the microprocessor  42  increases the temperature control set point by a predetermined amount up to the initial set point at step  314 . If the water temperature decreases by more than a predetermined amount, the microprocessor  42  reduces the temperature control set point by a predetermined amount down to a minimum set point at step  316 . 
   The control algorithm makes it possible to associate the cold water intake with the bottom portion of the water tank to increase hot water output. For example, the cold water intake may be an inlet tube associated with the bottom portion of the tank  16  designed to allow a minimum amount of cold water mixing with the heated water in the upper portion of the tank. The cold water inlet associated with the bottom portion of the tank improves hot water recovery by as much as about an additional 15 percent over the standard mode in conjunction with the control algorithm. 
   Our water heaters may also be equipped to monitor/check for so-called “dry-fire” conditions. In such a case, when the gas valve  30  is activated when there is a call for a need to heat water, the controller  40  activates burner  26  for about a 1 to 2 minute period of time. The activation period is based on tank thickness, burner location and sensor position, among other things. The controller monitors the lower sensor  38   b  (as opposed to the upper sensor  38   a ). If the lower thermistor temperature increases at a rate higher than a set rate such as 3° F. per minute, then controller  40  shuts gas valve  30  off, which also shuts off main burner  26 . The controller  40  can flash a so-called “error” code or provide other means of notification or alarm. The 3° F. degrees per minute rate of increase is also based on tank thickness, burner location and thermistor position. This indicates that the tank is dry under such a condition and that there is a problem with the water heater unit. 
   Although our water heaters have been described in connection with specific forms thereof, it will be appreciated that a wide variety of equivalents may be substituted for the specified elements described herein without departing from the spirit and scope of this disclosure as described in the appended claims. For example, water tank  16  may be made of a number of sizes and may be made from a wide variety of materials such as metals and/or plastics. Foam insulation  14  may similarly be made from any number of high energy efficient foam insulations well known in the art. 
   Also, the bottom of the water tank  16  may have various shapes, either with lower flanges as shown or as a flat construction. Other modifications may be made, including use of foam insulation between the bottom of tank  16  and bottom pan  20 . Also, outer jacket  12  may be made from any number of materials such as rolled metals, preferably steel, or extruded vinyl materials and the like. Also, top pan  18  and bottom pan  20  may be deep-drawn, stamped or the like, or be made from metal, plastic or other suitable materials. Various types of heating elements may be utilized so long as they are used in conjunction with sensors  38 . 
   The adjustment temperatures for the set point and the conditions necessary for set point adjustment are fully variable and the values used herein are examples for illustration purposes only. One skilled in the art will note that many set point usage combinations are possible without varying from the spirit and scope of the disclosure.
A water heater including a water reservoir container having a cold water inlet and a hot water outlet, a combustion chamber adjacent the water container, a burner associated with the container; a temperature probe associated with the container that detects a water temperature in an upper portion of the container, and a controller that activates the burner, the controller establishing an initial water temperature set point, comparing detected water temperature with the set point when the burner is activated, reducing the set point by a first selected amount when the water temperature decreases by less than a selected amount while the burner is activated or increasing the set point by a second selected amount when the water decreases by a selected amount of more while the burner is activated.