Abstract:
A method of controlling a gas-fired water heater. A resistance input is received from a sensor configured to sense flammable vapor near the heater tank. The resistance input is compared to one or more previously received inputs from the sensor. Based on the comparing, one or more functions of the heater are controlled. This control method can be used to compensate for gradual ageing of the sensor.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 10/863,319 filed on Jun. 8, 2004 now U.S. Pat. No. 7,032,542. The disclosure of the above application is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to gas furnaces and, more particularly, processor control of a water heater. 
     BACKGROUND OF THE INVENTION 
     In gas-powered furnace systems, sensors of various types are commonly used to provide information for controlling system operation. In residential water heaters, for example, an immersion sensor may be used inside a water tank to monitor water temperature. Commercial water heaters, which typically operate at higher temperatures than residential units, may have a pair of immersion sensors, one at the tank top and one at the tank bottom. Bottom and top sensors typically are monitored relative to a set-point temperature and a temperature range. Heating typically is stopped when the water temperature reaches the set-point temperature and is initiated when the temperature drops below the temperature range. 
     Water heaters also frequently are configured with flammable vapor (FV) sensors for detecting presence of a flammable vapor. Vapor presence may be detected by using a signal comparator to monitor the resistance level of an FV sensor. For example, where a typical FV sensor resistance might be approximately 10,000 ohms, such resistance could rapidly increase to approximately 50,000 ohms in the presence of a flammable vapor. If the FV sensor exhibits a high resistance as sensed by the signal comparator, gas supply to the heater typically is shut off. 
     The inventors have observed, however, that FV sensors may undergo changes in resistance due to general ageing, even in a mild environment. Chemical vapors, e.g., chlorines commonly found in household bleaches, can accelerate this process. Over time, a FV sensor may gradually exhibit increased resistance sufficient to cause a false shut-down of a furnace system. On the other hand, the inventors have observed that resistance of a FV sensor may diminish gradually over time, possibly to such a low level that it might not trip a shut-down of a heating system if a flammable vapor event were to occur. 
     In view of the foregoing, it has become apparent to the inventors that using processor-supplied logic to process sensor inputs and to control heater operation provides opportunities for improving the efficiency and safety of water heater operation. Heating systems are known in which operating power is supplied to a microprocessor by a thermoelectric generator connected to a pilot burner. Such a generator, however, might not be able to generate voltages high enough to operate the processor, unless energy output by the pilot burner is increased. 
     SUMMARY OF THE INVENTION 
     The present invention, in one embodiment, is directed to a method of controlling a gas-fired water heater having a tank. A resistance input is received from a sensor configured to sense flammable vapor near the tank. The resistance input is compared to one or more previously received inputs from the sensor. Based on the comparing, one or more functions of the heater are controlled. 
     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 embodiments 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 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a perspective view of a water heater according to one embodiment of the present invention; 
         FIG. 2  is a schematic diagram of a water heater controller according to one embodiment of the present invention; and 
         FIG. 3  is a flow diagram of a method of controlling a water heater according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description of embodiments of the invention is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     A gas water heater according to one embodiment of the present invention is indicated generally by reference number  20  in  FIG. 1 . The heater  20  has a tank  24  into which cold water enters via a cold water inlet pipe fitting  26  and cold water inlet  28 . Cold water entering the bottom  32  of the tank is heated by a gas burner (not shown) beneath the tank. The burner can be lighted, for example, using an igniter (not shown in  FIG. 1 ). Heated water rises to the top  40  of the tank and leaves the tank via a hot water pipe  44 . Combustion gases leave the heater via a flue  48 . An electrically operated solenoid gas valve (not shown in  FIG. 1 ) controls gas flow through a gas supply line  52  to the burner as further described below. 
     An apparatus for controlling the heater  20  includes a controller  56  positioned, for example, adjacent the tank  24 . As further described below, the controller  56  is configured to sense flammable vapors, water temperature at the top  40  of the tank  24 , and water being drawn from the tank. The controller  56  also can responsively activate or deactivate the igniter and the gas valve, as further described below. 
     A 24-volt plug-in transformer  60  is plugged into a line voltage source, e.g., a receptacle outlet  62  of a 120 VAC line  64 . Thus the transformer  60  can be plugged into a voltage source remote from the controller  56  and remote from the tank  24 . Conductive wiring  66  connects the transformer  60  with the controller  56 . The transformer steps down the line voltage to provide a stepped-down voltage to the controller  56 . In other embodiments, line and stepped-down voltages may differ from those described in the present configuration. 
     A surface-mounted temperature sensor  70  connected to the controller  56  senses water temperature near the top of the tank  24 . To prevent scalding, the controller  56  can shut off the heater  20  if the sensor  70  senses a temperature exceeding a predetermined maximum. A surface-mounted water-draw sensor  74  is configured with the controller  56  to sense water being drawn from the tank. More specifically, in the configuration shown in  FIG. 1 , the sensor  74  is a temperature sensor at the bottom of the tank  24  near the cold water inlet  28 . Cold water entering the tank  24  thus affects sensor  74  output. A flammable vapor (FV) sensor  78  is surface-mounted, for example, on the tank bottom  32  and connected with the controller  56 . 
     The controller  56  is shown in greater detail in  FIG. 2 . A board  110  includes an inlet  114  for connection of the transformer  60  to the board via the conductor  66 . The transformer  60  provides a stepped-down 24 VAC supply to a circuit  118  that provides operating power, for example, to an igniter  122  and a gas valve  126 . The gas valve  126 , for example, is solenoid-operated to control the flow of gas to a burner outlet (not shown). 
     The circuit  118  also provides operating power to a processor  134 , e.g., a microprocessor that receives input from the sensors  70 ,  74  and  78  and that controls activation of the igniter  122  and gas valve  126 . The processor  134  draws a low voltage, e.g., 5 VDC, from a 5-volt power supply  138  to control heater operation. Other voltages for the processor  134  and/or power supply  138  are possible in other configurations. In the present invention, the power supply is preferably a small transformer and zener diode circuit. 
     The processor  134  controls at least one solenoid gas valve switch, and in the present invention, controls a pair of switches  140  and  142  for operating the gas valve  126 . The processor  134  also controls an igniter switch  146  for operating the igniter  122 . A flammable vapor switch  150  can be activated by the processor  134  to interrupt the 24-volt power supply to the igniter  122  and gas valve  126 , in response to a signal from the FV sensor  78  indicative of undesirable flammable vapors. A thermal fuse  154  in the stepped-down voltage circuit  118  interrupts the 24-volt supply if water temperature exceeds a predetermined upper limit. Thus the fuse  154  serves as a backup for the temperature sensor  70  to prevent excessively high water temperatures. 
     The controller  56  monitors temperature change as signaled by the sensor  74 . If the controller  56  determines, for example, that a rapid drop in temperature has occurred, then the controller  56  determines that water is being drawn from the tank  24  and controls the heater  20  accordingly as further described below. What may constitute a “rapid” drop in temperature can be predefined and stored in the processor  134 . It can be appreciated that sensitivity can be programmed into the processor  134  to avoid a call for heat on every water draw. 
     In another configuration, the sensor  74  may be a temperature sensor surface-mounted on the cold water inlet fitting  26 . During a stand-by period (a period during which heating is not performed), temperature of the cold water inlet fitting  26  tends to be similar to temperature of hot water in the tank  24 . When cold water is drawn into the tank  24 , temperature of the cold water inlet fitting  26  tends to drop rapidly. What may constitute a “rapid” drop in temperature can be predefined and stored in the processor  134 . In other configurations, the sensors  70  and  74  could be positioned in other locations appropriate for monitoring temperature change indicative of water being drawn from the tank. 
     The controller  56  can control heater operation using an exemplary method indicated generally by reference number  200  in  FIG. 3 . At step  208 , the processor  134  uses input from the water-draw sensor  74  to determine whether water has been drawn from the tank  24 . If cold water is entering the tank, then at step  212  the processor  134  calls for heat and slightly increases a predetermined set-point at which heating is to be shut off and a stand-by mode is to be entered. In the present exemplary embodiment, to “slightly” increase the set-point means to increase the set-point by about 1 to 5 degrees F. The set-point is increased to provide for a case in which the temperature sensor  70  has already sensed the predetermined shut-off set-point temperature. At step  216  the processor uses input from the temperature sensor  70  to determine whether the increased set-point has been reached. If no, heating is continued. If yes, then at step  220  the processor  134  discontinues heating, restores the predetermined shut-off set-point and returns to step  208 . 
     An exemplary sequence shall now be described. A shut-off set-point may be predetermined to be 120 degrees F. with a 10-degree F. differential. The heater  20  is in stand-by mode and the top sensor  70  signals a temperature of 115 degrees F. A significant amount of water is drawn out of the tank  24  (“significant” having been predefined in the processor) and the sensor  74  senses a temperature change. The controller  56  starts an ignition sequence and increases the set-point to 125 degrees F. Temperature at the top  40  of the tank increases slowly until it reaches 125 degrees F. and the burner is shut down. The shut-off set-point is restored to 120 degrees F. with a 10-degree F. differential. 
     The processor  134  can control operation of the FV sensor  78 , for example, by keeping a running average of the FV sensor resistance. The running average could be updated, for example, each time the controller  56  performs a start-up. In another configuration, the running average may be updated every 24 hours. A running average of, for example, the last ten resistance measurements could be used to establish a new FV sensor resistance level. A change, for example, of 20 percent or more in ten seconds or less would cause the controller  56  to disconnect the gas supply and/or perform other function(s) for maintaining a safe condition. Of course, other limits may be placed on the FV sensor  78 . For example, if the running average were to reach a predetermined minimum or maximum value, the controller  56  could trigger a shut-down of the heater  20 . In an alternate embodiment, the controller  56  could also control activation of peripheral equipment for the appliance, such as an exhaust damper apparatus for preventing the loss of residual heat from the appliance. 
     In heating systems in which features of the present invention are incorporated, processor logic can be applied to sensor inputs to maintain heater efficiency and safety. The foregoing plug-in transformer provides power for microprocessor control, thus making it unnecessary to install, for example, a 120 VAC line to the water heater to power a processor. Using the above described heating controller can increase available hot water capacity in a heating tank. Since temperature changes occur relatively slowly at the top of the tank, accurate control can be achieved using a surface mount sensor at the top of the tank. In prior-art systems having an immersion sensor at the bottom of the tank, time must pass before water at the bottom registers a full temperature differential and thus before heating is initiated. Using an water-draw sensor in accordance with the foregoing embodiments can make more hot water available than would be available in a heater having standard temperature sensors at the bottom. There is no longer a need to prevent temperature stacking within the tank, and so hot water capacity can be increased. Because water temperature at the top of the tank is precisely controlled, chances of heating the water to excessively high temperatures are greatly reduced. Additionally, surface-mount sensing of water temperature is less costly and more efficient than immersion sensing. 
     The foregoing FV sensor control method can compensate for gradual ageing of a sensor due to its chemistry or due to environmental causes. The foregoing control method also allows a heating system to be shut down more quickly than previously possible in the event of a rapid sensor change. Configurations of the present apparatus and methods can allow a heating system to meet new high efficiency and safety standards applicable to atmospheric gas water heaters. Additionally, a prior art atmospheric gas water heater can be easily replaced with a new lower-voltage water heater in accordance with one or more embodiments of the present invention. Such replacement involves performing the simple additional steps of plugging in the foregoing transformer into a nearby line voltage receptacle and connecting the transformer to the foregoing controller. 
     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.