Automatic Ignition and Flame Detection System for Gas Fired Devices

This invention pertains to ignition systems for gas fired devices, and in particular to automatic ignition and heat detection systems for such devices.

In many conventional gas fired appliances, such as boilers, clothes dryer, ovens and the like, it is customary to provide heat by igniting gas emanating from a main burner. Commonly, gas flows through the main burner when the device is activated, the gas being ignited by a nearby pilot flame which is constantly burning. Recognizing the inefficiency and danger of a constantly burning pilot flame, automatic ignition systems which rely upon heat from a resistive element to ignite the main burner have been substituted for constantly burning pilot systems, the resistive element being energized only when the device calls for heat. In such systems, it is known to employ a silicon carbide resistive element having a negative temperature characteristic (i.e. the resistance of silicon carbide decreases with increasing temperature) as the igniter. One such prior art system is described in U.S. Pat. No. 3,282,324, the contents of which are hereby incorporated by reference in their entirety.

In the system disclosed in U.S. Pat. No. 3,282,324, a solenoid activated gas valve is employed, the solenoid winding being in a circuit with the igniter element. Because silicon carbide has a negative temperature characteristic, when the device calls for heat, current flow through the igniter heats the igniter thereby dropping its resistance. This continues until current flow through the circuit incorporating the solenoid winding increases sufficiently to energize the solenoid and open the gas valve.

To close the gas valve in the event of a flameout, the system includes a circuit which deenergizes the igniter element after the gas valve is opened. The igniter element then operates as a heat detector, the gas valve being closed if current flow through the igniter element drop below a predetermined value considered indicative of a sufficient drop in temperature to confirm a flameout.

It will be apparent that in both the ignition and heat detection modes, the system disclosed in U.S. Pat. No. 3,282,324 is based on the assumption that current flow through the igniter element, and hence its resistance, is an accurate indication of the igniter element temperature. Unfortunately, this assumption ignores the reality that the resistance/temperature characteristic for different silicon carbide igniter elements varies from one igniter element to the next. That is, one igniter element might display one temperature at a particular resistance, while another igniter element might display a quite different temperature at that resistance. Accordingly, relying on a predetermined igniter element resistance level as an indication that its temperature is sufficient to ignite gas results in a potentially inaccurate system. Furthermore, the time required for the system discussed in the patent to open and close the gas valve is relatively slow.

Another prior art system is disclosed in U.S. Pat. No. 3,933,419, the contents of which are also hereby incorporated by reference in their entirety. In the system disclosed in this patent, a heat sensing plate comprised of a magnetic alloy having a predetermined Curie temperature is employed to determine when the temperature of the igniter element is sufficient to ignite gas. In particular, the heat sensing plate exhibits magnetic properties at room temperature which are sufficient to attract a permanent magnet in a circuit operatively connected to the gas valve. As long as the permanent magnet is attracted to the plate, the valve remains closed. However, as the plate is heated by current flow through the igniter element, its Curie temperature is eventually reached at which point the plate loses its ability to attract the magnet. As a result, the magnet moves away from the plate under the urging of a spring whereupon the gas valve is opened. As usual, shortly after the gas valve is opened, the igniter element is deenergized, and as long as the heat of the flame keeps the temperature of the igniter element sufficiently high to maintain the plate above its Curie temperature, the gas valve remains open. In the event of a flameout, the temperature in the vicinity of the igniter element drops, and hence the temperature of the heat sensing plate also drops. As a result, the plate again acquires magnetic properties sufficient to attract the permanent magnet and close the gas valve.

It will be apparent, therefore, that the system disclosed in U.S. Pat. No. 3,933,419 relies upon the point at which the magnetic plate loses its magnetic attractability as an accurate indication of the temperature of the plate, and hence of the igniter element. This system is, therefore, based on the questionable assumption that the permanent magnet and heat sensing plate can be manufactured in commercial quantities with sufficiently uniform magnetic properties to insure that the gas valve will not be prematurely opened or closed. Furthermore, the response time of this system is also relatively slow.

According to the present invention I have developed an improved automatic ignition system for gas fired devices of the type including a resistance type igniter. The improved ignition system is capable of determining when the igniter is sufficiently hot to ignite gas, taking into account that different igniters have different temperature characteristics. Preferably, this is accomplished by incorporating in the system a microcomputer operatively connected to the igniter, the microcomputer being programmed to repeatedly measure the resistance of the igniter and to compare successive measurements. In this manner, the microcomputer is capable of determining when the igniter has reached the flattened portion of its temperature/resistance curve where the igniter is known to be sufficiently hot to ignite gas.

Preferably, the microcomputer is programmed to conduct two separate tests to confirm that the igniter has reached the flattened portion of its temperature characteristic. In the first test, referred to hereinbelow as the "warm" test, the microcomputer first establishes a threshold based on the resistance of the igniter element prior to energization. The igniter is then energized whereupon the microcomputer, after a predetermined delay, again measures the resistance of the igniter. If the new reading is below the threshold, the warm test is passed, as this indicates that the igniter is in the relatively steep portion of its temperature characteristic where increases in temperature result in relatively large decreases in resistance. If the warm test is not passed, the microcomputer, again after a predetermined delay, remeasures the igniter resistance for comparison with the threshold. This continues until either the warm heat is passed or a predetermined time interval expires. In the latter event, the system enters a FAULT mode to be described hereinafter. In the second test, referred to as the "hot" test, the microcomputer compares successive resistance measurements until the difference between successive measurements is below a predetermined maximum. This confirms that the igniter has reached the flattened portion of its temperature characteristic where its resistance changes relatively sightly with increasing temperatures. As noted, in the flattened portion of the temperature characteristic, the igniter is sufficiently hot to ignite gas. Upon confirming that the igniter has reached ignition temperature, the microcomputer is programmed to open the gas valve thereby effecting ignition as the gas passes through the burner in the vicinity of the igniter. The igniter element is then deenergized.

In the preferred system, the microcomputer is also programmed to detect a flameout. This is also preferably accomplished by comparing successive resistance measurements of the igniter. Specifically, in the flame detection mode, the microcomputer is programmed to conduct two separate tests, each of which is independently capable of confirming a flameout. In the first test, referred to hereinbelow as the threshold or level test, the microcomputer establishes a resistance threshold based on the resistance of the igniter just prior to ignition. Once the igniter is deenergized after ignition, the microcomputer continuously monitors the igniter resistance at regular intervals. If the igniter resistance exceeds the threshold, a flameout is confirmed, as this indicates that the temperature in the vicinity of the igniter is no longer sufficient to maintain the igniter on the flattened portion of its temperature characteristic. In the second test, referred to below as the "rate" test, the microcomputer compares successive resistance measurements and determines if the rate of change of the igniter resistance exceeds a predetermined rate. The rate is selected such that, if exceeded, a flameout is confirmed. In the event of a flameout, the microcomputer is preferably programmed for corrective action.

The preferred system also includes means for confirming that the gas pressure is above a predetermined minimum considered safe. For this purpose, the system preferably includes two independently operable, serially arranged gas valves. A conduit having a pair of spaced apart electrically conductive contacts at one end thereof communicates with the flow path between the valves, and a conductive member is disposed for sliding movement in the conduit. When the gas valve on the inlet side of the conduit is opened, gas flows into the conduit and, if gas pressure is sufficient, urges the conductive member upward until it completes an electrical circuit between the contacts. This is detected by the microcomputer, which is operatively connected to the contacts. If gas pressure is low, the electrical circuit between the contacts will not be completed by the conductive member, and this condition will also be detected by the microcomputer. In the event of a low gas pressure condition, the microcomputer is preferably programmed for corrective action. Preferably, the microcomputer monitors gas pressure both before and after ignition.

Another feature of the preferred system is the provision of means for indicating the status of the system and the nature of any malfunction. Preferably, such means comprises a self-powered module having a digital display thereon, the module being removably connectable to the microcomputer. When the module is connected to the microcomputer, the number on the digital display indicates the existence and nature of the particular system malfunction, or simply the status of the system. For example, the different numbers on the display may be utilized to indicate a faulty igniter, faulty valve circuitry, flameout, failure of the igniter to reach ignition temperature, etc. It is presently contemplated that the module will be utilized by service personnel during system inspection and repair.

The use of a microcomputer to control system operations also results in a reduction of system response time and therefore greater overall fuel efficiency and safety. Also, by reducing the number of moving parts, system reliability is increased. The preferred system also preferably includes means for modifying the programming of the microcomputer for particular applications, preferably by ungrounding specific inputs to the microcomputer.

The above as well as further features and advantages of the preferred automatic fuel ignition and detection system in accordance with the present invention will be more fully apparent from the following detailed description and annexed drawings of the presently preferred embodiment. In the following description the preferred system is described for use in connection with a gas-fired boiler. However, it will be apparent to those skilled in the art that it may be utilized for controlling a a wide range of gas-fired devices, such as domestic ranges, dryers, and the like, and that the system may be retrofitted on such gas-fired devices.

In the drawings:

FIG. 1 is a diagrammatic illustration of the preferred automatic ignition and heat detection system in accordance with the present invention;

FIG. 2 is an elevational view illustrating the preferred manner for supporting the igniter element in proximity to the burner;

FIGS. 3A and 3B schematically illustrate the preferred system shown in FIG. 1; and

FIGS. 4-9 are system logic flow diagrams for the preferred system.

Referring initially to FIG. 1 of the drawings, the presently preferred embodiment of the fuel ignition and heat detection system in accordance with the present invention is generally designated by the reference number 10. As shown, the system 10 preferably includes a microcomputer integrated circuit chip 12, such as a COP411L, manufactured and distributed by National Semiconductor Corp., which conventionally contains a microcprocessor, associated input/output devices, a read only memory and random access memory all in one chip. Such a microcomputer chip 12 is conventionally programmable in the associated machine language used with the chip such as, by way of example, what is termed COP assembly language.

The other components of the preferred system 10 illustrated in FIG. 1, the functions of which will be explained in greater detail hereinafter, are a burner 14, an igniter 16, a redundant valve arrangement comprised of a pilot valve assembly 18 and a secondary valve assembly 20, a pressure sensitive switch 22, a thermostat control 24, a high limit switch 26, a diagnostic plug-in module 28, and a power source 30. As diagrammatically illustrated in FIG. 1, the microcomputer 12 is supported within a module or housing 32. The housing 32 also contains the circuitry interfacing the microcomputer 12 with the other components of the system 10. This interfacing circuitry will be described in greater detail hereinafter with reference to FIG. 3, wherein the system 10 is schematically illustrated. Mounted on the housing 32 are an indicator light 34 and a reset switch 36, both of which are also connected to the microcomputer 12. The functions of the light 34 and switch 36 will also be explained in greater detail hereinafter. Power is supplied to the housing 32 and from there to the various components in the system 10 by a power source 30 which preferably comprises a standard 117 volt AC power line. The housing 32 is preferably mounted, as by screws, on a control panel adjacent the controlled apparatus, which may be a boiler.

The burner 14 is conventional and may comprise, for example, a burner of the type used in gas fired boilers. As usual, it comprises a tubular member 38 having a plurality of apertures 40 therein. Referring to FIGS. 1 and 2, the igniter 16 comprises an element 17 secured at one end in an insulating block 42 which is mounted, as by screw 44, on a bracket 46 extending from the burner 14. In this fashion, the element 17 is supported near the burner 14 so that the element can perform its dual functions of igniting the gas flowing from the burner and sensing the heat of the resulting flame. A pair of leads 48 extending from the outer end of the insulating block 42 connect the igniter element 17 with the module 32.

Igniter elements 17 suitable for incorporation in the system 10 are commercially available. The element 17 is comprised, for example, of silicon carbide, which has a negative temperature characteristic, i.e. the resistance of silicon carbide decreases with increasing temperature. Generally, the igniter element 17 is commercially available as a package including the insulating block 42 and leads 48. By way of example, the model no. 767A silicon carbide igniter manufactured by the White Rodgers division of Emerson Electric Company is suitable for incorporation in the system 10.

Those skilled in the art will appreciate that the temperature characteristic varies from one igniter element to the next. That is, one igniter element will exhibit a particular resistance at a temperature of 100.degree. F., while another igniter element may exhibit a different resistance at that temperature. Accordingly, for an automatic ignition system to be compatible with different igniter elements, it must be able to compensate for these differences. As will be explained in greater detail hereinafter, the system 10 is fully capable of doing so.

Before entering the burner 14, gas first flows through the redundant valve arrangement comprised of pilot valve assembly 18 and secondary valve assembly 20, such as the redundant valve arrangement model no. 36C84 manufactured by the White Rodgers division of Emerson Electric Co. As shown, the valve assemblies 18 and 20 are preferably of the solenoid variety and thus include cores 50, 52 and actuating coils 54, 56, respectively. As will be more fully apparent from FIG. 3, the coils 54, 56 are actuated by relays supported within the housing 32 and interfaced with the microcomputer 12. As shown, valves 58 and 60 are connected, respectively, to the cores 50, 52. The valves seats 62, 64 for the valves 58, 60 are formed in a preferably casted chamber 66 which defines the flow path for the incoming gas. It will be apparent from FIG. 1 that before incoming gas enters the burner 14, it must first flow through the openings defined by both of the valve seats 62, 64. In FIG. 1, the valves 58, 60 are shown in their closed positions wherein gas flow through the chamber 66 to the burner 14 is blocked. When the valves 58 and 60 are opened, gas flows into the burner 14 through a metered orifice 68.

The pressure sensitive switch 22 includes a conduit 70 which communicates with the gas flow path defined by the chamber 66 between the valve seats 62 and 64. The conduit 70 opens into a larger chamber 72 in which a diaphragm 74 is slidably supported. The diaphragm 74 has a conducting element 76 secured thereon which connects the contacts 78, 80 when the diaphragm 74 is in its uppermost position. The significance of this will be more fully apparent hereinafter. At this point, suffice it to say that the diaphragm will assume its uppermost position whenever the valve 58 is open and gas pressure is above a predetermined minimum considered safe.

The thermostat 24 may be of the type conventionally used for regulating the activation and deactivation of gas fired boilers and the like. As will be explained in greater detail hereinafter, when the thermostat 24 calls for heat, the system 10 is activated and the ignition sequence is commenced. The high limit switch 26 is a safety feature comprising a temperature sensitive switch preferably disposed to sense the temperature in the boiler chamber. As will be more fully explained hereinafter, if, for example, the temperature in the chamber is too hot, which may, for example, be caused by a fan malfunction, the high limit switch 26 opens, whereupon the microcomputer 12 automatically closes the valves 58 and 60 thereby blocking the further flow of gas to the burner 14.

The diagnostic plug-in module 28, which is connectable to the housing 32 via the receptacle 82, contains a digital display 84. As will be explained hereinafter, when the plug-in module 28 is connected to the housing 32, the display 84 provides information indicative of the status of the system 10, including the existence and nature of a malfunction, if any. The presence of a fault or malfunction in the system 10 is also indicated by the lighting of the indicator light 34.

A schematic representation of the system 10 is illustrated in FIG. 3 wherein typical component values and circuit elements are indicated. A detailed description of the schematic is deemed unnecessary, as the operation of the illustrated circuit will be fully apparent to the skilled art worker from this description. As regards the microcomputer 12, and as previously noted, it is preferably a conventional COP411L microcomputer of the type distributed by National Semiconductor Corp., which is conventionally programmed in COP assembly language. The preferred control program listing for the microcomputer 12 for operating the system 10 is as follows: ##SPC1## ##SPC2##

The operation of the system 10 will now be described with particular reference to the flow charts illustrated in FIGS. 4-9. In describing the operation of the system 10, it will be assumed that the flame is initially off. In this state, the microcomputer 12 maintains the system 10 in an IDLE mode (FIG. 4). In the IDLE mode, the microcomputer 12 continuously monitors the pilot valve relay driver input as well as the input connected to the thermostat 24 and high limit switch 26. As shown in FIG. 3, the thermostat 24 and high limit switch 26 are connected in series to a single input of the microcomputer 12.

As is apparent from FIG. 4, the microcomputer 12 maintains the system 10 in the IDLE mode until either the pilot valve relay driver fails shorted or the thermostat/high limit input becomes active, i.e. calls for heat. If the pilot relay shorts, the microcomputer 12 will enter a FAULT mode. The operation of the system 10 in the FAULT mode will be explained in greater detail hereinafter.

As shown, the microcomputer 12 is preferably programmed to establish a predetermined thermostat threshold current which must be exceeded before the microcomputer will attempt ignition. For example, a current threshold of 100 milliamperes may be set. This is done to accommodate programmable setback thermostats which "steal" current from the power circuit that might otherwise provide a false ignition signal to the microcomputer.

Assuming no fault occurs and the thermostat calls for heat, the microcomputer 12 effects a prepurge delay during which commencement of the ignition sequence is delayed for preferably thirty seconds. At the expiration of the thirty second delay, the microcomputer 12 enters the IGNITION mode (FIG. 5). Upon entering the IGNITION mode, the microcomputer 12 reduces the thermostat threshold current and then tests the igniter element 17 for a short by measuring its resistance. If the igniter element 17 is shorted, the microcomputer 12 enters the FAULT mode. Assuming no fault, the microcomputer activates the pilot valve relay driver circuit thereby opening the pilot valve 58. After a preferably one second delay, the microcomputer 12 again monitors the thermostat/high limit input. If the thermostat/high limit input is open, thereby indicating that heat is no longer called for, the microcomputer 12 enters the OFFGAS mode. As will be more fully apparent from the description of FIG. 8 below, when the system enters the OFFGAS mode, the microcomputer, after a ten second delay, returns to the IDLE mode whereupon the pilot valve 58 is closed. Assuming the thermostat/high limit input is still active, the microcomputer 12 next checks the gas pressure by monitoring the pressure sensitive switch 22. If gas pressure is normal, gas flow through the pilot valve 58 into the conduit 70 and connected chamber 72 will move diaphragm 74 upward until the conducting element 76 makes contact with the contacts 78, 80 thereby closing the circuit to the microcomputer 12. If the microcomputer 12 detects that the contacts 78, 80 are open, the microcomputer enters a LOWPRS mode. The operation of the system 10 in the LOWPRS mode will be explained in greater detail below. Assuming gas pressure is verified, the microcomputer 12 enters the TURNON mode.

At this point the microcomputer 12 readies the system 10 for gas ignition. This requires activating the igniter element relay driver circuit to energize the igniter element 17 and then opening gas flow to the burner 14 when the igniter element is sufficiently hot to effect gas ignition. As previously noted, to determine whether a particular igniter element has reached ignition temperature, the microcomputer 12 must be capable of distinguishing between different elements having different temperature characteristics. As shown in FIG. 6, to determine whether the element 17 has reached ignition temperature, the microcomputer 12 conducts two tests--the "warm" test and the "hot" test. First, the microcomputer establishes a threshold based on the resistance of the element 17 before the igniter 16 is energized, i.e. when the element 17 is still cold. The igniter 16 is then energized for preferably two seconds and the resistance of the element 17 again measured. If the resistance reading is below the threshold, the warm test is passed. If the warm test is not passed, the igniter 16 is again energized for preferably two seconds, the resistance of the element 17 is measured, and the new reading is compared to the reference cold reading. This process continues until either the warm test is passed or preferably one minute elapses. If the warm test is not passed after one minute, the microcomputer 12 enters the FAULT mode.

Assuming the warm test is passed, the microcomputer 12 next conducts the hot test. In this test, the microcomputer compares two consecutive resistance measurements of the element 17. If the difference between these readings is less than a predetermined value, the hot test is passed, as this indicates that the flat, i.e. high temperature, portion of the temperature characteristic for the element 17 has been reached. If the hot test is not passed, the element 17 is again energized for preferably two seconds, whereupon both the warm and hot tests are again conducted. This continues until the hot test is passed or until the one minute period expires. If the hot test is not passed within one minute, the microcomputer 12 enters the FAULT mode. By utilizing the above technique to confirm ignition temperature, ignition is achievable despite reduced line voltage.

When ignition temperature is confirmed, the microcomputer 12 enters the IGNOK mode (FIG. 7) whereupon the secondary valve relay driver is activated to open the secondary valve 60. Simultaneously, the element 17 is energized. At this point, gas flows through the chamber 66 and the metered orifice 68 into the burner 14 whereupon the gas is ignited as it passes through the apertures 40 in the vicinity of the element 17. Preferably four seconds later, the element 17 is deenergized.

At this point, the element 17 is utilized as a heat detector. To this end, the microcomputer 12 monitors the resistance of the element 17 for the presence or absence of a flame. As will be explained below, if a flameout occurs, the microcomputer is programmed for corrective action. For the microcomputer 12 to determine whether a flameout has occurred based on the resistance of the element 17, the microcomputer 12 must be capable of compensating for variations in the temperature characteristics of different igniter elements. To determine if a flameout occurs, the microcomputer 12 conducts two tests--a level test and a rate test. Referring to FIGS. 7 and 8, preferably two seconds after the igniter is deenergized the microcomputer 12 establishes a threshold resistance based on the measured resistance of the igniter element 17 just prior to ignition, i.e. when the igniter element is hot. The threshold resistance is preferably equal to the measured resistance increased by a predetermined value. That is, the threshold resistance is established such that if the resistance of the element 17 exceeds the threshold, this will indicate that the temperature in the vicinity of the element 17 has dropped sufficiently to confirm the occurrence of a flameout. The rate test is accomplished by comparing the rate of change of the resistance of the igniter element 17 with a preestablished rate. As preferred and shown in FIG. 8, if the rate of change of the igniter element resistance exceeds the preestablished rate twice in a row, thereby indicating a continuing drop in temperature in the vicinity of the igniter element, this too establishes a flameout.

In the event of a flameout, the microcomputer 12, after a ten second delay, returns to the IDLE mode. Assuming the thermostat 24 is still calling for heat, the microcomputer then repeats the ignition sequence described hereinabove in an effort to again ignite the flame. If flameout occurs three times in a row, as indicated by the flameout counter, the microcomputer 12 enters the FAULT mode.

When the microcomputer is in the MONITOR mode (FIG. 8), the microcomputer 12, in addition to monitoring the flame, also continuously monitors the igniter element 17, the thermostat/high limit input, the input from the pressure switch 22, and the pilot relay driver circuitry. As shown in FIG. 8, if the igniter element 17 shorts or the pilot valve relay driver circuitry fails, the microcomputer 12 enters the FAULT mode. If either the thermostat 24 or high limit switch 26 opens, the microcomputer 12 returns the system to the IDLE mode (FIG. 4) thereby closing the pilot valve 58 and shutting the flame. If the switch 22 opens, thereby indicating that gas pressure is low, the microcomputer enters the LOWPRS mode (FIG. 5).

In the LOWPRS mode, the microcomputer 12 closes the pilot and secondary valves 58, 60. As shown in FIGS. 4 and 5, after a thirty second delay, the microcomputer 12 then enters the IGNITION mode whereupon the microcomputer 12 runs through the ignition sequence more fully described above. This sequence concludes with a gas pressure check. As long as the pressure remains low, this sequence is repeated. As shown, when the switch 22 closes, thereby indicating that gas pressure has been restored, the microcomputer 12 enters the TURNON mode. Operation of the system 10 in the TURNON mode is more fully discussed above.

As is described above, the microcomputer 12 enters the FAULT mode in response to a malfunction, e.g. if the igniter element 17 fails shorted or open, the pilot valve relay driver circuitry shorts, three consecutive flameouts occur, etc. The flow chart for the FAULT mode is illustrated in FIG. 9. As shown, in the FAULT mode the pilot valve 58 and secondary valve 60 are closed to turn off the gas flow, and the igniter element 17 is deenergized. Assuming the external power source remains operative, these conditions will prevail until the reset switch 36, which preferably comprises a push button switch, is depressed, whereupon the microcomputer 12 is returned to START (FIG. 4). Of course, if the fault persists, the microcomputer 12 will re-enter the FAULT mode when the microcomputer again checks the faulty component. Whenever the system enters the FAULT mode, the indicator light 34 lights thereby visually indicating the presence of a fault. However, the light 34 may not light if the system 10 is completely down, which may result from a total loss of power.

In the event of a fault, it is preferable to provide means for indicating the nature of the fault. This function is accomplished by the plug-in fault analyzer 28 which also indicates the status of the system 10. The analyzer 28 is presently contemplated for use by service personnel. As previously noted, the analyzer 28 incorporates a conventional seven segment digital display 84. Referring to FIGS. 1 and 3, the self-powered analyzer 28 is connected to the microcomputer 12 by plugging the analyzer into the receptacle 82. The number on the digital display 84 then indicates the type of fault or a particular system status. In the above described preferred system 10, and referring to FIGS. 4-9, a reading of zero indicates that the power level is insufficient to operate the system, a reading of one indicates that the system is in the IDLE mode, a reading of two indicates that the thermostat/high limit input is active, a reading of three indicates that the system is in the IGNITION mode, a reading of four indicates that the system is in the IGNOK mode, a reading of five indicates that the gas pressure is low, a reading of six indicates that the pilot valve is improperly open in the IDLE mode, and a reading of seven indicates an igniter malfunction.

To accommodate particular applications, the preferred system 10 preferably incorporates means for modifying the functioning of microcomputer 12 for altering the mode of operation described above. Referring to FIG. 3, the functioning of the microcomputer 12 is preferably modifiable by ungrounding specific inputs to the microcomputer provided for this purpose. As shown in FIG. 3, such ungrounding is preferably accomplished by providing a removable conductive member ("strap a" in FIG. 3) which connects the input to ground. The conductive member is preferably factory installed and forms part of the interfacing circuitry within the housing 32. When strap a is removed, the mode of operation described above is modified as shown in the flow chart, FIGS. 4-9. Specifically, when strap a is removed, the thirty second prepurge delay before the microcomputer 12 enters the ignition mode is bypassed (see FIG. 4). This modification would be used, for example, where immediate heat is required. When strap a is removed, the microcomputer also preferably enters the FAULT mode in response to a single flameout, as opposed to three flameouts (FIG. 8). This prevents the accumulation of gas which might otherwise occur if ignition is attempted three times without a thirty second delay between attempts. If desired, the microcomputer may be programmed for still other options which would take effect upon removal of other straps not shown.

Throughout the specification and claims, reference is made to measuring the resistance of the igniter element 17 for determining the temperature characteristic thereof. Those skilled in the art will appreciate, however, that the relevant information may be obtained not only by actuatlly measuring the resistance of the igniter element, but also by measuring the current flow through the igniter element or the voltage drop across the igniter element. Accordingly, the phrase "measuring the resistance" or like phrases, when applied to the igniter element, should be understood throughout as contemplating current or voltage measurements which also yield information defining the temperature characteristic of the igniter. In the preferred system 10 described above, information defining the temperature characteristic of the igniter element is obtained by measuring the voltage drop across the igniter element.

While I have herein shown and described the preferred embodiment of the present invention and have suggested certain modifications thereto, it will be apparent that further changes and modifications may be made without departing from the spirit and scope of the invention. Accordingly, the above description should be construed as illustrative and not in the limiting sense, the scope of the invention being defined by the following claims.