Abstract:
A heater having a shut off device is provided which prevents false over temperature lockouts. The heater comprises a body having walls defining a volume for holding water, a heating element thermally coupled to the body for heating water within the body, and a temperature sensor for sensing temperature of the material. The heater also has a shut off switch for shutting off electric current in the heater when the sensed temperature of the water exceeds a predetermined maximum temperature limit, and a manually actuated reset input for resetting the shut off switch to allow current to be applied to the heater. The heater further includes a controller coupled to the shut off switch and the reset switch, wherein the controller determines the presence of a false over temperature event and overrides the need to manually actuate the reset input.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to electric heaters and, more particularly, to heaters, such as electric water heaters, employing over temperature shut off controls. 
     Electrically powered water heaters are commonly employed to heat a supply of water for use in jetted bathtubs, spas/hot tubs and other heated water applications by heating water flowing through a vessel. Electric water heaters typically include an electric powered heating element arranged in a heat transfer relationship with the water flowing within the vessel. In many conventional flow-through water heating systems, a thermostat is disposed within the hollow of the vessel or the tub to sense the temperature of the water, and the heating element is generally controlled based on the sensed water temperature so as to maintain a desired water temperature. One example of a water heater is disclosed in U.S. Pat. No. 6,080,973, the disclosure of which is hereby incorporated by reference. 
     Conventional electric water heaters employed in jetted bathtubs and spas/hot tubs are generally controlled in response to the sensed water temperature to maintain a user selectable water temperature in the heated water tub. In most jetted bathtubs, a maximum upper temperature limit of about 104° F. is typically established according to industry standards. In addition to controlling the heating element to achieve the selected water temperature, it is also desirable to ensure adequate operation of the water heater to prevent an excessive over temperature condition (i.e., overheating) and problems that can arise therefrom. For example, in the event that a failure occurs in the heater controls, the water temperature may exceed the maximum upper temperature limit. The water heater may overheat quickly when there is an inadequate amount of water present in the heater vessel due to an abnormally low water level. Advanced overheating may also occur when there is inadequate water flow through the heater vessel such as may be caused by the failure of a water pump or other water flow restriction. 
     In order to prevent the presence of an excessive over temperature condition, many conventional water heaters are generally equipped with a temperature actuated shut off device that discontinues power supplied to the heating element when a predetermined upper temperature limit is reached. Conventional temperature-based shut off devices include a snap disc thermal switch connected in series with the power input of the electrically operated heating element. The snap disc thermal switch is designed to switch from a closed position to an open position to open circuit the power line supplying electric current to the heating element upon detecting a predetermined upper temperature limit of about 117° F., according to one example. Some industries, such as the jetted bath tub and spa/hot tub industry, have established a requirement to also equip the water heater with a manually depressible reset button, and further require that a user must depress the reset button to reset the heater in order to allow the heater to be energized following an over temperature shut off. Typically, the reset button is located remote from the heated water tub, and thus requires that the user take additional action to reset the heater. 
     While it is desirable to equip heaters with over temperature shut off protection, there exist certain conditions where a false over temperature determination may occur. For example, if a user fills a spa/hot tub with excessively hot water having an elevated temperature above the upper temperature limit, the snap disc thermal switch may be tripped which, in turn, locks out (shuts off) use of the heater prior to the heater being energized, thus requiring that the user manually depress the reset button to reactivate and allow the heater to subsequently be energized. Therefore, it is desirable to provide for a heater control system that provides over temperature protection and yet reduces or minimizes the presence of false over temperature heater lockout events. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a heater having a shut off device is provided which prevents false over temperature lockouts. According to one aspect of the present invention, the heater comprises a body having walls defining a volume for holding material, a heating element thermally coupled to the body for heating material within the body, and a temperature sensor for sensing temperature of the material. The heater also has a shut off switch for shutting off the heater when the sensed temperature of the material exceeds a predetermined maximum temperature limit, and a manually actuated reset input for generating a reset signal to allow the heater to be turned on. The heater further includes a controller coupled to the shut off switch and the reset switch, wherein the heater is required to be reset by the reset input when an over temperature event is determined, and wherein the controller determines the presence of a false over temperature event and overrides the need to reset the heater during the false over temperature event. 
     According to another aspect of the present invention, a heater having a temperature sensitive shut off switch is provided. The heater includes a body having walls defining a volume for holding water to be heated, and an electric heating element thermally coupled to the body for heating water within the body. The heater also has a temperature sensitive switch, such as a snap disc thermal switch, connected in series with the heating element for sensing temperature of the water and shutting off electrical power supplied to the heating element when the sensed temperature of the water exceeds a predetermined maximum temperature limit. The heater further includes a controller connected to the temperature sensitive switch for monitoring voltage potential applied to the temperature sensitive switch and determining whether a failure of the heater has occurred as a function of the monitored voltage potential. 
     These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a block diagram illustrating an electric water heater in cross section and a heater control system according to the present invention; 
     FIG. 2 is a block/circuit diagram further illustrating the electric water heater control system for controlling the heater according to the present invention; 
     FIG. 3 is a flow diagram illustrating a methodology of controlling the heater by controlling switch K 1  according to the present invention; 
     FIG. 4 is a flow diagram illustrating a methodology of further controlling the heater by controlling switch K 2 ; and 
     FIG. 5 is a flow diagram illustrating a methodology of detecting an over temperature lockout condition for use in controlling the heater. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, an electric water heater  10  having a heater control system according to the present invention is generally illustrated for heating water for use in a heated water tub, such as a spa/hot tub  12  or a jetted bathtub. The heater  10  shown and described herein is a flow-through water heater in which water from the tub  12  is circulated in a known manner by way of a pump  14  into the inlet  16  of water heater  10 . During normal heating operation, the circulating water is heated in the heater  10  as it flows past an electric heating element  22 . The heated water then flows out of outlet  18  and is circulated back into the tub  12 . While the water heater  10  is illustrated and described herein as a flow-through water heater for use in heating water in a spa/hot tub  12  or jetted bathtub, it should be appreciated that the heater  10  may alternately include different types of heaters configured in various shapes and sizes and may be used in various other applications to heat various materials. 
     The heater  10  shown generally includes a body in the form of a hollow vessel  20  having cylindrical walls defining a volume for holding water or other material to be heated when the heater  10  is energized. The vessel  20  may be made of stainless steel or polymeric material, such as polyvinyl chloride (PVC), for example. An electrical heating element  22  is thermally coupled to the vessel  20  for transferring thermal energy to the water to heat the water within the vessel  20 . The heating element  22  may be disposed within vessel  20  and in direct contact with the water as shown. Alternately, heating element  22  may be disposed on the outer walls of a heat conductive vessel  20  for indirectly heating the water by heat conduction through vessel  20 . 
     The heating element  22  has an input terminal  24  and an output terminal  26  extending through a pair of openings in the walls of vessel  20 . The input terminal  24  is connected to a power supply that supplies an electric voltage input V IN . The output terminal  26  is connected in series to a snap disc thermal switch (S 1 )  42  which, in turn, is coupled to ground. Also coupled to the output terminal  26  is a detection circuit  40  which detects the voltage potential at the output terminal  26 . The detection circuit  40  further detects the state (i.e., open or closed positions) of the snap disc thermal switch  42  as described herein. 
     The heater  10  also employs a flow sensor  36  and a temperature sensor  38 . The flow sensor  36  senses water flow within the heater vessel  20  and produces a flow signal indicative thereof. The flow signal is processed and used to determine if insufficient water flow is present, such that the heater should be shut off to prevent overheating. The temperature sensor  38  senses temperature of the water within the vessel  20  and produces a temperature signal indicative thereof. The temperature signal is processed and used to determine the amount of heating required to achieve a set water temperature. 
     The heater  10  further includes a controller  30  having a microprocessor  32  and memory  34 . The controller  30  described herein is a digital controller programmed to process control routines that are stored in memory  34  and performed by microprocessor  32  for controlling the operation of the heater  10 . The controller has input/output pins P 1 -P 8 . Controller inputs include the flow signal at pin P 5 , the temperature signal at P 3 , and a reset signal at pin P 4  generated by a reset pushbutton  44 . The controller  30  is also connected to the detection circuit  40  via pins P 1  and P 2  for receiving the sensed voltage at output terminal  26  and further performing a routine to detect a shut off condition and set the lockout flag. The controller  30  also controls the input voltage V IN  from power supply  28  applied to heating element  22  by controlling switches K 1  and K 2  via pins P 6  and P 7  by keeping closed both of normally open switches K 1  and K 2  to apply voltage V IN  and allow current flow in the heating element  22 , and further open circuiting one or both of switches K 1  and K 2  to cut off power supplied to heating element  22 . The controller  30  controls both of switches K 1  and K 2  so as to turn off the heating element  22  during certain detected conditions. Based on certain detected conditions, the controller  30  provides a shut off to de-energize the heating element  22 . While a digital controller  30  is shown and described herein, it should be appreciated that the controller could otherwise include analog circuitry. 
     Referring particularly to FIG. 2, the pair of switches K 1  and K 2  are shown as relay controlled switches K 1  and K 2  connected in series to the input terminal  24  of heating element  22 . Switch K 1  is controlled in response to relay R 1  of a regulating relay drive circuit  48 . The regulating relay drive circuit  48  includes a pair of inputs coupled to pins P 7   a  and P 7   b  of controller  30 . In response to detecting certain conditions, controller  30  turns off relay R 1  of regulating relay drive circuit  48  to cause switch K 1  to switch from a closed position to an open position, thereby shutting off power to heating element  22 . Switch K 2  is controlled in response to relay R 2  of the limit relay drive circuit  50 . Limit relay drive circuit  50  likewise includes a pair of inputs coupled to pins P 8   a  and P 8   b  of controller  30 . Controller  30  turns off relay R 2  of limit relay drive circuit  50  so as to cause switch K 2  to switch from a closed position to an open position to thereby shut off power to the heating element  22 . Switches K 1  and K 2  are in a closed position during normal heater control, thus allowing the heating element  22  to be energized. Switch K 1  changes state from a closed position to an open circuit position whenever one of the following conditions is detected; the sensed temperature of the water exceeds a temperature limit of 104° F.; the snap disc thermo switch is open; insufficient water flow is detected, the reset button is depressed and not released; or a lockout event has occurred. Accordingly, switch K 1  turns off power to the heating element  22  whenever one of the aforementioned events occurs. Switch K 2  serves as a backup control switch that performs a redundancy check of certain conditions used to control switch K 1 . Switch K 2  changes state from a closed position to an open circuit position whenever one of the following conditions is detected: insufficient water flow is detected; the reset button is depressed and not released; or a lockout event has occurred. Accordingly, switch K 2  duplicates some of the function performed by switch K 1  to turn off power to the heating element  22  whenever such events are detected. 
     The snap disc thermal switch S 1  is a temperature sensitive switch that is in either an open circuit position or a closed circuit position depending on temperature. Snap disc thermal switches are well-known to those skilled in the art. One example of commercially available snap disc thermal switch includes Series No. Thermodisc 36T, commercially available from Thermodisc Inc. The aforementioned snap disc thermal switch is designed to change state from a closed position to an open circuit position whenever the temperature of the snap disc exceeds a predetermined temperature of about 117° F., and is further designed to reclose to the closed position when the temperature subsequently drops below a temperature of about 102° F. Sensors of this type generally have a tolerance of about ±4.5° F. Accordingly, the snap disc thermal switch S 1  de-energizes current flow through the heating element  22  when the temperature rises above a temperature of about 117° F., ±4.5° F. and keeps the heating element  22  shut off until the temperature drops to below 102° F., ±4.5° F. When the temperature exceeds an upper temperature limit of 117° F. sufficient to open circuit the snap disc thermal switch S 1 , the heater control requires a manual reset of the control circuit when an actual over temperature condition occurs prior to re-energizing the heating element  22 , but detects an event which provides a false over temperature indication, and thereby avoids the need for the manual reset. 
     To provide the manual reset, the heater  10  is further equipped with reset pushbutton  44  which is depressible by a user to reset the heater  10  following an over temperature shut off. The reset pushbutton  44  includes a contact for close circuiting an input to controller  30  via pin P 4  to produce a reset signal. The controller  30  checks for both a closing of the reset pushbutton  44  followed by the release of the pushbutton  44  prior to acknowledging a reset event. By requiring both closing and release of the reset pushbutton  44 , the controller  30  ignores the reset signal until release is detected to prevent users from keeping the reset pushbutton  44  fully depressed in an attempt to by pass the reset function. One example of a reset pushbutton  44  may include a miniature mechanical key switch having Part No. B3F-1052, commercially available from Omron Electronics. 
     Also shown in FIG. 2 is an AC reference circuit  46  which continuously checks for the presence of an AC line (e.g., 120 volts A.C.) supplied by voltage V IN . If controller  30  determines that an AC line has not been detected for three continuous cycles, the controller  30  determines that a faulty control signal is present, and shuts down the heater control system by opening one or both of switches K 1  and K 2 . Coupled in parallel to the heater element  22  is an indicator light  52  which provides a visual indication when the heating element  22  is energized. 
     The detection circuit  40  is connected to the output terminal  26  of heater element  22  and one end of snap disc thermal switch  42  via a pair of high impedance lines  54  and  56 . The detection circuit  40  is further coupled to controller  30  via pins P 1  and P 2 . The detection circuit  40  includes high impedance resistors R in each of high impedance lines  54  and  56 . The other end of snap disc thermal switch  42  is connected to a ground reference common with the ground reference employed by the controller  30  and associated circuitry. In addition, each of the high impedance lines  54  and  56  has a capacitor C coupled to ground. High impedance line  56  further has a resistor R coupled to ground. High impedance line  54  is coupled to input pin P 1  of controller  30 , while high impedance line  56  is coupled to input line P 2  of controller  30 . The ground connections employed by detection circuit  40  are common to the ground connected to one end of snap disc thermal switch  42 . The controller  30  applies a signal to one of the pins P 1  or P 2  and receives a signal on the other of pins P 1  and P 2 , to detect whether the snap disc thermal switch S 1  is open as described herein. By applying a voltage signal on one of the high impedance lines  54  and  56 , via pins P 1  or P 2 , respectively, the voltage signal on the other of the high impedance lines  54  and  56  may be sensed. If the snap disc is closed, the sensed signal received by one of pin P 1  or P 2  will be substantially the same as ground. Whereas if the snap disc  42  is open, the voltage potential received at the other of pin P 1  or P 2  will have a higher voltage potential. 
     Referring to FIG. 3, a method  60  of controlling switch K 1  to control power supplied to the heating element  22  is described therein. Methodology  60  checks for a number of conditions to determine whether to open or close switch K 1 . Included is decision step  64  for determining if the sensed water temperature is below an upper temperature limit of 104° F. and, if not, methodology  60  proceeds to turn switch K 1  off to open circuit the power supplied to heating element  22 . In decision step  66 , methodology  60  determines if the snap disc thermal switch S 1  is opened and, if so, turns switch K 1  off (open). Otherwise, methodology  60  proceeds to decision step  68  to check if water is flowing and, if not, turns switch K 1  off in step  62 . Otherwise, methodology  60  proceeds to decision step  70  to determine if the reset button is depressed and has not been released and, if so, proceeds back to step  62  to turn switch K 1  off. Otherwise, methodology  70  proceeds to decision step  72  to check if the lockout flag (e.g., bit) is set equal to true and, if so, turns switch K 1  off in step  66 . Otherwise, methodology  60  proceeds to step  74  to turn switch K 1  on to thereby close the power supply circuit and allow heating element  22  to be energized. Thereafter, methodology  60  returns to decision step  64 . Accordingly, if the temperature is below the upper temperature limit of 104° F., the snap disc thermal switch S 1  is not open, water is flowing, the reset button is not depressed without being released, and the lockout flag is set equal to false, switch K 1  is turned on (closed). 
     A methodology  76  for controlling switch K 2  to open circuit or close circuit power supplied to heating element  22  is illustrated in FIG.  4 . Methodology  76  likewise includes decision steps  68 ,  70 , and  72  which check for whether water is flowing, the reset button is depressed without being released, and a lockout event has occurred, respectively. If water is not flowing, the reset button is depressed and has not been released, or if a lockout event has been detected, methodology  76  proceeds to step  78  to turn switch K 2  off (open) to thereby open circuit the power supply to heating element  22 . Otherwise, if water is flowing, the reset button is not depressed and released, and no lockout event is detected, methodology  76  proceeds to step  77  to turn switch K 2  on (closed) to thereby close the power supply circuit and allow heating element  22  to be energized. Accordingly, methodology  76  controls switch K 2  to perform duplicative functions similar to those performed by switch K 1 , thus serving as a backup control in the event that a relay or switch failure occurs. 
     Referring to FIG. 5, a methodology  80  of detecting a lockout condition and setting the lockout flag is illustrated therein. Methodology  80  includes step  82  of setting pin P 1  output high. Next, in decision step  84 , methodology  80  determines if pin P 2  is set high and, if so, determines that the snap disc thermal switch S 1  is open in step  86 . In step  88 , pin P 1  is set as an input, and then in decision step  90 , methodology  80  checks whether pin P 1  or pin P 2  is set high and, if so, sets the lockout flag equal to true in step  92 , and then returns to step  82 . Otherwise, methodology  80  proceeds to decision step  94  to check if the reset button is depressed and has not been released and, if not, returns to step  82 . If the reset button has been depressed and has not yet been released, methodology  80  proceeds to step  96  to set the lockout flag equal to false. Accordingly, by setting the lockout flag equal to true in step  92 , a lockout event is determined, whereas by setting the lockout flag equal to false in step  96 , no such lockout event is determined. When the lockout flag is set equal to true, the controller  30  prevents the heater from being energized until the manual reset event occurs. As long as the lockout flag is set equal to false, the requirement for a manual reset is overridden by controller  30 , and thus the heating element  22  may be energized. Thus, with the lockout flag set equal to false, closing of the snap disc combined with a sensed temperature of less than the preset upper temperature limit, will cause the controller  30  to turn on the relays R 1  and R 2  to close switches K 1  and K 2  to allow current to flow through heating element  22 . 
     Accordingly, the heater  10  of the present invention advantageously detects the state of the snap disc thermal switch  42  and determines the presence of an over temperature condition. If a failure occurs in the control system, the resulting over temperature condition will be detected and a manual reset by the user will be required. If the over temperature condition is a false over temperature condition, the need for a manual reset is overridden. For example, if the heated water tub is filled with water having a temperature exceeding the upper maximum over temperature limit sufficient to open the snap disc thermal switch  42  and at least one of the switches K 1  and K 2  are open, when the water temperature drops sufficiently low enough to reclose the snap disc thermal switch  42 , normal control of the heater  10  may be resumed without requiring actuation of the manual reset pushbutton  44 . 
     It will be understood by those who practice the invention and those skilled in the art, that various modifications and improvements may be made to the invention without departing from the spirit of the disclosed concept. The scope of protection afforded is to be determined by the claims and by the breadth of interpretation allowed by law.