Abstract:
A fluid-heating apparatus for heating a fluid and method of operating the same. The fluid-heating apparatus includes a heating element for heating a fluid surrounding the heating element and a control circuit connected to the heating element and connectable to a power source. The control circuit is configured to determine whether a potential dry-fire condition exists for the heating element. The method includes applying a first electric signal to the heating element, detecting a first value of an electrical characteristic during the application of the first electric signal, applying a second electric signal to the heating element, applying a third electric signal to the heating element, detecting a second value of the electrical characteristic during the application of the third electric signal; and determining whether a potential dry-fire condition exists based on the first and second values.

Description:
BACKGROUND 
   The invention relates to a fluid-heating apparatus, such as an electric water heater, that can determine an operating condition of the apparatus, and a method of detecting a dry-fire condition and preventing operation of the fluid-heating apparatus when a dry-fire condition exists. 
   When an electric-resistance heating element fails in an electric water heater, the operation of the heater is diminished until the element is replaced. This can be an inconvenience to the user of the water heater. 
   SUMMARY 
   Failure of the electric-resistance element may not be immediate. For example, the element typically has a sheath isolated from an element wire by an insulator, such as packed magnesium oxide. If the sheath is damaged, the insulator can still insulate the wire and prevent a complete failure of the element. However, the insulator does become hydrated over time and the wire eventually shorts, resulting in failure of the element. The invention, in at least one embodiment, detects the degradation of the heating element due to a damaged sheath prior to failure of the heating element. The warning of the degradation to the element prior to failure of the element allows the user to replace the element with little downtime on his appliance. 
   A heating element generates heat that can be transferred to water surrounding the heating element. Water can dissipate much of the heat energy produced by the heating element. The temperature of the heating element rises rapidly initially when power is applied and then the rate of temperature rise slows until the temperature of the heating element remains relatively constant. Should power be applied to the heating element prior to the water heater being filled with water or should a malfunction occur in which the water in the water heater is not at a level high enough to surround the heating element, a potential condition known as “dry-fire” exists. Because there is no water surrounding the heating element to dissipate the heat, the heating element can heat up to a temperature that causes the heating element to fail. Failure can occur in a matter of only seconds. Therefore, it is desirable to detect a dry-fire condition quickly, before damage to the heating element occurs. 
   In one embodiment, the invention provides a method of detecting a dry-fire condition of an electric-resistance heating element. The method includes applying a first electric signal to the heating element and detecting a first value of an electrical characteristic during the application of the first electric signal. The first electric signal is then disconnected from the heating element and a second electric signal, substantially different from the first electric signal, is applied to the heating element. The second electric signal is disconnected from the heating element and a third electric signal, substantially different from the second electric signal, is applied to the heating element. A second value of the electrical characteristic is detected during the application of the third electric signal, and a determination is made of the potential for a dry-fire condition based on the first and second values of the electrical characteristic. 
   In another embodiment, the invention provides a fluid-heating apparatus for heating a fluid. The fluid-heating apparatus includes a vessel, an inlet to introduce the fluid into the vessel, an outlet to remove the fluid from the vessel, a heating element, and a control circuit. The control circuit is configured to apply a first electric signal to the heating element, read a first value of an electrical characteristic, apply a second electric signal to the heating element, the second electric signal being substantially different than the first electric signal, apply a third electric signal to the heating element, the third electric signal being substantially different than the second electric signal, read a second value of the electrical characteristic, determine whether a potential dry-fire condition exists based on the first and second values, and apply a fourth electric signal to the heating element if the potential dry-fire condition does not exist, the fourth electric signal being substantially different than the first third signal. 
   Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a partial exposed view of a water heater embodying the invention. 
       FIG. 2  is a partial exposed, partial side view of an electrode capable of being used in the water heater of  FIG. 1 . 
       FIG. 3  is a partial block diagram, partial electric schematic of a first control circuit capable of controlling the electrode of  FIG. 2 . 
       FIG. 4  is a partial block diagram, partial electric schematic of a second control circuit capable of controlling the electrode of  FIG. 2 . 
       FIG. 5  is a partial block diagram, partial electric schematic of a third control circuit capable of controlling the electrode of  FIG. 2 . 
       FIG. 6A  is a chart of a temperature curve of the electrode of  FIG. 2  submerged in water. 
       FIG. 6B  is a chart of a temperature curve of the electrode of  FIG. 2  exposed to air. 
       FIG. 7  is partial block diagram, partial electric schematic of a fourth control circuit capable of controlling the electrode of  FIG. 2  and detecting a dry-fire condition. 
       FIG. 8  is a flowchart of the operation of the control circuit of  FIG. 7  for detecting a dry-fire condition. 
       FIG. 9A  is a chart of a resistance curve of the electrode of  FIG. 2  submerged in water. 
       FIG. 9B  is a chart of a resistance curve of the electrode of  FIG. 2  exposed to air. 
   

   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 arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limited. 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 “mounted,” “connected,” “supported,” and “coupled” are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings, and can include electrical connections or couplings, whether direct or indirect. 
     FIG. 1  illustrates a storage-type water heater  100  including an enclosed water tank  105  (also referred to herein as an enclosed vessel), a shell  110  surrounding the water tank  105 , and foam insulation  115  filling the annular space between the water tank  105  and the shell  110 . A typical storage tank  105  is made of ferrous metal and lined internally with a glass-like porcelain enamel to protect the metal from corrosion. However, the storage tank  105  can be made of other materials, such as plastic. A water inlet line or dip tube  120  and a water outlet line  125  enter the top of the water tank  105 . The water inlet line  120  has an inlet opening  130  for adding cold water to the water tank  105 , and the water outlet line  125  has an outlet opening  135  for withdrawing hot water from the water tank  105 . The tank may also include a grounding element (or contact) that is in contact with the water stored in the tank. Alternatively, the grounding element can be part of another component of the water heater, such as the plug of the heating element (discussed below). The grounding element comprises a metal material that allows a current path to ground. 
   The water heater  100  also includes an electric resistance heating element  140  that is attached to the tank  105  and extends into the tank  105  to heat the water. An exemplary heating element  140  capable of being used in the water heater  100  is shown in  FIG. 2 . With reference to  FIG. 2 , the heating element  140  includes an internal high resistance heating element wire  150 , surrounded by a suitable insulating material  155  (such as packed magnesium oxide), a metal jacket (or sheath)  160  enclosing the insulating material, and an element connector assembly  165  (typically referred to as a plug) that couples the metal jacket  160  to the shell  110 , which may be grounded. For the construction shown, the connector assembly  165  includes a metal spud  170  having threads, which secure the heating element  140  to the shell  110  by mating with the threads of an opening of the shell  110 . The connector assembly  165  also includes connectors  175  and  180  for electrically connecting the wire  150  to the control circuit (discussed below), which provides controlled power to the wire  150 . While a water heater  100  having the element  140  is shown, the invention can be used with other fluid-heating apparatus for heating a conductive fluid, such as an instantaneous water heater or an oil heater, and with other heater element designs and arrangements. 
   A partial electrical schematic, partial block diagram for one construction of a control circuit  200  used for controlling the heating element  140  is shown in  FIG. 3 . The control circuit  200  includes a microcontroller  205 . As will be discussed in more detail below, the microcontroller  205  receives signals or inputs from a plurality of sensors or circuits, analyzes the inputs, and generates one or more outputs to control the water heater  100 . In one construction, the microcontroller  205  includes a processor and memory. The memory includes one or more modules having instructions. The processor obtains, interprets, and executes the instructions to control the water heater  100 . Although the microcontroller  205  is described as having a processor and memory, the invention may be implemented with other controllers or devices including a variety of integrated circuits (e.g., an application-specific-integrated circuit) and discrete devices, as would be apparent to one of ordinary skill in the art. Additionally, the microcontroller  205  and the control circuit  200  can include other circuitry and perform other functions not discussed herein as is known in the art. 
   Referring again to  FIG. 3 , the control circuit  200  further includes a current path from a power supply  201  to the heating element  140  back to the power supply  201 . The current path includes a first leg  202  and a second leg  203 . The first leg  202  connects the power source  201  to a first point  206  of the heating element  140  and the second leg  203  connects the power source  201  to a second point  207  of the heating element  140 . A thermostat, which is shown as a switch  210  that opens and closes depending on whether the water needs to be heated, is connected in the first leg  202  between the power source  201  and the heating element  206 . When closed, the thermostat switch  210  allows a current from the power source  201  to the heating element  140  and back to the power source  201  via the first and second legs  202  and  203 . This results in the heating element  140  heating the water to a desired set point determined by the thermostat. The heating of the water to a desired set point is referred to herein as the water heater  100  being in a heating state. When open, the thermostat switch  210  prevents a current flow from the power source  201  to the heating element  140  and back to the power source  201  via the first and second legs  202  and  203 . This results in the water heater  100  being in a non-heating state. Other methods of sensing the water temperature and controlling current to the heating element  140  from the power source  201  are possible (e.g., an electronic control having a sensor, the microcontroller  205  coupled to the sensor to receive a signal having a relation to the sensed temperature, and an electronic switch such as a triac controlled by the microcontroller in response to the sensed temperature). 
   As just stated, the thermostat switch  210  allows a current through the heating element  140  when the switch  210  is closed. A variable leakage current can flow from the element wire  150  to the sheath  160  via the insulating material  155  when a voltage is applied to the heating element  140 . The variable resistor  215  represents the leakage resistance, which allows the leakage path. The resistance between the wire and ground drops from approximately 4,000,000 ohms to approximately 40,000 ohms or less when the heating element  140  degrades due to a failure in the sheath  160 . This will be discussed in more detail below. 
   The control circuit  210  further includes a voltage measurement circuit  220  and a current measurement circuit  225 . The voltage measurement circuit  220 , which can include a filter and a signal conditioner for filtering and conditioning the sensed voltage to a level suitable for the microcontroller  205 , senses a voltage difference between the first and second legs  202  and  203 . This voltage difference can be used to determine whether the thermostat switch  210  is open or closed. The current measurement circuit  225  senses a current to the heating element  140  with a torroidal current transformer  230 . The torroidal current transformer  235  can be disposed around both legs  202  and  203  to prevent current sense signal overload during the heating state of the water heater  100 , and accurately measure leakage current during the non-heating state of the water heater  100 . The current measurement circuit  225  can further include a filter and signal conditioner for filtering and conditioning the sensed current value to a level suitable for the microcontroller  205 . 
   During operation of the water heater  100 , the sheath  160  may degrade resulting in a breach (referred to herein as the aperture) in the sheath  160 . When the aperture exposes the insulating material  155 , the material  155  may absorb water. Eventually, the insulating material  155  may saturate, resulting in the wire  150  becoming grounded. This will result in the failure of the element  140 . 
   When the insulating material  155  absorbs water, the material  155  physically changes as it hydrates. The hydrating of the insulating material  155  decreases the resistance  215  of a leakage path from the element wire  150  to the grounded element (e.g., the heating element plug  165  and the coupled sheath  160 ). The control circuit  200  of the invention recognizes the changing of the resistance  215  of the leakage path, and issues an alarm when the leakage current increases to a predetermined level. 
   More specific to  FIG. 3 , it is common in the United States to apply  240  VAC to the element wire  140  by connecting a first  120  VAC to the first leg  202  and a second  120  VAC to the second leg  203 . The thermostat switch  210  removes the first  120  VAC from being applied to the heating element  140 , thereby having the water heater  100  enter a non-heating state. However, as shown in  FIG. 3 , the second  120  VAC through the second leg is still applied to the heating element  140 . As a consequence, a leakage current can still flow through the leakage resistance  215 . The voltage measurement circuit  220  provides a signal to the microcontroller  205  representing, either directly or through analysis by the microcontroller  205 , whether the thermostat switch  210  is in an open state, and the current measurement circuit  230  provides a signal to the microcontroller  205  representing, either directly or through analysis by the microcontroller  205 , the current through the circuit path including the leakage current. The microcontroller  205  can issue an alarm when the measured leakage current is greater than a threshold indicating the heating element  140  has a degrading sheath  160 . The threshold value can be set based on empirical testing for the model of the water heater  100 . The alarm can be in the form of a visual and/or audio alarm  250 . It is even envisioned that the alarm can be in the form of preventing further heating of the water until the heating element  140  is changed. 
   In another construction of the water heater  100 , the voltage measurement circuit  220  may not be required if the control of the current to the heating element  140  is performed by the microcontroller  205 . That is, the voltage measurement circuit  220  can inform the microcontroller  205  when the water heater  100  enters a heating state. However, in some water heaters, the microcontroller  205  receives a temperature of the water in the tank  105  from a temperature sensor and controls the current to the heating element  140  via a relay (i.e., directly controls the state of the water heater  100 ). For this construction, the voltage measurement circuit  220  is not required since the microcontroller knows the state of the water heater  100 . 
   In yet another construction of the water heater  100 , the microcontroller  205  (or some other component) may control the current measurement circuit  225  to sense the current through the heating element  140  only during the “off” state. This construction allows the current measurement circuit  225  to be more sensitive to the leakage current during the non-heating state. 
   Referring to TABLE 1, the table provides the results of eight tests performed on eight different elements. Each of the elements where similar in shape to the element  140  shown in  FIG. 2 . The elements were 4500 watt elements secured in 52 gallon electric water heaters similar in design to the water heater  100  shown in  FIG. 1 . Various measurements of the elements were taken during the tests. The measurements include the “Power ‘On’ Average Measured Differential Current”, the “Power ‘On’ Maximum Measured Differential Current”, the “Power ‘Off’ Average Measure Differential Current (ma)”, and the “Power ‘Off’ Maximum Measured Differential current.” Aperture were introduced to the sheath  160  of elements E, F, G, and H. The apertures resulted in the degradation of the insulating materials  155 . Measurements for the elements EFGH were taken while the insulators degraded. The data in TABLE 1 shows that the current measurements of elements with intact sheaths  160  taken during the “on” state (or heating state), overlap with the current measurements of elements with a damaged sheath  160 . For example, the element “Edge Hole G”, has a lower average current than the good element C and the good element D. In contrast, the current measurements made during the “off” state (or non-heating state) indicate a wide gap in current readings for an element with a damaged sheath  160  versus the element with an intact sheath  160 . For example, the lowest average current measured for a degraded sheath  160 , Edge Hole G at 12.5 ma, is over six times higher than the highest average current measured for an uncompromised element, i.e., Good D. 
   
     
       
             
           
             
             
             
             
             
           
             
             
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               DIFFERENTIAL CURRENT MEASUREMENTS 
             
           
        
         
             
                 
               POWER “ON” 
               POWER “ON” 
               POWER “OFF” 
               POWER “OFF” 
             
             
                 
               AVERAGE 
               MAXIMUM 
               AVERAGE 
               MAXIMUM 
             
             
                 
               MEASURED 
               MEASURED 
               MEASURED 
               MEASURED 
             
             
                 
               DIFFERNITAL 
               DIFFERENITAL 
               DIFFERNTIAL 
               DIFFERENTIAL 
             
             
               ELEMENT 
               CURRENT(ma) 
               CURRENT (ma) 
               CURRENT(ma) 
               CURRENT(ma) 
             
             
                 
             
           
        
         
             
               GoodA 
               0.45 
               2.78 
               0.56 
               3.15 
             
             
               GoodB 
               3.78 
               4.19 
               0.15 
               1.72 
             
             
               GoodC 
               4.41 
               5.15 
               0.10 
               0.12 
             
             
               GoodD 
               8.38 
               9.73 
               2.07 
               2.90 
             
             
               Center 
               59.9 
               &gt;407 
               218.8 
               &gt;407 
             
             
               HoleE 
             
             
               Center 
               79.8 
               &gt;407 
               144.3 
               378 
             
             
               HoleF 
             
             
               Edge 
               4.38 
               24.5 
               12.5 
               78.2 
             
             
               HoleG 
             
             
               Edge 
               9.44 
               14.7 
               13.8 
               15.2 
             
             
               HoleH 
             
             
                 
             
           
        
       
     
   
   A partial electrical schematic, partial block diagram for another construction of the control circuit  200 A used for controlling the heating element  140  is shown in  FIG. 4 . Similar to the construction shown in  FIG. 3 , the control circuit  200 A includes the microcontroller  205 , the thermostat switch  210 A, the voltage measurement circuit  220 , and the current measurement circuit  225 . However, for the construction of the control circuit in  FIG. 4 , the first leg  202 A of the circuit  200 A is connected to 120 VAC or 240 VAC and the second leg  203 A of the control circuit  200  is connected to ground. As further shown in  FIG. 4 , the double pole thermostat switch  210 A is electrically connected between the current measurement circuit  225  and 120 VAC or 240 VAC. The operation of the control circuit  200 A for  FIG. 4  is similar to the control circuit  200  for  FIG. 3 . TABLE 2 demonstrates a comparison between a heating element  140  initially having no apertures and the element  140  having an aperture at the edge of the element  140 . As can be seen, TABLE 2 demonstrates a large difference in current between the degraded element and the good element during the non-heating state. 
   
     
       
             
           
             
             
             
           
         
             
               TABLE 2 
             
           
           
             
                 
             
             
               DIFFERENTIAL CURRENT MEASUREMENTS DURING 
             
             
               POWER “OFF” CONDITION (240 VAC) 
             
           
        
         
             
               ELEMENT ID 
               Starting Current (mA) 
               Current at 1 Hour (mA) 
             
             
                 
             
             
               Good 
               0.04 mA 
               0.15 mA 
             
             
               Center Hole 
                560 mA 
                693 mA 
             
             
                 
             
           
        
       
     
   
   Before proceeding further, it should be understood that the constructions described thus far can include additional circuitry to allow for intermittent testing. For example and as shown in  FIG. 2 , a second switch  255  controlled by the microcontroller  225  can be added to attach the power source  201 A to the heating element  140  when thermostat switch  210 A is open, allowing the microcontroller  225  to perform a leakage current calculation. 
   A partial electrical schematic, partial block diagram for yet another construction of the control circuit  200 B used for controlling the heating element  140  is shown in  FIG. 5 . Similar to the construction shown in  FIG. 3 , the control circuit  200 B includes the microcontroller  205 , a thermostat switch  210 B, the voltage measurement circuit  220 , and a current measurement circuit  225 B. However, for the construction of the control circuit  200 B in  FIG. 5 , the arrangement and operation of the circuit  200 B shown in  FIG. 5  is slightly different than the arrangement of the circuit  200  shown in  FIG. 3 . As shown in  FIG. 5 , the current measurement circuit  225 B includes a current resistive shunt  500  that is electrically connected between a 12 VDC (or 12 VAC) power supply  505  and the thermostat switch  210 B. The thermostat switch  210 B is controlled by the thermostat temperature sensor and switches between the 120 VAC (or 240 VAC) power source and the 12 VDC (or 12VAC) power supply  505 . The voltage measurement circuit  220  is electrically connected in parallel with the heating element to determine the state of the water heater  100 . The operation of the control circuit  200 B for  FIG. 5  is somewhat similar to the control circuit  200  for  FIG. 3 . However, unlike the control circuit  200  for  FIG. 3 , when the control circuit  200 B moves to the non-heating state, the thermostat switch  210 B applies the voltage of the low-voltage power supply  505  to the heating element  140 . TABLE 3 demonstrates a comparison between a heating element  140  initially having no apertures and the element  140  having an aperture at the edge of the element  140 . As can be seen, TABLE 3 demonstrates a large difference in current between the degraded element and the good element during the non-heating state. 
   
     
       
             
           
             
             
             
           
         
             
               TABLE 3 
             
           
           
             
                 
             
             
               DIFFERENTIAL CURRENT MEASUREMENTS DURING 
             
             
               POWER “OFF” CONDITION (12 VDC) 
             
           
        
         
             
               ELEMENT ID 
               Starting Current (mA) 
               Current at 1 Hour (mA) 
             
             
                 
             
             
               Good 
               0.0 mA 
               0.0 mA 
             
             
               Center Hole 
                18 mA 
                18 mA 
             
             
                 
             
           
        
       
     
   
   When the temperature in the water heater  100  drops below a predetermined threshold the water heater  100  attempts to heat the water to a temperature greater than the predetermined threshold plus a dead band temperature by applying power to the heating element  140 . The heating element  140  generates heat that can be transferred to water surrounding the heating element  140 . Much of the heat energy produced by the heating element  140  can be dissipated by the water.  FIG. 6A  illustrates the temperature of a heating element  140  following application of power to the heating element  140  and wherein the heating element  140  is surrounded by water. The temperature of the heating element  140  rises rapidly initially and then the temperature rise slows until the temperature of the heating element  140  remains relatively constant. The constant temperature maintained by the heating unit  140  can be below a temperature wherein the heating element  140  fails. 
   Should power be applied to the water heater  100  prior to the water heater  100  being filled with water or should a malfunction occur in which the water in the water heater  100  is not at a level high enough to surround the heating element  140 , applying power to the heating element  140  creates a condition known as “dry-fire.” As shown in  FIG. 6B , during a dry-fire condition the heating element  140  heats up and, because there is no water surrounding the heating element  140  to dissipate the heat, continues to heat up to a temperature that causes the heating element  140  to fail. Failure of the heating element  140  during a dry-fire condition can occur in only a matter of seconds. It is, therefore, desirable to detect a dry-fire condition quickly, before damage occurs to the heating element  140 . 
     FIG. 7  illustrates a partial block diagram, partial schematic diagram of a construction of a fourth control circuit  600  that detects a dry-fire condition and prevents power from being applied to the heating element  140  when a dry-fire condition exists. 
   In some constructions, the control circuit  600  includes a relatively high-voltage power source (e.g., 120 VAC, 240 VAC, etc.)  201 B, a heating element  140 , a relatively low voltage power source (e.g., +12 VDC, 12 VAC, +24 VDC, etc.)  605 , a current sensing circuit  610 , a controller  205 , a temperature sensing circuit  615 , an alarm  620 , a normally open switch  625 , and a double-pole, double-throw relay  630   
   As shown in the construction of  FIG. 7 , the normally closed (“NC”) contacts of the relay  630  are coupled to the high-voltage power source  201 B through switch  625 . The normally open (“NO”) contracts of the relay  630  are coupled to the low-voltage power supply  605 . The output contacts of the relay  630  are coupled to the heating element  140 . When the switch  625  is closed and power is not applied to the coil (indicated at  635 ) of the relay  630 , the relay  630  remains in a state wherein the normally closed contacts remain closed and high voltage is applied to the heating element  140  enabling the heating element  140  to generate heat. When power is applied to the coil  635  of the relay  630 , the relay  630  closes the NO contacts and +12VDC is applied to the heating element  140 . The voltage of the low-voltage power supply  605  can be selected such that the heating element  140  would not be harmed from prolonged exposure in a dry-fire condition. 
   In this construction, the controller  205  is coupled to the temperature sensor  615  and the current sensor  610 , and receives indications of the temperature in the water heater  100  and the current drawn from the low-voltage power supply  605  from each sensor respectively. The controller  205  is also coupled to the alarm  620 , the switch  625 , and the relay  630 . 
     FIG. 8  represents a flow chart of an embodiment of the operation of the control circuit  600  for detecting a dry-fire condition. When the water heater  100  is powered on (block  700 ), the controller  205  applies power (block  705 ) to the coil  635  of the relay  630 . This opens the NC contacts of the relay  630  and closes the NO contacts of the relay  630 . Closing the NO contacts of the relay  630  couples the low-voltage power supply  605  to the heating element  140 . 
   In some constructions, the controller reads (block  710 ), from the current sensor  610 , a first current being supplied by the low-voltage power supply  605  to the heating element  140 . Other constructions of the dry-fire detection system  600  can read other electrical characteristics (e.g., voltage via a voltage sensor) of the circuit created by the low-voltage power supply  605  and the heating element  140 . 
   Next, the controller  205  closes (block  715 ) the switch  625  and couples the high-voltage power supply  201 B to the NC contacts of the relay  630 . The controller  205  also removes (block  720 ) power from the coil  635  of the relay  630 . This opens the NO contracts of the relay  630  which decouples the low-voltage power supply  605  from the heating element  140  and closes the NC contacts of the relay  630  coupling the high-voltage power supply  201 B to the heating element  140 . Coupling the high-voltage power supply  201 B to the heating element  140  causes the heating element  140  to heat up. The controller  205  delays (block  725 ) for a first time period (e.g., three seconds). 
   Following the delay (block  725 ), the controller  205  applies (block  730 ) power to the coil  635  of the relay which opens the NC contacts of the relay  635  and decouples the high-voltage power supply  201 B from the heating element  140 . The first time period can be a length of time that allows the heating element  140  to heat up but can be short enough to ensure the heating element  140  does not achieve a temperature at which it can fail if a dry-fire condition were to exist. Applying power to the coil  635  of the relay  630  also enables the NO contacts of the relay  630  to close and couples the low-voltage power supply  605  to the heating element  140 . 
   The controller  205  delays (block  735 ) for a second time period (e.g. ten seconds). During the delay, the heating element  140  begins to cool. The rate at which the heating element  140  cools can be faster if the heating element  140  is surrounded by water. The controller  205  reads (block  740 ), from the current sensor  610 , a second current being supplied by the low-voltage power supply  605  to the heating element  140 . The controller  205  compares (block  745 ) the first sensed current to the second sensed current and determines if the second sensed current is greater than the first sensed current by more than a threshold. If the second sensed current is not greater than the first sensed current by more than the threshold, the controller  205  determines that a dry-fire condition does not exist and continues (block  750 ) normal operation. 
   If the second sensed current is greater than the first sensed current by more than the threshold, the controller  205  determines that a dry-fire condition exists and opens (block  755 ) the switch  625 . Opening the switch  625  ensures that the high-voltage power supply  201 B is decoupled from the heating element  140  and prevents the heating element from being damaged. The controller  205  then signals (block  760 ) an alarm to inform an operator of the dry-fire condition. 
     FIGS. 9A and 9B  illustrate the resistance of the heating element  140  at different points during the dry-fire detection process for a wet-fire condition ( FIG. 9A ) and a dry-fire condition ( FIG. 9B ). At block  720 , the high-voltage power is applied to the heating element  140 . The temperature of the heating element  140  rises which increases the resistance of the heating element  140 . After a delay (block  725 ) the high-voltage power is disconnected from the heating element  140  (block  730 ). In a wet-fire condition,  FIG. 9A , the heating element  140  cools relatively rapidly causing the resistance of the heating element  140  to drop relatively rapidly to near the level of resistance of the heating element  140  prior to originally applying the high voltage as shown at block  740 . 
   Referring to  FIG. 9B , the resistance of the heating element  140  in a dry-fire condition is similar to the resistance of the heating element  140  in a wet-fire condition ( FIG. 9A ) for blocks  720  to  730 . Following disconnection of the high-voltage power at block  730  the heating element  140 , in a dry-fire condition, retains more heat and has a higher resistance for a relatively longer period of time. Testing an electrical characteristic of a circuit including the heating element  140  as explained at block  740  results in, when a dry-fire condition exists, a relatively large differential between the first reading at block  710  and the second reading at block  740 . 
   The control circuit  600  can execute the dry-fire detection process once, when power is first applied to the water heater  100 , each time the temperature sensing circuit  615  indicates that heat is needed, or at some other interval. Other constructions of the control circuit  600  can execute the dry-fire detection process at other times where it is determined that the potential for a dry-fire condition exists (e.g., following a period of time wherein the heating element  140  has been coupled to the high power signal). 
   Thus, the invention provides, among other things, a new and useful water heater and method of controlling a water heater. Various features and advantages of the invention are set forth in the following claims.