Patent Publication Number: US-8111980-B2

Title: Water heater and method of controlling the same

Description:
RELATED APPLICATIONS 
     This application is a division of U.S. patent application Ser. No. 10/808,686, filed Mar. 25, 2004, which is a continuation-in-part of U.S. patent application Ser. No. 10/453,163, filed Jun. 3, 2003, issued as U.S. Pat. No. 6,795,644, which is a division of U.S. patent application Ser. No. 09/752,477, filed Jan. 2, 2001, issued as U.S. Pat. No. 6,633,726, which is a continuation-in-part of U.S. patent application Ser. No. 09/361,825, filed Jul. 27, 1999, issued as U.S. Pat. No. 6,374,046, all of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to water heaters and method of controlling water heaters. 
     BACKGROUND 
     A storage-type water heater typically comprises a permanently enclosed water tank, a cylindrical shell coaxial with and radially spaced apart from the water tank to form an annular space between the outer wall of the water tank and the inner wall of the shell, and insulating material in at least a portion of the annular space for providing thermal insulation to the water tank. The water tank has various appurtenances such as inlet, outlet and drain fittings. Additionally, the water heater is provided with a water heating and temperature control system. In an electrical water heater, the water heating and temperature control system includes an electrical resistance heating element. The electrical resistance heating element extends through a fitting in the wall of the water tank such that the heating element is inside the tank. The heating element is connected to an electrical power source outside the water tank. In a gas water heater, the water heating and temperature control system includes a combustion chamber, typically beneath the tank, and a gas heating element (e.g., a gas burner) in the combustion chamber. An air plenum upstream of the combustion chamber provides air to the heating element, and a flue for discharge extends upward from the combustion chamber and through the water tank. 
     Conventional water heating and temperature control systems typically further include a mechanical thermostat. For the electrical water heater, the mechanical thermostat closes a switch to allow electrical power through the electrical resistance heating element when water in the tank is sensed to be below a selected set-point temperature, and opens the switch to stop electrical power from passing through the electrical resistance heating element when the water in the tank is at or above the set point temperature. Electrical power through the electrical resistance heating element is either fully on, passing full electrical current, or completely off. 
     Due to variations in manufacture and hysteresis of the mechanical thermostat, the temperature of the water will “overshoot” the desired set-point temperature. In other words, the water heating and temperature control system allows the heating element to continue heating water in the water tank even when the water temperature is above the set point temperature. It would be beneficial to prevent or limit the amount of overshoot of the conventional water heater. 
     SUMMARY 
     Accordingly, the invention provides, in some embodiments, a water heater having a controller for modulating power from a heating element in controllable pulses or bursts. Having a heating element provide power in pulses or bursts allows an amount of water to be heated to a selected temperature at substantially the same rate as with a mechanical temperature controller of the prior art, yet uses substantially less power to heat the water. Therefore, modulating the power improves the efficiency of the water heater by using less electricity or gas. 
     One way for modulating power in short bursts to the heating element is by using a temperature controller that takes into account the unique signature of the water heater. That is, when determining the amount of modulation between a burst of power being supplied to a heating element and a period during which no power is supplied to the element, the water heater may vary the amount of modulation based on a number of variables or water heater characteristics. The variables can include the number of elements, the location of the heating element(s), inlet water temperature, water capacity of the water tank, ambient room temperature of the physical environment in which the heater is installed, and usage patterns of the facility in which the heater is being used. By combining all of these aspects with burst or pulsing technology, significantly greater energy savings are achieved over conventional water heaters. 
     In another embodiment, the invention provides a water heater including a tank for holding water, a water inlet line having an inlet opening that introduces cold water to the tank, a water outlet line having an outlet opening that withdraws heated water from the tank, and a heating element. The water heater further includes a control circuit operable to control power issued by the heating element. In one construction, the heating element issues power in bursts. Each burst is followed by a period during which power is not supplied to the heating element. In another construction, the heating element decreases the issued power as the temperature of the water approaches a set point temperature. 
     In another embodiment, the tank has a tank characteristic, the heating element has an element characteristic, and the control circuit includes a temperature sensor operable to sense a temperature of the water within the tank. The control circuit further includes a controller in communication with the heating element and the temperature sensor. The controller is operable to receive the sensed temperature from the temperature sensor, to determine a heating strategy for the water heater based in part on the element characteristic and/or the tank characteristic, and to generate a signal activating the heating element in response to the heating strategy. In yet another embodiment, the control circuit is further operable to change the on to off time in response to the sensed water temperature and at least one of an element characteristic, a tank characteristic, an external water tank temperature, a water consistency, and a history of water use. 
     In even yet another embodiment, the invention provides a gas-powered water heater including a water tank, a combustion chamber having a thermal relation to the water tank, and a gas heating element disposed in the combustion chamber. The gas heating element including a first combustive section and a second combustive section separately controlled from the first combustive section. 
     In another embodiment, the invention provides a storage-type water heater including a water tank, at least one water temperature sensor operable to sense a water temperature, a combustion chamber having a thermal relation to the water tank, a gas heating element disposed in the combustion chamber, a valve connectable to a fuel source and connected to the gas heating element, and a controller in communication with the valve and the temperature sensor. The controller is operable to receive the sensed temperature, to determine a ratio of the maximum amount of fuel or energy deliverable to the heating element based on the sensed temperature, the ratio being determined from a plurality of available ratios including a ratio between zero and one hundred percent, and to selectively generate a control signal to control the valve based on the determination. 
     The invention also provides a method of controlling the temperature of water in a water heater. In one embodiment, the method includes the acts of sensing a temperature of the water in the tank, determining an amount of power to be provided to the heating element based at least in part on the sensed temperature, and transmitting the amount of power from the power source to the heating element. In one construction, the method further includes determining a heating strategy. The act of determining the heating strategy can be based at least in part on an element characteristic, a tank characteristic, an environment (i.e., ambient) temperature, a water characteristic (i.e., temperature, hardness, etc.), or a combination thereof. 
     In another embodiment, the invention provides a method of controlling a temperature of water in a storage-type water heater. The method includes the acts of sensing a temperature of the water, determining a ratio of the maximum power deliverable by the heating element over a time period based on the sensed temperature, and controlling the valve to deliver an amount of power corresponding to the determined ratio. In one construction, the ratio is determined from a plurality of available ratios including a ratio between zero and one hundred percent. 
     Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims, and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of a water heater embodying the invention, and showing the arrangement of an exemplary temperature controller in relation to other components of the water heater. 
         FIG. 2  is an electrical schematic of a temperature controller capable of being used with the water heater of  FIG. 1 . 
         FIG. 3  is a plot of energy usage data of a mechanical temperature controller and a relational band temperature controller. 
         FIG. 4  is a plot of energy consumption rate data of the mechanical temperature controller and a relational band temperature controller. 
         FIG. 5  is a sectional view of another water heater embodying the invention and having multiple heating elements. 
         FIG. 6  is a sectional view of yet another water heater embodying the invention and having multiple heating elements. 
         FIG. 7  is a partial sectional view of the water heater shown in  FIG. 6 . 
         FIG. 8  is a sectional view of yet another water heater embodying the invention. 
         FIG. 9  is a partial perspective view of the controller shown in the water heater of  FIG. 8 . 
         FIG. 10  is a schematic representation of the control circuit of the water heater shown in  FIG. 8 . 
         FIG. 11  is an electrical schematic of a power supply of the control circuit shown in  FIG. 10 . 
         FIG. 12  is an electrical schematic of a zero crossing detector of the control circuit shown in  FIG. 10 . 
         FIG. 13  is an electrical schematic of a low-voltage reset circuit of the control circuit shown in  FIG. 10 . 
         FIG. 14  is an electrical schematic of a temperature sensing circuit of the control circuit shown in  FIG. 10 . 
         FIG. 15  is an electrical schematic of a thermostat of the control circuit shown in  FIG. 10 . 
         FIGS. 16(   a ) and  16 ( b ) are an electrical schematic of portions of the control circuit shown in  FIG. 10 . 
         FIG. 17  is an electrical schematic of an oscillator for the control circuit shown in  FIG. 10 . 
         FIG. 18  is a flowchart representing an exemplary method of controlling the water heater shown in  FIG. 8 . 
         FIG. 19  is a flowchart representing an exemplary method of performing a test to determine whether a heating element is submerged. 
         FIGS. 20   a ,  20   b ,  20   c  and  20   d  are portions of a flowchart representing an exemplary method of performing the acts of gathering sensor samples, computing water temperature, computing a thermostat setting, changing operating mode if necessary, setting a heating cycle state, and setting a heating priority. 
         FIG. 21  is a flowchart representing an eight hundred microsecond interrupt event. 
         FIG. 22  is a perspective view of another water heater embodying the invention. 
         FIG. 23  is a sectional view of the bottom portion of the water heater of  FIG. 22 . 
         FIG. 24  is a partial side view, partial sectional view of a fuel control system capable of being used with the water heater of  FIG. 22 . 
         FIG. 25  is a schematic view of another construction of a fuel control system capable of being used with the water heater of  FIG. 22 . 
         FIG. 26  is a top view of a gas burner capable of being used with the water heater of  FIG. 22 . 
         FIG. 27  is a sectional view of the gas manifold tube shown in  FIG. 26 . 
         FIG. 28  is a top view of another gas burner capable of being used with the water heater of  FIG. 22 . 
     
    
    
     DETAILED DESCRIPTION 
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. The terms “connected,” “coupled,” and “mounted” and variations thereof herein are used broadly and, unless otherwise limited, encompass both direct and indirect connections, couplings, and mountings. In addition, the terms “connected,” “coupled,” and “mounted” and variations thereof herein are not restricted to physical and mechanical connections or couplings. 
     The use of a relational (e.g., proportional) band temperature controller in a water heater having a heating element has the unexpected advantage of heating water in the water heater to a preselected set point temperature while consuming less power than is consumed when heating the same amount of water to the same set point temperature in the same water heater using a mechanical temperature controller of the prior art. As used herein, the term “heating element” is broadly defined as one or more elements and/or one or more sections of an element that produces or radiates thermal energy. For example and as will be shown below, the heating element can include an electrical resistance heating element, and in another example, can include a gas heating element. Before proceeding further, the term “gas heating element” and related terms used herein (“gas burner,” “gas manifold,” etc.) refer to elements or components that receive a fuel, where the fuel can be in a state other than a gas. That is, a gas heating element is not limited to receiving only a gas fuel (e.g., a gas heating element can receive a liquid fuel). Additionally, other types of heating elements can be used with a water heater embodying the invention. While the invention will be described below in connection with a storage-type water heater, aspects of the invention can be used in an instantaneous-type water heater and/or in other apparatus (e.g., a boiler). 
     An exemplary relational band temperature controller is an electronic device which comprises a water temperature sensing device (e.g., a thermistor), a temperature set point device (e.g., a variable rheostat), a gated thyristor for switching electric power to an electrical resistance heating element, and a logic circuit for controlling the thyristor in response to signals from the water temperature sensing device and the temperature set point device. The logic circuit receives a voltage input from the water temperature sensing device and the temperature set point device which indicates the differential of the water temperature from the set point temperature. The logic circuit, in response to the voltage inputs from the water temperature sensing device and the temperature set point device, signals the gated thyristor. At large temperature differentials between the water temperature sensing device and the temperature set point device, the logic circuit signals the gated thyristor to conduct electricity during a major portion, about 94%, of each cycle of the AC current, and signals the gated thyristor to stop conducting electricity for about 6% of each AC cycle. As the temperature differential between the water and the set point narrows, the water temperature enters a relational (e.g., proportional) control band where the logic circuit begins to exert more control over the gated thyristor to limit electric power to the resistance heating element. As the water temperature enters the relational control band, the logic circuit establishes a new control cycle period and signals the thyristor to conduct electric power for 85% of each cycle and to stop conducting for 15% of each cycle. As the water temperature gets closer to the set point temperature the logic circuit signals the thyristor to conduct for less of each cycle period. When the water temperature reaches the set point temperature, the logic circuit closes the thyristor and electric power is not supplied to the resistance heating element until the water temperature again falls below the set point temperature. To prevent undue cycling about the set point temperature, the logic circuit is set to require the water temperature to drop 5° to 10° F. below the set point temperature before the thyristor is again signaled to conduct electric power and heat the water back to the set point temperature. 
     This improvement in the efficiency of heating water in the water heater using a relational band temperature controller is not completely understood. In theory, essentially all the electrical power supplied to a resistance heating element will be converted to heat, and that heat should be transferred to the water surrounding the resistance heating element. The same amount of electric power should heat the same weight of water the same number of degrees temperature. As shown in the example below, a water heater having a relational band temperature controller requires about 10% less electrical power to heat a tank of water to a selected set point temperature than the same water heater having a mechanical temperature controller of the prior art. The improved accuracy of a relational band temperature controller for bringing water to a set point temperature with little overshoot accounts for some of the improved efficiency over a mechanical temperature controller, but does not appear to account for all. 
     While not wishing to be bound, I suggest that the improvement in heating efficiency when using a relational band temperature controller arises from physical conditions within the water tank affecting the transfer of heat from the resistance heating element to the water. A relational band temperature controller conducts electric power to the resistance heating element in short bursts followed by short periods during which electric power is not conducted until the water in the water tank reaches a selected set point temperature. The relational band temperature controller accurately stops conducting electric power to the resistance heating element when the water reaches the set point temperature. On the other hand, a mechanical temperature controller of the prior art conducts electric power to the resistance heating element continuously at full power as the water is heating. When the water reaches the set point temperature, mechanical characteristics of the bimetallic thermocouple may cause the mechanical temperature controller to overshoot and heat the water to a temperature above the set point temperature before it stops conducting electric power to the resistance heating element. 
     A resistance heating element, as is used in domestic water heaters, heats in a few seconds to a temperature in the range of 800° F. to 900° F. Water, in contact with such a hot resistance heating element, may vaporize depending on tank pressure, may form a layer of vapor around the resistance heating element and reduce the transfer of heat from the resistance heating element to the water. With a mechanical temperature controller, the resistance heating element is so heated and remains at a high temperature until the bimetallic thermocouple cuts off electric power. Heat from a resistance heating element controlled by a mechanical temperature controller may be radiated to the wall of the water tank, or may be transported by vaporization convection currents to the top of the water tank where the excess heat is absorbed in the topmost layer of water which is located away from the temperature sensing bimetallic thermocouple. 
     With a relational band temperature controller, the resistance heating element is heated during each burst of electric power and is cooled by contact with the water during periods between bursts. This cooling of the resistance heating element between each burst of electric power reduces the temperature to which the resistance heating element is raised and reduces the potential for accumulation of vaporization around the hot resistance heating element. Consequently, heat transfer from the resistance heating element to the water is increased. Supplying electric power to a resistance heating element in a water heater in discrete short bursts, each burst followed by a period with the electric power shut off, improves the efficiency of heat transfer from the resistance heating element to the water in the water heater. 
     Relational band temperature controllers are well known and widely used in many commercial applications, including to control water temperature in such appliances as coffee makers. Relational band temperature controllers have not, to my knowledge, been used to control the temperature of a large volume of water in a storage water heater. 
       FIG. 1  of the drawing shows a sectional view of a water heater  10  comprising a permanently enclosed water tank  11 , a shell  12  surrounding water tank  11 , and foam insulation  13  filling the annular space between water tank  11  and shell  12 . Water inlet line or dip tube  14  and water outlet line  15  enter the top of water tank  11 . The water inlet line  14  has an inlet opening  22  for adding cold water near the bottom of water tank  11 . Water outlet line  15  has an outlet opening  24  for withdrawing hot water from near the top of water tank  11 . Resistance heating element  16  extends through the wall of water tank  11 . The relational band control circuitry in control box  17  is connected to resistance heating element  16 . Thermistor  18 , in contact with the outer wall of water tank  11  for sensing the temperature of water in water tank  11 , is connected to the logic circuit by electrical wire  19 . Electric alternating current (AC) power is supplied to the gated thyristor through line  20 . A customizable operator interface can be mounted on the outside of the water heater to permit communication with the control box  17  and provides security protected access for control of the resistance heating element. The operator interface can be operable to provide direct or remote control of the resistance heating element. 
       FIG. 2  of the drawings is a schematic drawing of an exemplary relational band temperature control circuit  100  for heating water in the water heater  10 . In  FIG. 2 , resistance heating element  125  is a 4,500 watt heating element for heating water in a water heater. Temperature set point device  101  is a variable rheostat for setting the temperature set point in the range of about 90° F. to 180° F. Thermistor  102  is for sensing temperature of water in the water heater. In an alternative embodiment, a plurality of thermistors could be placed through the tank to measure water temperature at a plurality of locations. The output of the thermistors could be averaged. 
     Gated thyristor  103  is a triac, manufactured by Motorola, Inc., for controlling electric power to the resistance heating element  125 . Logic chip  104  is a proportional band temperature controller UAA1016A manufactured by Motorola, Inc. However, other relationships can be used by the band temperature controller. Two hundred forty volt electric power is supplied to the relational band temperature control circuit 100 through lines  105  and  106 . Opto-electric coupler  108 , as will be described below, is for controlling the amount the water temperature must decrease from the set point temperature before the relational band temperature control circuit  100  will reactivate. 
     A stabilized supply voltage of about −8 Volts is delivered to the relational band temperature control circuit  100  from line  106  through Zener diode  107  and resistor  109  into line  110 . Voltage drops through temperature set point device  101  and temperature sensor  102  produce a signal voltage at point  111 . The signal voltage is proportional to the temperature difference between the set point temperature and the sensed water temperature. The sensed voltage is transmitted via line  112  to one leg of a voltage comparator  113  within logic chip  104 . A reference voltage, the magnitude of which is determined by voltage drops through resistors  114  and  115 , is generated at point  116 . A saw tooth voltage, generated in saw tooth generator  118  in logic chip  104 , is imposed upon the reference voltage at point  119 . The reference voltage, modified by the saw tooth voltage passes via line  117  to the second leg of voltage comparator  113 . 
     The saw tooth voltage imposed upon the reference voltage causes the voltage at the second leg of voltage comparator  113  to vary, in a saw tooth pattern, over a cycle of about 0.85 seconds from a minimum to a maximum voltage. In voltage comparator  113 , the signal voltage at the first leg is compared to the modified reference voltage at the second leg. The comparison result is transmitted via line  120  to logic circuit  121 . In logic circuit  121 , a signal is generated for passing via line  122 , amplifier  123 , and line  124  for controlling thyristor  103 . When the signal voltage at the first leg of comparator  113  is greater than the maximum value of the reference voltage at the second leg of comparator  113 , the signal to thyristor  103  is to conduct and allow electric power to flow through resistance heating element  125  for heating water in the water tank. Logic chip  104  is arranged such that the signal in line  124  causes thyristor  103  to conduct electricity for 96% of each AC current cycle and stop conducting for 4% of each current cycle. 
     The signal voltage at the first leg of voltage comparator  113  will fall to a value less than the maximum value of the reference voltage at the second leg of voltage comparator  113  as the water temperature sensed by temperature sensor  102  approaches the set point temperature selected on set point temperature device  101 . When the signal voltage is in the range between the maximum value of the reference voltage and the average of the reference voltage value, the temperature control circuit  100  is within the proportional band control range. Thus, when the signal voltage is greater than the value of the reference voltage at the second leg of the voltage comparator , logic circuit  121  signals amplifier  123  to signal thyristor  103  to conduct electric power to resistance heating element  125 . Then, as the saw tooth voltage causes the reference voltage at the second leg of voltage comparator to increase to a value greater than the value of the signal voltage at the first leg of the voltage comparator, logic circuit  121  signals amplifier  123  to signal thyristor  103  to stop conducting electric power to resistance heating element  125 . As the signal voltage at the first leg of voltage comparator approaches closer to the average value of the reference voltage at the second leg of voltage comparator  113 , thyristor  103  is not conducting for greater percentages of each cycle of the generated saw tooth voltage. When the water temperature sensed by temperature sensor  102  is equal to the set point temperature of temperature set point device  101  the signal voltage at the first leg of voltage comparator  113  will equal the average reference voltage value at the second leg of voltage comparator  113  and logic circuit  121  signals amplifier  123  to turn off thyristor  103 , shutting off electric power to resistance heating element  125 . Thyristor  103  remains in the non-conducting state until the water temperature sensed by temperature sensor  102  falls below the set point temperature by a preset amount, as is described below. 
     The signal voltage at the first leg of voltage comparator  113  and the reference voltage at the second leg of voltage comparator  113  must have values which allow logic circuit  121  to produce a signal to amplifier  123  which will properly control thyristor  103  to heat the water to the desired temperature. Temperature set point device  101  is a variable rheostat the resistance of which can be adjusted manually for changing the set point temperature. Temperature sensor  102  is a thermistor in which the resistance decreases as the sensed temperature of the water increases. The values of resistors  126  and  127  are selected such that the signal voltage at point  111  will be proportional to the difference between the set point temperature and the sensed water temperature. The reference voltage at point  116  is determined by the value of resistors  114  and  115 , and the magnitude of the saw tooth voltage imposed upon the reference voltage at point  119  is determined by the values of resistors  128  and  129 . The values for these resistors must be adjusted to accommodate the characteristics of the particular temperature set point device  101 , temperature sensor  102 , and logic chip  104  selected for the relational band temperature control circuit  100 . 
     As described above, opto-electric coupler  108  is included in relational band temperature control circuit  100  to prevent undue cycling of thyristor  103  when the sensed water temperature is at about the set point temperature. When the sensed water temperature equals the set point temperature, logic circuit  121  signals amplifier  123  to cut off thyristor  103  and stop conduction of electric power to resistance heating element  125 . Without opto-coupler  108 , when the sensed water temperature drops a small amount, for example, less than 1° C., below the set point temperature, logic circuit  121  will signal amplifier  123  to open thyristor  103  and conduct electric power to resistance heating element  125  until the sensed water temperature is again heated to the set point temperature. This action results in rapidly turning thyristor  103  off and on, to control the sensed water temperature as closely as possible to the set point temperature. 
     Opto-electric coupler  108 , connected electrically across resistance heating element  125  by lines  130  and  131 , operates to make the sensed temperature appear to be about 5° C. higher than it actually is when electric current is flowing through resistance heating element  125 . So, when the water temperature sensed by temperature sensor  102  reaches the set point temperature, thyristor  103  is stopped from conducting electric current through resistance heating element  125  and opto-electric coupler  108 . With no current flowing through opto-electric coupler  108 , the signal voltage at point  111  is determined by voltage drop through temperature sensor  102  and voltage drop through set point device  101 , resistor  126 , and resistor  127 . Resistor  127  produces a voltage drop equivalent to the voltage drop caused by about a 5° C. temperature change in the sensed temperature. Consequently, the sensed temperature appears to be about 5° C. higher than it actually is, and the sensed temperature must drop an additional 5° C. before the signal voltage at the first leg of voltage comparator  113  will indicate that the sensed temperature is below the set point temperature. When voltage comparator  113  signals logic circuit  121  that the sensed temperature is below the set point temperature, logic circuit  121  signals amplifier  123  to open thyristor  103  and allow electric current to flow through resistance heating element  125 . With electric current flowing through resistance heating element  125 , electric current flows through opto-electric coupler  108  via lines  130  and  131 . With electric current flowing through opto-electric coupler  108 , resistor  127  is bypassed and the 5° C. bias to the apparent sensed water temperature is removed. Logic circuit  121  then signals amplifier  123  to open thyristor  103  until the sensed water temperature again reaches the set point temperature. This action of opto-electric coupler  108  allows the sensed temperature to fall about 5° C. below the set point temperature before thyristor  103  again conducts electric power through resistance heating element  125 , and allows the sensed water temperature to be heated to the set point temperature before electric power is cut off from resistance heating element  125 . This action prevents cycling of electric current through resistance heating element  125  when the sensed water temperature is at about the set point temperature. 
     In an alternative embodiment, the temperature control circuit  100  could include a programmable real time clock wherein peak or off-peak energy demand periods or vacation operation cycles could be programmed into the control cycle for the heating element. Additionally, a pressure sensor, temperature sensor, mineral deposit sensor and/or sensor for detecting the presence of water could be added. The control circuit would be programmed to disconnect power from the water heater and/or the heating element when predetermined conditions or limits are detected. Further, the control circuit could include means for automatically adjusting the set point in response to various conditions such as amount of water used, or whether it is a peak or off-peak energy demand period. 
     EXAMPLE 
     In a first example, an electric water heater having a 4,500 Watt resistance heating element was operated for heating water from 60° F. to 120° F. using 240 Volt AC current. In a first run, a commercially available bimetallic thermostat, as described in the introduction to this application, was used to sense the water temperature and control electric current to the resistance heating element. In a second run, the relational band temperature control circuit, as shown in  FIG. 2  and described in this application, was used to sense the water temperature and control flow of electric current to the resistance heating element. Results of the two comparative runs are shown in  FIG. 3  of the drawings. 
     For Run  1 , tension on a bimetallic thermostat was adjusted with a threaded stud such that the bimetallic thermostat would snap from a flat configuration to a domed configuration at a set point temperature of 120° F. The bimetallic thermostat was placed in contact with the outer wall of the water heater water tank at a position about three inches above the electric resistance heating element. The bimetallic thermostat was connected, via an insulating rod, to an electric switch in a line supplying electric power to the resistance heating element. The water tank was filled with 60° F. water and the electric power connected to the line supplying the resistance heating element. The bimetallic thermostat remained in a flat position and the electric switch was closed. Electric current passed through the resistance heating element at a rate of 19.7 amperes for about 27 minutes until the water was heated to about 122° F. The bimetallic thermostat then snapped into a domed shape, activating the switch to cut off electric current to the resistance heating element. A graph of water temperature versus time for this first run is shown in  FIG. 3 . 
     For Run  2 , a relational (e.g., proportional) band temperature control circuit, as shown in  FIG. 2  and described above in this application, was used. The temperature set point device  101  was calibrated for a set point of 120° F., and the thermistor temperature sensing device  102  was attached to the water tank about three inches above the resistance heating element  125 . Thyristor  103  was connected to resistance heating element  125 . The water tank of the water heater was drained and refilled with 60° F. water and the relational band temperature control circuit  100  was connected to the electric power main. The relational band temperature control circuit  100  initially supplied 18.8 amperes of electricity to the resistance heating element  125 , i.e. about 95% of the amperes supplied by the mechanical thermostat of Run  1 . After about four minutes (at 68° F.), the relational band temperature control circuit  100  reduced the electricity supplied to resistance heating element  125  to 18.6 amperes, i.e. about 91% of the amperes supplied by the mechanical thermostat of Run  1 . After about 21 minutes (at 104° F.), the sensed water temperature entered the relational band temperature range and the relational band temperature control circuit  100  began to slowly reduce electric current to resistance heating element  125 , until after 27 minutes the sensed water temperature reached the set point temperature and the relational band temperature circuit  100  shut off electric current to the resistance heating element  125 . 
     Inspection of  FIG. 3  shows that the same amount of water was heated to substantially the same temperature in the same amount of time in Run  1  and Run  2 . However, in Run  1 , 19.7 amperes of electricity were required and in Run  2 , only about 18.6 amperes of electricity were required over the heating period. That is, heating water in a water heater equipped with the relational band temperature control circuit shown in  FIG. 2 , which supplies electricity to the resistance heating element  125  in short bursts followed by short periods with electricity shut off, requires about 9% less electric power than heating the same amount of water to the same temperature in the same water heater, but using a mechanical temperature controller. This is an unexpected result. 
     The pulsing of current to the load by the relational band temperature control circuit permits the water temperature to minutely rise and fall rapidly in response to the applied current. A brief interruption of current applied to the heater element each cycle allows for a more efficient transfer of radiation energy to the water from the heater element. 
     As a second example, a test was performed to determine the actual amount of energy a consumer would use during a typical hot water heater operating cycle. Referring to  FIG. 4 , the actual kilowatt hours (kWh) is plotted versus time for a mechanical thermostat and an electronic thermostat including relational band control logic. 
       FIG. 4  illustrates that during a typical heating cycle, approximately 3% less energy is being used as a direct result of using the relational band control logic. It is possible that this percentage could be increased to approximately 5-5.5% by changing the conduction angle of the triac&#39;s firing quadrants, without adversely affecting the performance of the water heater. 
     Additionally, by limiting the current to the heater element using relational band control logic and by supplying the current to the heater in pulses, gradually coasting to the temperature set point without overshooting the desired temperature offers an additional 15% energy reduction. 
     The combination of current modulation and preventing the overshooting of the temperature set points offers the consumer a combined energy savings of nearly 10% over the cost of operation of a similar heater using a bimetal mechanical thermostat. 
     Overheating water past a reasonable temperature of 125° F. -130° F. generally wastes energy. A typical two inch thick layer of insulation loses its capacity to effectively retain heat at temperatures above 130° F. or so. This energy loss in standby mode is wasteful and potentially causes the heater to cycle more often than necessary. 
     The relational band control circuit shown in  FIG. 2  prevents overshooting and allows the water temperature to drop only 10° F. or so to cycle only the needed difference to return the water temperature to a desired setpoint. 
     An additional advantage to the relational band control circuit shown in  FIG. 2  is its suitability for a flammable vapor environment. For example, such an environment may exist in a garage, workshop, or basement storage area wherein solvents, gasoline, propane or other highly flammable or explosive vapors are present. Mechanical thermostats and contact type switching devices can arc when an electrical contact is made or broken, depending on the amount of current being switched. The electrical arc can ignite a flammable vapor if the vapor is sufficiently volatile. In contrast, the relational band control circuit is totally solid state, has no moving parts, and would not ignite flammable vapors. 
     While implementing relational (e.g., proportional) band control as described above is advantageous, even greater heating efficiency can be achieved in a water heater with multiple, controlled resistive heating elements. An exemplary water heater  150  with such elements is shown in  FIG. 5 . The water heater  150  shares many common elements with the water heater  10 , and common elements are designated with the same reference numerals in  FIGS. 1 and 5 . However, unlike the water heater  10 , the water heater  150  has multiple electrical resistance heating elements  16  and  16 ′. Resistance heating element  16  is located in the lower portion of the tank and resistance heating element  16 ′ is located in the upper portion of the tank. The resistance heating element  16 ′ can be controlled by control circuitry stored in a control box  17 ′ which receives input from a thermistor or temperature sensor  18 ′ through a communication link  19 ′, such as an electrical wire. Alternatively, although not shown, the sensor  18 ′ and resistance heating element  16 ′ could communicate with control circuitry stored in the control box  17  and just one controller, rather than multiple circuits, could be employed. Communication between the sensor  18 ′ and heating element  16 ′ could be accomplished through a communication link (not shown) running physically parallel to line  20 . In the case of controlling two resistance heating elements with a single controller, the control circuitry in box  17  might take the form of a programmable microprocessor. Of course, more than two heating elements could be installed in the water heater  150  and controlled by such a controller if desired. 
     Regardless of the exact control circuitry used, or whether a single controller or multiple controllers are implemented, the heating elements in  FIG. 5  are activated sequentially or at some predetermined frequency or fashion so that heat energy being transferred to the tank  150  is distributed in a balanced or uniform manner. Thus, for example, the heating element  16  might be active for a first period of time T 1  during which power is supplied to it in the pulsed or multiple-burst manner described above. Subsequently, the element  16 ′ might be activated in a pulsed manner for a period of time T 2 . Times T 1  and T 2  may or may not be of equivalent lengths, and may or may not overlap one another depending on the specific heating application and conditions. Moreover, feedback mechanisms employing the temperature sensors  18  and  18 ′ may be used to trigger activation of the specific resistance heating elements depending upon the temperature sensed in the upper and lower portions of the tank  11 . 
     Whatever specific sequencing is employed, the use of a relational band temperature controller to control multiple elements in a water heater helps to avoid uneven heating of the water in the tank. Uneven heating generally occurs in conventional heating systems where the bulk of water heating is accomplished with a resistance heating element positioned near the bottom of the heater tank. This configuration often results in the creation of “stacking,” where water that is heated rises to the top of the tank and becomes super-heated, while non-uniform temperature strata are formed in the lower portion of the tank. To make matters worse, the heat accumulation at the top of the tank tends to rapidly dissipate because the insulation  13  in the tank cannot effectively retain the high energy heat from the super heated water. With sequential pulse or burst heating of water as described herein, water in the tank  11  is more uniformly heated. This reduces the occurrence of hot or cold spots in the strata from the top to the bottom of the tank. The creation of super heated water is also reduced and efficiency is increased. 
     The sequencing described above can also be combined with controlled introduction of cold water through an outlet or conduit  155  of a mixing valve  157  positioned in the dip tube  14 . The valve  157  can be controlled through a communication link V I/O  coupled to the control circuitry in box  17 ′ or, alternatively (and not shown), the circuitry in box  17  when it is configured to control multiple heating elements. Thus, for example, if super heating is sensed by the sensor  18 ′ in the upper portion of the tank, an amount of cold water may be introduced into the top portion of the tank  11  through the outlet  155  to lower the temperature of the heated water. 
     Yet another water heater  160  embodying the invention is shown in  FIG. 6 . The water heater  160  shares many common elements with the water heaters  10  and  150 , and common elements are designated with the same reference numerals in  FIGS. 1 ,  5  and  6 . For the embodiment shown in  FIG. 6 , the water tank  160  defines a volume  165  having an approximately upper two-thirds volume  170  and an approximately lower one-third volume  175 . The inlet opening  22  is disposed in the lower one-third volume  175  and introduces cold water into the tank  11 . The outlet opening  24  is disposed within the upper two-thirds volume  170 . 
     As shown in  FIG. 6 , both electrical resistance heating elements  16  and  16 ′ extend into the lower one-third volume  175  of the tank  11 . The heating elements  16  and  16 ′ are controlled by control circuitry stored in control box  17  which receives input from temperature sensors  18  and  18 ′. Alternatively, the water heater  160  can include more than one control box, can include more than two resistance heating elements, and can include more than two temperature sensors. 
     Similar to what was disclosed for water heater  150 , the resistance heating elements  16  and  16 ′ are activated sequentially or at some predetermined frequency or fashion so that heat is transferred to the tank  11  in a balanced or uniform manner. Additionally, resistance heating elements  16  and  16 ′ can be activated by controller  17  utilizing relational (e.g., proportional) band control techniques. 
     In one construction of water heater  160 , the resistance heating elements  16  and  16 ′ are arranged in a plane  180  substantially orthogonal to the longitudinal axis  185  of the tank  11  (i.e., in a substantially “horizontal” plane) (see  FIG. 7 ). However, the resistance heating elements  16  and  16 ′ can be placed in any other configuration in the approximately lower one-third volume  175  as long as both elements are in the approximately lower one-third volume  175  (See  FIG. 6 ). Also, if additional heating elements are used, they too are located in the approximately lower one-third volume  175 . 
     Typically, water heaters of the prior art rarely utilize the upper resistance heating element. The upper resistance heating element is typically active only when the water heater is first installed, when the water heater has not been used for a long period of time, or when a large amount of hot water has been extracted from the tank in a short period of time. Except for these rare occurrences, the upper resistance heating element of the prior art is rarely used. Thus, most of the water heated over the life of the unit is heated using only the lower resistance heating element. The use of only the lower resistance heating element is energy inefficient, requires a large period of time for recovery of the water temperature to set point temperatures, and often requires a large reserve storage tank of heated water to insure that an adequate supply of hot water is present when needed. The water heater  160  overcomes the above-described deficiencies by placing the second resistance heating element  16 ′ in the approximately lower one-third volume  175  of the tank  11 . Arranging the elements  16  and  16 ′ this way and controlling the operation of the elements  16  and  16 ′ by generating sequential pulses having relational band control allows the water heater  160  to utilize more efficient water heating strategies. This results in the resistance heating elements  16  and  16 ′ having an improved effective transfer of heat energy to the water. Furthermore, resistance heating elements  16  and  16 ′ more evenly distribute watt densities, which reduces vaporization losses. Consequently, the water heater  160  has a faster recovery time while using less energy than conventional heaters of the prior art. Moreover, the water heater  160  can have a more compact tank size for comparable hot water demands than the prior art. 
       FIG. 8  illustrates another water heater  200  embodying the invention. Water heater  200  includes a permanently enclosed water tank  205 , a shell  210  surrounding water tank  205 , and foam insulation  210  filling the annular space between the water tank  205  and the shell  210 . The water tank  205  has an outer surface  206 . Water inlet line or dip tube  215  and water outlet line  220  enter the top of water tank  205 . The water inlet line  215  has an inlet opening  225  for adding cold water near the bottom of water tank  205 . Water outlet line  220  has an outlet opening  230  for withdrawing hot water from near the top of water tank  205 . 
     The water heater  200  further includes a first resistance heating element  235  and a second resistance heating element  240  extending through the wall of the water tank  205 . It is envisioned that the resistance heating elements  235  and  240  can be placed anywhere within the tank  205  and can be of any particular shape. However, for the construction shown, the first and second resistance heating elements  235  and  240  are in a lower one-third volume of the tank  200 , and are in a plane substantially orthogonal to a longitudinal axis (similar to  FIG. 7 ). In addition, although the invention will be described with two heating elements  235  and  240 , the water heater  200  can include additional heating elements or can contain just one heating element  235 . For example, a commercial tank water heater (as compared to a residential tank water heater) can contain as many as fifteen heating elements. 
     The water heater  200  includes a first water temperature sensor  245  and a second water temperature sensor  250 . Both water temperature sensors  245  and  250  are mounted on the outer surface  206  of water tank  205 . For the construction shown, the water temperature sensors  245  and  250  are thermistors and are thermodynamically coupled to the water in the water tank  205 . The water temperature sensor  250  is located on a lower half of the tank  205  and the temperature sensor  245  is located on an upper half of the tank  205 . However, it is envisioned that the water temperature sensors  245  and  250  can be mounted on the same half of the tank  205 . Additionally, the water heater  200  can include additional temperature sensors or can contain only one temperature sensor  245 . 
     The water heater  200  can include an ambient or room temperature sensor  255 . The ambient temperature sensor  255  is located external to the water heater  200 , but is located within the surrounding environment of the water heater  200  and senses the temperature of the surrounding environment of the water heater  200 . Of course, the water heater  200  can include additional ambient temperature sensors and can include other sensors (e.g., a water consistency sensor). 
     The water heater  200  includes a controller or control unit  260  electrically connected to the first and second heating elements  235  and  240 , the first and second water temperature sensors  245  and  250 , and ambient temperature sensor  255 . In general terms, the controller  260  receives a two-hundred-forty volt alternating current (AC) signal from power line  265 ; modulates first and second signals provided to the first and second resistance heating elements  245  and  250 , respectively; receives first and second water temperature signals from the first and second temperature sensors  245  and  250 , respectively; and receives an ambient temperature signal from ambient sensor  255 . 
     As shown in  FIG. 9 , the controller  260  includes a housing  267  having a visual display area  270  and a user entry area  275 . The visual display area  270  includes a plurality of light-emitting diodes (LEDs). The LEDs include a first element LED 2 , a second element LED 3 , a system LED 4 , a heat LED 5 , an alert LED 6  and a power LED 7 . Power LED 7  is preferably a red LED and lights any time the electronics are active (i.e., “on”). System LED 4  is preferably green and is used to indicate the overall status of the system. During normal operation, the system LED 4  blinks approximately one blink per second. The fact that the system LED 4  is blinking regularly indicates that the water heater is working properly. Heat LED 5  blinks in unison with the system LED 4  when the controller  260  is in a “heating” mode (i.e., the water heater is heating the water to a desired). First element LED 2  and second element LED 3  activate whenever the respective heating elements are active. Alert LED 6  and heating LED 5  are in the same package. Alert LED 6  works in conjunction with the system LED 4  to indicate the status of the water heater  200 . 
     During normal operation, if the controller  260  is in a “Stand-by” mode (i.e., the temperature of the water is equal to or greater than the desired water temperature), only the system LED 4  blinks. If the controller  260  is in the heating mode, the controller  260  blinks the system LED 4  and the heating LED 5  in unison. If for any reason there is an error state, then the heating LED 5  changes to the Alert LED 6 , which is red. During the error state, the system LED 4  blinks an error code indicating the type of error. Of course, other LEDs can be added, and any of the disclosed LEDs can be removed or modified. Additionally, an audible speaker can be included to provide audible indication, or the information provided by the LEDs can be communicated by other visual indicators (e.g., a liquid crystal display). 
     The user entry area  280  includes an entry dial  283  for a user to enter a desired water temperature. The entry dial  283  includes an off position (i.e., the water heater  200  is “off”), a vacation position, and a plurality of positions between a low or cold water temperature and a high or hot water temperature. If the entry dial  285  is in the vacation position, then the controller is in a “vacation” mode. The “vacation” mode heats the water to a preset temperature lower than the normal temperature range of the water heater. Alternatively, the user entry area  275  can include other possible devices for entering a desired water temperature state including a plurality of push buttons with a digital LCD display. Of course, the visual display area  275  and the user entry area  280  can be mounted in a second control box located remotely from the water heater  20  (i.e., not mounted on the water heater  20 ). The second control box in communication with the controller  260  either through a hard-wired connection, or through RF or other appropriate communication scheme. 
     The controller  260  includes a control circuit  285 , which is schematically represented in  FIG. 10 . In general terms, the control circuit  285  includes a power supply  290 , a zero crossing detector  295 , a low-voltage reset circuit  300 , a temperature sensing circuit  305 , a thermostat circuit  310 , an LED control circuit  312 , a microcontroller U 1 , a memory unit  315 , a first driving circuit  320 , a second driving circuit  325 , and a dry fire circuit  330 . 
     As shown in  FIGS. 10 , the power supply  290  receives a high-voltage AC signal (e.g., AcIn=240 VAC) from line  260  ( FIG. 8 ) and creates a low voltage AC signal (e.g., AcOut=9 VAC), an unregulated direct current (DC) signal (e.g., V-SNS=5 VDC), and a regulated direct-current signal (e.g., Vcc=5 VDC). An exemplary power supply  290  is shown in greater detail in  FIG. 11 . 
     As shown in  FIG. 11 , the power supply  290  includes a transformer T 2  having a primary coil and a secondary coil for transforming the high-voltage AC signal (AcIn) to the low-voltage AC signal (AcOut). The resulting low-voltage AC signal (AcOut) is provided to the zero-crossing detector  295  ( FIG. 10 ) and to a switch S 1 , which is a single-throw, single pole (SPST) switch connected to the high side of the secondary coil. When the switch S 1  is closed, the control circuit  285  is active. 
     The power supply further includes a full-wave bridge rectifier D 8 , a capacitor C 26 , a zener diode D 9 , a voltage regulator U 9 , and capacitors CU 1 , CU 2 , CU 4 , CU 7  and CU 8 . The bridge rectifier D 8  rectifies the low-voltage AC signal (AcOut) and the capacitor C 26  filters the signal resulting in the unregulated DC signal (VSNS). The zener diode D 9  caps the unregulated DC signal (VSNS) and protects the input of the voltage regulator U 9  from short-term, over-voltage transients. The voltage regulator U 9  regulates the voltage to a Vcc signal of five volts and each of the capacitors CU 1 , CU 2 , CU 4 , CU 7  and CU 8  on voltage regulator U 9  are decoupling capacitors dedicated to a respective integrated circuit. For example, capacitor CU 1  is a decoupling capacitor for integrated circuit U 1 . 
     Referring back to  FIG. 10 , the power supply  290  provides the low voltage AC signal (AcOut) to zero-crossing detector  295 . An exemplary zero-crossing detector  295  is shown in greater detail in  FIG. 12 . Zero-crossing detector  295  provides an output signal (ZeroCross) which indicates each time the detector  295  detects that the low voltage signal (AcOut) has changed polarity. The zero-crossing detector  295  includes resistors R 55 , R 61  and R 53 , capacitor C 21 , diode D 1 , and transistor Q 8 . The resistor R 55  receives the low-voltage AC signal (AcOut). The diode D 1 , capacitor C 21 , and resistor R 61  are connected in parallel with one end connected to resistor R 55  and the base of transistor Q 8  and the other end connected to the emitter of transistor Q 8 . Resistor R 53  has one end connected to Vcc and the other end connected to the collector of transistor Q 8 . The zero-crossing signal (ZeroCross) is generated at the collector of transistor Q 8 . As the AC voltage changes polarity, Q 8  goes back and forth between the off state and saturation, generating a series of pulses having a front edge. The front edge of each pulse corresponds to a zero crossing. 
     Referring back to  FIG. 10 , the control circuit  285  includes a low-voltage reset circuit  300 . An exemplary low-voltage reset circuit  300  is shown in greater detail in  FIG. 13 . The low voltage reset circuit includes an integrated circuit U 3 , which is preferably a Motorola MC34064P-5 (although other circuits can be used) connected to a capacitor C 18 , and resistors R 45  and R 46 . The integrated circuit U 3  provides an under voltage reset protection signal to the microcontroller U 1 . In the event that power should fail or “brown” out, integrated circuit U 3  causes the microcontroller U 1  to reset. Preferably, this occurs as soon as the requested DC signal drops below four and one-half volts. The low-voltage reset circuit ensures that the control circuit  285  safely operates and does not malfunction due to low-line power. 
     Referring back to  FIG. 10 , the control circuit  285  includes a temperature sensing circuit  305 . The temperature sensing circuit  305  in combination with first and second water temperature sensors  245  and  250  transmits a water temperature for the water heater  200  to the microcontroller. As shown in greater detail in  FIG. 14 , the temperature sensing circuit includes resistors R 70  and R 71 , and thermistors RT 1  and RT 2 , which have a negative temperature coefficient. Resistor R 70  and thermistor RT 1  form a first voltage divider resulting in a first temperature signal (First-Sensor), and resistor R 71  and thermistor RT 2  form a second voltage divider resulting in a second temperature signal (Second-Sensor). Since the first and second voltage dividers are preferably the same, only the first voltage divider will be discussed in detail. As the temperature on the outside of the water tank  205  increases, the resistance in the thermistor RT 1  decreases causing the output voltage (First-Sensor) to increase. The voltage (First-Sensor) is read by an analog-to-digital (A/D) converter in microcontroller U 1  resulting in an eight-bit number. The eight-bit number is used as an index to a lookup table that has a plurality of corresponding sensed temperatures. Based on the eight-bit number, a sensed temperature results. 
     As the water inside the tank  205  increases in temperature, there is an increasing error in what the temperature sensor  245  or  250  senses. That is, the thermal conductive path from the water through the material of the water tank  205  has a lag time differential. To correct this, the sensed temperature value read from the lookup table is “corrected” by a linear equation. The corrected temperature is used in making water heating decisions by the microcontroller U 1 . 
     Referring back to  FIG. 10 , the control circuit includes a thermostat  310 . As shown in greater detail in  FIG. 15 , the thermostat is a potentiometer R 65  wired as a voltage divider and having a resistance range (e.g., 20 kOhms). The output signal of the voltage divider (Thermostat) is converted to an eight-bit number by the microcontroller U 1  and then scaled to produce a set-point temperature value. The set-point temperature value is the temperature to which the water will be heated. 
     Referring back to  FIG. 10 , the control circuit  285  includes an LED control circuit  312 . The LED control circuit  312  controls the activation of the light-emitting diodes LED 2 , LED 3 , LED 4 , LED 5 , LED 6  and LED 7 . As shown in greater detail in  FIG. 16(a) , the LED controller  312  includes resistors R 56 , R 57 , R 58 , R 59 , R 60 , R 47 , R 48 , R 49 , R 50 , R 51  and R 52 , and transistors Q 3 , Q 4 , Q 5 , Q 6  and Q 7 . When switch S 1  ( FIG. 11 ) is closed, the power supply  290  generates a regulated low-voltage DC signal (Vcc) that is provided to LED 7  and resistor R 52 . The provided low-voltage regulated DC signal (Vcc) lights LED 7 . For controlling LED 2 , LED 3 , LED 4 , LED 5  and LED 6 , a five-bit signal is provided to resistors R 56 , R 57 , R 58 , R 59  and R 60 . If any of the bits are high, a low-voltage DC signal is provided to the respective resistor R 56 , R 57 , R 58 , R 59  or R 60  resulting in a base current sufficient to allow current flow through the respective transistor Q 3 , Q 4 , Q 5 , Q 6  or Q 7 . The current flows from Vcc through the transistor Q 3 , Q 4 , Q 5 , Q 6  or Q 7 , through the respective light emitting diode LED 2 , LED 3 , LED 4 , LED 5  or LED 6 , to ground. 
     Referring back to  FIG. 10 , the control circuit includes a microcontroller or processor U 1  and a memory unit  315 . The microcontroller U 1 , which is also shown in  FIG. 16(   a ), is preferably a 28-pin Motorola MC68HC705P6A (although other microcontrollers can be used). The microcontroller U 1  includes an eight-bit input/output port (pins  3 - 10 ), a three-bit serial interface (pins  11 - 13 ), a four-bit analog to digital converter (pins  15 - 19 ), memory for storing a software program that operates the microcontroller, and two pins (pins  26  and  27 ) for receiving a signal from an oscillator  317  ( FIG. 17) . The memory unit  315  includes a two hundred fifty six byte Electrically Erasable Programmable Read Only Memory (EEPROM) chip U 4 . Of course, the memory for storing the software program and the memory unit  315  can be combined to form a single memory (or memory unit). The EEPROM U 4  is used to store configuration data, such as water heater construction specifics (e.g., operating voltage, tank water capacity, resistances of various elements, etc.), user usage pattern data, element type data, and other related data. With the EEPROM data and real-time sensory data (e.g., the sensed temperature of the first and second water temperature sensors  245  and  250 ), the microcontroller U 1  implements a software program to control the heating elements to heat and maintain water temperature. In addition, the software program includes at least one subroutine to determine whether water is surrounding each heating element. 
     Referring back to  FIG. 10 , the control circuit includes a first driving circuit  320  and a second driving circuit  325  that control the power being provided to the first and second resistance heating elements  235  and  240 , respectively. The driving circuits are identical and, thus, only driving circuit  320  will be discussed in detail. As shown in  FIG. 16(   b ), the first driving circuit  320  includes resistors R 66  and R 86  a triac Q 1 , and an opto coupled zero-cross triac opto-driver U 5 . The triac opto-driver U 5  is driven as determined by pulses being received from the output of the microcontroller U 1 . A pulse train is generated by the microcontroller U 1 , which determines the power levels being delivered to the resistance heating element  235  ( FIG. 10) . For example, the microcontroller U 1  can provide a pulse train to the driver U 5  resulting in a sixty-six percent power transfer (i.e., sixty-six percent of the available power (over a time period) is transferred to the heating element), or can provide a pulse train to the triac driver U 5  resulting in a forty percent power transfer. The driver U 5  is coupled to the zero-crossing detector  295  to insure that the triac turns completely off. Without the use of driver U 5 , the triac Q 1  could remain partially open in a conduction state and potentially effect the reliability of the control circuit  285 . 
     Referring back to  FIG. 10 , the control circuit includes a dry fire circuit  330 . As shown in greater detail in  FIGS. 16(   a ) and  16 ( b ), the dry fire circuit  330  includes data latch U 2  ( 16 ( a )), a first resistor ladder  335  ( 16 ( a )), a second resistor ladder  340  ( 16 ( a )), a voltage sensing amplifier  345  ( 16 ( b )), a current sensing amplifier  350  ( 16 ( b )), resistors R 90 , R 91 , R 92 , R 97 , R 98  and R 100  (all in  16 ( b )), transistors Q 9  and Q 10  (both in  16 ( b )), a current sensor T 1  ( 16 ( b )), and a resistor R 44  ( 16 ( b )). The data latch U 2  is preferably a Motorola 74HC374 data latch (other data latches can be used) and is used to hold a five-bit data word that controls the first and second resistor ladders  335  and  340 . The first resistor ladder  335  generates a voltage that is used as a reference by the voltage sensing amplifier  345 . Once this reference voltage has been set or calibrated, the data latch U 2  is used to control the second resistor ladder  340  to generate a voltage that is used as a reference by the current sensing amplifier  350 . The latch also holds three additional data bits. The first data bit (bit  7 ), controls one of the display LEDs; the second data bit (bit  6 ), selects the EEPROM; and the third data bit (bit  5 ), enables communication with off-board testing equipment. The current sensor T 1  and the resistor R 44  create a voltage that is proportional to the current being provided to the resistance heating elements. Transistors Q 9  and Q 10  select which amplifier is currently providing a signal to the microcontroller U 1 . 
     The basis for the “DryFire” test is the measurement of the peak voltage and peak current on an “almost” cycle by cycle basis. The reason that the measurement is not exactly cycle-by-cycle is that the voltage is measured after it has been rectified and filtered. Changes in the AC line voltage manifest as changes in the rectified DC voltage. Because of the time constant of the capacitor C 26 , with the resistance in the secondary windings of the power transformer, voltage and current samples are taken on a cycle-by-cycle basis and stored in a buffer. When the buffer is full, the voltage samples are examined to determine whether the voltage was stable during the time period it took to fill the buffers. If the variance is within acceptable limits, the voltage and current samples are average and a simple resistance calculation is performed (i.e., R=V/I). 
     When the manufacturer assembles the water heater  200 , the manufacturer programs into the memory unit  315  the components used for assembly of the water heater  200 , the capacity of the water tank  205 , and/or product information about particular components of the water heater  200 . For example, the manufacturer can program one or more tank characteristics and/or one or more element characteristics into the memory unit. The tank characteristics can include, but are not limited to, tank diameter, tank height, tank storage capacity, etc. The tank characteristics determine heating convection current flow patterns within the tank  205  that create different temperature water strata layers in the tank  205 . The element characteristics can include, but are not limited to, number of elements, element type, voltage of an element, physical location of an element (e.g., upper and lower, or side-by-side), element watt density, etc. The element characteristics help to provide information on how effectively the elements  235  and  240  will heat the water. 
     In addition, some of the tank or element characteristics can be determined by the microcontroller U 1 . For example, the microcontroller can calculate an element wattage for a particular element by applying a voltage to the element and calculating a resistance for the element over time. 
     Preferably, all of the water heater tank characteristics and element characteristics are programmed into the memory unit  315 . Based on the variables and characteristics, the microcontroller U 1  obtains from a lookup table a code specific to the water heater  200 . The software of the microcontroller U 1  creates a heating strategy for the water heater  200  based in part on the water heater code (discussed below). The microcontroller U 1  can update the water heater code if it senses that an element has been replaced or if a repairperson reprograms the data stored in the memory unit  315 . Additionally, although the manufacturer programs each variable or characteristic into the memory unit  315 , it is envisioned that the manufacturer can directly program the code or heating strategy into the memory unit  315 . 
     Because there are a diversity of tank characteristics and elements used in the manufacture and construction of electric water heaters, one heating strategy alone is unable to account for the numerous constructions. Instead, the software or manufacturer assigns a code to the water heater  200  based on the variables and characteristics of the water heater  200 . The variables and characteristics define a water heater signature and, when used with a water heater usage pattern, create a more reliable effective and energy efficient water heater. 
     In operation of water heater  200  and referring now to  FIG. 18 , a user “turns-on” the water heater  200  (act  500 ) by turning the thermostat  310  clockwise from the off position. This closes switch S 1 . Upon closing switch S 1 , the power supply  290  generates the low-voltage AC signal (AcOut), the unrectified DC signal (V-SNS) and the rectified DC signal (Vcc). Once the power source generates a Vcc greater than four and one-half volts, the low voltage reset  300  brings the microcontroller U 1  out of reset. If at any time the voltage drops below four and one-half volts (e.g., a user turns the system off, a “black-out” occurs, or a “brown-out” occurs), the low voltage reset  300  provides a signal to the microcontroller U 1  resetting the microcontroller U 1 . 
     At act  505 , after the microcontroller U 1  comes out of reset, the software initializes the microcontroller U 1 . The software resets all variables to their default values, and resets all outputs to their respective default states. 
     At act  510 , the microcontroller performs a “DryFire” test. The term “DryFire” refers to the heating of a resistance heating element  235  or  240  that is not submerged in water. Usually, a “DryFire” will destroy or burn-out the resistance heating element  235  or  240  in less than a minute. The control circuit  285  performs the “DryFire” test to determine whether the heating element is surrounded by water. 
     In general terms, the control circuit  285  performs the “DryFire” test by measuring the peak current and the peak voltage being applied to each resistance heating element  235  and  240  and making a resistance calculation based on the measurement. For example, by applying a voltage to one of the resistance heating elements  235  or  240  for a specific period of time and measuring the resistance at the beginning and end of the test period, the status of the resistance heating element  235  or  240  can be determined. As the element  235  or  240  heats up, its resistance increases. If the element is in water, the element reaches equilibrium (i.e., a steady temperature and resistance), very quickly. Conversely, if the resistance heating element  235  or  240  is “dry”, it continues heating and reaches high temperatures (and resistances) in a very short time. At the end of the test, the beginning and ending resistances are compared. For a “wet” element, the difference between the beginning and ending resistances is small, while for a “dry” element, the difference between the beginning and ending resistances is many times larger than when the element is wet. 
     In addition, by varying the length of the DryFire test, the resistance of the heating element  235  or  240  can be accurately measured. Based on the results, the microcontroller U 1  can update the water heater code. 
     An exemplary method for performing the DryFire test is shown in  FIG. 19 . At act  605 , the microcontroller U 1  deactivates the system LED during the DryFire test. Deactivating the system LED ensures that the blinking of the LEDs does not affect the test. At act  610 , the software sets an element number indicating the first resistance heating element  235  is being tested. At act  615 , the software sets the operating mode for the microcontroller U 1  to a DryFire mode which informs all subroutines that the microcontroller U 1  is performing a DryFire test. At act  620 , the software clears all DryFire error flags. The DryFire error flags indicate whether the most recent DryFire test (if one occurred) resulted in an error. For example, if the previous DryFire test resulted in an error flag corresponding to the first element being “dry”, then the microcontroller U 1  resets the error flag pending the results of the current test. 
     At act  625 , the microcontroller U 1  calibrates the voltage amplifier  345 . Before any voltage samples can be taken for DryFire calculations, the voltage amplifier  345  must be calibrated using a variable reference voltage generated by data latch U 2  and resistor ladder  335 . To accomplish this calibration, the microcontroller U 1  first selects the output of the voltage sensing circuit by driving Q 10  into saturation (Q 9  is off). The reference voltage (V-REF) is then set to its highest value. Next, the reference voltage (V-REF) is incrementally reduced until the output of the voltage amplifier (Dry-Out) reaches a predetermined value. The reference voltage is then left at this value. 
     For example, V-SNS is a non-regulated DC signal having a steady-state component and a small “alternating current” component. Any increases or decreases in the signal being provided to the transformer (AcIn) will reflect in the small “AC” component of V-SNS signal. In order for the microcontroller U 1  to notice any changes of significance, the voltage amplifier  345  amplifies small “AC” component changes. If, for example, the steady state is 2.0 volts, any reference voltage (V-REF) feeding resistor R 88  ( FIG. 16(   b )) above 2.0 volts will result in no amplification taking place and the output of the amplifier will be zero. If the reference voltage (V-REF) is below 2.0 volts, amplification will take place. The reference voltage (V-REF) is adjusted so the output of U7B is somewhere in the middle of its output swing (e.g., 0-3.5 volts). The microcontroller U 1  continues to reduce the reference voltage (V-REF) in steps until a desired output is reached (e.g., reference voltage is equal to 1.5 volts). Thus, any changes in the line voltage are exaggerated by a factor equal to the gain of U7B. 
     At act  630 , microcontroller U 1  calibrates the current amplifier  350 . As with the voltage amplifier  345 , the second stage, U8B ( FIG. 16(   b )), must be calibrated before sampling can begin. The current sensing circuit is selected by driving Q 9  into saturation (Q 10  is off) and then incrementally adjusting the reference voltage (I-REF) similar to the reference voltage (V-REF). 
     At act  635 , the software determines whether the voltage and current amplifiers  345  and  350  were properly calibrated. If there was an error in the calibration, then the software sets a calibration error flag(s) (act  640 ) to a positive result and proceeds to act  660 . If the calibration did not result in any errors, then the microcontroller U 1  proceeds to act  645 . 
     At act  645 , the microcontroller U 1  performs a DryFire test for the first resistance heating element  235 . For the test, instantaneous voltages and currents are measured at their peak values. This is accomplished by sampling the signal from the voltage and current amplifying circuits  345  and  350  (Dry-Out) relative to a zero crossing of the low-voltage AC signal (AcOut). At the appropriate zero crossing, a timer is started for each of the amplifying circuits  340  and  350 . A time-out variable is used to take the voltage or current samples at a predetermined time period with respect to the zero crossing when the voltage and current waveforms are at their peak. The instantaneous voltage and current samples are each loaded into separate buffers within the microcontroller U 1 . When the buffers are full, the data is analyzed to determine if the line voltage has been stable during the sampling period. If the sampled voltage is stable, an average voltage and current is computed, and a resistance calculation is made. Calculations continue in this manner for the duration of the DryFire test. At the end of the test, the beginning and ending resistance values are subtracted to find out how much the resistance has changed over the course of the test. The basis of the test is not the actual value of resistance (which is different for each type of heating element), but the difference in resistances from the beginning of the test to the end of the test. 
     At act  650 , the microcontroller U 1  determines whether the first resistance heating element  235  is dry. If the calculated resistance difference is greater than a set resistance change value (which can vary depending upon the heating element used) then the microcontroller U 1  determines that the element is not surrounded by water (i.e., “dry”) and proceeds to act  655 . If the microcontroller U 1  determines that the calculated resistance change is equal to or less than a set resistance change value, then the microcontroller U 1  determines that the element is surrounded by water and proceeds to act  660 . 
     At act  655 , the software sets a first element error flag to a positive result. A positive first element error flag informs subsequent subroutines that the first resistance heating element  235  is not surrounded by water. Consequently, later subroutines will not use this element to heat the water. The microcontroller U 1  will also set a ReCheck timer to 180 minutes. The ReCheck timer will decrease in time until it reaches zero minutes. When the ReCheck timer reaches zero, the microcontroller U 1  will perform another DryFire test on that element. 
     At act  660 , the microcontroller U 1  sets the element number to the second resistance heating element. At act  665 , the microcontroller U 1  repeats acts  625 ,  630 ,  635 ,  640 ,  645 ,  650  and  655  for the second resistance heating element to determine whether the second resistance heating element is dry. If the microcontroller U 2  determines the second resistance heating element is dry, it will set a second resistance heating element error flag to a positive result. Of course, if the water heater includes more than two resistance heating elements, then the microcontroller U 2  performs a dry test for the remaining elements. Additionally, if the water heater contains only one resistance heating element, then the microcontroller U 2  will not perform acts  660  or  665 . 
     Referring back to  FIG. 18 , at act  515 , the software determines whether a “ReCheck” timeout is greater than zero. The ReCheck timeout is a timer (e.g., twenty milliseconds) used by the software to inform the software when to sample the temperature sensors  245 ,  250  and  255 , and create or modify a heating strategy for heating the water contained within the water heater  200 . If the ReCheck timeout is greater than zero, then the software proceeds to act  520 . If the ReCheck timeout is less than or equal to zero, then the software proceeds to act  525 . 
     At act  520 , the microcontroller U 1  “blinks” the system LED 4 , the heat LED 5  and the alert LED 6 . That is, the software performs a subroutine that activates appropriate LEDs depending on the mode the software is in or if an error flag has occurred. For example, during normal operations, microcontroller  305  generates a signal resulting in the system LED 4  to blink on and off. If the software is in a heating mode (discussed below), then the heat LED 5  blinks in unison with the system LED 4 . If the software has a positive error flag, the alert LED 6  works in conjunction with the system LED 4  to indicate the status of the water heater  200  to an operator or repairperson. 
     If the ReCheck timeout is less than or equal to zero, then the microcontroller U 1  proceeds to Act  525 . In general terms, the microcontroller U 1  samples temperature sensor samples (act  525 ), computes a water temperature (act  530 ), computes the thermostat setting (act  535 ), establishes an operating mode (act  540 ), sets a heating cycle state (act  545 ), and sets a heating priority (act  550 ). An exemplary method implementing acts  525 ,  530 ,  535 ,  540 ,  545  and  550  is shown in  FIG. 18 . In addition, the microcontroller U 1  stores data for creating a usage history (act  555 ) and blinks the LEDs ( 560 ). 
     At act  705  ( FIG. 20(   a )), the microcontroller U 1  samples temperature sensor  245  and loads a resulting first voltage into the software for processing. At act  710 , the microcontroller U 1  samples temperature sensor  250  and loads a resulting second voltage into the software for processing. At act  715 , the microcontroller U 1  converts the first and second sampled voltages to a first and second sensed temperatures, respectively, using a temperature lookup table. The look-up table contains a plurality of voltage ranges having a respective associated temperature. For example, if the first temperatures sensor generates a 2.1 volt signal, the associated temperature may be 110 degrees Fahrenheit. The look-up table can vary depending on the sensor used. After obtaining the first and second sensed temperatures, the software modifies the sensed temperatures to take into account any lag time in obtaining the temperature. That is, as the water inside the tank  205  increases in temperature, there is an increasing error in what the temperature sensor  245  or  250  senses. The thermal conductive path from the water through the material of the water tank  205  has a lag time differential. To correct this, the temperature values read from the lookup table are “corrected” for the lag. The corrected first and second temperatures are used in making water heating decisions by the software. 
     At act  720 , the microcontroller U 1  loads or samples a signal from the thermostat  310 . If the microcontroller U 1  determines that the thermostat voltage corresponds to the thermostat being in off position (act  725 ), then the software sets an operating mode equal to an off state (act  730 ) and returns to act  555  of  FIG. 18 . For example, if the thermostat voltage is less than 0.1 volts, then the software determines the thermostat is in an off position and turns off the controller  260 . If the thermostat voltage is greater than a voltage corresponding to an off position (act  725 ), then the software proceeds to act  735 . 
     At act  735 , the software determines whether the operating mode was previously set to off (i.e., the system was just turned on). If the operating mode was previously off, then the software changes the operating mode to “stand-by” (act  740 ). As will be discussed in more detail below, when the water heater  200  is in a stand-by mode, the controller  260  is not increasing the temperature of the water. If the operating mode is in a mode other than the off operating mode, then the software proceeds to act  745 . 
     At act  745 , the software compares the thermostat voltage with a set voltage representing the vacation position of the thermostat. For example, if the thermostat voltage is less than 0.7 volts, then the software determines that the thermostat is set to the vacation position and proceeds to act  750 . If the thermostat voltage is greater than 0.7 volts, then the software determines that a user has set the water heater to a desired temperature and proceeds to act  755 . 
     At act  750 , the software sets the set point temperature equal to a vacation temperature (e.g., 90 degrees Fahrenheit). The vacation temperature can be a manufacturer-determined value, or can be preset by a user. After setting the set-point temperature, the software proceeds to act  760  ( FIG. 20(   b )). 
     At act  755  ( FIG. 20(   b ), the software computes a set point temperature based on the sampled thermostat voltage. The microcontroller U 1  preferably uses a second lookup table, but can alternatively use a formula based on the input voltage. 
     At act  760 , the software computes a heater-on temperature. The heater-on temperature is the temperature at which one or more resistance heating elements receive a power signal. The heater-on temperature is the set-point temperature minus a hysteresis temperature. The hysterisis temperature is the number of degrees Fahrenheit (e.g., 10 degrees Fahrenheit) that the water temperature drops below the set-point temperature before heating occurs. Thus, by calculating a heater-on temperature, the microcontroller U 1  avoids “under cycling”. 
     At act  765 , the software determines whether the operating mode is in a “stand-by” mode or a “heating” mode. If the operating mode is set to stand-by, the software proceeds to act  770 . If the operating mode is set to heating, then the software proceeds to act  775 . 
     At act  770 , the software determines whether the lower-tank temperature (from temperature sensor  250 ) is less than or equal to the heater-on temperature. If the lower-tank temperature is less than or equal to the heater-on temperature, then the software determines that the water should be heated and proceeds to act  780 . If the lower-tank temperature is greater than the heater-on temperature, then the software determines that the water should not be heated and proceeds to act  800 . 
     At act  780 , the software sets the operating mode to the heating mode indicating that the water should be heated. After setting the operating mode to heating, the software resets all operating state variables and timeouts for another heating cycle (act  785 ). For example, the software resets the ReCheck timeout (e.g., to twenty milliseconds.) 
     If, at act  765 , the software determines the operating mode is set to heating, the software proceeds to act  775 . At act  775 , the software determines whether the lower tank temperature is greater than or equal to the set point temperature. If the lower tank temperature is greater than or equal to the set point temperature, then the software determines that the water should continue to be heated, and therefore stays in the heating mode and proceeds to act  800 . If the lower tank temperature is less then the set point temperature, than the software determines that the water has been properly heated and proceeds to act  785 . 
     At act  785 , the software changes the operating mode to stand-by (i.e., indicating that the water temperature no longer should increase). At act  790 , the software determines whether the first heating element  235  is surrounded by water (this is assuming the first element is above the second  235 ). If the first heating element  235  is not surrounded by water (i.e., the element is dry), then the software sets the ReCheck timeout variable to two minutes (act  795 ). By changing the length of the ReCheck timeout variable, the software allows the water tank to fill with more water before heating with the first element. Of course, the amount of time the software sets the ReCheck timeout variable to can vary, and a specific value is not required for purposes of the invention to work. If the first element does have water surrounding the element (i.e., a wet state has resulted), then the software proceeds to act  800 . 
     At act  800  (see  FIG. 20(   c )), the software determines whether a temperature slope calculation period has elapsed. If the period has elapsed, then the software resets the timer and computes a temperature slope (act  805 ). Computing the temperature slope allows the determination of whether a water draw is occurring. At regular intervals (e.g., 90 seconds), the most recent temperature sample of the tank is compared with previous samples stored in the memory unit ( 315 ). Based on the temperature values, a temperature slope or rate of change of temperature is calculated for the water. If the user is drawing water, a large negative slope value will inform the software that a draw of water is in progress. 
     At act  810 , the software sets a duty cycle that determines the amount of power to be transferred to each heating element. The amount of power varies depending on the temperature of the water and the water heater code for the water heater  200 . In addition, the amount of power can take into account a water heater usage pattern (which is stored in the memory unit  315 ), the ambient temperature, a water consistency value, or other information. 
     For act  810 , the software obtains from the memory unit  315  the water heater code and past records of data stored by the water heater. The past records are stored each time the software completes act  555  ( FIG. 18 ), and each record includes the time of day, duration of past heatings, rate of change (slope) in water temperature decline and rise, and can additionally include other information such as ambient room temperature. As the controller  260  heats the water, it looks into the memory unit  315  for recorded information of similar circumstances during the same time period in previous days and/or weeks. If it appears that the user is using about the same amount of water during any given period then the water will be heated at a standard rate for the water heater code that will satisfy the anticipated consumption rate of heated water. If the stored data would indicate no further usage after the present heating cycle, the water then will be heated very slowly at a lower duty cycle to minimize energy consumption. If there is an abrupt and rapid decline (i.e., negative temperature slope) in water temperature, the software will calculate a new duty cycle according to the present usage condition of the water heater. As usage patterns change, the old records will be modified to reflect the current operating conditions. For the preferred embodiment, the base line formula in considering what minimum water temperature flow rates will be acceptable is a minimum recovery equal to ten gallons per hour at sixty degree Fahrenheit temperature rise. 
     With this formula, product code information and usage records, the power input ratios versus temperature rate change are used in determining heating strategies. The strategies provide input power levels to meet or exceed the minimum recovery rate, while keeping energy efficiency to a maximum. As conditions change in usage patterns the strategy is modified to maintain the minimum recovery standard. 
     For example, a first heating strategy for a first water heater code having a first element wattage will differ when compared to a second heating strategy for a second water heater code having a second element wattage. Two exemplary heating strategies for the resistance second element  240  are shown in Tables 1 and 2. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Water Heating Strategy for a First Heater Code 
               
            
           
           
               
               
               
            
               
                   
                   
                 Power or Duty Cycle of the 
               
               
                   
                 Water Temperature 
                 Second Element 
               
               
                   
                   
               
               
                   
                 &lt;115° F. 
                 100%  
               
               
                   
                 115° F. to 120° F. 
                 66% 
               
               
                   
                 120° F. to 125° F. 
                 57% 
               
               
                   
                 125° F. to 130° F. 
                 50% 
               
               
                   
                 130° F. to 135° F. 
                 40% 
               
               
                   
                 135° F.&gt; 
                 20% 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Water Heating Strategy for a Second Heater Code 
               
            
           
           
               
               
               
            
               
                   
                   
                 Power or Duty Cycle of the 
               
               
                   
                 Water Temperature 
                 Second Element 
               
               
                   
                   
               
               
                   
                 &lt;115° F. 
                 100%  
               
               
                   
                 115° F. to 120° F. 
                 80% 
               
               
                   
                 120° F. to 125° F. 
                 66% 
               
               
                   
                 125° F. to 130° F. 
                 50% 
               
               
                   
                 130° F. to 135° F. 
                 40% 
               
               
                   
                 135° F.&gt; 
                 20% 
               
               
                   
                   
               
            
           
         
       
     
     For water heater  200 , the duty cycle or power applied to the resistance heating elements  235  or  240  is based at least in part on the sensed water temperature and the water heater code. The concept of a heating strategy dependent on a water heater code is unlike the method of heating water for water heaters  10  and  150 . For water heaters  10  and  150 , the duty cycle or power applied to the heating elements  16  and/or  16 ′ is based on the difference between the sensed water temperature and the desired water temperature. However, it has been determined that increasing the power to an element submerged in water at a given water temperature may not result in an optimum water temperature gain when compared to the power input. For example, assuming all other conditions are the same, it has been determined that more heat can be transferred from an element to water when the water is at a cooler temperature. As the water temperature increases, less power needs to be provided to the heating element  235  or  240  regardless of the difference between the sensed temperature and the desired temperature (i.e., the excess power will not result in an optimum transfer when compared to the power input). Therefore, the software does not need to take into account the difference between the desired temperature and the sensed temperature for heating the water. But it is envisioned that under some circumstances (e.g., the usage pattern changes resulting in the water needing to be heated as fast as possible without a concern for efficiency) a heating strategy may want to include a difference measurement. It should also be understood that, while the heating strategies of Tables 1 and 2 use discrete levels, the heating strategy can take the form of an equation. 
     At act  815 , the software determines the “draw down” state. The draw down state indicates whether a user is currently drawing water and at what rate the user is drawing the water. In one construction, the draw down state has four values: “tank is heating”, “draw-down-one”, “draw-down-two”, and “recovering”. If the draw down state is “tank-is-heating”, then the software proceeds to act  820 . If the draw down state is “draw-down-one”, then the software proceeds to act  825 . If the draw down state is “recovering”, then the software proceeds to act  830 . If the draw down state is “draw-down-two”, then the software proceeds to act  835 . 
     At act  820 , the software determines whether the temperature slope is less than or equal to a threshold for a draw down. For example, if the calculated temperature slope is less than ten degrees Fahrenheit then the software determines a draw down is in progress and sets the draw down state to “draw-down-one” (act  840 ). If the temperature slope is greater than the draw down threshold then the software determines a draw is not in progress and proceeds to act  870 . 
     If the draw down state is currently “draw-down-one”, then the water heater had previously been in a draw down (i.e., a user is using hot water). At act  825 , the software determines whether the temperature slope is positive. If the temperature slope is positive, then the software determines that the water heater is recovering and sets the draw down state to recovering (act  845 ). If the temperature slope is still negative, then the software determines the water heater is still in a draw down and proceeds to act  870 . 
     If the draw down state is currently set to “recovering”, then the water heater is recovering from a draw down. At act  830 , the software determines whether there has been another draw down (i.e., the temperature slope is less than or equal to the threshold for a draw down). If there was another draw down, then the software sets the draw down state to “draw-down-two” (act  850 ). If the software determines the water heater is still recovering, the program proceeds to act  870 . 
     At act  835 , the software determines whether the lower tank temperature is greater than or equal to a heater-on temperature. If the lower tank temperature is greater than or equal to a heater-on temperature, then the software sets the draw down state to recovering and resets the temperature slope. If the lower tank temperature is less than the heater-on temperature, then the microcontroller U 1  sets the duty cycle to full power (act  760 ). Of course, other duty cycles can be used depending upon the particular water heater and environmental circumstances. 
     At act  870 , the software determines the heating priority for the water heater. If the heating priority is “fifty-fifty” (discussed below), then the software sets the duty cycle to full power (act  875 ) regardless of the water temperature. Of course, other duty cycles can be used depending upon the particular water heater and environmental circumstances. If the heating priority is not in the fifty-fifty mode, then the software proceeds to act  880  ( FIG. 20(   d )). 
     At act  880 , the software selects a case based on the previously determined heating priority. The heating priority is used for determining which elements are active. For example, if the first element is an upper element and the second element is a lower element (similar to  FIG. 5 ), then under certain conditions both elements can be used. For this arrangement, if both elements are being used, then the heating priority will be fifty-fifty. If only one element is used, then the heating priority is zero-one-hundred. Alternatively, if the elements are in a substantially horizontal plane, both elements can be used in a fifty-fifty arrangement (vs. only one element being used) to heat the water. 
     At act  885 , the software determines if the upper tank temperature has fallen (i.e. the temperature slope of the upper element is less than or equal to a threshold). If the upper tank temperature has fallen, then the software sets the heating priority to “fifty-fifty” (act  887 ), resulting in both elements heating the water. If the upper tank temperature has not fallen, then the software proceeds to act  555  ( FIG. 16 ). 
     At act  890 , the software determines whether the upper tank temperature has recovered (i.e., the temperature slope of the upper element is greater than a threshold). If the upper temperature tank has recovered, then the software sets the priority to “zero-one-hundred” (act  895 ), resulting in only the second element  240  heating the water. If the upper tank temperature has not recovered, then the software proceeds to act  555  ( FIG. 16 ). 
     Every eight hundred microseconds, the software performs a timer interrupt event. The timer interrupt is used as a time base for various timeouts (e.g., the “ReCheck” timeout). During each interrupt, the microcontroller&#39;s timer is reset and the timeout variables are decreased if their value is still greater than zero. Once a timeout value reaches zero, the associated routine can be performed at that time, or can be performed during the main loop. As shown in  FIG. 21 , at act  905 , the software resets the timer for the next scheduled interrupt. At act  910 , the software services timeouts (i.e., decrease each timeout) and delays variables. At act  915 , the software executes event-related routines as required. At act  920 , the software returns from the interrupt to the act it was previously implementing. 
     Every time the signal (AcOutHI) crosses zero volts, the micro controller U 1  performs a zero crossing event interrupt. When transistor Q 8  ( FIG. 12 ) turns on, it goes into saturation causing a falling edge that generates an interrupt to the microcontroller U 1 . The falling edge is used as a reference edge for activating triacs Q 1  and Q 2  ( FIG. 16(   b )). When the reference edge occurs, the timer interrupt ( FIG. 21)  is adjusted so that it will correspond exactly to when a zero crossing occurs. In this way, the zero crossing interrupt fires the triacs at precisely the right time. 
     To control the power transmitted to the resistance heating elements  235  and  240 , the microcontroller U 1  generates an output signal (first-element or second-element) which is provided to the zero-cross triac drivers U 5  and U 6 , respectively. The zero-cross triac drivers U 5  and U 6  in combination with triacs Q 1  and Q 2  control the high-voltage AC signal (AcIn) being provided to the resistance heating elements  235  and  240 . 
     For controlling the power transmitted to the heating elements  235  and  240 , triac Q 1  or Q 2  is fired for a sequence of four sequential half AC cycles. The triac Q 1  or Q 2  fired is based on the heating priority and the status at the software relative to the heating cycle. For example, if the heating priority is “zero-one-hundred”, then only one triac Q 2  will be fired. Alternatively, if the heating priority is “fifty-fifty” and the resistance heating elements  235  and  240  are being fired sequentially, then the software includes a variable specifying which heating element  235  or  240  is being activated. After firing a sequence of four sequential AC half cycles, the software delays firing, i.e. does not fire the triac Q 1  or Q 2  for a number of cycles. The number of cycles the triac Q 1  or Q 2  does not fire is determined by the amount of power to be transmitted to the heating elements  235  or  240 . For example, if 100% power is to be transmitted, then the software will not delay the firing at all. If 50% power is to be transmitted, then the software will delay the firing of the triac Q 1  or Q 2  for four half AC cycles. Table 3 discloses an exemplary power transfer table. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Lookup Table for Various Duty Cycles based 
               
               
                 on an Initial Four Cycle Firing 
               
            
           
           
               
               
               
            
               
                   
                 Delay (half-cycle) 
                 Power Transfer 
               
               
                   
                   
               
               
                   
                 0 half cycle delay 
                 100% Power  
               
               
                   
                 1 half cycle delay 
                 80% Power 
               
               
                   
                 2 half cycle delay 
                 66% Power 
               
               
                   
                 3 half cycle delay 
                 57% Power 
               
               
                   
                 4 half cycle delay 
                 50% Power 
               
               
                   
                 6 half cycle delay 
                 40% Power 
               
               
                   
                 16 half cycle delay  
                 20% Power 
               
               
                   
                   
               
            
           
         
       
     
     Of course, other half cycle delays can be used and the initial four cycle firing can vary to obtain different power transfer ratios. 
     Another exemplary water heater  1010  incorporating aspects of the invention is shown in  FIGS. 22 and 23 . Before proceeding further, it should be noted that the water heater  1010  shown in  FIG. 22 and 23  is an example storage-type gas water heater incorporating aspects of the invention and that other constructions for a gas water heater  1010  are possible. The water heater  1010  includes a base pan  1012  supporting a water tank  1014  ( FIG. 23 ), insulation  1016  surrounding the tank  1014 , and an outer jacket  1018  surrounding the insulation  1016  and the water tank  1014 . The base pan  1012  may be constructed of stamped metal or plastic. A cold water inlet tube  1022  ( FIG. 22 ) and a hot water outlet tube  1026  extend through a top wall  1034  of the water tank  1014 . 
       FIGS. 22-23  illustrate the bottom of the water heater  1010 . The tank  1014  is defined by a tank bottom wall  1038 , side wall  1042 , and the top wall  1034 . A flue  1044  extends from the tank bottom wall  1038  up through the tank portion  1014  of the water heater  1010 . The water contained in the tank  1014  surrounds the flue  1044 . 
     The bottom of the water heater  1010  defines a combustion chamber  1046  having therein a gas burner  1048 . The water heater  1010  includes a seal  1050 , a skirt  1054 , a radiation shield  1058 , a retainer member  1062 , first and second flame arrestor seals  1066  and  1070 , a flame arrestor  1074 , an inner plate or flame arrestor support  1078 , and a plenum seal  1082 . Optionally, the skirt  1054  and flame arrestor support  1078  may be formed as one piece. 
     The flame arrestor  1074  has an upper surface  1074   a  and a lower surface  1074   b . The flame arrestor  1074  permits substantially all flammable vapors that are within flammability limits to burn near its top surface  1074   a  while preventing substantially all flames from passing from the top surface  1074   a , through the flame arrestor  1074 , out the bottom surface  1074   b , and into the plenum  1086 . The flame arrestor  1074  is constructed of materials that resist thermal conduction from the upper surface  1074   a  to the lower surface  1074   b  to further reduce the likelihood of ignition of flammable vapors in the air plenum  1086 . 
     There are a number of forms that the flame arrestor  1074  may take. For example, the flame arrestor  1074  may have through-holes or a random pattern of interconnected voids. A conglomeration of randomly-oriented fibers or particles may form the random pattern of interconnected voids. The air that is necessary for combustion of the fuel during normal operation of the water heater  1010  is allowed to flow from void to void from the bottom surface  1074   b  to the top surface  1074   a of the flame arrestor  1074 . The arduous air-flow path through the flame arrestor  1074  reduces the thermal conductivity of the flame arrestor  1074 , and substantially ensures that the bottom surface  1074   b  of the flame arrestor  1074  will be below the ignition temperature of the flammable vapors entering the flame arrestor  1074 , even when vapors are burning on the top surface  1074   a  of the flame arrestor  1074 . 
     The base pan  1012  is best illustrated in  FIG. 23 . The base pan  1012  is configured to provide the primary structural support for the rest of the water heater  1010 . Elevated temperatures and heat cycling do not compromise the structural stability of the materials from which the base pan  1012  is constructed. The plenum seal member  1082 , which may be made of fiberglass or another suitable material, creates a substantially airtight seal between the flame arrestor support  1078  and the base pan  1012 . The weight of the water tank  1014  is transferred through the base pan  12  to dimples  1098  on the bottom of the base pan  1012 . The dimples  1098  may be replaced with a formed ring in the bottom of the pan  1012  or by any other suitable supporting structure. The dimples  1098  reduce the amount of surface contact between the base pan  1012  and the floor to inhibit the formation of rust. The dimples  1098  are designed to retain the overall structural stability of the water heater  1010 . 
     The base pan  1012  and the flame arrestor support  1078  together define the air plenum  1086 . The base pan  1012  includes an air intake aperture or air inlet  1100  to the air plenum  1086 . The air inlet  1100  is covered by a screen  1102 . The screen  1102  is positioned upstream of the flame arrestor  1074 , and is made of a wire mesh material that acts as a lint or bug screen so that undesired objects or particles are not allowed to enter the plenum  1086  leading to the combustion space  1046 . The illustrated screen  1102  is located on the front side of the water heater  1010  to facilitate cleaning. The location provides high visibility and reminds operators not to block the air inlet  1100 , and to inspect or clean the screen  1102  whenever other components of the water heater  1010  are adjusted. 
     As indicated by the arrows in  FIG. 23 , air flows through the screen  1102 , into the plenum  1086 , through the flame arrestor  1074 , and around the radiation shield  1058  or through apertures  1104  in the radiation shield  1058 . Substantially all of the air that is necessary for combustion must pass through the flame arrestor  1074 . The hot products of combustion rise up through the flue  1044 , and heat the water by convection and conduction through the flue  1044 . 
     Referring again to  FIGS. 22 and 23 , the seal  1050 , which may be an O-ring, provides an airtight seal between the top of the skirt  1054  and the bottom wall  1038  of the water tank  1014 . The skirt  1054  includes an inner door or access door  1114  covering an access opening  1116 . The access door  1114  includes one or more apertures. The first aperture accommodates a sight glass  1118  that is made of a transparent material to permit viewing of the pilot light (if present). A grommet  1122  is disposed within the second aperture and has channels or holes through which various burner operating conduits, such as wires and tubes  1126  extend so that the grommet  1122  seals these components to the door  1114 . The grommet  1122  is made of a material that will not degrade when exposed to elevated temperatures or cyclical heating. The grommet  1122  has slits extending from the holes to an outer edge of the grommet  1122  so that the wires and tubes  1126  may be inserted into respective openings via respective slits. In another construction, the grommet  1122  is assembled with the wires and tubes  1126  in place so that the slits would not be necessary. For example, the grommet  1122  could be molded around the components  1126 . The grommet  1122  can be designed with a peripheral or circumferential groove to snap into place in the access door  114  during assembly. 
     A gas manifold tube  1138  extends through the third aperture. A boot  1142  surrounds a portion of the manifold tube  1138  and forms a substantially airtight compression seal around the manifold tube  1138 , and between the manifold tube  1138  and the access door  1114 . The manifold tube  1138  supplies fuel to the burner  1048  (discussed below). The boot  1142  includes a plurality of folds that create an undulating surface and allow the manifold tube  1138  to move with respect to the access door  1114 , while maintaining the airtight seal. The boot  1142  includes a peripheral groove  1146  ( FIG. 5 ) that receives an edge defining the third aperture to seal the boot  1142  to the inner door  1114  or some other surface that is penetrated. The undulating surface of the boot  1142  allows the manifold tube  1138  to be positioned in a location relative to the hole in the inner door  1114  or combustion chamber  1046  that is within an acceptable tolerance range. 
     The radiation shield  1058  includes a plurality of feet  1158  that contact the flame arrestor support  1078  and support the radiation shield  1058  above the flame arrestor support  1078  to permit the air flowing through the flame arrestor  1074  to flow between the flame arrestor support  1078  and the radiation shield  1058 , or through the apertures  1104  before reaching the burner  1048 . Alternatively, the skirt  1054  may include projections which support the radiation shield  1058  above the flame arrestor support  1078 . 
     It should be noted that the position and orientation of the flame arrestor  1074  is not limited to those shown in the drawings. The flame arrestor  1074  may be positioned anywhere and in an orientation, provided the screen  1102  is upstream of the flame arrestor  1074 , and preferably, an air plenum  1086  is disposed between the flame arrestor  1074  and screen  102 . 
       FIG. 24  shows a partial side view, partial sectional view of one construction of a fuel control system  1200  used in the water heater  1010 . For the construction shown, the fuel control system  1200  includes a gas valve  1202  and a controller or control unit  1204  that controls the gas valve  1202 . The valve  1202  includes a gas inlet connection  1205  that interconnects with the gas main supply. The gas inlet connection  1205  receives the fuel from the gas main and provides the fuel to an inlet passageway  1210 . The inlet passageway  1210  delivers the fuel from the gas inlet connection  1205  to a valve spool  1215 . While only one inlet passageway  1210  is shown in  FIG. 24 , the valve  1202  can include more inlet passageways  1210 . The valve spool  1215  includes one or more channels  1220  that control the flow of the fuel from the inlet passageway to one or more outlet passageways  1225 . In the construction shown in  FIG. 24 , the outlet passageways  1225  include four passageways. However, the number of outlet passageways  1225  can vary. The channels  1220  can be designed such that fuel is delivered to the one or more outlet passageways  1225  depending on the position of the valve spool  1215 . For example and in one construction, the channels  1220  can be designed such that each outlet passageway  1225  is either “open” or “closed.” Accordingly, while only one spool valve  1215  is used to control the flow of fuel from the inlet passageway  1210  to the outlet passageways  1225 , the valve spool  1215  can be viewed as providing individual valve control to each outlet passageway  1225 . It is also envisioned, in another construction of the water heater  1010 , the channels  1220  provide between zero percent (0%) and one hundred percent (100%) flow control for each outlet passageway or any combination thereof. 
     The outlet passageways  1225  provide the issued fuel from the valve spool  1215  to a multiport fuel outlet connection  1230 . In the construction shown, the multiport fuel outlet connection  1230  includes four outlet ports  1235 . However, the number of outlet ports  1235  can vary. 
     Referring again to the construction shown in  FIG. 24 , a spool shaft  1240  of the valve spool  1215  is connected to a linear force motor  1245 , which controls the movement of the valve spool  1215 . The linear motor  1245  includes a coil  1250  having windings that produce a magnetic field, thereby controlling the positioning of the spool shaft  1240 . The spool shaft  1240  is connected to the valve spool  1215 . Accordingly, the coil  1250  produces a magnetic field that ultimately controls the position of the valve spool  1215 . It is envisioned that other constructions of the fuel control system  1200  can include other motor types (such as a servo motor) to control the valve spool  1215 . 
     A position transducer  1260  is connected to the shaft  1240  of the valve spool  1215 . The position transducer  1260  acquires a position of the valve spool  1215  and provides information to the controller  1204 . The controller  1204  receives inputs from the position transducer  1260 , a user interface  1225  ( FIG. 22 ), and external sensors (e.g., temperature sensors), and provides outputs to control the linear force motor  1245 . The controller  1204  can also provide electronic ignition to the gas burner  1048 . For the construction shown in  FIG. 24 , the controller  1204  is directly coupled with the valve  1202 . However, in other constructions, the controller  1204  can be indirectly coupled with the valve  1202  (e.g., combined with the user interface  1225 ). 
     Before proceeding further, it should be noted that the control of the fuel from the source to the burner  1048  for  FIG. 24  uses a single valve  1202  having a single valve spool  1215 . However, it is envisioned that other arrangements are possible. For example and in another construction, the fuel control system can include more than one valve for controlling the flow of fuel. As schematically shown in  FIG. 25 , four individual valves  1270  control the flow of fuel from a source to the burner  1048 . Additionally, the single spool valve  1215  of  FIG. 24  can conceptually be viewed as shown in  FIG. 25 ; i.e., the spool valve  1215  can be viewed as providing four-valve control to the outlet passageways  1225 . 
     The burner  1048  shown in  FIG. 23  is a circular burner. A top view of the burner  1048  is shown in  FIG. 26  and a sectional view of the manifold tube  1138  is shown in  FIG. 27 . The manifold tube  1138  includes one or more manifold passageways  1285 . The number of passageways  1285  shown in  FIG. 27  is four, and each passageway  1285  corresponds to a respective port  1235  of the multiport burner outlet  1230  ( FIG. 24 ). However, the number of passageways  1285  can vary. The burner  1048  ( FIG. 26 ) is divided into one or more combustive sections  1290 . The number of combustive sections  1290  is schematically shown in  FIG. 26  as four, and each combustive section corresponds to a respective passageway  1285  of the second gas manifold  1280 . However, the number of combustive sections  1290  can vary. When fuel is supplied to one of the combustive sections  1290 , the fuel mixes with air flowing through the flame arrestor  1074  ( FIG. 23 ). A pilot light or electronic igniter (electronic igniters  1295  are shown in  FIG. 26 ) lights or ignites the mixture, resulting in a flame. In the construction shown in  FIG. 26 , one igniter  1295  ignites two adjacent sections. The combustion resulting from the combustive sections  1290  can be individually controlled with the valve spool  1215  or the valves  1270 . 
     As an alternative to the single burner  1048 , the gas water heater  1010  can include individual burners  1300  ( FIG. 28 ) for each respective passageway  1285  of the second gas manifold tube  1280  ( FIG. 27 ). Each burner  1300  of  FIG. 28  provides an individually controlled combustive section. Accordingly, as used herein, the term “combustive section” comprises a gas burner (or similar apparatus) that supports combustion or a section of a gas burner that supports combustion. Additionally, the term “gas heating element” comprises one or more combustive sections (e.g., one or more gas burners and/or one or more sections of a gas burner). 
     For the construction shown in  FIG. 24 , the controller  1204  is a modulation controller (e.g., a relational band controller) that regulates the flow of fuel through the valve  1202  or valves  1270 . The fuel contains stored energy that results in a power when combusting. Therefore, the control of the fuel through the valve is conceptually similar to controlling current in the electric water heaters described earlier. That is, the electric controllers discussed above control the current supplied to the electrical resistance heating element(s). The controlled current results in an amount of energy being delivered to the resistance heating element(s) over a time period (an instantaneous time period, a time period used for defining a duty cycle, etc.). The energy applied to the resistance heating element(s) produces an amount of power that heats the water. Accordingly, the control of current, the amount of energy delivered to the heating element(s), and the amount of power applied to the heating element(s) are all related. Similarly, the fuel control systems discussed herein control the fuel supplied to the gas heating element(s). The controlled fuel results in an amount of energy being delivered to the gas heating element(s) over a time period. The fuel delivered to the gas heating element(s) produces an amount of power that heats the water. Accordingly, the control of fuel, the amount of energy delivered to the heating element(s), and the amount of power applied to the heating element(s) are all related. 
     The discussions above relating to relational band temperature control circuit  100  and to the portion of the intelligent control circuit  285  used for controlling the resistance heating element(s) apply equally to the controller  1204  except, rather than controlling electricity, the controller  1204  controls the delivery of fuel. For example, the controller  1204  can control an instantaneous amount of fuel delivered to the heating elements (e.g., the burners or combustive sections) and/or an amount of fuel delivered over a time period (i.e., burst an amount a fuel having a varying duty cycle). In one construction, the control of fuel flow and volume (or energy or power) is based on a heating strategy and a water temperature. The heating strategy can be determined by the controller  1204  (e.g., using a water heater code) or can be set by the manufacturer. The control of fuel can be based on a number of other factors (e.g., a usage pattern, a water use history, a sensed ambient temperature, etc.), which were discussed in connection with the earlier electric water heaters. Additionally, the controller  1204  can include a programmable device and memory (similar to control circuit  285 ), can be logic based (similar to control circuit  100 ), or a combination thereof. 
     The fuel control system  1200  can control the fuel to multiple combustive sections, which can be viewed conceptually similar to the electric water heater controlling multiple resistive heating elements (see, e.g., water heaters  150 ,  160 , and  200 ). 
     In one example operation, the controller  1204  receives inputs from various temperature sensors located at key position(s) on the tank. As the controller  1204  senses the need to heat the water, the controller  1204  can determine the difference between the set point temperature and the water temperature. If the difference is within a specified range, the controller  1204  opens all four ports at 100% flow to each port. As the temperature rises in the tank, the controller  1204  reduces the flow of fuel to the combustive sections until the set point temperature has been achieved (i.e., the controller can operate as a relational (e.g., proportional) band controller having a heating strategy). Alternatively, the controller  1204  can control the temperature based only on the sensed temperature and a heating strategy (e.g., using Tables 1 and 2). 
     The reducing of fuel can be an instantaneous control or a bursting control. Additionally, the control of fuel can be the same for each combustive section or independent for each combustive section (e.g., the flow to a first section is 100% and to a second section is 50%). The controller  1204  can also learn a usage pattern for the water heater  1010  or create a usage history for the water heater  1010 , and can look ahead (e.g., an hour) to determine if hot water will be required. If hot water is unlikely in the future hour and the controller  1204  is calling for heat, the controller  1204  can trickle heat to the water similar to what was discussed for some of the electric water heater(s) above. The water heater can also be designed with a vacation mode as discussed earlier. Additionally, the water heater can operate at constant full power if hot water is immediately required and the water temperature is significantly below the set point. 
     While particular embodiments of the invention have been shown and described herein, changes and modifications can be made without departing from the spirit and scope of the invention. For example, logic chips other than the Motorola UAA 1016A logic chip can be used to control the on-off cycle of thyristor  103 . Also, a temperature sensing device other than the thermistor used as temperature sensing device  102  can be employed. Also, a thyristor other than a Motorola TRIAC can be used as thyristor  103  and multiple heating elements and other alternative control circuits, as noted above, can be utilized. Therefore, no limitation of the invention is intended other than limitations contained in the appended claims. 
     Various other features and advantages of the invention are set forth in the following claims.