Patent Abstract:
A water heater control system that prevents the accumulation and accidental ignition of dangerous quantities of unwanted flammable vapors. The system intermittently generates a spark at a predetermined interval such that if unwanted flammable vapors are present, they are burned in a controlled manner. The system obviates the need for flammable vapor sensors.

Full Description:
BACKGROUND OF THE INVENTION 
   The device of the present invention pertains to a safety device for preventing the accumulation and unsafe combustion of flammable vapors by a water heater. 
   The Consumer Product Safety Commission (CPSC) has been working to reduce the risk of injuries and deaths from gas-fired water heaters. One solution involved a redesign of the water heaters to eliminate the ignition of flammable vapors by installing a flame arrestor on the inlet of the burner compartment. The flame arrestor allows the passage of combustible mixtures but prohibits the passage of flames. Thus, when a liquid capable of giving off flammable vapors is spilled under a water heater, the flammable vapors can go into the combustion chamber and the pilot flame inside the burner will ignite the vapors, but the flame arrestor prohibits this flame from going back towards the spilled gas and causing a large explosion. The flame arrestor is a sufficient solution if there is a pilot to light the spilled gas once it makes its way into the combustion chamber. 
   However, modern water heaters are becoming equipped with electronic ignition systems. These systems use an electronic spark triggered by a signal from a control circuit when the control system determines there is a need for heat. This intermittent system obviates the need for a pilot light but creates new safety concerns related to the accumulation of combustible vapors. During an off cycle, the hot water in a water heater will heat the air in the flue assembly. The air will rise, creating a draft that will draw in makeup air from the surrounding room. If there is a flammable fluid spill anywhere near the water heater, this draft will pull combustible vapors from the spill into the burner compartment. If a sufficient amount of time elapses between ignition events, a dangerous concentration of vapor could accumulate in the combustion chamber. When a demand for heat occurs, the resulting ignition event could trigger a violent explosion in the combustion chamber. 
   Water heaters that include a power vent exacerbate the drafting problem. A power vent incorporates a blower that can cause vapors from a flammable liquid spill to form and accumulate more quickly. 
   Solutions to this problem are being developed. One solution is to provide a flammable vapor sensor attached to the water heater that prevents an ignition event if it detects a flammable vapor. These vapor sensors must pass stringent tests including scenarios involving clogged flame arrestors. This solution not only requires relatively expensive flammable vapor sensors, it does not address the issue of vapor evacuation. In the case of a relatively large spill, the vapors could be present long enough to allow the water in the water heater to cool off, depriving the residents of hot water until the spill is either cleaned up or completely evaporates. 
   There is a need for a solution to the problem presented by electronically ignited water heaters that avoids the aforementioned vapor accumulation problem. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention provides a water heater control system for a water heater with an electronic ignition. The control system includes a feature that prevents the accumulation of flammable vapors in the combustion chamber without using a pilot light or flammable vapor sensors. 
   The water heater control system includes a timing circuit and an ignitor that causes an ignition spark at a regular, predetermined interval, independent of heat demand. The intermittent spark is used to ignite whatever flammable gasses may have accumulated in the combustion chamber since a previous intermittent spark. The ignitor is appropriately positioned to best accommodate this function. The predetermined interval is preferably about one minute or less, ensuring that any vapors present are burned off in a safe, controlled manner. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of the control system of the present invention; 
       FIG. 2  is a flow chart of a preferred logic sequence followed by the microprocessor of the present invention; and, 
       FIG. 3  is a graph of a preferred interval schedule of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , there is shown one embodiment of a control system  10  of the present invention. The control system  10  includes a microprocessor  12  which is configured to receive a number of inputs which either provide necessary signals or indicate operating conditions of the water heater. In order to provide more efficient operation, most inputs to the microprocessor  12  are received from various application-specific integrated circuits (ASICs). 
   For example, an ignition control ASIC  14  has numerous connections to the microprocessor  12 . The ignition control ASIC  14  is connected to a power source  26  in order to provide power to the control system  10 . The ignition control ASIC  14  includes a power conditioning circuit  16  to condition this power supply and convert it in order to produce all necessary signals used by control system  10 . For example, the necessary 5 volt DC power signal used by typical digital circuitry is generated. Furthermore, a 60 Hz square wave is generated in order to provide a timing signal  86  for the microprocessor  10 . 
   The ignition control ASIC  14  is also attached to a flame sensor  30  which is utilized to determine if an existing flame is present in the water heater combustion chamber. The ignition control ASIC  14  has a flame sense conditioning circuit  20  that appropriately conditions the signal from the flame sensor  30  in order to provide a flame sense input  84  to microprocessor  12 . 
   In addition to above-mentioned capabilities, ignition control ASIC  14  further includes a watchdog control circuit  22 , which is capable of shutting down power to the control system whenever appropriate signals are not received. More specifically, the watchdog circuit  22  will remove power from the microprocessor  12  in the event that the watchdog circuit  22  does not receive an AC input at a fixed, predetermined frequency, thereby ensuring that the power conditioning circuit  16 , and the timing signal  86 , are functioning properly. This watchdog function is achieved via watchdog output  82  generated by microprocessor  12 . 
   Also attached to microprocessor  12  is condition sensing circuit  32  which is utilized to detect certain operating conditions. Attached to condition sensing circuit  32  is a pressure sensor  39  and thermal limit switch  38 . Thermal limit switch  38  will operate to identify an over-temperature condition. More specifically, once a desired temperature is exceeded, normally-closed thermal limit switch  38  will open, thus signifying a high temperature condition. Condition circuit  32  will then produce a limit input  90  to microprocessor  12  indicating this condition. Similarly, pressure sensor  39  is utilized to determine the presence of air flow into the combustion chamber. A pressure input  92  is provided to microprocessor  12  in order to communicate this information. 
   An A/D circuit  40  is an ASIC, or alternatively discreet circuit, that receives analog inputs from a tank temperature sensor  42  and a thermostat  44 , useable to select a desired temperature set point. The A/D circuit  40  converts these inputs to digital signals  94  and  96 , which are useable by the microprocessor  12 . 
   A relay output circuit  28 , is attached to the microprocessor  12  and receives outputs  102  and  104  therefrom. The output  102  is a command signaling an induced fan relay  50  to allow 120VAC power (not shown) to be aligned to an induced draft fan  54 . The output  104  is a command signaling a gas valve relay  52  to energize a gas valve actuator  56 . The relay output circuit  28  acts as a failsafe circuit, allowing the microprocessor  12  to close the gas valve by cutting power to the gas valve relay  52 , which removes power from the gas valve actuator  56 . This setup also ensures that if the microprocessor  12  loses power for any reason, including by operation of the watchdog circuit  22 , the gas valve will close. 
   A spark generation circuit  48 , is attached to microprocessor  12  at spark drive output  106 . Spark control circuitry  58  is utilized to operate an igniter  60 , in accordance with the operating parameters outlined below. 
   Having described the various circuits feeding and receiving signals from the microprocessor  12 , it is now possible to detail a preferred logic sequence  99  followed by the microprocessor  12 . Looking at  FIG. 2 , it can be seen that the sequence  99  is a loop. For convenience, description of the sequence will begin at  100  with the first vapor accumulation preventative spark. 
   Thus, at  100 , the spark circuit  48  is activated, causing the capacitor  58  to dump  160  VDC to the spark generator  60 , thereby creating a 15 KV spark. Upon creating this spark, the microprocessor  12  resets a timer function that the microprocessor  12  generates using input from a time base circuit  18  of the ignition control circuit  14 . The timer function, having been reset, begins measuring the amount of time that has elapsed since being reset. 
   At this point, the microprocessor  12  has entered the “Off State” of operation, whereby the gas valve  56  is closed and the fan  54  is off. Thus, at  110 , the microprocessor  12  sets the timer function to activate the spark circuit  48  at the off state interval. The off state interval is preferably between 10 and 90 seconds, more preferably between 30 and 75 seconds, and even more preferably between 55 and 65 seconds. 
   While in the off state at  110 , the microprocessor  12  checks the inputs  94  and  96  from the A/D circuit  40  to determine whether the temperature T in the water tank has dropped below a temperature Ts selected on the thermostat of the water heater at  115 . If T has not dropped below Ts, the logic sequence  99  returns to  100 , where the spark circuit  48  continues to be activated according to the off state interval. 
   If at  115  the temperature T has dropped below Ts, the logic sequence  99  begins preparations for igniting gas burners of the water heater to bring the temperature in the water tank above the desired selected temperature Ts. However, a check is first made at  120  to ensure that the input from the 24 VAC input conditioning circuit  32  does not indicate that the limit switch  38  has tripped. Preferably, tripping this switch  38  at any point in the sequence  99  will cause a shutdown at  125 . 
   If the switch  38  has not tripped at  120 , the next step  130  of the sequence  99  is to command the microprocessor  12  to enter a “Fan Proving/Purging State” whereby the timer is set to a faster interval, preferably between 0.5 and 10 Hz, such that a spark is created anywhere from once every couple of seconds, to ten times per second. 
   With the spark interval increased, next the induced draft fan  54  is energized at  135 . The microprocessor  12  energizes the fan  54  by sending an on signal to the fan relay  50  of the relay output circuit  28 . The fan relay  50  closes, thereby connecting the fan  54  to 120VAC power (not shown). This step only occurs in the event that the water heater to which the system  10  is attached has an induced draft fan  54 . 
   Next, at  140 , the microprocessor waits until it gets an indication from the 24VAC input conditioning circuit  32  that the pressure switch input circuit  36  has changed state, indicating a sufficient draft has been established by the induced draft fan  54 . Then, at  145 , the microprocessor  12  opens the gas valve  56  by sending an open command to the gas valve relay  52  of the relay output circuit  28 . In the event that the water heater is not equipped with a fan  54 , a pressure switch circuit  36  is not necessary. 
   With the gas valve  56  open at  145 , and the timer set to fan proving state from step  130 , thereby causing a spark at an increased interval, the flame sense amplifier circuit  20  of the ignition control ASIC  14  is used to detect that the sparks have successfully lit the gas at  150 . Preferably, when the gas valve is opened, the timer enters an ignition state whereby sparks are generated almost continuously, such as on the order of 10 Hz to 60 Hz. Also, it is preferable that the microprocessor starts a timer at  140 , when the gas valve is opened, and establishes a time limit for successful ignition. Thus, as part of the proving ignition step  150 , if the timer elapses, indicating a possible problem with the spark circuitry or the gas flow, the gas valve is closed and the spark is turned off as part of the shutdown sequence at  125 . If the time limit is not reached, at  155  the spark is turned off once the flame sense amplifier circuit  20  of the ignition control ASIC  14  sends a positive flame sensed signal to the microprocessor  12  indicating that the gas from the gas valve  56  has been successfully lit. 
   Step  160  of the sequence  90  is provided to clarify that, preferably, the microprocessor  12  is continually looking for abnormal conditions such as a tripped limit switch. If the microprocessor  12  receives an indication that an abnormal condition exists, the system will be shut down at  125  and will not restart until it is serviced. 
   At  165 , the burners remain lit until the temperature T in the water tank exceeds the selected temperature Ts by a predetermined amount. Then, at  170 , the microprocessor  12  sends a valve close command to the gas valve relay  52  of the relay output circuit  28 , thereby causing the gas valve  56  to close, and the sequence  90  repeats at  100 . Additionally, when the valve  56  is closed, the fan is turned off. 
   It is contemplated that features disclosed in this application can be mixed and matched to suit particular circumstances. For example, the present invention is suitable for use with a system that does not include a force draft fan  54 . Water heaters without fans experience a natural draft that is strongest when the heater is in operation. The draft is caused by hot air rising up the flue and drawing cool air into the bottom of the water heater. When the heating cycle is complete and the burner is off, the draft decreases as the temperature in the flue drops. As heat flow between two mediums is proportional to the temperature difference, the draft decreases at an exponential rate. Thus, the present invention can be utilized to provide intermittent sparks after the burner goes into an off state between heating periods. Preferably, the interval between sparks grows at an exponentially decreasing rate, until the next heating cycle commences. 
   For example, the draft flow rate decreases at a rate that causes it to be reduced by 63% of its original rate after a first time constant passes, 86% after the second time constant, 95% after the third time constant, 98% after the fourth time constant, and nearly 100% after the fifth time constant. So, if it is determined that a spark should occur every 30 seconds as a precautionary measure regardless of flow, then a 30 second interval should be achieved by the fifth minute after a heating cycle. Following the aforementioned curve, the spark schedule shown in  FIG. 3  might be appropriate. 
   Various other modifications and changes will be apparent to those of ordinary skill in the art without departing from the spirit and scope of the present invention. Accordingly, reference should be made to the claims to determine the scope of the present invention.

Technology Classification (CPC): 5