Patent Publication Number: US-11022305-B2

Title: Control system and method for a solid fuel combustion appliance

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a continuation of U.S. patent application Ser. No. 15/424,485 filed on Feb. 3, 2017, which is a continuation-in-part of U.S. patent application Ser. No. 13/113,669 filed on May 23, 2011, now U.S. Pat. No. 9,803,862, which claims priority to U.S. Provisional Patent Application No. 61/351,477 filed on Jun. 4, 2010. U.S. patent application Ser. No. 15/424,485 also claims priority to U.S. Provisional Patent Application No. 62/290,752 filed Feb. 3, 2016, all of which are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to computerized control systems and methods for solid fuel combustion appliances, e.g., wood stoves. 
     2. Description of the Related Art 
     Wood burning stoves have a long and distinguished history for providing heating for houses and enclosures of every sort. The efficiency of such stoves has been steadily increasing in recent years, especially with the addition of catalysts to lower the burning temperature of the solid fuel. However, there still remains the possibility of peak efficiency and greater temperature control over such stoves. 
     BRIEF SUMMARY 
     A solid fuel combustion appliance, methods for operating the same, and a control system implemented by the same are provided. The appliance includes a housing defining a combustion chamber, an inlet, an outlet, and an opening. A door is operatively connected to the housing and positionable in a closed position to block the opening. An inlet damper is movable between a plurality of positions and is configured to control airflow into the inlet. A drive mechanism is operatively coupled to the inlet damper and is configured to control positions of the inlet damper. A controller is in communication with the drive mechanism and is configured to control operation of the drive mechanism. An exhaust temperature sensor is coupled to the controller and is configured to produce measurements of an exhaust gas temperature of airflow through the outlet. The controller is configured to receive measurements from the exhaust temperature sensor, determine a derivative of the exhaust gas temperature with respect to time, and compare the derivative of the exhaust gas temperature with respect to time to a predetermined threshold. The controller modulates the inlet damper in response to determining that the derivative of the exhaust gas temperature with respect to time reaches the predetermined threshold. 
     As such, the appliance, control system, and method utilize calculus computations relating to the derivative of the exhaust gas temperature with respect to time to provide more sophisticated and intelligent control over the progress of the combustion process. For instance, the predetermined threshold may be associated with a derivative value that indicates a property of the load of fuel in the combustion chamber, e.g., that the load of fuel undergoing combustion in the chamber has been successfully ignited. As such, the techniques provide control over the appliance in a way that is optimized for the particular load of fuel being consumed. The calculus computations are determined to learn about the progress of the combustion and to apply that information to the inlet damper adjustments suitable for any mode of operation. Computation of the derivative, by its nature, will accommodate many variables involved in control without reliance on preset relationships, e.g., between the damper and temperature. Accordingly, the techniques provide peak efficiency and greater temperature control. In this way, the appliance, control system, and method provide heating, while burning a load of fuel correctly and according to a manufacturer&#39;s specification. Advantages other than those described herein may be realized by these techniques. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other advantages of the disclosed subject matter will be readily appreciated, as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein: 
         FIG. 1  is a perspective view of an exemplary solid fuel combustion appliance for use with the control system and method; 
         FIG. 2  is a cross-sectional view of an exemplary solid fuel combustion appliance; and 
         FIG. 3  is an electrical block diagram of the control system. 
         FIGS. 4A and 4B  describe an operation of the control system using a flow chart and a corresponding data table. 
         FIG. 5  is an exemplary chart of a temperature of airflow through an outlet of the solid fuel combustion device with respect to time of during an operation of the control system. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to the Figures, wherein like numerals indicate like parts throughout the several views, a solid fuel combustion appliance  12 , and a control system  10  and methods employed by the same are described and shown herein. 
     The control system  10  is used in conjunction with a solid fuel combustion appliance  12 , as shown in  FIG. 1 . The appliance  12  may be alternatively referred to as a stove, a fireplace, a burner, or other name as appreciated by those skilled in the art. The solid fuel (not shown) burned with the appliance  12  may be wood, biomass, coal, charcoal, or other solid known to those skilled in the art. The solid fuel may be in log, pellet, chip, powder, briquette, or other suitable form known to those skilled in the art and typically dependent on the specific design and configuration of the appliance  12 . 
     Referring now to  FIG. 2 , the appliance  12  includes a housing  14  defining a combustion chamber  16 . The combustion chamber  16  may also be referred to by those skilled in the art as a “firebox”. The housing  14  defines an inlet  18  and an outlet  20 , each in fluidic communication with the combustion chamber  16 . The inlet  18  supplies air to the combustion chamber  16  while the outlet  20  serves to exhaust combustion gases. In the illustrated embodiment, a chimney  21  is fluidly connected to the outlet  20  to exhaust the combustion gases to atmosphere, outside of a structure (not shown) where the appliance  12  is located, as is well known to those skilled in the art. 
     The housing  14  may further define an opening  22  in fluidic communication with the combustion chamber  16 . The opening  22  may be utilized to add the solid fuel to the combustion chamber  16 . In the illustrated embodiment, as shown in  FIG. 1 , a door  24  is operatively connected to the housing  14 . For instance, the door  24  may be connected to the housing  14  with hinges (not shown). The door  24  is positionable in a plurality of positions including a closed position to block the opening  22 . The opening  22  may be completely or at least partially blocked by the door  24  depending on the design and configuration of the appliance  12 . 
     In one embodiment, the door  24  is manually opened by a user for adding solid fuel to the combustion chamber  16 . In other embodiments, the solid fuel may be added automatically. For instance, an auger (not shown) may feed the solid fuel, especially in pellet form, through the opening  22  and to the combustion chamber  16 . 
     Referring again to  FIG. 2 , the appliance  12  further includes an inlet damper  26 . The inlet damper  26  is in fluidic communication with the inlet  18  and movable between a plurality of positions for controlling the flow of air into the inlet  18  and, as such, controlling the flow of air into the combustion chamber  16 . The appliance  12  may also include an outlet damper (not shown) for closing off the outlet  20 , e.g., when the appliance  12  is not in use. 
     The appliance  12  may also include a catalyst  28  fluidically disposed between the combustion chamber  16  and the outlet  20 . As such, combustion gases pass through the catalyst  28  prior to being exhausted through the outlet. Those skilled in the art realized that the catalyst  28 , often referred to as a catalytic converter, changes the rate of the chemical reaction, which, in this case, is the combustion or burning of the solid fuel. In particular, the catalyst  28  of the appliance  12  lowers the temperature at which smoke can catch fire. The appliance  12  may further include a catalyst damper  30  to allow the combustion gases to pass through the catalyst  28  or to bypass the catalyst  28 . 
     The appliance  12  may also include a fan  32  for blowing air from the combustion chamber  16  to a space outside the housing  14 . That is, the fan  32  may blow heated air from inside the housing  14  to outside the housing  14 . Control of the fan  32  will be described in further detail hereafter. 
     Referring now to  FIG. 3 , the control system  10  includes a controller  40 . The controller  40  controls various aspects of the combustion performed by the solid fuel combustion appliance  12  as described herein. In the illustrated embodiment, the controller  40  is programmable and executes a software program. The controller  40  may be implemented as a processor, microcontroller, microprocessor, application specific integrated circuit, or other suitable device or combination of devices capable of performing the functions described herein. The control system  10  may also include an analog-to-digital converter (“ADC”) and a digital-to-analog converter (“DAC”) for converting signals as is well known to those skilled in the art. The ADC and DAC may be integrated with the controller  40  or separate therefrom. 
     The control system  10  includes at least one temperature sensor  42 . The temperature sensor  42  may be implemented as a thermocouple, a resistive temperature detector (“RTD”), infrared thermometer, or other suitable device as appreciated by those skilled in the art. The one temperature sensor  42  is coupled to the controller  40 . Typically, the temperature sensor  42  is electrically connected to the ADC, which produces a digital value corresponding to the measured temperature to the controller  40 . Of course, other techniques for transferring temperature data from the temperature sensor  42  to the controller  40  are realized by those skilled in the art. 
     The temperature sensor  42  may be implemented as an exhaust temperature sensor  42   a . The exhaust temperature sensor  42   a  measures the temperature of air exhausted through the outlet  20 . In the illustrated embodiment, the exhaust temperature sensor  42   a  is disposed in the chimney  21  adjacent the outlet  20 . However, other suitable locations for positioning the exhaust temperature sensor  42   a  will be realized by those skilled in the art. 
     Alternatively or additionally, the temperature sensors  42  may be a catalyst temperature sensor  42   b . The catalyst temperature sensor  42   b  measures the temperature of air passing through the catalyst  28 . Accordingly, the catalyst temperature sensors  42   b  is disposed adjacent to the catalyst  28  or integrated within the catalyst  28 . 
     In yet another embodiment, the temperature sensor  42  is a surface temperature sensor  42   c . The surface temperature sensor  42   c  measures the temperature of surface of the solid fuel combustion appliance  12 . Accordingly, the surface temperature sensor  42   c  is disposed onto the housing  14  of the appliance  12 . 
     The control system  10  may employ any one or more of these temperature sensors  42   a ,  42   b ,  42   c  to make determinations as described herein. 
     The control system  10  also includes a drive mechanism  44  operatively connected to the inlet damper  26 . The drive mechanism  44  controls the position of the inlet damper  26 . As just one example, the drive mechanism  44  may control the position of the inlet damper  26  at five degree increments (e.g., 0% open, 5% open, 10% open, . . . 95% open, 100% open). The drive mechanism  44  may be a motor (not separately numbered) having a mechanical linkage (not shown) to the inlet damper  26 . However, other devices may be implemented as the drive mechanism  44 . The drive mechanism  44  is in communication with the controller  40  such that the controller  40  issues commands and/or signals to the drive mechanism  44  for controlling the position of the inlet damper  26 . 
     The control system  10  may further include a detector  46  for signaling a certain condition of the solid fuel in the combustion chamber  16 . The detector  46  is in communication with the controller  40  such that the controller  40  receives a signal when the certain condition of the solid fuel is ascertained. In the illustrated embodiment, the certain condition is the addition of solid fuel. 
     The detector  46  of the illustrated embodiment is implemented as a switch  48  electrically connected to the controller  40 . In one technique, the switch  48  is coupled to the housing  14  to operatively engage the door  24  to signal when the door  24  has been opened and reclosed. The opening and reclosing of the door  24  thus signals the addition of solid fuel to the combustion chamber  16 . In another technique, the switch  48  is disposed in a position allowing the user to manually depress the switch  48 , thus signaling the addition of solid fuel to the combustion chamber  16 . In yet another technique, the switch  48  is operatively connected to the auger to sense when the auger is adding solid fuel to the combustion chamber  16 . 
     The detector  46  may be implemented with devices other than the switch  48  in other embodiments. In one example, an optical device (not shown) may be utilized to sense when the door  24  is opened and reclose or when additional solid fuel is added to the combustion chamber  16 . In another example, a capacitive sensor (not shown) may be implemented to sense the amount of solid fuel in the combustion chamber  16  and thus determine whether additional sold fuel has been added. 
     The controller  40  may also be in communication with the fan  32  for controlling operation of the fan  32 . For example, the controller  40  may operate a relay (not shown) for turning the fan  32  on and off. Alternatively, the controller  40  may be electrically connected to a motor (not shown) of the fan  32  to more precisely control the speed of the fan  32 , and thus the airflow produced by the fan  32 . 
     The control system  10  of the illustrated embodiment further includes an annunciator  50  in communication with the controller  40 . The annunciator  50  may be implemented as any device capable of providing information to the user. For instance, the annunciator  50  may be implemented as a light, a display, and/or a speaker. Those skilled in the art will realize other techniques to implement the annunciator  50 . 
     The control system  10  may further include a remote control device  52  in communication with the controller  40  such that commands and/or data may be sent back-and-forth between the remote control device  52  and the controller  40 . The communications between the controller  40  and the remote control device  52  may be implemented via radio frequency (“RF”) signals, optical signals (e.g., infrared or ultraviolet), or a combination of RF and optical signals. Those skilled in the art realize other techniques for facilitating communications between the remote control device  52  and the controller  40 . 
     The remote control device  52  allows the user to control operation of the controller  40  and to receive information from the controller  40 . The remote control device  52  of the illustrated embodiment includes a plurality of pushbuttons  54  for receiving input from the user and a display  56  for providing information to the user. Of course, other techniques for receiving input from the user and providing information to the user may alternatively be implemented. 
     In addition to or as a substitute to the remote control device  52 , the control system  10  may also include pushbuttons, switches, keypads, or other controls (none of which are shown) electrically connected to the controller  40 . For instance, DIP switches (not shown) may be mounted on a printed circuit board (not shown) which also supports the controller  40 . 
     In the illustrated embodiment, the controller  40  operates an automatic mode or a manual mode. In the manual mode, the user may control some or all of the control elements of the control system  10  manually. For example, the user may utilize the remote control device  52  to manually open and close the inlet damper  26  to maintain control over the temperature output from the appliance  12 . In the automatic mode, the controller  40  generally attempts to control for output temperature of the combustion. The mode of the controller  40  may be selected or controlled utilizing the remote control device  52 . 
       FIG. 4A  is a flow chart to describe an operation of the controller  40  in an embodiment of automatic mode. In the embodiment of automatic mode shown in  FIG. 4A , the automatic mode is divided into three main stages: (1) STARTUP, (2) REGULATION, and (3) BURNOUT. During the first stage, (1) STARTUP, solid fuel is added to the combustion chamber  16  and the combustion process begins. During the second stage, (2) REGULATION, the controller  40  actively modulates control of the inlet damper  26  to achieve a desired result. In this stage, however, the controller  40  attempts to control for an output temperature of the combustion, also known as an exhaust gas temperature of airflow through the outlet  20 , or EGT. During the third stage, (3) BURNOUT, the combustion process ends. 
     During the first stage of automatic mode, (1) STARTUP, solid fuel is added to the combustion chamber  16  and the controller  40  positions the inlet damper  26  to not only ensure full combustion of the solid fuel, but to mitigate variables contributing to an inefficient combustion. For example, during complete oxidation, almost all carbon in the solid fuel bonds with available oxygen (O 2 ), triggering a creation of carbon dioxide (CO 2 ) and ultimately, heat. In contrast, incomplete combustion occurs when there is not enough O 2  to allow the solid fuel to react completely and produce CO 2 , leaving only some O 2  to bond with carbon, creating carbon monoxide (CO). Heat produced during the creation of CO 2  is almost double that of CO, making complete oxidation a more efficient form of combustion. Therefore, to allow complete combustion of the solid fuel the controller  40  positions the inlet damper  26  to provide maximum airflow to the combustion chamber  16 , allowing a plethora of available O 2 . Furthermore, once the solid fuel begins to combust, the process may be interrupted if the inlet damper  26  closes too early and the flow of O 2  to the combustion chamber  16  is limited. When the combustion process is interrupted, complete combustion does not occur as CO levels begin to rise in the combustion chamber and the amount of heat produced declines. 
     Referring to  FIG. 4A , (1) STARTUP, features two methods of operation that ensure complete and uninterrupted combustion startup of the solid fuel in the combustion chamber  16 . In the first method of operation, represented by step  404  in  FIG. 4A , the controller  40  ensures the combustion of the fuel by controlling the drive mechanism  44  to position the inlet damper  26  at a predetermined position for a predetermined amount of time. In the second method of operation, represented by step  406  in  FIG. 4A , the controller  40  controls the drive mechanism  44  to position the inlet damper  26  at predetermined or variable positions and computes a derivative of EGT with respect to time to determine the progress of the combustion process. The derivative is the rate of change of the EGT over time, and not simply the change of the EGT. In the embodiment shown in  FIG. 4A , the controller  40  chooses between these two methods of operation by determining if the predetermined time is specified to maintain the inlet damper  26  at the predetermined position. This determination is shown in step  402  of  FIG. 4A . As shown, the step of determining the derivative occurs in response to determining that no predetermined time is specified to maintain the inlet damper  26  at a predetermined position. In other words, step  406  occurs for a variable amount of time that is not predetermined. This variable amount of time will depend on the specific characteristics/properties of the fuel undergoing combustion and therefore provides a more dynamic and higher order approach to control. 
     At step  404 , the controller  40  reacts to the certain condition of the solid fuel sensed by the detector  46 , for instance. In response to the certain condition of the solid fuel, the controller  40  controls the drive mechanism  44  to position the inlet damper  26  at a predetermined position for a predetermined period of time. The predetermined position and period of time are chosen such that a complete combustion of the solid fuel will occur when the inlet damper  26  is at the predetermined position for the period of time. For example, in one embodiment, the controller  40  controls the drive mechanism  44  to position the inlet damper  26  at a fully-open position for about one minute, ensuring a complete combustion of the solid fuel. After the predetermined period of time has expired, the controller  40  enters the second stage, (2) REGULATION, and begins to modulate the inlet damper  26 . 
     At step  406 , the controller  40  may similarly react initially to the certain condition of the solid fuel sensed by the detector  46 . In response to the certain condition of the solid fuel, the controller  40  controls the drive mechanism  44  to position the inlet damper  26  at predetermined or variable positions and computes the derivative of EGT with respect to time to determine the progress of the combustion process. Specifically, the controller  40  measures EGT, determines a derivative of EGT with respect to time, and compares the derivative of EGT to a predetermined threshold. These steps are repeated until the derivative of EGT with respect to time reaches the predetermined threshold thereby indicating a complete combustion of the solid fuel. 
     The controller  40  may employ any suitable techniques, software, programming, and/or electrical or electronic components for calculating the derivative of EGT. In one example, the signals from the temperature sensor  42  are converted into digital signals. The digital signals may be any suitable value depending on the configuration of the temperature sensor  42  or measurements derived therefrom. For example, the signals may be voltage, current, capacitance or the like. These values may be logged over time and stored in memory coupled to the controller  40 . In memory, these values may be associated with a specific EGT in a look-up table, wherein the association is based on predetermined or calibrated data about the temperature sensor  42 . The controller  40  may employ logic means or other computation programming for calculating dEGT/dt. 
     For reference,  FIG. 4B  and  FIG. 5  show an exemplary determination of the derivative of EGT with respect to time. In the embodiment shown in  FIG. 4B , the predetermined threshold is zero or approximately zero. As shown, the derivative of EGT reaches the predetermined threshold at time t=7. After the derivative of EGT with respect to time reaches the predetermined threshold, the controller  40  enters the second stage, (2) REGULATION, and begins to modulate the inlet damper  26 . 
       FIG. 5  is an exemplary chart of EGT with respect to time of the embodiment of automatic mode where the controller  40  controls the inlet damper  26  according to the second method of operation (i.e., calculating EGT).  FIG. 5  also illustrates the relationship between the three stages, (1) STARTUP, (2) REGULATION, and (3) BURNOUT. As shown in  FIG. 5 , once the derivative of EGT with respect to time reaches the predetermined threshold (shown at t=7 for example) in this embodiment, the predetermined threshold is again zero or approximately zero—the controller  40  enters the second stage, (2) REGULATION. 
     Thereafter, the controller  40  controls the drive mechanism  44  to position the inlet damper  26  to maintain a predetermined EGT. The predetermined EGT may be one temperature or a range of temperatures. For instance, in one implementation, the predetermined EGT may range from 260° C. to 280° C. As such, the controller  40  may incrementally close the inlet damper  26  as the temperature rises and approaches or exceeds 280° C. to reduce the amount of air, and consequently oxygen, that is available to the fire. Likewise, the controller  40  may incrementally open the inlet damper  26  as EGT falls and approaches or passes 260° C. In one embodiment, the controller  40  controls the inlet damper  26  in response to selecting one of a plurality of operation settings. The control to maintain EGT may be implemented with the controller  40  utilizing a closed-loop technique, such as proportional-integral (“PI”) or proportional-integral-derivative (“PID”) techniques, or the like. 
     In one embodiment, the controller  40  controls the drive mechanism  44 , during (2) REGULATION, to position the inlet damper  26  in response to the user selecting one of a plurality of operation settings where the plurality of operation settings specify a set point for EGT. The controller  40  uses this set point as the predetermined EGT for controlling the inlet damper  26 . Referring to  FIG. 5 , four operation settings of automatic mode are shown in the second stage, (2) REGULATION: “High Set Point”, “Medium Set Point”, “Low Set Point”, and “Eco Set Point”. In  FIG. 5 , the user selects the “Medium Set Point” operation setting and the controller  40  maintains EGT according to the specified set point. It is to be noted that, while  FIG. 5  represents the embodiment of automatic mode where the controller  40  controls the inlet damper  26  according to the second method of operation, the above described embodiment of the second stage, (2) REGULATION, can apply to any automated method of operation/control. 
     Other embodiments of automatic mode provide additional operation settings, selectable by the user. For instance, in a “long-burn” operation setting of automatic mode, the set point for EGT is set very low, but still high enough to support combustion. In another instance, in a “high output” operation setting of automatic mode, the set point for EGT is at or near a maximum safe operating temperature (labeled “Overheat Limit” in  FIG. 5 ). 
     During the third stage of automatic mode, (3) BURNOUT, the combustion process ends. The third stage, (3) BURNOUT, may be induced by the controller  40 , or may occur naturally. In one embodiment, the controller  40  induces the third stage, (3) BURNOUT, by restricting the amount of oxygen delivered to the combustion chamber to end the combustion process, e.g. the controller  40  may position the inlet damper  26  to a closed position. In another embodiment, the controller  40  induces the third stage, (3) BURNOUT, in response to detecting that the fuel from the combustion chamber  16  has been removed thereby ending the combustion process. In yet another embodiment of automatic mode, the combustion process ends naturally when the fuel is burned up. Any components of the control system  10  may be utilized to make determinations regarding when to trigger the (3) BURNOUT stage. 
     As previously discussed, the controller  40  is able to control the inlet damper  26  in response to a predetermined time or in response to the derivative of EGT during the first stage of automatic mode, (1) STARTUP. Furthermore, the controller  40  is able to control the inlet damper  26  to maintain a set point for EGT during the second stage of automatic mode, (2) REGULATION, and induce an end of the combustion process in the third stage, (3) BURNOUT. However, it is contemplated that, in other embodiments of automatic mode, the controller  40  may control the inlet damper  26  in accordance with at least one of a variety of other parameters and during any stage of automatic mode. The variety of other parameters may include a set point for the surface temperature of the solid fuel combustion appliance  12 , an amount of CO gas output by the solid fuel combustion appliance  12  during combustion, an O 2  concentration within the combustion chamber  16 , or an energy efficiency of the solid fuel combustion appliance  12 . It is also contemplated the user may select which of the variety of parameters the controller  40  will control for via a selectable operation setting. The control of the variety of other parameters may be implemented with a proportional-integral (“PI”) or proportional-integral-derivative (“PID”) techniques, or other suitable techniques. 
     In a specific example of one such embodiment, the controller  40  controls the drive mechanism  44  based on temperature of the room, i.e., the area outside of the appliance  12  itself. This is accomplished with a thermostat (not shown) or other device in communication with the controller  40 . Furthermore, the controller  40  may also provide for different conditions of the solid fuel. For instance, the controller  40  may include a “wet wood” automatic mode. In this mode, the controller  40  will control for a higher temperature output due to the wet nature of the solid fuel. Similarly, the controller  40  can control for a higher temperature output when compensating for larger loads. 
     In other embodiments, the controller  40  receives both the temperature of the air passing through the outlet  20  and the temperature of the air passing through the catalyst  28 . By analyzing these two temperatures, the controller  40  determines when the solid fuel is expiring. Specifically, when both temperatures fall by a predetermined amount for a predetermined period of time, the controller  40  ascertains that the solid fuel is near the end of its combustible life. In response to the solid fuel expiring, the controller  40  communicates the expiration via the annunciator  50 . For instance, in one embodiment, the controller  40  may activate an LED (not shown) affixed to the housing. Furthermore, it is to be noted that this embodiment, along with all of the previous embodiments pertaining to automatic or manual mode, can be executed using just two thermocouples. 
     Additionally, it is to be appreciated that the embodiments of automatic mode in  FIG. 5  and  FIG. 4A  have shown a sequential operation of the controller  40 . That is, the controller  40  has only been shown to transition from the first stage, (1) STARTUP, to the second stage, (2) REGULATION, and then to the third stage (3) BURNOUT. In other embodiments, the controller  40  is able to transition between the stages in a more indeterminate fashion. For example, if, during the second stage, (2) REGULATION, the detector  46  detects that additional solid fuel is added to the combustion chamber  16 , the controller  40  may transition back to the first stage, (1) STARTUP. As another example, if the user opts to turn off the solid fuel combustion appliance  12  prior to combustion, the controller  40  may transition from the first stage, (1) STARTUP, directly to the third stage, (3) BURNOUT. 
     The present invention incorporates by reference U.S. patent application Ser. No. 13/113,669 filed on May 23, 2011, which claims priority to U.S. Provisional Patent Application No. 61/351,477 filed on Jun. 4, 2010 for features disclosed therein, which are common support for the appended claims. The disclosure of U.S. patent application Ser. No. 13/113,669 filed on May 23, 2011, which claims priority to U.S. Provisional Patent Application No. 61/351,477 filed on Jun. 4, 2010 is hereby fully incorporated by reference. 
     The present invention has been described herein in an illustrative manner, and it is to be understood that the terminology which has been used is intended to be in the nature of words of description rather than of limitation. Obviously, many modifications and variations of the invention are possible in light of the above teachings. The invention may be practiced otherwise than as specifically described within the scope of the appended claims.