Abstract:
A control for an electric water heater detects a condition of a heating element when the heating element is not being energized. A switching module is operable to interrupt power to the heating element, which de-energizes the heating element. A detector module detects the condition of the heating element when the heating element is de-energized. The detector module senses current flowing through the heating element and generates a detection signal that is indicative of the current.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to electric water heater control, and more particularly to an electric water heater control employing a method for detecting high temperature conditions in electric water heaters. 
       BACKGROUND OF THE INVENTION 
       [0002]    This application relates to the art of controls and methods for operating electric water heaters. The invention is particularly applicable to a control and method that uses a control module running software and will be described with specific reference thereto. However, it will be appreciated that the invention has broader aspects and can be carried out with other types of controls. 
         [0003]    An electric water heater energizes one or more heating elements located within the water heater tank to heat water. Electrical power to the heating elements is managed through the operation of a control module, which controls the opening and/or closing of electrical relays connected in series between a power source and the heating elements. The thermal energy generated by the heating elements dissipates in the water, thereby heating the water according to a desired or preset water temperature. The control module is operable to interrupt power to the heating elements by opening one or more of the electrical relays. 
         [0004]    Certain circumstances may cause the heating elements to malfunction or burn out, causing an open circuit. When this occurs, the control module is unable to use the heating element to heat the water. Operation of the electric water heater with an open heating element may result in further damage to one or more additional components of the electric water heater. Therefore, it is desirable to detect an open heating element prior to providing power to the heating element. 
       SUMMARY OF THE INVENTION 
       [0005]    A control for an electric water heater detects a condition of a heating element. A switching module has an open state and a closed state and is connected between a first voltage potential and a second voltage potential. When the switching module is in the open state, the heating element is not energized. When the switching module is in the closed state, the heating element is energized. A detector module is connected in parallel to the switching module. The detector module senses current flowing through the heating element when the switching module is in the open state. The detector generates a detection signal that is indicative of the current. 
         [0006]    Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
           [0008]      FIG. 1  is a schematic diagram of an electric water heater according to the prior art; 
           [0009]      FIG. 2  is a functional block diagram of a water heater control according to the prior art; 
           [0010]      FIG. 3  is a functional block diagram of a water heater control that provides element out protection according to the invention; 
           [0011]      FIG. 4  is a functional block diagram of a water heater control including a detector module referenced to earth ground according to the invention; 
           [0012]      FIG. 5  is a schematic diagram of a water heater control including a current limiting resistor according to the invention; 
           [0013]      FIG. 6  is a schematic diagram of a water heater control including a current limiting capacitor according to the invention; 
           [0014]      FIG. 7  is a schematic diagram of a water heart control including an element out detection circuit referenced to earth ground according to the invention; and 
           [0015]      FIG. 8  illustrates an element out detection algorithm according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0016]    The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. As used herein, the term module refers to an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality. 
         [0017]    With reference to  FIG. 1 , the electric water heater  10  is shown and includes a tank  14 , an upper heating element  16 , and a lower heating element  18 . The tank  14  defines an inner volume  11  and includes an inlet  22  and an outlet  23 , both fluidly coupled to the inner volume  11 . The inlet  22  is fluidly coupled to a water supply  24  while the outlet  23  is connected to building fixtures such as faucets and showers, schematically represented as  26  ( FIG. 1 ). In this manner, the inlet  22  receives a constant supply of cold water under pressure from the building supply  24  such that the inner volume  11  of the tank  14  is always full of water. Water only exits the tank  14  via outlet  23  when water is consumed at one of the fixtures  26  throughout the building. Therefore, cold water only enters the tank  14  when hot water is consumed (i.e., exits the tank  14  via outlet  23 ). 
         [0018]    The upper heating element  16  extends through a side wall  28  of the tank  14  and generally into the inner volume  11 . The upper heating element  16  is electrically connected to a building power supply  30  and is disposed near to an upper wall  32  of the tank  14 . The upper heating element  16  receives current from the power supply  30  via control module  12  such that the control module  12  regulates the upper heating element  16  between an ON state and an OFF state. 
         [0019]    The lower heating element  18  extends through the side wall  28  of the tank  14  and generally into the inner volume  11 . The lower heating element  16  is electrically connected to the building power supply  30  and is disposed near to a lower wall  34  of the tank  14  such that the lower heating element  18  is generally closer to the lower wall  34  of the tank  14  than the upper heating element  16  is to the upper wall  32 . The lower heating element  18  receives current from the power supply  30  via control module  12  such that the control module  12  regulates the lower heating element  18  between an ON state and an OFF state. 
         [0020]    The electric water heater  10  also includes an upper temperature sensor  36  and a lower temperature sensor  38 , each in communication with the control module  12 . The upper and lower temperature sensors  36  and  38  are in communication with the control module  12  such that readings from the upper and lower temperature sensors  36  and  38  are transmitted to the control module  12  for processing. 
         [0021]    The upper temperature sensor  36  is disposed adjacent to the upper heating element  16  to monitor a temperature of water within the tank  14  generally between the upper heating element  16  and the upper wall  32 . The lower temperature sensor  38  is disposed adjacent to the lower heating element  18  to monitor a temperature of water within the tank  14  generally between the lower heating element  18  and the upper heating element  16 . The temperature sensors  36  and  38  are preferably thermistors, such as an NTC thermistors, but could be any suitable temperature sensor that accurately reads the temperature of the water within the tank  14 . 
         [0022]    During operation, the control module  12  receives information from the sensors  36  and  38  for use in selectively actuating the upper heating element  16  and/or lower heating element  18  to the ON state. Furthermore, the sensor module  35  could also include a flow sensor  37  disposed at the inlet  22  or the outlet  23  of the tank  14  to monitor a flow of water entering or exiting the tank  14 . The flow sensor  37  can be used to indicate exactly how much water has been consumed over a predetermined amount of time and can therefore be used in determining when the upper and lower heating elements  16 ,  18  should be toggled to the ON state to thereby heat water disposed within the tank  14 . 
         [0023]    An exemplary electric water heater control  50  is shown in  FIG. 2 . The water heater control  50  includes a control module  12  and a relay module  52 . The control module  12  is an electronic circuit and/or memory, such as a processor, that execute one or more software or firmware programs. For example, the control module  12  may include one or more software modules. The control module  12  generates one or more relay control signals  54  to determine a status of the relay module  52 . For example, if the water temperature exceeds a particular threshold, the control module  12  opens or closes one or more relays of the relay module  52 . In this manner, the control module  12  interrupts power between a power module  56  and one or more heating elements, represented schematically at  58 . 
         [0024]    Referring now to  FIG. 3 , the electric water heater control  70  of the invention provides element out detection of one or more heating elements. The electric water heater control  70  includes a control module  72  and a detector module  74 . The electric water heater control  70  may also include a current limiting module  76  and an output conditioning module  78 . The detector module  74  includes a device for detecting a current through the detector module  74 . For example, the detector module  74  may include a relay, hall effect current sensor, current transformer, optoisolator, or any other suitable device. 
         [0025]    When a relay  82  is closed and a heating element  84  is functioning properly, current flows between voltage sources  86  and  88 , and through the heating element  84 , thereby energizing the heating element  84 . The voltage potential across the current limiting module  76  and the detector module  74  is minimal. When the relay  82  is open (i.e. before the heating element  84  is energized) and the heating element is functioning properly, current flows through the detector module  74  and the heating element  84 . The detector module  74  detects the current and generates a detector output  90  that is indicative of the current. If the relay  82  is open and the heating element  84  is not functioning properly (e.g. the heating element  84  is out or open), current does not flow through the detector module  74  and the heating element  84 . In other words, the detector output  90  is indicative of whether the heating element  84  is functioning properly. 
         [0026]    The output conditioning module  78  receives the detector output  90  and outputs a signal  92  indicative of the detector output  90  to the control module  72 . The output conditioning module  78  may include any device operable to interface between the detector output  90  and the control module  72 . For example, the output conditioning module  78  may include a pull-up resistor, rectification circuit, integrator, pulse counter, amplifier, or any other suitable device. 
         [0027]    The current limiting module  76  limits current through the detector module  74  and the heating element  84  when the relay  82  is open. For example, the voltage difference between the voltage sources  86  and  88  may be 240 VAC for energizing the heating element  84 . Therefore, the current limiting module  76  may be used to protect the circuitry of the detector module  74  and limit current through the heating element  84 . The current limiting module  76  may include a resistor, capacitor, or any other AC impedance device. 
         [0028]    The electric water heater control module  50  may also include a diode  94 . The diode  94  may function as a reverse bias relief device that protects reverse bias breakdown in polarized devices. For example, if one or more devices of the detector module  74  is polarized, the diode  94  may be included. If detector module  74  does not include a polarized device, the diode  94  may be omitted. 
         [0029]    Referring now to  FIG. 4 , an alternative implementation of an electric water heater control  100  includes first and second element out detection modules  102  and  104 , respectively, that are referenced to an earth ground  106 . The first element out detection module  102  includes a detector module  74 - 1 , a current limiting module  76 - 1 , an output conditioning module  78 - 1 , and a diode  94 - 1 . Similarly, the second element out detection module  104  includes a detector module  74 - 2 , a current limiting module  76 - 2 , an output conditioning module  78 - 2 , and a diode  94 - 2 . When relays  108  is closed, relays  110 ,  112 , and  114  are open. Current flows between the second voltage source  88  and the earth ground  106 , through the upper and lower heating elements  116  and  118 . 
         [0030]    In this manner, the current flowing between the second voltage source  88  and the earth ground  106  is significantly less than the current flowing between the first voltage source  86  and the second voltage source  88 . Therefore, the current limiting modules  76 - 1  and  76 - 2  can be designed to accommodate less than the full 240 VAC potential between the first voltage source  86  and the second voltage source  88 . In other words, the current limiting modules  76 - 1  and  76 - 2  provide an impedance for 120 VAC rather than an impedance for 240 VAC. 
         [0031]    Referring now to  FIG. 5 , a first implementation of an element out detection circuit  120  is shown according to the implementation described in  FIG. 3 . The element out detection circuit  120  includes an optoisolator  122 , a current limiting resistor  124 , and a reverse bias relief diode  126 . A resistor  128  conditions an output  130  of the optoisolator  122  for the control module  72 . First and second voltage sources  130  and  132  provide current to a heating element  134  when a relay  136  is closed as described above, and current through the element out detection circuit  120  is minimal. When the relay  136  is open and the heating element  134  is functioning properly, optoisolator  122  is ON, and current flows between a potential  138  and ground  140 , through the resistor  128 . In this manner, the control module  72  receives a detection signal  142  indicative of the current flowing through the element out detection circuit  120 . In the present implementation, the first and second voltage sources  130  and  132  provide alternating current, and therefore the detection signal  142  will pulse accordingly. 
         [0032]    Conversely, if the relay  136  is open and the heating element  134  is out, current does not flow through element out detection circuit  120 , and the optoisolator  122  is OFF. Therefore, the detection signal  142  indicates that there is no current flowing through the element out detection circuit  120 . In other words, the detection signal  142  will remain at one of a high or low logic level, and will not pulse. Although only one element out detection circuit  120  is shown, those skilled in the art can appreciate that any number of element out detection circuits  120  may be implemented for one or more heating elements as described above and in  FIG. 3 . 
         [0033]    Referring now to  FIG. 6 , a second implementation of the element out detection circuit  150  replaces the current limiting resistor  124  with a current limiting capacitor  152 . A phase shift of the current through the current limiting element (e.g. the current limiting resistor  124  or capacitor  152 ) relative to the voltage generates heat. The current limiting capacitor  152  reduces the power dissipation of the current limiting element. 
         [0034]    Referring now to  FIG. 7 , a third implementation of the invention including first and second element out detection circuits  160  and  162  is shown according to the implementation described in  FIG. 4 . The element out detection circuits  160  and  162  include optoisolators  164 - 1  and  164 - 2 , referred to collectively as optoisolators  164 , current limiting resistors or capacitors  166 - 1  and  166 - 2 , referred to collectively as capacitors  166 , and reverse bias relief diodes  168 - 1  and  168 - 2 , referred to collectively as diodes  168 . Resistors  170 - 1  and  170 - 2  condition outputs  172 - 1  and  172 - 2  of the optoisolators  168  for the control module  72 . 
         [0035]    When relays  172  and  174  are closed, relays  176  and  178  are open, and heating elements  180 - 1  and  180 - 2  are functioning properly, current flows between a first voltage source  182  and earth ground  184 , through the heating elements  180 . The optoisolators  164  are ON, and the control module  72  receives one or more detection signals  186 - 1  and  186 - 2  indicative of the current flowing through the element out detection circuits  160  and  162 . If one or more of the heating elements  180  is out, current through one of the optoisolators  164  is interrupted. The corresponding signal  186  then indicates that a heating element is out. For example, the detection signal  186 - 1  indicates when the heating element  180 - 1  is out, and the detection signal  186 - 2  indicates when the heating element  180 - 2  is out. 
         [0036]    The element out detection circuits  160  and  162  may also be used to detect a condition of one or more of the relays. For example, regardless of whether the heating element  180 - 2  is functioning properly, current will flow through the optoisolator  164 - 2  when the relays  172  and  178  are closed. In other words, when the relays  172  and  178  are closed, current will flow between a second voltage source  188  and the earth ground  184 . However, if one or more of the relays  172  and  178  are supposed to be open (i.e. the control module  72  is attempting to open the relay  178 ), the detection signal  186 - 2  indicates the actual state of the relay. For example, if the relay  178  fuses closed, the control module  72  is no longer able to open the relay  178 . The detection signal  186 - 2  indicates that the relay  178  is closed notwithstanding the control of the control module  72 . 
         [0037]    The control module implements an element out detection method  200  as shown in  FIG. 8  (and in reference to  FIG. 4 ). In step  202 , the method  200  starts with all relays open. In step  204 , the method  200  determines whether pulses are detected from one or more of the detector modules (i.e. current is flowing through one or more of the detector modules). If true, the method  200  continues to step  206 . If false, the method  200  continues to step  208 . In step  206 , the method  200  determines that the relay  108  is closed due to a malfunction. For example, the relay  108  may be fused closed. Additionally, the coil drive circuit of the relay may be malfunctioning. In other words, although all relays should be open, current is flowing from the second voltage source  88 , through the relay  108 , to the element out detection modules  102  and  104 . In step  210 , the method  200  terminates. For example, because the relay  108  is closed due to a malfunction, the method  200  aborts power-up of the electric water heater control. 
         [0038]    In step  208 , the method  200  closes the relays  112  and  114 . In step  212 , the method  200  determines whether pulses are detected from one or more of the detector modules. If true, the method  200  continues to step  214 . If false, the method continues to step  216 . In step  214 , the method  200  determines that the relay  110  is closed due to a malfunction. The method  200  terminates in step  218 . 
         [0039]    In step  216 , the method  200  opens the relays  112  and  114 , and closes the relay  110 . In step  220 , the method  200  determines whether pulses are detected from the detector module  74 - 1 . If true, the method  200  continues to step  222 . If false, the method  200  continues to step  224 . In step  222 , the method  200  determines that the relay  112  is closed due to a malfunction. The method  200  terminates in step  226 . 
         [0040]    In step  224 , the method  200  determines whether pulses are detected from the detector module  74 - 2 . If true, the method  200  continues to step  228 . If false, the method  200  continues to step  230 . In step  228 , the method determines that the relay  114  is closed due to a malfunction. The method  200  terminates in step  232 . 
         [0041]    In step  230 , the method  200  closes the relay  112 . In step  234 , the method  200  determines whether pulses are detected from the detector module  74 - 1 . If true, the method  200  continues to step  236 . If false, the method  200  continues to step  238 . In step  238 , the method  200  determines that the relay  112  is open due to a malfunction. The method  200  terminates in step  240 . 
         [0042]    In step  236 , the method  200  opens the relay  112  and closes the relay  114 . In step  242 , the method  200  determines whether pulses are detected from the detector module  74 - 2 . If true, the method  200  continues to step  244 . If false, the method  200  continues to step  246 . In step  246 , the method  200  determines that the relay  114  is open due to a malfunction. The method  200  terminates in step  248 . 
         [0043]    In step  244 , the method  200  opens the relay  114  and closes the relay  108 . In step  250 , the method  200  determines whether pulses are detected from the detector module  74 - 1 . If true, the method  200  continues to step  252 . If false, the method  200  continues to step  254 . In step  254 , the method  200  determines that the upper heating element  116  is open (e.g. burned out). The method  200  terminates in step  256 . In step  252 , the method  200  determines whether pulses are detected from the detector module  74 - 2 . If true, the method  200  continues to step  258 . If false, the method  200  continues to step  260 . In step  260 , the method  200  determines that the lower heating element  118  is open. The method  200  terminates in step  262 . In step  258 , the method  200  determines that all relays and heating elements are functioning properly and then terminates. 
         [0044]    The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.