Patent Publication Number: US-9410719-B2

Title: Systems and methods for controlling gas powered appliances

Description:
FIELD 
     The field of the disclosure relates generally to gas powered appliances, and more particularly, to systems and methods for controlling operation of a gas powered water heater. 
     BACKGROUND 
     Storage water heaters may be utilized domestically and industrially in various applications. Domestically, a storage water heater is used for generation of hot water that may be used for bathing, cleaning, cooking, space heating, and the like. 
     A conventional gas fired water heater includes a water storage tank and gas fired burner assembly for heating water within the tank. In operation, combustion gases generated by the firing of the burner assembly may be directed upwardly through a flue pipe via a hood. The combustion gases serve to transfer heat to the water contained within the storage tank. The top of the water heater may include suitable fittings for connection to a supply of water and a water distribution system with a water inlet provided with a dip tube, which serves to direct the inflow of cold water to the bottom of the tank. 
     This Background section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art. 
     SUMMARY 
     In one aspect, a control system for controlling a gas powered appliance including at least one electrically actuated gas valve for selectively providing gas to a burner is described. The control system includes a power source to provide electrical power to control the at least one electrically actuated gas valve, a valve control system configured to selectively couple electrical power from the power source to the electrically actuated gas valve, and a controller. The valve control system includes a first switch having an on state to permit current to pass through the first switch and an off state to prevent current from passing through the first switch, and a second switch having an on state to permit current to pass through the second switch and an off state to prevent current from passing through the second switch, the first switch and the second switch electrically connected in series between the power source and the electrically actuated gas valve. The controller is operatively connected to the first switch and the second switch and configured to control the first switch and the second switch to selectively couple power from the power source to the electrically actuated gas valve. 
     In another aspect, a water heater includes a storage tank, a main burner configured to burn gas to heat water in the storage tank, a main gas valve, and a control system configured to control operation of the main burner to provide water in the storage tank substantially at a setpoint temperature. The main gas valve is coupled to the main burner and has an open position permitting gas flow through the main gas valve and a closed position preventing gas flow through the main gas valve. The main gas valve is an electrically actuate gas valve. The control system includes a power source to provide electrical power, a valve control system configured to selectively couple electrical power from the power source to the main gas valve, and a controller. The valve control system includes a first switch having an on state to permit current to pass through the first switch and an off state to prevent current from passing through the first switch, and a second switch having an on state to permit current to pass through the second switch and an off state to prevent current from passing through the second switch, the first switch and the second switch electrically connected in series between the power source and the main gas valve. The controller is operatively connected to the first switch and the second switch and configured to control the first switch and the second switch to selectively couple power from the power source to the main gas valve. 
     Various refinements exist of the features noted in relation to the above-mentioned aspects. Further features may also be incorporated in the above-mentioned aspects as well. These refinements and additional features may exist individually or in any combination. For instance, various features discussed below in relation to any of the illustrated embodiments may be incorporated into any of the above-described aspects, alone or in any combination. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cut-away view of a water heater including one embodiment of a control system for controlling operation of the water heater. 
         FIG. 2  is a block diagram of a computing device for use in the water heater shown in  FIG. 1 . 
         FIG. 3  is a schematic block diagram of the control system shown in  FIG. 1 . 
         FIG. 4  is a schematic block diagram block of an embodiment of the control system shown in  FIG. 3 . 
         FIGS. 5A-5D  is a circuit diagram of an embodiment of the control system shown in  FIG. 3 . 
         FIG. 6  is a circuit diagram of part of a valve control system for use in the control system shown in  FIGS. 5A-5D . 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     The embodiments described herein generally relate to water heaters. More specifically, embodiments described herein relate to methods and systems for controlling operation of a gas powered water heater. 
     Referring initially to  FIG. 1 , a control system  100  is provided for controlling operation of a water heater  20  to maintain a desired temperature of water in the water heater  20 . The water heater  20  has a storage tank  22  that stores heated water and receives cold water via a cold water inlet  26 . Cold water entering a bottom portion  28  of the storage tank  22  is heated by a fuel-fired main burner  30  beneath the storage tank  22 . Water leaves the storage tank  22  via a hot water outlet pipe  34 . Combustion gases from the main burner  30  leave the water heater  20  via a flue  36 . The control system  100  provides for control of gas flow via a gas supply line  40  and one or more valves (not shown) to the main burner  30 , as described herein. The gas burned by the water heater  20  may be natural gas, liquid propane (LP) gas, or any other suitable gas for powering a water heater. Moreover, the control system  100  controls a standing (i.e., continuously lit) pilot burner  41  that operates as an ignition source for the main burner  30 . The control system  100  also controls gas flow via gas line  40  and one or more valves (not shown in  FIG. 1 ) to the pilot burner  41 . Alternatively, the ignition source may be a piezoelectric lighter or any other suitable ignition source. In some embodiments, a piezoelectric lighter is used to ignite the pilot burner  41 . 
     The control system  100  includes a sensor  102  that provides an output or value that is indicative of a sensed temperature of the water inside of the storage tank  22 . For example, the sensor  102  may be a tank surface-mounted temperature sensor, such as a thermistor. Alternatively, in other embodiments, the sensor  102  may be a temperature probe or any other sensor suitable for measuring the water temperature in storage tank  22 . In the embodiment shown in  FIG. 1 , sensor  102  is positioned proximate bottom portion  28  of the storage tank  22 . Alternatively, the sensor  102  may be positioned to detect the temperature of the water in the storage tank  22  at any other suitable portion or portions of the storage tank, such as a middle portion  31 , an upper portion  32 , or a combination of bottom, middle, and/or upper portions. Moreover, the control system  100  may include more than one sensor  102 . For example, the control system  100  may include two or more temperature sensors  102  for detecting the water temperature at one or more locations in the storage tank  22 . In one example, the control system  100  include two sensors  102  that are thermistors mounted on a circuit board positioned within a watertight tube near the bottom of the storage tank  22 . The two thermistors detect the temperature of the water near the bottom portion  28  of the storage tank  22 . 
     The control system  100  is positioned, for example, adjacent the storage tank  22 . Alternatively, the control system  100  is located underneath the storage tank  22 , in a watertight compartment within the storage tank  22 , or in any other suitable location. Sensor  102  is in communication with control system  100 , and provides control system  100  an output or value indicative of the water temperature in storage tank  22 . In some embodiments, a second sensor (not shown) may be disposed at an upper portion  32  of the water heater  20 , to provide an output or value that is indicative of a sensed temperature of the water in upper portion  32  of storage tank  22 . 
     Various embodiments of the control system  100  may include and/or be embodied in a computing device. The computing device may include, a general purpose central processing unit (CPU), a microcontroller, a reduced instruction set computer (RISC) processor, an application specific integrated circuit (ASIC), a programmable logic circuit (PLC), and/or any other circuit or processor capable of executing the functions described herein. The methods described herein may be encoded as executable instructions embodied in a computer-readable medium including, without limitation, a storage device and/or a memory device. Such instructions, when executed by a processor, cause the processor to perform at least a portion of the methods described herein. 
       FIG. 2  is an example configuration of a computing device  200  for use in the control system  100 . The computing device  200  includes a processor  202 , a memory area  204 , a media output component  206 , an input device  210 , and communications interfaces  212 . Other embodiments include different components, additional components, and/or do not include all components shown in  FIG. 2 . 
     The processor  202  is configured for executing instructions. In some embodiments, executable instructions are stored in the memory area  204 . The processor  202  may include one or more processing units (e.g., in a multi-core configuration). The memory area  204  is any device allowing information such as executable instructions and/or other data to be stored and retrieved. The memory area  204  may include one or more computer-readable media. 
     The media output component  206  is configured for presenting information to user  208 . The media output component  206  is any component capable of conveying information to the user  208 . In some embodiments, the media output component  206  includes an output adapter such as a video adapter and/or an audio adapter. The output adapter is operatively coupled to the processor  202  and operatively coupleable to an output device such as a display device (e.g., a liquid crystal display (LCD), organic light emitting diode (OLED) display, cathode ray tube (CRT), or “electronic ink” display) or an audio output device (e.g., a speaker or headphones). 
     The computing device  200  includes, or is coupled to, the input device  210  for receiving input from the user  208 . The input device is any device that permits the computing device  200  to receive analog and/or digital commands, instructions, or other inputs from the user  208 , including visual, audio, touch, button presses, stylus taps, etc. The input device  210  may include, for example, a variable resistor, an input dial, a keyboard/keypad, a pointing device, a mouse, a stylus, a touch sensitive panel (e.g., a touch pad or a touch screen), a gyroscope, an accelerometer, a position detector, or an audio input device. A single component such as a touch screen may function as both an output device of the media output component  206  and the input device  210 . 
     The communication interfaces  212  enable the computing device  200  to communicate with remote devices and systems, such as sensors, valve control systems, safety systems, remote computing devices, and the like. The communication interfaces  212  may be wired or wireless communications interfaces that permit the computing device to communicate with the remote devices and systems directly or via a network. Wireless communication interfaces  212  may include a radio frequency (RF) transceiver, a Bluetooth® adapter, a Wi-Fi transceiver, a ZigBee® transceiver, a near field communication (NFC) transceiver, an infrared (IR) transceiver, and/or any other device and communication protocol for wireless communication. (Bluetooth is a registered trademark of Bluetooth Special Interest Group of Kirkland, Wash.; ZigBee is a registered trademark of the ZigBee Alliance of San Ramon, Calif.) Wired communication interfaces  212  may use any suitable wired communication protocol for direct communication including, without limitation, USB, RS232, I2C, SPI, analog, and proprietary I/O protocols. Moreover, in some embodiments, the wired communication interfaces  212  include a wired network adapter allowing the computing device to be coupled to a network, such as the Internet, a local area network (LAN), a wide area network (WAN), a mesh network, and/or any other network to communicate with remote devices and systems via the network. 
     The memory area  204  stores computer-readable instructions for control of the water heater  20  as described herein. In some embodiments, the memory area stores computer-readable instructions for providing a user interface to the user  208  via media output component  206  and, receiving and processing input from input device  210 . The memory area  204  includes, but is not limited to, random access memory (RAM) such as dynamic RAM (DRAM) or static RAM (SRAM), read-only memory (ROM), erasable programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), and non-volatile RAM (NVRAM). The above memory types are example only, and are thus not limiting as to the types of memory usable for storage of a computer program. 
     A functional block diagram of the control system  100  is shown in  FIG. 3 . The control system includes a safety system  302 , a power system  304 , a controller  306 , sensors  102 , a valve control system  308 , and a valve picking system  310 . The control system is coupled to and controls a first valve  314  and a second valve  312 . The second valve  312  and the first valve  314  are solenoid actuated gas valves for selectively coupling gas to the main burner  30  and the pilot burner  41 , respectively. An electrical current through the coil of the valve  312  or  314  causes the valve  312  or  314  to open. As shown in  FIG. 4 , gas flows from a gas source to first valve  314 . Gas the passes through the first valve  314  is provided to the pilot burner  41  and the second valve  312 . Gas passing through the second valve  312  is provided to the main burner  30 . 
     With reference again to  FIG. 3 , the power system  304  provides power to the other components of the control system  100 . Specifically, the power system  304  provides power to the controller  306  and the valve control system  308 . The power system  304  provides an output to the valve control system  308  at a first voltage that is lower than a second voltage output to the controller  306 . The power system  304  may include and/or receive power from any suitable alternating current (AC) or direct current (DC) power source, such as one or more batteries, thermoelectric generators, photovoltaic cells, AC utilities, and the like. In an exemplary embodiment, the power system includes an unregulated DC power source (not shown in  FIG. 3 ) with a source resistance between about two and five ohms. In some embodiments, the unregulated DC power source is a thermoelectric generator in thermal communication with the pilot burner  41 . The thermoelectric generator can be ideally represented by a 650-850 mV Thevenin equivalent voltage source with a 2 to 5 ohm Thevenin equivalent source resistance. 
     The safety system  302  is configured to selectively extinguish and/or prevent ignition of the main burner  30  and/or the pilot burner  41 . Specifically, the safety system  302 , under the direction of the controller  306 , prevents the power system from providing sufficient voltage, current, and/or power to hold open the first valve  314  or the second valve  312 . When the valves  312  and  314  are closed, gas flow to the main burner  30  and the pilot burner  41  is prevented and ignition of the main burner  30  and the pilot burner  41  is thereby prevented. When the controller  306  determines to shut down the water heater  20  using the safety system  302 , the controller  306  outputs a signal to safety system  302 . In response to the signal, the safety system  302  causes the valves  312  and  314  to close (if open) and prevents them from being opened (if already closed). In other embodiments, the safety system  302  operates in response to a lack of an expected signal from the controller  306 . Thus, if the controller does not send (or the safety system  302  otherwise does not receive) the expected signal, whether continuously or periodically, the safety system  302  causes the valves  312  and  314  to close. 
     Responsive to signals from the controller  306 , the valve control system  308  selectively couples power from the power system  304  to the valves  312  and  314  to selectively hold them open. The valve control system  310  is responsive to signals from the controller  306  to couple power to one of the valves  312  or  314  and to signals that instruct it to decouple the valve  312  or  314  from the power system  304 . Moreover, when the valve control system is holding one of the valves  312  or  314  open, the valve control system  308  ceases coupling power to the valves  312  and  314  if it does not receive an expected signal from the controller  306 . Thus, if the controller  306  stops sending the expected signal (or sends an incorrect signal) the valve control system decouples the valve(s)  312  and/or  314  from the power system  304 , thereby causing the valves  312  and/or  314  to close. The expected signal may be a continuous signal, a signal repeated at a particular interval, a signal with a particular duty cycle or frequency, or any other suitable signal. 
     The valve pick system  310  receives power at the second voltage from the controller  306  and opens (also sometimes referred to as “picking” or “picking open”) the main valve  312  when commanded to do so by the controller  306 . The valve pick system  310  does not open the pilot valve  314 . The pilot valve  314 , in this embodiment, is a manually opened valve, which may be held open by the valve control system  308  after it is manually opened. Alternatively, the valve pick system  310  may also be operable to pick the pilot valve  314 . 
     The sensors  102  are temperature sensors operable to provide a signal indicative of the temperature the water in the storage tank  22 . The sensors  102  provide their signals to the controller  306 . As described above, the sensors  102  are any suitable sensor, such as thermistors, probes, and the like, for detecting the temperature of the water within the storage tank. Additionally, or alternatively, the sensors  102  may include any other suitable types of sensors, such as oxygen sensors, ambient air temperature sensors, moisture sensors, etc. 
     The controller  306  controls operation of the water heater  20  and the control system  100 . The controller  306  operates the water heater to provide water heated to a desired temperature, such as a temperature setpoint that is set by a user via the input  210 . The controller  306  includes a computing device, such as computing device  200 . In some embodiments, the controller  306  is a microcontroller. Alternatively, the controller  306  includes any combination of digital and/or analog circuitry that permits the controller  306  to function as described herein. 
     In general, the controller  306  controls the water heater  20  based on the inputs from the sensors  102  and the temperature setpoint. Under normal operations, the controller  306  utilizes the valve control system  308  to hold open the pilot valve  314  to permit gas to flow to the pilot burner  41  and the main valve  312  When the water temperature detected by the sensors  102  drops below the a threshold slightly below the temperature setpoint, the controller  306  opens the main valve  312  using the valve pick system  310 . After the main valve  312  is picked open, the controller  306  holds the main valve open by coupling power from the power system  304  to the main valve  312  through the valve control system  308 . When the controller  306  determines, based on the temperature set point and the input from the temperature sensors  102 , to turn off the main burner  30 , it decouples the main valve  312  from the power system  304  to close the main valve  312 , thereby interrupting the flow of gas to the main burner  30  and extinguishing the main burner  30 . If an abnormal condition occurs at any point during operation, the safety system prevents the power system  304  from opening and/or holding open the valves  312  and  314 . 
       FIG. 4  is a block diagram of an example embodiment of the control system  100  shown in  FIG. 3 .  FIGS. 5A-5D  show a circuit diagram of one implementation of the control system  100  shown in  FIG. 4 . Particular components as shown in  FIGS. 5A-5D  produce the voltage values and timings described herein. It should be understood that different components with the same or different characteristics and/or values may be used in other implementations. 
     The power system  304  includes a thermoelectric generator  402 , a power converter  404 , and a voltage switch  406 . The thermoelectric generator  402  is thermally coupled to the pilot burner  41 . The thermoelectric generator  402  provides a direct current (DC) electrical output (voltage V 1 ) in response to a flame on the pilot burner  41 . Although the output voltage V 1  will vary based on load, temperature, and other factors, under steady state conditions the voltage V 1  will be around 450 mV. The output of the thermoelectric generator  402  is input to the power converter  404 . The power converter  404  is a modified Colpitts oscillator that is self-starting and self-oscillating. The converter  404  automatically begins operating in response to the electrical output from the thermoelectric generator  402 . The power converter  404  produces a DC output with a voltage (V 2 ) greater than its input voltage V 1 . In an example embodiment, the maximum value of voltage V 2  output by the converter  404  varies between about seventeen times V 1  to about ten times V 1  depending on the magnitude of the voltage V 1  input to the converter  404 . In other embodiments, the maximum voltage V 2  may have any other suitable relationship or range of relationships to the voltage V 1 . At steady state, the converter  404  will provide an output voltage of approximately 5 volts. When the voltage V 2  is coupled to the controller  306 , the controller  306  turns on and begins controlling operation of the water heater  20 . 
     The control system  100  includes a flame loss feedback safety feature. The thermoelectric generator&#39;s thermal communication with the pilot burner  41  produces the current to hold open the pilot valve  314 . If the flame on the pilot burner  41  is lost, the output voltage from the thermoelectric generator  402  will decrease until there is insufficient current to hold open the pilot valve  314 . Because gas flows through the pilot valve  314  to the main valve  312  (and the main burner  30 ), the loss of flame on the pilot burner  41  causes the pilot valve  314  to close and interrupt gas flow to both the pilot burner  41  and the main burner  30 . This may help prevent gas from being delivered to the pilot burner  41  or the main burner  30  when there is no ignition source available for the gas. 
     The voltage switch  406  is located between the converter  404  and the controller  306 . The voltage switch  406  defaults to an OFF (non-conducting) state and turns ON when its supply voltage (i.e., the output of converter  404 ) reaches a first threshold. The voltage switch  406  also turns OFF if its supply voltage falls below a second, lower threshold. The voltage switch  406  selectively connects the voltage V 2  to the controller  306  to power the controller  306 . At startup, the thermoelectric generator  402  output V 1  will be zero and it will ramp toward its steady value over several minutes. When voltage V 1  reaches approximately 50-100 mV, the power converter  404  will turn on and its output voltage V 2  will begin ramping toward its steady state value of 5V. The ramp to 5V can take 30-60 seconds depending on the V 1  ramp rate. When the converter  404  output voltage V 2  reaches the first threshold, the voltage switch  406  turns ON and the power supply voltage of the controller  306  will immediately rise to a voltage substantially equal to the first threshold. The voltage output from the voltage switch  406  will be slightly less than the voltage V 2  because there is a small voltage drop across the voltage switch  406 . The voltage drop depends on the particular device used for the voltage switch  406  and the ambient temperature. In an example embodiment, the voltage drop is between about 0.1 volts and 0.2 volts. This provides a “hard-edge” to the controller  306  power supply pin and other systems that use the controller  306  power supply voltage. The voltage switch  406  also provides a reference for software timings as the software can assume the supply voltage of the controller  306  is roughly equal to the first threshold at the start of code execution. The voltage switch  406  includes hysteresis so that it will not turn OFF if the voltage V 2  falls back below the first threshold value. The OFF threshold for the voltage switch  406  is set to a second, lower threshold value that is below the brown-out voltage for the controller  306 . In the example embodiment, the first threshold value is about 3.5 volts, the brownout voltage of the controller  306  is about 1.8 volts, and the second threshold value is less than 1 volt. If V 2  drops below 1.8V, the controller  306  will brown-out before the voltage switch  406  turns off. Alternatively, the second threshold may be a value that is not below the brown-out voltage of the controller  306 . For example, the second threshold voltage may be set at 2.5V. The voltage V 2  could then vary between 5 volts and 2.5 volts without the voltage switch  406  turning off. Because the second threshold is above the brownout voltage, the voltage switch  406  will be turned off by a decreasing voltage V 2  before the brownout voltage of the controller  306  is reached. 
     The safety system  302  includes a safety switch control circuit  408  and a safety switch  410 . In the illustrated embodiment, the safety switch control circuit  408  is coupled to the output of the voltage switch  406 , the safety switch  410 , and a control pin of the controller  306 . The safety switch  410  is also coupled between the output of the thermoelectric generator  402  and ground. In the example embodiment, at startup, the pin of the controller  306  that is coupled to the safety switch control circuit  408  is held in a high impedance (Hi-Z) state. The safety switch control circuit  408  includes a timing circuit, e.g., an RC circuit defining an RC time constant, that is enabled by placing the controller  306  pin in the Hi-Z state. When the voltage switch  406  turns on, the safety switch control circuit  408  will slowly charge toward the voltage V 2 . If the voltage of the safety switch control circuit  408  reaches a threshold value, the safety switch control voltage will cause the safety switch  410  to turn on. When the safety switch  410  is turned on, the thermoelectric generator output is substantially shorted to ground and there is insufficient power available to hold open the main valve  312 , hold open the pilot valve  314 , operate the converter  404 , and operate the controller  306 . If the pin of the controller  306  that is coupled to the safety switch control circuit  408  is switched to a logical low state before the safety switch control circuit  408  reaches the threshold value, the timing circuit is disabled and the safety switch  410  does not turn on. Alternatively, the safety switch control circuit  408  may not be coupled to the voltage switch  406  and the pin of the controller  306  that is coupled to the safety switch control circuit  408  is not held in a Hi-Z state at startup. In such embodiments, the pin of the controller  306  coupled to the safety switch control circuit  408  is driven high or low to turn the safety switch  410  on or off. 
     The thermoelectric generator  402  is an unregulated DC power source that can be represented by a 650 mV to 850 mV Thevenin equivalent voltage source with a 2 to 5 ohm source resistance at optimal steady state. The Thevenin equivalent voltage generally decreases as ambient temperature around the generator  402  increases, such as after the main burner  30  has been on for a long time. Because of the thermoelectric generator  402  power supply characteristics, the size of its load (in ohms) will determine the voltage over the load. Substantially lowering the overall load on the thermoelectric generator  402 , by switching in a parallel low resistance load (e.g., resistor  506  shown in  FIG. 5D ) or shorting directly to ground (e.g., resistor  506  is substantially 0 ohms) via the safety switch  410 , substantially lowers the voltage (V 1 ) because of the voltage divider created with the source resistance and the new lower overall load. The safety switch  410  load is sized so that when it is switched on it will lower the voltage V 1  below the voltage required to hold open the valves  312  and  314  and below the voltage required to start the converter  404 . Moreover, the size of the safety switch load (and its presence or absence) is determined according to the source impedance of the power source. If the source impedance of the power source is relatively low, the safety switch load should be greater than 0 ohms to limit the current and drop the output voltage substantially across the safety switch load. In the example embodiment, the safety switch  410  load is sized to drop the load resistance to about 0.24 ohms and the voltage V 1  drops to about 40 mV. Alternatively, because the thermoelectric generator  402  has a relatively high source impedance, the safety switch  410  couples the output of the thermoelectric generator  402  directly to ground without inclusion of a parallel low resistance load. In one example, the safety switch  410  load is sized to drop the load resistance to about 0 ohms and the voltage V 1  to between about 10 mV and about 15 mV. 
     In normal startup operation, the controller  306  will change the output of its safety switch control pin to a low state within a preset amount of time, preventing the voltage of the safety switch control circuit  408  from reaching the threshold to turn on the safety switch  410 . The controller  306  changes the output of the safety switch pin to a low state after the controller  306  passes all internal microprocessor and hardware checks (internal microprocessor checks can take from 4 to 6 seconds after the voltage switch  406  turns on and the controller  306  begins executing instructions). In embodiments in which the safety switch control circuit  408  is not coupled to the voltage switch  406 , the safety switch control pin begins in the low state during normal startup operations. During normal operation of the water heater  20 , the controller  306  will maintain the output pin coupled to the safety switch control circuit  408  in a low state, thus keeping the voltage of the safety switch control circuit  408  from reaching the threshold to turn on the safety switch  410 . If the controller  306  determines to shut the valves  312  and  314  of the water heater  20  for safety reasons, the controller  306  switches the safety circuit output pin to a high state. When the output pin is high, the safety switch circuit  408  charges to the threshold to turn on the safety switch  410  at a rate that is faster than the rate when the pin is in the Hi-Z state. 
     In some embodiments, the controller also sets the safety switch enable pin to a high impedance state (thus allowing the safety switch control voltage to charge) before providing signals to hold open the valves  312  and  314 . The safety switch enable pin is then driven low once the signals are completed. In this way if the controller  306  malfunctions and becomes stuck in the state when signaling to the valves is ON, the safety switch  410  will eventually charge and shut the system down. 
     The valve control system  308  includes a first main switch  412 , a second main switch  414 , a main charge pump  416 , a pilot switch  418 , and a pilot charge pump  420 . As described above, the controller  306  selectively holds open the main valve  312  and the pilot valve  314  via the valve control system  308 , which may also be referred to as a valve holding system. The controller  306  holds the pilot valve  314  open by closing the pilot hold switch  418  to couple the pilot valve  314  to the thermoelectric generator  402  output. Specifically, the controller  306  supplies periodic bursts of pulse width modulated (PWM) signals to the pilot charge pump  420 . The PWM signals are square waves with an amplitude that switches from 0 volts to substantially the voltage V 2 . The burst of PWM signals charge the pilot charge pump  420  to a voltage V 3  sufficient to turn on the pilot switch  418 . In the exemplary embodiment, the voltage V 3  is less than the voltage V 2 . The magnitude of the voltage V 3  will vary with the varying of voltages V 1  and V 2 . When the voltage V 2  is about 5 volts, the exemplary voltage V 3  will be about 3 volts. In other embodiments, the voltage V 3  may be the same as or greater than the voltage V 2  depending on the voltage needed to turn on the pilot switch  418 . In one embodiment, V 3  is about 3.25 volts. The controller  306  periodically provides PWM signal bursts to maintain the output of the charge pump at about V 3 . If the controller  306  ceases providing the PWM signal bursts or delays too long before providing a burst, the charge pump will not output a voltage V 3  sufficient to turn on the pilot switch  418 . The pilot switch  418  will turn off (or stay off), the pilot valve  314  will be closed, the pilot burner  41  will not receive gas through the pilot valve  314 , and the pilot burner  41  will be extinguished. A generally similar control procedure is used to hold open the main valve  312  using the first main switch  412  and the main charge pump  416 . The addition of the second main switch  414  and the pick circuit  310  change the operation as described below. 
     The valve pick system  310  includes a pick switch  422  and a pick circuit  424 . The pick circuit  424 , the pick switch  422 , and both main valve switches  412  and  414  are utilized for picking open the main valve  312 . The controller  306  outputs the voltage V 2  to the pick circuit  424  to charge a pick circuit capacitor (not shown) to, ideally, the voltage V 2 . In reality, the pick circuit capacitor may be charged to a voltage that is slightly less than V 2 . The pick circuit capacitor will take time to charge. The controller  306  monitors the voltage of the pick capacitor. When the pick capacitor is charged to a voltage greater than a picking threshold voltage, the controller  306  may pick open the main valve  312 . The picking threshold voltage is less than the voltage V 2 , but more than the minimum voltage needed to open the main valve  312 . In one example, the minimum voltage needed to open the main valve  312  is between about 1.7 volts and 2.0 volts, and the picking threshold voltage is about 3 volts. In other embodiments, the picking threshold voltage is a voltage between about 1V and 5V. Alternatively, the picking threshold voltage may be any voltage greater than the minimum voltage sufficient to open the main valve  312 . Thus, the output of the pick circuit  424  may be any voltage between about 3 volts and about 5 volts. To pick the main valve, the controller  306  sends a burst of PWM signals to the main charge pump  416  to charge the charge pump  416  to a voltage V 4  sufficient to turn on the first main switch  412 . In the example embodiment, the magnitude of the voltage V 4  will vary with the varying of voltages V 1  and V 2 . For example, when the voltage V 2  is about 5 volts, the voltage V 4  will be about negative 2 volts. In another embodiment, the voltage V 4  will be about negative 3.15 volts. In other embodiments, the voltage V 4  is any other voltage suitable for turning on the first main switch  412 . The controller  306  periodically provides PWM signal bursts to maintain the output of the main charge pump  416  at about V 4 . If the controller  306  ceases providing the PWM signal bursts or delays too long before providing a burst, the main charge pump  416  will not output a voltage V 4  sufficient keep the first main switch  412  turned on. The second main switch  414  is initially off. After the first main switch  412  is turned on, the controller  306  turns the pin connected to the pick switch  422  to a high output in order to activate the pick switch  422 . The energy stored in the pick circuit capacitor is coupled to the main valve  312  through the pick switch  422  and the main valve  312  opens. The second main switch  414  is closed briefly before the pick switch  422  is opened. Closing the second main switch  414  couples the thermoelectric generator  402  voltage V 1  to the main valve  312  through the first and second main switches  412  and  414  to hold the main valve  312  open so the main burner  30  remains lit. To keep the main burner  30  lit, the controller  306  keeps the main switches  412  and  414  on by maintaining the output pin coupled to the second main switch  414  high and periodically sending bursts of PWM signals to the main charge pump  416 . To turn off the main burner  30 , the controller  306  opens both main switches  412  and  414 , thereby interrupting the connection between the main valve  312  and the thermoelectric generator  402 . 
     The second main switch  414  is used in both picking and holding open the main valve  312  and can be considered part of both the valve pick system  310  and the valve control system  308 . The second main switch  414  ensures that substantially all of the picking voltage is directed from the pick circuit  424  to the main valve  312 . The first main switch  412  and the second main switch  414  are MOSFETS with internal body diodes. The first main switch  412  has an internal body diode with its cathode pointed toward the thermoelectric generator  402 . The second main switch  414  has its body diode with the cathode pointed toward the main valve  312  (and away from the first main switch  412 ). Without the second main switch  414 , when the pick switch  422  is turned ON, the pick voltage would appear on the main valve  312  and simultaneously on the first main switch  412 . Even with the first main switch  412  turned off, the 3 to 5V pick spike may be sufficient to forward bias the internal body diode of first main switch  412 , allowing current to flow through the first main switch  412  to discharge through the thermoelectric generator  402  source resistance to ground. This could have an adverse effect on the thermoelectric generator  402  and it is a loss of power that could be used for picking the main valve  312 . The second main switch  414 , however, has its internal body diode oriented opposite of the first main switch  412 . When the second main switch  414  is off, the pick voltage reverse biases the internal body diode of the second main switch  414 , preventing the flow of current to the thermoelectric generator  402  and permitting substantially all of the pick current to travel to the main valve  312 . Alternatively, the second main switch  414  may be eliminated and the first main switch  412  may be oriented as the second main switch  414 , i.e., with its internal body diode&#39;s cathode pointed toward the main valve  312  and its anode toward the thermoelectric generator  402 . In such an embodiment, the first main switch&#39;s body diode will be reverse biased by the pick voltage and substantially all of the pick current travels to the main valve  312 . 
     When it is determined that picking of the main valve  312  will occur, the main charge pump  416  is activated for 30 ms and first main switch  412  is turned on. The controller  306  will then go to sleep for 2 seconds to conserve power to let the voltage on the pick circuit capacitor rise. Upon waking at t=0 ms, the controller  306  turns on the pick switch  422 . The pick circuit capacitor&#39;s voltage will begin decaying and current begins flowing through the main coil of the main valve  312 . As the current through the main coil increases the main valve  312  will eventually open. At a time between about t=20 ms and t=30 ms (depending on the main valve&#39;s specific coils) the voltage from the pick circuit capacitor is close to zero. The second main switch  414  is turned on to couple the thermoelectric generator  402  output voltage to the main valve  312  to hold the valve  312  open. At t=30 ms, the pick switch  422  is turned off. At t=30 ms to 60 ms, the controller provides a PWM burst to the main charge pump  416  to keep the voltage V 4  sufficient to keep the first main switch  412  turned on. 
       FIG. 6  is a circuit diagram of another embodiment of portion  600  of the valve control system  308 . The portion  600  may replace portion  500  (shown in  FIG. 5D ) of the valve control system  308 . The portion  600  includes the pilot hold switch  418 , charge pump  420 , and a discharge circuit  602 . 
     The discharge circuit  602  is coupled to and controlled by the controller  306 . The controller  306  controls the discharge circuit  602  to selectively and quickly drain capacitor  604  to open pilot hold switch  418 . Thus, the controller can quickly open the pilot hold switch  418  to close the pilot valve  314  with or without using the safety system  302 . 
     The discharge system  602  is also used during switch checks of the system  100 . During normal operation, the controller  306  periodically checks the functionality of at least some of the switches of the system  306 . In particular, the controller checks the functionality of the safety switch  410 , the pilot hold switch  418 , and the first and second main switches  412  and  414 . The first and second main switches  412  and  414  are checked for functionality by reading a main monitor  502  (shown in  FIG. 5C ) during normal cycling of the main burner  30 . To check the safety switch  410  and the pilot hold switch  418 , the conductive state of each switch is briefly (e.g., for about 1 ms) changed from its present state and interrupter monitor  504  (shown in  FIG. 5D ) is read. When the safety switch  410  is ON or the pilot hold switch  418  is OFF, changing the state of either switch removes the voltage over the coil in the pilot valve  314 . The magnetic field over the coil cannot, however, change instantaneously. If the switches  410  and  418  are returned to their original states before the magnetic field over the coil collapses, the pilot valve  314  will not close and the functionality may be tested without interrupting normal operation of the system  100 . The discharge circuit  602  allows the controller  306  to turn the pilot hold switch  418  off quickly so that functionality may be checked without closing the pilot valve  314 . 
     Embodiments of the methods and systems described herein achieve superior results compared to prior methods and systems. The dual main switch configuration limits or eliminates the flow of main valve picking current back to the thermoelectric generator without needing a large resistor between the thermoelectric generator and the main valve. This may prevents potential adverse consequences of the revers current on the thermoelectric generator. Moreover, the dual main switch configuration simplifies the timing for applying the valve picking current and applying the main valve holding current. Furthermore, the example safety switch configuration allows the controller to shut down the power supply to prevent the main valve and the pilot valve from being held open. Moreover, the safety switch configuration provides a different failure mode for the safety switch. For example, whether all switches of the control system fail shorted or fail open, no voltage is applied to the coils of the main and pilot valves. 
     Example embodiments of systems and methods for controlling a water heater are described above in detail. The system is not limited to the specific embodiments described herein, but rather, components of the system may be used independently and separately from other components described herein. For example, the controller and processor described herein may also be used in combination with other systems and methods, and are not limited to practice with only the system as described herein. 
     When introducing elements of the present disclosure or the embodiment(s) thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” “containing” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. The use of terms indicating a particular orientation (e.g., “top”, “bottom”, “side”, etc.) is for convenience of description and does not require any particular orientation of the item described. 
     As various changes could be made in the above constructions and methods without departing from the scope of the disclosure, it is intended that all matter contained in the above description and shown in the accompanying drawing(s) shall be interpreted as illustrative and not in a limiting sense.