CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This Application claims the benefit of U.S. Provisional Application No. 62/112,300 filed on Feb. 5, 2015. U.S. Provisional Application No. 62/112,300 is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates generally to heating, ventilation, and air conditioning (HVAC) systems and, more particularly, but not by way of limitation, to gas flame control and sensing presence of a gas flame in furnaces of the HVAC systems. 
       BACKGROUND 
       [0003]    HVAC systems can be used to regulate an environment within an enclosure. Typically, a circulating fan is used to pull air from the enclosure into the HVAC system through ducts and push the air back into the enclosure through additional ducts after conditioning the air (e.g., heating or cooling the air). For example, a gas furnace, such as a residential gas furnace, is used in a heating system to heat the air. 
         [0004]    Flame rectification to sense presence or absence of a flame is conventional in gas furnace controls technology. Typically, a 120 volt AC power is coupled to a flame-probe through a first capacitor. When no flame is present, a second capacitor coupled to the flame-probe is charged to a selected value of, for example, 5 volts DC, through a resistor connected to a DC voltage source. A change of state device, such as an inverter, has an output connected to a microprocessor and an input connected to the second capacitor. When no flame is present, the second capacitor maintains the voltage at an input of the inverter above a threshold value so that an output of the inverter is low, thereby providing an indication to the microprocessor that there is no flame. When a flame is present, the second capacitor discharges to ground through the flame which acts as a poor diode connected in series with a resistor. When the second capacitor discharges to a level below the threshold, the inverter changes state with its output going high thereby providing an indication to the microprocessor that a flame is present. 
       SUMMARY 
       [0005]    A method of determining presence of a flame in a furnace of a heating, ventilation, and air conditioning (HVAC) system. The method comprises determining, using a controller, whether a processor signal (G) is active, responsive to a determination that the processor signal (G) is active, determining, using the controller prior to assertion of a flame-test input control signal, an output state of a first comparator, responsive to a determination that the output state of the first comparator is high, determining, using the controller prior to assertion of the flame-test input control signal, an output state of a second comparator, and responsive to a determination that the output state of the second comparator is low, transmitting, using the controller, a notification that a flame is present. 
         [0006]    A heating, ventilation, and air conditioning (HVAC) system comprising circuitry for determining presence of a flame. The circuitry comprises a flame detect circuit, a tank circuit, a first comparator, a second comparator, and a controller operatively coupled to the flame detect circuit, the tank circuit, the first comparator, and the second comparator. The controller is configured to determine whether a processor signal (G) is active, responsive to a determination that the processor signal (G) is active, determine, prior to assertion of a flame-test input control signal, an output state of a first comparator, responsive to a determination that the output state of the first comparator is high, determine, prior to assertion of the flame-test input control signal, an output state of a second comparator, and responsive to a determination that the output state of the second comparator is low, transmit a notification that a flame is present. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  illustrates an exemplary HVAC system employing a heating system; 
           [0008]      FIG. 2A  is an exemplary simulator diagram of a circuit for flame detection; 
           [0009]      FIG. 2B  is an exemplary circuit for flame detection; 
           [0010]      FIGS. 3A-3F  illustrate exemplary voltage amplitude waveforms relative to time of signals generated using the circuit of  FIG. 2A ; and 
           [0011]      FIG. 4  is a flow chart illustrating an exemplary process for detecting presence of a flame. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    Embodiment(s) of the invention will now be described more fully with reference to the accompanying Drawings. The invention may, however, be embodied in many different forms and should not be construed as limited to the embodiment(s) set forth herein. The invention should only be considered limited by the claims as they now exist and the equivalents thereof. 
         [0013]    A problem exists with the prior approach described above, in view of the low level of current flow. If the inverter or the second capacitor develops too much leakage current to ground, an indication of the presence of a flame can occur even at times when, in fact, no flame is present. This can happen because of age, static damage, faulty components or the like. 
         [0014]    Sensing presence of a flame is important for safety and effectively controlling operation of furnaces and other apparatuses using natural gas or another combustible fluid as a flame fuel source. For example, an absence or loss of the flame while fuel is being delivered causes a safety risk. Conversely, avoiding unnecessary shut-down of the furnace and other apparatus is important for continued, effective operation. It is desirable to reduce or eliminate the risk of erroneously sensing the presence of the flame in furnaces and the resulting delivery of fuel to burners without the fuel being burned. Accumulating unburned fuel is hazardous, in addition to being wasteful and inefficient. Exemplary embodiments disclose a method of and system for detecting presence or absence of the flame in furnaces and other apparatuses where a flame is generated. 
         [0015]      FIG. 1  illustrates an exemplary HVAC system  100  employing a heating system  101 . The heating system  101  is, for example, a gas fired combustible fuel-air burning furnace. The furnace may be for a residence or for a commercial building (i.e., a residential or commercial unit), for example a rooftop unit (RTU). The heating system  101  includes a burner assembly  112  having at least one burner  114 , a heat exchanger  116 , an air circulation fan  118 , a combustion air-inducer or combustion air-blower (CAB)  120 , a gas valve  122 , and a furnace controller  126 . The furnace controller  126  is operationally connected for example to the CAB  120 , the gas valve  122 , a thermostat  128 , and a discharge air sensor (DAS)  130 . The heating system  101  may be utilized in single or multiple zoned systems. Portions of the heating system  101  may be contained within a cabinet  132 . In some embodiments, the furnace controller  126  may be included in the cabinet  132 . One skilled in the art will also understand that the heating system  101  disclosed herein may include additional components and devices that are not presently illustrated or discussed. 
         [0016]    The furnace controller  126  may include a memory section  103  having a series of operating instructions stored therein that direct the operation of the furnace controller  126  (e.g., the processor) when initiated thereby. The series of operating instructions may represent algorithms that are used to prevent or reduce temperature overshooting in the conditioned space. The furnace controller  126  also includes a printed circuit board (PCB)  104 . As illustrated in  FIG. 1 , the furnace controller  126  is coupled to the DAS  130 , the thermostat  128  and components of the heating system  101 . The controller  126  may also be connected to other elements and systems, such as a zone controller. In some embodiments, the connections are through a wired-connection. A conventional cable and contacts may be used to couple the furnace controller  126  to the various components of the heating system  101 . In some embodiments, a wireless connection may also be employed to provide at least some of the connections. 
         [0017]    The burner assembly  112  includes the at least one burner  114  that is configured for burning a combustible fuel-air mixture (e.g., gas-air mixture) and to provide a combustion product to the heat exchanger  116 . The heat exchanger  116  includes a plurality of tubes  117 , for example a tube corresponding to each of the at least one burner  114 . The heat exchanger  116  is configured to receive the combustion product from the burner assembly  112  and use the combustion product to heat air that is blown across the heat exchanger  116  by the air circulation fan  118 . The air circulation fan  118  is configured to circulate air through the cabinet  132 , whereby the circulated air is heated by the heat exchanger  116  and supplied to the conditioned space. The CAB  120  is configured to supply combustion air to the burner assembly  112  (i.e., the at least one burner  114 ) by an induced draft and is also used to exhaust waste products of combustion from the furnace through a vent  134 . The burner assembly  112  also includes a flame sensing rod  106 . The flame sensing rod  106  is configured to determine presence or absence of the flame. In some embodiments, the flame sensing rod  106  is positioned in the burner assembly  112  in front of the at least one burner  114 . When an ignition source lights the at least one burner  114 , the flame sensing rod  106  being in the path of the flame energizes circuitry that detects presence or absence of the flame. 
         [0018]      FIG. 2A  is an exemplary simulator diagram of a circuit  200  for flame detection. For illustrative purposes, the circuit  200  will be described relative to  FIG. 1 . In a typical embodiment, the circuit  200  is utilized in the printed circuit board (PCB)  104  of the HVAC system  100  or another apparatus requiring flame detection and control. The circuit  200  is configured to monitor the burner assembly  112  to determine presence or absence of the flame during ON and OFF cycles. The circuit  200  includes an LC circuit  210 , also referred to herein as a tank circuit. In the exemplary embodiment shown, the tank circuit  210  includes an inductor (L 1 ) and a capacitor (C 1 ) connected in parallel. In some embodiments, a voltage of the tank circuit  210  is adjusted by, for example, varying the inductor (L 1 ) and capacitor (C 1 ) values of the tank circuit  210 , as well as a duty cycle of a processor signal (G). In some embodiments, the tank circuit  210  may be tuned to approximately 20 kHz and may be periodically recharged from a 20-25V DC power supply (not explicitly illustrated). In a typical embodiment, the processor signal (G) is configured to gate a photovoltaic field-effect transistor (FET)  220  to allow a direct-current (DC) input signal to feed the tank circuit  210  from the 20-25V DC power supply at predefined intervals such as, for example, every 1 ms. 
         [0019]    The circuit  200  further includes a flame detect circuit  230 , a flame simulation circuit  232 , a relay circuit  240 , a first comparator  260 , and a second comparator  270 . In a typical embodiment, the flame detect circuit  230  is configured to determine presence or absence of the flame. In some embodiments, the flame sensing rod  106  is positioned in the burner assembly  112  in front of the at least one burner  114 . When an ignition source lights the at least one burner  114 , the flame sensing rod  106  is in the path of the flame and energizes the flame detect circuit  230 . In some embodiments, the photovoltaic FET  220  within the relay circuit  240  is configured to decouple the voltage of the processor signal (G) from the input DC signal (Vpump) to enhance an accuracy of voltage control at a rate of G control pulses. In some embodiments, the tank circuit  210  may be pumped to a peak voltage of approximately 60V and may rapidly decay as the tank circuit  210  discharges through the flame detect circuit  230  when a flame is present. 
         [0020]    The circuit  200  utilizes a plurality of input control signals and a plurality of output-detect signals to determine presence or absence of the flame. For example, the plurality of input control signals include the processor signal (G) and a flame-test input control signal (FLMTST). The plurality of output-detect signals include a first-output-detect signal (FLMSNS_CMP 1 ), which is an output signal of the first comparator  260 , and a second-output-detect signal (FLMSNS_CMP 2 ), which is an output signal of the second comparator  270 . In various embodiments, a voltage of the tank circuit  210  is adjusted, for example, by varying the inductor (L 1 ) and capacitor (C 1 ) values of the tank circuit  210 , as well as a duty cycle of the processor signal (G). In some embodiments, a peak current through the relay circuit  240  may be adjusted, for example, by varying a value of a series resistor  204  between the relay circuit  240  and the tank circuit  210  so that the peak current remains below a peak current rating of the relay circuit  240 . In a typical embodiment, the tank circuit  210  generates an alternating current (AC) signal of approximately 120 volts peak to peak. 
         [0021]    In a typical embodiment, when a flame is present, the tank circuit  210  and a test pulse capacitor  250  within the flame detect circuit  230  discharge through the flame via the flame detect circuit  230 . In various embodiments, the rate of discharge depends on the strength of the flame. For example, the stronger the flame, the faster is the rate of discharge. In a typical embodiment, the flame-test input control signal (FLMTST) may be used to inject, for example, a 5V pulse to charge the test pulse capacitor  250  so that the first comparator  260  may sense, for example, the flame strength. This may be done by a processor (e.g., furnace controller  126 ) measuring a time period from a time of removal of the flame-test input control signal (FLMTST) to a rising edge of a first comparator output signal (FLMSNS_CMP 1 ) as a first comparator input signal (FLMUB) decays to 3V from the fully charged 5V of the test pulse capacitor  250 . A second comparator  270  generates a second comparator output signal (FLMSNS_CMP 2 ), which serves as a functionality check of the circuit  200  to determine a component failure. The functionality check may avoid an indication that a flame is sensed, and present, when no flame is present. 
         [0022]    In various embodiments, a functionality check of the circuit  200  performed by, for example, the furnace controller  126 , may be as follows:
       Upon proper detection of a flame and prior to assertion of the flame-test input control signal (FLMTST), the first comparator output signal (FLMSNS_CMP 1 ) is high while the second comparator output signal (FLMSNS_CMP 2 ) is low.   Upon assertion of the flame-test input control signal (FLMTST), the first comparator output signal (FLMSNS_CMP 1 ) goes low while the second input signal (FLMSNS_CMP 2 ) goes high then flip states again after a delay proportional to the flame strength.   When the tank circuit  210  is not actively running such as, for example, when the processor signal (G) is low, the first comparator output signal (FLMSNS_CMP 1 ) and the second comparator output signal (FLMSNS_CMP 2 ) are both high prior to the flame-test input control signal (FLMTST) being asserted.   Upon assertion of the flame-test input control signal (FLMTST), the first comparator output signal (FLMSNS_CMP 1 ) goes low while the second comparator output signal (FLMSNS_CMP 2 ) remains high. The first comparator output signal (FLMSNS_CMP 1 ) flips state back to high after a short delay.   When no flame is detected, the first comparator output signal (FLMSNS_CMP 1 ) is low and the second comparator output signal (FLMSNS_CMP 2 ) is high regardless of the action of the flame-test input control signal (FLMTST).   If both the first comparator output signal (FLMSNS_CMP 1 ) and the second comparator output signal (FLMSNS_CMP 2 ) are low, a problem with the comparator circuit exists. Appendix A of U.S. Provisional Application No. 62/112,300 illustrates a number of flame-sense simulations, including simulations of various potential problem conditions.   If the flame sensing rod  106  is shorted to ground, the first comparator output signal (FLMSNS_CMP 1 ) and the second comparator output signal (FLMSNS_CMP 2 ) behaves similar to when the processor signal (G) is active (e.g., high). When the processor signal (G) is inactive (e.g., low), the second comparator output signal (FLMSNS_CMP 2 ) does not remain high as it did before under normal operating conditions but changes state upon assertion of the flame-test input control signal (FLMTST).       
 
         [0030]    As stated above, the flame-test input control signal (FLMTST) is configured to charge the test pulse capacitor  250  by injecting, for example, a 5V pulse, so that the first comparator  260  may sense the flame strength. This may be done by the furnace controller  126  measuring the time period from a time of removal of the flame-test input control signal (FLMTST) to a rising edge of a first comparator output signal (FLMSNS_CMP 1 ) as a first comparator input signal (FLMUB) decays to 3V from the fully charged 5V of the test pulse capacitor  250 . 
         [0031]      FIG. 2B  is an exemplary circuit  280  for flame detection. In  FIG. 2B , like reference numerals are used to indicate like components. In  FIG. 2B , the flame simulation circuit  232  is not shown because an actual flame condition would be sensed. 
         [0032]      FIG. 3A  illustrates a signal V[tank] generated by the tank circuit  210 , when the processor signal G causes the relay circuit  240  to pump the tank circuit  210  to a peak voltage of approximately 60V. 
         [0033]      FIG. 3B  illustrates a signal V[g] of the processor signal G, which causes the relay circuit  240  to pump the tank circuit  210 . 
         [0034]      FIGS. 3C and 3D  illustrate signal V[flmtst] (in solid lines) charging the capacitor  250  within the flame detect circuit  230  and signal V[flmub] (in wavy lines) from the capacitor  250  during charge and during decay in the presence of a flame when signal V[flmst] drops to zero. 
         [0035]      FIG. 3E  illustrates signal V[flmsns_cmp_ 1 ] from the first comparator  260 , as signal V[flmub] from the capacitor  250  varies during charge and during decay in the presence of a flame. In some embodiments, the first comparator  260  may be set to output a signal V[flmsns_cmp_ 1 ] when signal V[flmub] indicates that the capacitor  250  has discharged to a predetermined level or value. 
         [0036]      FIG. 3F  illustrates signal V[flmsns_cmp 2 ] from the second comparator  270 , as signal V[flmub] from the capacitor  250  varies during charge and during decay in the presence of a flame. In some embodiments, the second comparator  270  may be set inversely to the first comparator  260  to discontinue signal V[flmsns_cmp 2 ] output when signal V[flmub] indicates that the capacitor  250  has discharged to a predetermined level or value. 
         [0037]    Referring now to  FIGS. 1-2B and 3C-3E , in various embodiments, the furnace controller  126  is configured to determine the strength of the flame in an absolute sense and relative to other flame settings and other flame operations, by measuring a time lapse between flame test signal V[flmtst] dropping to zero and the first comparator  260  signal V[flmsns_cmp_ 1 ] output. This results in the capacitor  250  discharging more rapidly in the presence of a stronger flame. Accordingly, a shorter time lapse would indicate a stronger flame, and a longer time lapse would indicate a weaker flame. 
         [0038]    Referring again to  FIGS. 1-2A and 3E-3F , the first and second comparators  260  and  270  may be set to output and discontinue output of their respective signals (V[flmsns_cmp_ 1 ], V[flmsns_cmp 2 ]) at different charge levels or values of the capacitor  250 . In various embodiments, the furnace controller  126  is configured to measure the time lapse between such events to determine the rate of discharge of the capacitor  250  and thereby determine the strength or weakness of the flame. Furthermore, setting of the first and second comparators  260  and  270  at different charge levels of the capacitor  250  causes their respective signals to change at different times, thus providing a further indication to, for example, the furnace controller  126  that the first and second comparators  260  and  270  are operating correctly. 
         [0039]      FIG. 4  is a flow chart illustrating an exemplary process  400  for detecting presence of a flame. For illustrative purposes, the process  400  will be described relative to  FIGS. 1-3F . The process  400  starts at step  402 . At step  404 , the furnace controller  126  determines whether the processor signal (G) is active (e.g., high) or inactive (e.g., low). If it is determined at step  404  that the processor signal (G) is active, the process  400  proceeds to step  406 . At step  406 , the furnace controller  126  determines, prior to assertion of a flame-test input control signal (FLMTST), whether an output of the first comparator  260  is low or high. If it is determined at step  406  that the output of the first comparator  260  is low, the process  400  proceeds to step  408 . At step  408 , the furnace controller  126  determines, prior to assertion of the flame-test input control signal (FLMTST), whether an output of the second comparator  270  is low or high. If it is determined at step  408  that the output of the second comparator  270  is low, the process  400  proceeds to step  420 . At step  420 , the furnace controller  126  provides an indication that a problem exists in the circuit  200 . 
         [0040]    However, if it is determined at step  408  that the output of the second comparator  270  is high, the process  400  proceeds to step  410 . At step  410 , the furnace controller  126  provides an indication that no flame is present. As described above, the flame-test input control signal (FLMTST) may be used to inject, for example, a 5V pulse to charge the test pulse capacitor  250  so that the first comparator  260  may sense, for example, the flame strength. This may be done by the processor (e.g., furnace controller  126 ) measuring a time period from the time of removal of the flame-test input control signal (FLMTST) to the rising edge of the first comparator output signal (FLMSNS_CMP 1 ) as the first comparator input signal (FLMUB) decays to 3V from the fully charged 5V of the test pulse capacitor  250 . 
         [0041]    From step  410 , the process proceeds to step  412 . At step  412 , the flame-test input control signal (FLMTST) is asserted. From step  412 , the process  400  proceeds to step  416 . At step  416 , the furnace controller  126  determines whether the output of the first comparator  260  flips from low to high and the output of the second comparator  270  flips from high to low. If it is determined at step  416  that the output of the first comparator  260  flips from low to high or the output of the second comparator  270  flips from high to low, the process  400  proceeds to step  420  indicating that a problem exists in the circuit  200 . However, if it is determined at step  416  that neither condition described in step  416  is true, the process  400  proceeds to step  418 . At step  418 , the furnace controller  126  provides an indication that flame is not present and the circuit  200  is working correctly. 
         [0042]    However, if it is determined at step  406  that the output of the first comparator  260  is high, the process  400  proceeds to step  422 . At step  422 , the furnace controller  126  determines, prior to assertion of the flame-test input control signal (FLMTST), whether the output of the second comparator  270  is low or high. If it is determined at step  422  that the output of the second comparator  270  is high, the process  400  proceeds to step  420  indicating that a problem exists in the circuit  200 . However, if it is determined at step  422  that the output of the second comparator  270  is low, the process  400  proceeds to step  424 . At step  424 , the furnace controller  126  provides an indication that flame is present. From step  424 , the process  400  proceeds to step  426 . 
         [0043]    At step  426 , the flame-test input control signal (FLMTST) is asserted. From step  426 , the process  400  proceeds to step  428 . At step  428 , the furnace controller  126  determines whether the output of the first comparator  260  flips from high to low and the output of the second comparator  270  flips from low to high. If it is determined at step  428  that the output of the first comparator  260  flips from high to low and the output of the second comparator  270  flips from low to high, the process  400  proceeds to step  430 . However, if it is determined at step  428  that the at least one condition described in step  428  is not true, the process  400  proceeds to step  420  indicating that a problem exists in the circuit  200 . At step  430 , the flame-test input control signal (FLMTST) is deasserted. From step  430 , the process proceeds to step  432 . At step  432 , the furnace controller  126  determines whether the outputs of the first comparator  260  and second comparator  270  return to the original state. If it is determined at step  432  that the at least one condition described in step  432  is not true, the process  400  proceeds to step  420  indicating that a problem exists in the circuit  200 . However, if it is determined at step  432  that the outputs of the first comparator  260  and second comparator  270  return to the original state, the process  400  proceeds to step  434 . At step  434 , the furnace controller  126  provides an indication that flame is present and the circuit  200  is working correctly. From steps  418  and  434 , the process  400  returns to step  402 . 
         [0044]    However, if it is determined at step  404  that the processor signal (G) is inactive, the process  400  proceeds to step  438 . At step  438 , the furnace controller  126  determines, prior to assertion of a flame-test input control signal (FLMTST), whether an output of the first comparator  260  is low or high. If it is determined at step  438  that the output of the first comparator  260  is low, the process  400  proceeds to step  440 . At step  440 , the furnace controller  126  determines, prior to assertion of the flame-test input control signal (FLMTST), whether an output of the second comparator  270  is low or high. If it is determined at step  440  that the output of the second comparator  270  is low, the process  400  proceeds to step  420 . At step  420 , the furnace controller  126  provides an indication that a problem exists in the circuit  200 . However, if it is determined at step  440  that the output of the second comparator  270  is high, the process  400  proceeds to step  442 . At step  442 , the furnace controller  126  provides an indication that no flame is present. 
         [0045]    From step  442 , the process  400  proceeds to step  444 . At step  444 , the flame-test input control signal (FLMTST) is asserted. From step  444 , the process  400  proceeds to step  446 . At step  446 , the furnace controller  126  determines whether the output of the first comparator  260  flips from low to high and the output of the second comparator  270  flips from high to low. If it is determined at step  446  that the output of the first comparator  260  flips from low to high or the output of the second comparator  270  flips from high to low, the process  400  proceeds to step  420  indicating that a problem exists in the circuit  200 . However, if it is determined at step  446  that the both conditions described in step  446  are not true, the process  400  proceeds to step  448 . At step  448 , the furnace controller  126  provides an indication that flame is not present and the circuit  200  is working correctly. From step  448 , the process  400  proceeds to step  464 . At step  464 , the processor signal (G) is active (e.g., high). 
         [0046]    However, if it is determined at step  438  that the output of the first comparator  260  is high, the process  400  proceeds to step  452 . At step  452 , the furnace controller  126  determines, prior to assertion of the flame-test input control signal (FLMTST), whether the output of the second comparator  270  is low or high. If it is determined at step  452  that the output of the second comparator  270  is low, the process  400  proceeds to step  420  indicating that a problem exists in the circuit  200 . However, if it is determined at step  452  that the output of the second comparator  270  is high, the process  400  proceeds to step  454 . At step  454 , the furnace controller  126  provides an indication that flame is present. From step  454 , the process  400  proceeds to step  456 . 
         [0047]    At step  456 , the flame-test input control signal (FLMTST) is asserted. From step  456 , the process  400  proceeds to step  458 . At step  458 , the furnace controller  126  determines whether the output of the first comparator  260  flips from high to low. If it is determined at step  458  that the output of the first comparator  260  does not flip from high to low, the process  400  proceeds to step  420  indicating that a problem exists in the circuit  200 . However, if it is determined at step  458  that the output of the first comparator  260  flips from high to low, the process  400  proceeds to step  460 . At step  460 , the furnace controller  126  determines whether the output of the second comparator  270  remains high. If it is determined at step  460  that the output of the second comparator  270  flips from high to low, the process  400  proceeds to step  420  indicating that a problem exists in the circuit  200 . However, if it is determined at step  460  that the output of the second comparator  270  remains high, the process  400  proceeds to step  462 . At step  462 , the furnace controller  126  provides an indication that flame is present and the circuit  200  is working correctly. From step  424 , the process  400  proceeds to step  464 . 
         [0048]    For purposes of this patent application, the term computer-readable storage medium encompasses one or more tangible computer-readable storage media possessing structures. As an example and not by way of limitation, a computer-readable storage medium may include a semiconductor-based or other integrated circuit (IC) (such as, for example, a field-programmable gate array (FPGA) or an application-specific IC (ASIC)), a hard disk, an HDD, a hybrid hard drive (HHD), an optical disc, an optical disc drive (ODD), a magneto-optical disc, a magneto-optical drive, a floppy disk, a floppy disk drive (FDD), magnetic tape, a holographic storage medium, a solid-state drive (SSD), a RAM-drive, a SECURE DIGITAL card, a SECURE DIGITAL drive, a flash memory card, a flash memory drive, or any other suitable tangible computer-readable storage medium or a combination of two or more of these, where appropriate. 
         [0049]    Particular embodiments may include one or more computer-readable storage media implementing any suitable storage. In particular embodiments, a computer-readable storage medium implements one or more portions of the furnace controller  126 , one or more portions of the system memory, or a combination of these, where appropriate. In particular embodiments, a computer-readable storage medium implements RAM or ROM. In particular embodiments, a computer-readable storage medium implements volatile or persistent memory. In particular embodiments, one or more computer-readable storage media embody encoded software. 
         [0050]    In this patent application, reference to encoded software may encompass one or more applications, bytecode, one or more computer programs, one or more executables, one or more instructions, logic, machine code, one or more scripts, or source code, and vice versa, where appropriate, that have been stored or encoded in a computer-readable storage medium. In particular embodiments, encoded software includes one or more application programming interfaces (APIs) stored or encoded in a computer-readable storage medium. Particular embodiments may use any suitable encoded software written or otherwise expressed in any suitable programming language or combination of programming languages stored or encoded in any suitable type or number of computer-readable storage media. In particular embodiments, encoded software may be expressed as source code or object code. In particular embodiments, encoded software is expressed in a higher-level programming language, such as, for example, C, Python, Java, or a suitable extension thereof. In particular embodiments, encoded software is expressed in a lower-level programming language, such as assembly language (or machine code). In particular embodiments, encoded software is expressed in JAVA. In particular embodiments, encoded software is expressed in Hyper Text Markup Language (HTML), Extensible Markup Language (XML), or other suitable markup language. 
         [0051]    Depending on the embodiment, certain acts, events, or functions of any of the algorithms described herein can be performed in a different sequence, can be added, merged, or left out altogether (e.g., not all described acts or events are necessary for the practice of the algorithms). Moreover, in certain embodiments, acts or events can be performed concurrently, e.g., through multi-threaded processing, interrupt processing, or multiple processors or processor cores or on other parallel architectures, rather than sequentially. Although certain computer-implemented tasks are described as being performed by a particular entity, other embodiments are possible in which these tasks are performed by a different entity. 
         [0052]    Conditional language used herein, such as, among others, “can,” “might,” “may,” “e.g.,” and the like, unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements and/or states. Thus, such conditional language is not generally intended to imply that features, elements and/or states are in any way required for one or more embodiments or that one or more embodiments necessarily include logic for deciding, with or without author input or prompting, whether these features, elements and/or states are included or are to be performed in any particular embodiment. 
         [0053]    While the above detailed description has shown, described, and pointed out novel features as applied to various embodiments, it will be understood that various omissions, substitutions, and changes in the form and details of the devices or algorithms illustrated can be made without departing from the spirit of the disclosure. As will be recognized, the processes described herein can be embodied within a form that does not provide all of the features and benefits set forth herein, as some features can be used or practiced separately from others. The scope of protection is defined by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.
A method of determining presence of a flame in a furnace of a heating, ventilation, and air conditioning (HVAC) system. The method comprises determining, using a controller, whether a processor signal (G) is active, responsive to a determination that the processor signal (G) is active, determining, using the controller prior to assertion of a flame-test input control signal, an output state of a first comparator, responsive to a determination that the output state of the first comparator is high, determining, using the controller prior to assertion of the flame-test input control signal, an output state of a second comparator, and responsive to a determination that the output state of the second comparator is low, transmitting, using the controller, a notification that a flame is present.