Abstract:
A combustion chamber is disposed below the water container of a water heater and formed at least partially by a shell. A burner disposed within the combustion chamber and a fuel supply line is connected to the burner. A valve associated with the fuel supply line. A combustion chamber sensor is disposed within the combustion chamber and adapted to sense a rise in temperature indicative of an abnormality in the combustion chamber. A circuit connected to the sensor and the valve such that the circuit triggers the valve to shut off fuel to the burner in response to a sensed temperature by the sensor.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to fuel-fired water heaters. 
       BACKGROUND 
       [0002]    A commonly used gas-fired water heater is the storage type, generally comprising an assembly of a water tank, a main gas burner to provide heat to the tank, a standing pilot burner to initiate the main burner on demand, an air inlet adjacent the burner near the base of the jacket, an exhaust flue and a jacket to cover these components. Another type of gas-fired water heater is the instantaneous type which has a water flow path through a heat exchanger heated, again, by a main burner initiated from a pilot burner flame. For convenience, the following description is in terms of storage type water heaters. However, the invention is not limited to this type. 
         [0003]    A particular difficulty with many locations for water heaters is that they are also used for storage of other equipment such as lawn mowers, trimmers, snow blowers and the like. It is common for such machinery to be refueled in such locations. 
         [0004]    There have been a number of reported instances of spilled gasoline and associated fumes being accidentally ignited. There are many available ignition sources, such as refrigerators, running engines, electric motors, electric light switches and the like. However, gas water heaters have sometimes been suspected because they often have a continuously burning pilot flame and combustion air inlets disposed at or near floor level, where spillage may occur. 
         [0005]    To contain ignitions that may occur due to the accidental spillage of fuel near a gas fired water heater, many manufacturers have incorporated flame traps into the design of their water heater. An example of such a design is disclosed in U.S. Pat. No. 6,293,230 to Valcic et al. The flame traps used in such designs comprise ports sized and shaped to cause air and extraneous fumes to pass through the ports at a velocity higher than the flame velocity of the extraneous fumes, thereby confining ignition and combustion of the extraneous fume species within the combustion chamber. 
         [0006]    One potential problem associated with the ports of the flame arresters is that the ports may become clogged with lint, dust, oil or any other element that may become disposed in or around the ports. When the ports become clogged, there is a potential for the combustion of the burner to burn inefficiently and produce increased levels of CO. 
         [0007]    One general consequence to both the emission of CO and the ignition of vapors is that the temperature in the combustion chamber rises above a normal operating level. It would be beneficial to provide a water heater with an improved system for detecting a rise in temperature in the combustion chamber and cut the fuel to the burner, thereby terminating combustion in the combustion chamber. 
       SUMMARY OF THE INVENTION 
       [0008]    This invention relates to field of fuel-fired water heaters and devices for sensing combustion chambers of fuel-fired water heaters. 
         [0009]    One aspect of the invention relates to a water heater having a water container; a combustion chamber disposed below the water container and formed at least partially by a shell having an interior surface; a burner disposed within the combustion chamber; a fuel supply line connected to the burner; a valve associated with the fuel supply line; a movable combustion chamber sensor disposed interiorly of the shell proximate to the interior surface of the shell, and adapted to sense a rise in temperature indicative of an abnormality in the combustion chamber; and a switch associated with the sensor and operatively associated with the valve such that the switch triggers the valve to shut off fuel to the burner in response to a sensed temperature by the sensor. 
         [0010]    Another aspect relates to a combustion chamber temperature sensing system including a casing having a sensing extension, and a barrel portion; a sensor disposed within the barrel portion and adapted to operate from a concave to convex position upon reaching a predetermined temperature; a switch including a member having a fixed first end portion connected to a first terminal and a movable second end portion biased against a second terminal; and a shaft portion is disposed between the sensor and the member and adapted to move the second end portion away from the second terminal when the sensor shifts from a concave to a convex position. 
         [0011]    A further aspect of the invention includes a water heater including a water container; a combustion chamber disposed below the water container and formed at least partially by a shell having an interior surface; a burner disposed within the combustion chamber; a fuel supply line connected to the burner; a valve associated with the fuel supply line; a movable combustion chamber sensor disposed interiorly of the shell proximate to the interior surface, and adapted to sense a rise in temperature indicative of a selected amount of carbon monoxide present in the combustion chamber; and a switch associated with the sensor and operatively associated with the valve such that the switch triggers the valve to shut off fuel to the burner in response to a sensed temperature by the sensor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    For the purpose of illustrating the invention, there is shown in the drawings a form which is presently preferred; it being understood, that this invention is not limited to the precise arrangements and instrumentalities shown. 
           [0013]      FIG. 1  is a side elevational view, taken partly in section, of a gas water heater embodying aspects of the invention. 
           [0014]      FIG. 2  is a front elevational view, taken partly in section, of the gas water heater shown in  FIG. 1 . 
           [0015]      FIG. 3  is a front elevational view of selected parts of the lower portion of the gas water heater shown in  FIG. 2 . 
           [0016]      FIG. 3A  is an exploded view of a portion of the structure shown in  FIG. 3 . 
           [0017]      FIG. 4  is a side view of a fuel supply line assembly with the burner removed for ease of understanding. 
           [0018]      FIG. 5  is a top plan view of the assembly shown in  FIG. 4 . 
           [0019]      FIG. 6  is an exploded side elevational view of a sensing system according to an embodiment of the present invention. 
           [0020]      FIG. 7  is a top plan view of a casing portion shown in  FIG. 6 . 
           [0021]      FIG. 8  is a top plan view of a spacer shown in  FIG. 6 . 
           [0022]      FIG. 9  is a bottom plan view of a circuit portion shown in  FIG. 6 . 
           [0023]      FIG. 10  is a cross sectional view of an embodiment of the sensing system of  FIG. 6  in an assembled condition. 
           [0024]      FIG. 11  is a cross sectional view of the sensing system of  FIG. 7  in a closed circuit condition, inserted in an access plate according to an embodiment of the invention. 
           [0025]      FIG. 12  is a cross sectional view of the sensing system of  FIG. 8  in an open circuit condition. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    It will be appreciated that the following description is intended to refer to specific aspects of the invention selected for illustration in the drawings and is not intended to define or limit the invention, other than in the appended claims. 
         [0027]    Turning now to the drawings in general and  FIGS. 1 and 2  in particular, the number “2” designates a storage type gas water heater  2 . The water heater  2  includes a jacket  4  which surrounds a water tank  6 , and a main burner  14  in a combustion chamber  15 . Passing through the center of the tank  6  is a flue  10 , which incorporates a series of baffles  12  to better transfer heat generated by the main burner  14 . The water tank  6  is preferably capable of holding heated water at a pressure at or exceeding that of any water main that may feed the water heater  2 . The water tank  6  is preferably insulated by foam insulation  8 . Alternative insulation may include fiberglass or other types of fibrous insulation, as is known to those skilled in the art. Preferably, fiberglass insulation  9  surrounds combustion chamber  15  and the lowermost portion of water tank  6 . It is possible that heat resistant foam insulation can be used if desired. A foam dam  7  separates the foam insulation  8  and the fiberglass insulation  9 . 
         [0028]    Located underneath the water tank  6  is the main burner  14  which uses natural gas or other gases such as LPG, for example. Other suitable fuels may be substituted, as is known to those skilled in the art. The main burner  14  combusts a gas and air mixture and the hot products of combustion resulting rise up through flue  10 , possibly with heated air. Preferably, the water tank  6  is lined with a glass coating for corrosion resistance. The bottom portion  5  of the water tank  6  is preferably coated on both its interior facing surface  3  and exterior facing surface  11 . The thickness of the coating of exterior facing surface  11  is preferably about half of the thickness of the coating on the interior facing surface  3 . Also, the lower portion of flue  10  is preferably coated on both of its opposing surfaces. The surface exposed to the flue gases preferably has a thickness about half the thickness of the surface exposed to water in water tank  6 . The glass coating helps to prevent scaling of the flue and water tank surfaces. 
         [0029]    Referring now to  FIGS. 1-5 , the combustion chamber  15  also contains a pilot burner  49  connected to a gas control valve  48  by a pilot fuel supply line  47 . A sheath  52 , preferably made of copper, containing wires (not shown) from a flame detecting thermocouple  51  to ensure that, in the absence of a flame at the pilot burner  49 , the gas control valve  48  shuts off the gas supply. The thermocouple  51  may be selected from those known in the art. Robertshaw Model No. TS 750U is one preferred thermocouple. The gas control valve  48  supplies fuel to the burner  14  by way of a fuel supply line  21 . 
         [0030]      FIGS. 1-5  show the fuel supply line  21  and pilot fuel supply line  47  extending outwardly from a plate  25 . The plate  25  is removably sealable to a skirt  60  that forms the side wall of the combustion chamber  15 . The plate  25  is held into position by a pair of screws  62  or by any other suitable means. The pilot fuel supply line  47  and fuel supply line  21  preferably pass through plate  25  in a substantially fixed and sealed condition. The sheath  52  also extends through the plate  25  in a substantially fixed and sealed condition as does an igniter line  64 . The igniter line  64  connects on one end to an igniter button  22  and on a second end to a piezo igniter  66  (see  FIGS. 3 and 5 ). The igniter button  22  can be obtained from Channel Products, for example, however those skilled in the art will recognize that many variations of the igniter button  22  may be used. Each of the pilot fuel supply line  47 , the fuel supply line  21  and the sheath  52  are removably connectable to the gas control valve  48  by compression nuts  68 ,  70  and  72 , respectively. Each of the compression nuts  68 ,  70  and  72  are threaded and threadingly engage the control valve  48 . 
         [0031]    Referring now to  FIGS. 1-5 , the products of combustion pass upwards and out the top of the jacket  4  via a flue outlet  16  after heat has been transferred from the products of combustion to water contained in the water tank  6 . The flue outlet  16  discharges conventionally into a draft diverter  17  which in turn connects to an exhaust duct leading outdoors, as is well known to those skilled in the art. 
         [0032]    The water heater  2  is preferably mounted on legs  24 . The water heater has a bottom pan  26 , which is raised off of the floor by the legs  24 . The bottom pan  26  preferably has one or more apertures  28  or some other means (not shown) for receiving combustion air, and allowing the combustion air to pass therethrough. 
         [0033]    The gas control valve  48  is preferably electronically operated, as is well known to those skilled in the art. Preferably, when power is supplied to the gas control valve  48 , the valve  48  is operable to the open position. Preferably, the valve  48  controls the flow of gas through both the fuel supply line  21  and pilot fuel supply line  47 . Preferably, the valve  48  is connected to a fuel source (not shown) by an external fuel supply line (not shown) as is well known in the art. Power may be provided in mill-volts, generated by a thermocouple. However, those skilled in the art will recognize that the power may come from any suitable source. The power may be measured in milli-volts up to 240 Volts AC. 
         [0034]    Preferably, the valve  48  is adapted to close when a source of power to the valve  48  is terminated. Closure of the valve  48  occurs in a manner that is well known. By way of example only, the valve  48  may be biased in the closed position by a spring and opened by an electronic actuator. When power to the electronic actuator is terminated, the spring may force the valve  48  to the closed position. 
         [0035]    Referring now to  FIGS. 2 ,  3  and  5 , a combustion chamber sensing system  100  is shown. The combustion chamber sensing system  100  is shown as being disposed on the plate  25 , although it need not be. For example, it may be disposed on skirt  60  if desired. Preferably, the sensing system  100  may be electronically connected to the valve  48  by a wire  86 . For purposes of describing the sensing system  100 , the terms proximal and distal, respectively, refer to the directions closer to and away from the burner  14  disposed within the combustion chamber  15 . 
         [0036]    Referring now to  FIGS. 6 to 12 , the system  100  preferably comprises a sensor casing  102 , a sensor  104 , a shaft  106 , a spacer  108 , and a switch portion  110 . The system further comprises a proximal end portion  103 , a distal end portion  105  and a longitudinal axis  101  extending therethrough from the proximal end portion  103  to the distal end portion  105 . 
         [0037]    Preferably, the sensor  104  is disposed within the casing  102 . The spacer  108  is disposed distally of the sensor  104 , and the shaft  106  is inserted through a central passageway  148  in the spacer  108 . The switch portion  110  is disposed distally to the spacer  108 , on an opposite side of the spacer  108  from the casing  102 . When the sensing system  100  is disposed through the plate  25 , the casing  102  is the part of the sensing system that is disposed closest to the burner  14 . Correspondingly, the switch portion  110  is the part of the sensing system  100  that is disposed furthest away from the burner  14 . While the sensing system  100  having a casing  102 , sensor  104 , and switch portion  110  is described herein, those skilled in the art will recognize that a variety of other specific structures may be utilized without departing from the scope of the invention. 
         [0038]    The casing  102 , moving from a distal to proximal portion thereof, preferably comprises a barrel portion  120 , a flange portion  122  and a sensing extension portion  124 . The sensing extension  124  extends proximally from the flange portion  122  in a direction away from the barrel portion  120 . Preferably, the barrel portion  120 , is generally cylindrical and hollow. The barrel portion  120  comprises a generally circumferential exterior wall  126  that defines an interior cavity  128 . A proximal end of the interior cavity  128  is further defined by an internal wall  130 . The internal wall  130  is generally perpendicular to the longitudinal axis  101 . A circumferential ridge  132  is disposed within the interior cavity  128 . The circumferential ridge  132  extends substantially circumferentially around the outer edge of the internal wall  130 , along the inner surface of the exterior wall  126 . Preferably, a distal casing lip  166  extends distally and towards the longitudinal axis  101  from the exterior wall  126 . While a circumferential barrel portion  120  is disclosed here, those skilled in the art will recognize that the barrel portion  120  may be any suitable shape. 
         [0039]    The flange portion  122  is preferably disposed proximally of the barrel portion  120 . Preferably, the flange portion  122  has a proximal flange wall  134  located on a proximal surface thereof and generally perpendicular to the longitudinal axis  101 . Preferably, an exterior surface  136  of the flange portion  122  is generally hexagonal. The exterior surface  136  is made up of a plurality of exterior flat portions  136   a  and exterior corners  136   b.  The corners  136   b  of the hexagonal exterior surface  136  extend generally farther from the longitudinal axis  101  than the circumferential exterior wall  126  of the barrel portion  120 . While a hexagonal shaped flange portion  122  is shown here, those skilled in the art will recognize that the flange portion  122  may have many different shapes without departing from the scope of the present invention. 
         [0040]    Preferably, the sensing extension portion  124  extends proximally from the proximal flange wall  134  along the longitudinal axis  101 . The sensing extension portion  124  comprises a generally elongated post. Preferably, an exterior surface  138  of the sensing extension portion  124  is threaded, although those skilled in the art will recognize that the exterior surface  138  may be smooth or have some other suitable texture without departing from the scope of the invention. One advantage of the threaded exterior surface  138  is that other articles may be connected to the sensing extension  124  with relative ease. Once installed in the water heater  2 , the sensing extension is preferably the closest part of the sensing system  100  to the burner  14 . 
         [0041]    The sensor  104  is disposed within the interior cavity  128  and completely inwardly of skirt  60 . The sensor  104  is a generally circular disc. The sensor  104  is preferably a bimetallic snap disc, which is well known in the art. The sensor  104  comprises an outer circumferential portion  140  and a central portion  142 . The sensor  104  is generally biased in a concave position when viewed from the distal direction and convex when viewed from the proximal direction. For purposes of this disclosure, the concave position shall be interchangeably used with the unsnapped position. 
         [0042]    Preferably, when the sensor  104  is inserted into the interior cavity  128 , the outer circumferential portion  140  engages the circumferential ridge  132 . The circumferential ridge  132  is raised enough from the internal wall  130  that when the outer circumferential portion  140  engages the circumferential ridge  132 , the central portion  142  does not contact the internal wall  130 . The sensor  104 , while generally biased in a concave position, preferably operates to a convex position upon reaching a predetermined temperature. 
         [0043]    The spacer  108  is generally circular, disc shaped, has a central passageway  148  and adapted to partially fit within the internal cavity  128  of the casing portion  102 . Preferably, the spacer  108  is disposed generally perpendicular to the longitudinal axis  101 . The spacer  108  comprises a proximal spacer surface  144  and a distal spacer surface  146 . The spacer  108  comprises a central spacer passageway  148  adapted to allow the shaft  106  to pass therethrough along the longitudinal axis  101 . A distal lip  150  extends distally from the distal spacer surface  146  and away from the skirt  60  toward the jacket  4 . Preferably, the distal lip  150  extends circumferentially around the central spacer passageway  148 . A circumferential reveal  152  is disposed around the outer edge of the distal spacer surface  146 . A proximal lip  154  extends proximally from the proximal spacer surface  144 . Preferably, the proximal lip  154  is disposed circumferentially around the outer edge of the proximal surface  144 . When the sensing system  100  is assembled, the proximal lip  154  engages the circumferential ridge  132  of the casing portion  102 . A proximal ridge  156  is disposed around an inner edge  155  of the proximal lip  154  and the proximal surface  144 . 
         [0044]    When the sensor  104  is inserted into the internal cavity  128  and the spacer  108  is placed above or distally of the sensor  104 , the proximal lip  154  surrounds the sensor  104 , thereby restricting lateral movement of the sensor  104 . The proximal ridge  156  does not compressibly engage the sensor  104  so as to restrict movement of the sensor  104  along the longitudinal axis  101 . Rather, the proximal ridge  156  is disposed just distally of the sensor  104  to loosely restrict longitudinal movement of the circumferential portion  140  of the sensor  104 . Those skilled in the art will recognize that the proximal ridge  156  may compressibly engage the sensor  104 , thereby pressing the sensor  104  against the circumferential ridge  132  of the casing portion  102 . Preferably, the proximal ridge  156  is disposed proximally enough away from the proximal surface  144  of the spacer  108  that when the sensor  104  operates from a concave position to a convex position, the central portion  142  of the sensor  104  does not come into contact with the proximal surface  144  of the spacer  108 . 
         [0045]    The shaft portion  106  is preferably a generally elongated solid cylindrical piece. The shaft portion  106  comprises a proximal shaft end portion  106   a  and a distal shaft end portion  106   b.  The shaft  106  is preferably adapted to pass through the central passageway  148 . Those skilled in the art will recognize that although the shaft  106  and central passageway  148  are shown here having a generally cylindrical profile, any profile shape may be used. 
         [0046]    Preferably, during assembly of the sensing system  100 , the shaft  106  is inserted through the central passageway  148 , engaging the central portion  142  of the sensor  104 . The shaft  106  is preferably slidable through the central passageway  148  without much, if any frictional resistance. 
         [0047]    The switch portion  110  comprises a generally cylindrical switch casing  158 , having a proximal end  160  comprising a proximal lip  162 . The proximal lip  162  extends circumferentially around the proximal end  160  of the switch casing  158 . Preferably, the outer diameter of the proximal lip  162  is slightly smaller than the inner diameter of the exterior wall  126  of the barrel portion  120 . Preferably, the inner diameter of the proximal lip  162  is larger than the outer diameter of the distal spacer surface  146 , thereby allowing the proximal lip  162  to contact the distal surface of the circumferential reveal  152  while surrounding the distal spacer surface  146 . A distal edge  164  of the proximal lip  162  defines the distal terminus of the proximal lip  162 . The distal edge  164  of the proximal lip  162  is disposed proximally enough along the switch casing  158  that, when the switch portion  110  is inserted into the internal cavity  128  of the casing portion  102 , the distal casing lip  166  exterior wall  126  extends distally of the distal edge  164  of the proximal lip  162 . 
         [0048]    Inside of the switch portion  110  is a circuit comprised of a first lead  168 , operatively connected to a first terminal  170 . The first terminal  170  is disposed within the switch casing  158 . The first terminal  170  is conductively and fixedly connected to a conductive member  172  that has a fixed portion and a flexible, movable portion. The conductive member  172  preferably comprises a fixed first contact end  174 , a movable second contact end  176  and a “U” shaped spring section  178  disposed between the fixed first contact end  174  and the movable second contact end  176 . The conductive member  172  is connected to the first terminal  170  at the first contact end  174 . 
         [0049]    A second lead  180  is operatively connected to a fixed second terminal  182 . The movable second contact end  176  is biased towards the fixed second terminal  182  by the “U” shaped spring section  178 . When the movable second contact end  176  contacts the fixed second terminal  182 , there is a continuous electrical connection between the first lead  168  and the second lead  180 . In such an instance, there is a closed circuit between the first lead  168  and the second lead  180 . The movable second contact end  176  is operable away from the second terminal  182  by applying force to the movable second contact end  176  distally, thereby compressing the “U” shaped spring section  178 . 
         [0050]    As shown in  FIG. 10 , when the sensing system  100  is assembled, the distal shaft end  106   b  is disposed just below the second contact end  176 . A raised convex contact surface  184  is disposed on the proximal surface of the movable second contact end  176 . The raised convex contact surface  184  is adapted to contact the distal shaft end  106   b  in the event that the shaft  106  is translated in a distal direction, towards the conductive member  172 . When the shaft  106  is translated in a distal direction, the distal shaft end  106   b  contacts the raised convex contact surface  184 . In other words, convex contact surface  184  moves relative to the balance of switch portion  110  substantially in concert with the movability of shaft  106 . If the shaft  106  is translated further distally, the second contact end  176  is translated distally and away from the fixed second terminal  182 . Thus, movable contact end  176  also moves relative to the balance of switch portion  110  substantially in concert with shaft  106 . When the movable second contact end  176  is translated away from the fixed second terminal  182 , the conductive connection between the first lead  168  and the second lead  180  is broken, thereby rendering the switch portion  110  open, as best seen in  FIG. 12 . 
         [0051]    It is preferable that the casing portion  102  be constructed from brass, or some other metal with similar heat conducting properties. The sensor  104  is made from materials known to those skilled in the art for bimetallic snap discs. The spacer  108  and the shaft  106  are preferably constructed from ceramic material. The switch portion  110  preferably comprises a combination of materials, each adapted to serve a specific purpose. By way of example, it is preferable that the leads  168 ,  180  the terminals  170 ,  182  and the flexible conductive member  172  conduct electricity. Preferably, the switch casing  158  and the reset shaft  186  are made from materials that generally insulate against conducting electricity and do not facilitate the flow of electricity therethrough, such as ceramic. 
         [0052]    In assembly, as best seen in  FIG. 10 , where the bottom of the figure is the proximal direction and the top of the figure is the distal direction, the sensor  104  is first inserted into the interior cavity  128  of the casing portion  102 . The sensor  104  is inserted  104  in a concave position, when viewed from the top or distal direction. This results in the central portion  142  of the sensor  104  being disposed closer to the interior wall  130  than the circumferential portion  140 . 
         [0053]    The spacer  108  is inserted into the interior cavity  128 , above the spacer  104 , so that the proximal lip  154  of the spacer engages the circumferential ridge  132  of the casing portion  102 . When the proximal lip  154  of the spacer engages the circumferential ridge  132  of the casing portion  102 , the sensor  104  is disposed between the casing portion  102  and the spacer  108 . There should be sufficient space between the casing portion  102  and the spacer  108  to allow the sensor  104  to operate between concave and convex dispositions. 
         [0054]    The shaft  106  is inserted through the central passageway  148  so that the proximal shaft end  106   a  engages the distal side of the central portion  142  of the sensor  104 . When the proximal shaft end  106 a engages the distal side of the central portion  142  of the sensor  104 , the proximal shaft end  106 b extends proximally from the distal lip  150  of the spacer  108 . 
         [0055]    The proximal lip  162  of switch portion  110  is then inserted into the internal cavity  128  of the barrel portion  120 . The switch portion  110  is inserted far enough into the internal cavity  128  that the distal edge  164  of the proximal lip  162  is proximal of the distal casing lip  166 . During assembly, the distal casing lip  166  is rolled towards the longitudinal axis  101 , thereby retaining the switch portion  110  partially within the casing portion  102 . The switch portion  110  is further partially retained within the casing portion  102  by sizing the pieces so that a press fit exists between the outer circumferential surface of the proximal lip  162  and the inner surface of the exterior wall  126  of the barrel portion  120 . 
         [0056]    A movable reset shaft  186  extends through the switch portion  110  along the longitudinal axis  101 . A proximal end  188  of the reset shaft  186  is adapted to engage a distal surface  177  of the movable second circuit end  176 . Thus, the reset shaft moves relative to the balance of switch portion  110  substantially in concert with shaft  106 , convex contact surface  184  and movable contact end  176 . A distal end  190  if the reset shaft  186  extends distally beyond a distal surface  111  of the circuit portion  110 . 
         [0057]    When the sensing system  100  is assembled, and the sensor  104  is in a concave position when viewed from the distal direction, it is preferable that the movable second contact end  176  is in contact with the fixed second terminal  182 . It is preferable that the shaft  106  is disposed between the distal surface of the central portion  142  of the sensor  104  and the raised convex contact surface  184 , without engaging the raised convex contact surface  184  at all, or alternatively, without applying enough force the raised convex contact surface  184  to move the movable second circuit end  176  away from the fixed second terminal  182 . 
         [0058]    Preferably, the assembled system  100  is installed into the access plate  25  by inserting the system  100 , distal end  105  first, through an aperture in the plate  25 . The system  100  is preferably inserted from an interior side of the plate  25 , when the plate  25  is installed on the water heater  2 . The installation of the system  100  into the plate may be done before the plate  25  is installed onto the water heater  2 . Referring to  FIG. 3A , a die contacts plate  25  to punch a hole for system  100  and forms tabs  200  that extend outwardly from plate  25 . The system  100  is then placed into plate  25  through the newly formed hole from the interior. A press then contacts tabs  200  and forms them over the round cap portion  202  of switch portion  110 . 
         [0059]    Referring now to  FIGS. 11 and 12 , the system  100  is inserted through the plate  25  only far enough that the switch portion  110  extends through the plate  25 . Preferably all, or at least a portion, of the barrel portion  120  is disposed either within the aperture of the plate  25  or on the interior (proximal) side of the plate  25 . There is a space between the distal side of the flange portion  122  and a proximal face  25   a  of the plate  25 . A spacer  190  may be disposed between the flange portion  122  and the plate  25  to restrict the distal movement of the sensing assembly  100 . The switch portion  110  is preferably disposed entirely outside of the plate  25 , although those skilled in the art will recognize that all or a portion of the switch portion  110  may be disposed within plate  25  or the combustion chamber  15 . 
         [0060]    The sensing assembly  100  may be retained in place, in relation to the plate  25  by a slip ring fastener  192 , or push nut fastener, as is known to those skilled in the art. The fastener  192  preferably compressibly engages the circuit casing  158 , applying inward and distal force on the sensing assembly  100 . Preferably, the fastener  192  biases the sensing system distally, so that the flange  122  compressibly engages the spacer  190  against the proximal face  25   a  of the plate  25 . Various alternative methods of mounting the system  100  to the plate  25  are possible. By way of example, a portion of the casing  158  or the exterior wall  126  of the barrel portion  120  may be threaded. Correspondingly, mating threads (not shown) may be disposed on the plate  25 . Additionally, the assembly  100  and a corresponding recess (not shown) in the plate  25  may be shaped to create a mechanical engagement, such as a quarter-turn lock, between the assembly and the plate. The assembly  100  may also be retained in relation to the plate  25  through the use of “C” or “E” clips, or through spot welding a portion of the assembly to the plate  25 . Alternatively, there may be at least one, and preferably two, holes in the switch portion  110  and corresponding hole(s) in the plate  25 . The assembly  100  may be retained to the plate  25  using a stud or other well known fasteners. 
         [0061]    Because the barrel portion  120  is disposed within the plate  25  or proximally of the plate  25 , the sensor  104  is mounted interiorly of the plate  25 . The sensor  104  is disposed at a point along the longitudinal axis  101  that is generally even with the distal edge of the flange  122 . This disposition ensures that the sensor  104  is disposed interiorly of the plate  25 . The distance between the proximal face  25   a  of the plate  25  and the sensor  104  is large enough that even when the sensor  104  is operated from a concave to a convex disposition, the entirety of the sensor  104  is disposed interiorly of the plate  25 . 
         [0062]    In operation, the switch portion  110  is connected in series to a power source on one end and the valve  48  on the other end. Generally, since the switch portion  110  is normally disposed in the closed position, the switch portion  110  facilitates the flow of electrical current from the power source to the gas control valve  48 . The valve  48  is adapted to close when power is interrupted via the opening of the circuit. When the valve  48  closes, the flow of fuel to the burner  14  is stopped. 
         [0063]    Generally, combustion occurs in the combustion chamber  15  at a predetermined temperature. This temperature is set according to ways known to those skilled in the art. Those skilled in the art will also recognize that certain events may cause the temperature in the combustion chamber  15  to rise above the predetermined level. Such a rise in combustion chamber  15  temperature may be indicative of a change in the operating characteristics in the combustion chamber  15  such as a flammable vapor event, or the accumulation of the combustion air intake area with lint, dust, oil or other debris, thereby causing the burner  14  to burn in an inefficient fuel-rich condition. When the inefficient, fuel-rich combustion occurs, undesirable levels of carbon monoxide may be released. There may also be other undesirable conditions indicated by an elevated temperature in the combustion chamber  15  as is known to those skilled in the art. 
         [0064]    The casing portion  102  is the part of the sensor system  100  that is disposed closest to the burner  14 . Therefore, the casing portion is directly exposed to the heat of the combustion chamber  15 . Heat is conducted through the casing portion  102  to the interior cavity  128 . The sensor  104  senses the temperature of the interior cavity  128 . By sensing the temperature of the interior cavity  128 , the sensor  104  senses the temperature of the combustion chamber  15 . 
         [0065]    When the sensor  104  reaches a predetermined temperature, the sensor  104  operates from a concave position, as shown in  FIG. 11 , to a convex position, as shown in  FIG. 12 . When the sensor  104  operates from concave to convex, the distal movement of the central portion  142  of the sensor  104  translates the shaft  106  distally such that the shaft  106  does not extend or project into combustion chamber  15  at all, thereby translating the movable second circuit end  176  distally and away from the fixed second terminal  182 . Generally, for natural gas models, the predetermined temperature at which the sensor  104  operates from a concave to a convex disposition is in the range between 400 and 460 degrees Fahrenheit. A preferred embodiment of a sensor  104  is adapted to operate from a concave to a convex disposition at 450 degrees Fahrenheit. For models using propane as a fuel, it is preferable to have the predetermined temperature between 300 and 350 degrees Fahrenheit. Those skilled in the art will recognize that the predetermined temperature at which the sensor  104  operates from a concave to a convex disposition may vary outside of the above-mentioned range. 
         [0066]    When the movable second circuit end  176  is moved away from the fixed second terminal  182 , the circuit is opened and current no longer flows through the switch portion  110  from the first lead  168  to the second lead  180 . This interruption in the flow of current through the switch portion  110  to the valve  48  triggers the valve  48  to close and restrict the flow of fuel to the burner  14 . The closing of the valve  48  when power is terminated thereto is a procedure that is well known to those skilled in the art. 
         [0067]    Preferably, when the circuit is opened, and power to the gas control valve  48  is terminated, gas is no longer permitted to flow to the burner. When gas ceases to flow to the burner  14 , combustion in the combustion chamber  15  is stopped. 
         [0068]    The sensing system may be reset by pushing the reset shaft  186  proximally. When pushed proximally, the reset shaft  186  engages the movable second circuit end  176 , which engages the shaft  106 , which engages the sensor  104 . When a user applies proximal force to the reset shaft  186 , the above-described chain of engagement ultimately applies force to the central portion  142  of the sensor  104  and “flip” the sensor  104  from a convex disposition back to a concave disposition. 
         [0069]    A variety of modifications to the aspects described will be apparent to those skilled in the art from the disclosure provided herein. Thus, aspects of the invention may be embodied in other specific forms without departing from the spirit or attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of the invention.