Abstract:
Provided herein are embodiments of a multistage gas furnace, a controller therefor and a computer-usable medium having non-transitory computer readable instructions stored thereon for execution by a processor to perform a method for operating a gas furnace. In one embodiment, the gas furnace includes: (1) a burner, (2) a circulation fan and (3) a furnace controller. The furnace controller having: (3A) an interface configured to receive heating calls and a blower control signal, the blower control signal corresponding to an operating speed of the circulation fan and (3B) a processor configured to respond to the heating calls and the blower control signal by setting and adjusting a gas input rate for the burner that is based on the blower control signal and that corresponds to a discharge air temperature determined by a dedicated discharge air sensor associated with the furnace.

Description:
TECHNICAL FIELD 
       [0001]    This application is directed, in general, to furnaces and, more specifically, to controlling the operation of furnaces. 
       BACKGROUND 
       [0002]    HVAC systems can be used to regulate the 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. Some gas furnaces are modulating or two-stage gas furnaces that can operate at different speeds compared to a single stage furnace that runs at one speed, i.e., full speed. The modulating furnaces can operate more efficiently compared to conventional single stage furnaces and reduce energy costs. 
         [0003]    In addition to modulating furnaces, some HVAC systems also use zone controls. A zone controlled system allows a user to independently control the temperature in various designated zones of an enclosure, such as a house. A zone control panel or zone controller manages the movement of conditioned air to the various zones using electronic dampers and thermostats dedicated to each of the zones. Harmony III™ Zone Control System available from Lennox Industries, Inc. of Richardson, Tex., is an example of a zoning system that manages the distribution of conditioned air to designated zones. 
       SUMMARY 
       [0004]    In one aspect, the disclosure provides a controller for a multistage gas furnace having a circulation fan and a dedicated discharge air sensor associated therewith. In one embodiment, the controller includes: (1) an interface configured to receive heating calls and a blower control signal, the blower control signal corresponding to an operating speed of the circulation fan; and (2) a processor configured to respond to the heating calls and the blower control signal by setting a gas input rate for the furnace that is based on the blower control signal and that corresponds to a discharge air temperature determined by the discharge air sensor. 
         [0005]    In another aspect, the disclosure provides a computer-usable medium having non-transitory computer readable instructions stored thereon for execution by a processor to perform a method for operating a gas furnace. In one embodiment the method includes: (1) igniting the furnace in response to receiving a heating call, (2) operating the circulation fan at an operating speed according to a blower control signal, (3) setting a gas input rate for the furnace that is based on the blower control signal and that corresponds to a discharge air temperature determined by a discharge air sensor associated with the furnace and (4) adjusting the gas input rate to maintain the discharge air temperature in response to the blower control signal. 
         [0006]    In yet another aspect, the disclosure provides a multistage gas furnace. In one embodiment, the gas furnace includes: (1) a burner, (2) a circulation fan and (3) a furnace controller. The furnace controller having: (3A) an interface configured to receive heating calls and a blower control signal, the blower control signal corresponding to an operating speed of the circulation fan and (3B) a processor configured to respond to the heating calls and the blower control signal by setting and adjusting a gas input rate for the burner that is based on the blower control signal and that corresponds to a discharge air temperature determined by a dedicated discharge air sensor associated with the furnace. 
     
    
     
       BRIEF DESCRIPTION 
         [0007]    Reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
           [0008]      FIG. 1  illustrates a diagram of an embodiment of a furnace constructed according to the principles of the disclosure; 
           [0009]      FIG. 2  illustrates a block diagram of an embodiment of a controller constructed according to the principles of the disclosure; and 
           [0010]      FIG. 3  illustrates a flow diagram of an embodiment of method of operating a furnace carried out according to the principles of the disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    In a zone controlled system, a zone controller generates a blower control signal to control the operating speed of a circulation fan. As such, the blower control signal is used to control the blower volume (e.g., cubic feet per minute (CFM)) of the circulation fan. The blower control signal typically changes in a zone controlled system when demand changes to different or more zones. For example, a thermostat in a first zone may demand heat. As such, the furnace initiates and provides heat for the first zone based on the heating call and an operating speed for the circulation fan for the first zone. A thermostat for a second zone then demands heat. Accordingly, the furnace continues to provide heat with an operating speed of the circulation fan based on the blower control signal generated by the zone controller for both the first and second zones. The blower control signal is typically an electrical signal generated by a zoning control panel in response to thermostat demands from different zones. The blower control signal can be an analog or a digital signal. Considering the Harmony III™ Zone Control System, a pulse width modulated (PWM) signal is used for a blower control signal and a change in the duty cycle of the PWM signal indicates a change in the operating speed of the circulation fan. In other embodiments, the blower control signal can be a data signal including a messaging protocol signal, such as a controller area network (CAN) signal, or an output of a transducer. 
         [0012]    Disclosed herein is a heating system configured to maintain a discharge air temperature of a furnace by adjusting the gas input rate thereof. By maintaining the discharge air temperature through adjusting the gas input rate, the furnace can prevent or at least reduce the occurrence of overshooting a desired temperature in a particular zone. In one embodiment, a control scheme for a furnace is disclosed that reacts to a first stage heating call W1 by determining a gas input rate for a discharge air temperature based on a blower control signal. The gas input rate can be determined from a pre-programmed look-up table that corresponds to an operating point (or targeted discharge air temperature) based on the blower volume requested by the blower control signal. In one embodiment, the operating point is determined from a predefined temperature rise of return air to supply air, such as a midpoint of the temperature rise. A single table or multiple tables may be used when determining the gas input rate. 
         [0013]    In response to a second heating call W2, the gas input rate can be increased after a delay timer expires and reduced after expiration of the delay timer when the W2 heating call is not present. In one embodiment, the W2 heating call input may not be connected to the furnace controller. In this embodiment, the furnace is prevented from increasing the input rate within the same duty cycle heating call. In such embodiments, the firing rate of the furnace can be adjusted for a given duty cycle to provide a hotter discharge air temperature. Embodiments of a controller, such as an integrated furnace controller, a heating system and a method of operating a furnace based on maintaining a discharge air temperature are disclosed herein. 
         [0014]      FIG. 1  is a block diagram of an embodiment of a heating system  100  constructed according to the principles of the disclosure. The heating system  100  includes a furnace  101 , a zone controller  190 , a discharge air sensor (DAS)  194 , and thermostats  196 ,  198 . The heating system  100  includes two operating zones, Zone 1 and Zone 2. As such, the heating system  100  is a zoned system. One skilled in the art will understand that the principles of the disclosure also apply to heating systems having more than two zones. 
         [0015]    The furnace  101  is a combustible fuel-air burning furnace, such as, a natural gas furnace or a propane furnace. The furnace  101  may be for a residence or for a commercial building (i.e., a residential or commercial unit). Additionally, the furnace  101  is a two-stage furnace having a two-stage control that uses a two-stage adjustment of the furnace input rate in response to changes in the heating load. Based on thermostat demand, the two-stage control cycles the burners between a reduced heat input rate and off or between the maximum heat input rate and off. 
         [0016]    The furnace  101  includes a burner assembly  110 , a heat exchanger  120 , an air circulation fan  130 , a combustion air inducer  140 , a low pressure switch  152 , a high pressure switch  154 , a low fire gas valve  162 , a high fire gas valve  164  and a furnace controller  170 . Portions of the furnace  101  may be contained within a cabinet  180 . In some embodiments, the furnace controller  170  may also be included in the cabinet  180 . One skilled in the art will understand that the furnace  101  may include additional components and devices that are not presently illustrated or discussed but are typically included in a furnace. 
         [0017]    The burner assembly  110  includes a plurality of burners that are configured for burning a combustible fuel-air mixture (e.g., gas-air mixture) and provide a combustion product to the heat exchanger  120 . The heat exchanger  120  is configured to receive the combustion product from the burner assembly  110  and use the combustion product to heat air that is blown across the heat exchanger  120  by the indoor circulation fan  130 . The indoor circulation fan  130  is configured to circulate air through the cabinet  180 , whereby the circulated air is heated by heat exchanger  120  and supplied to conditioned space. The combustion air inducer  140  is configured to supply combustion air to the burner assembly  110  by an induced draft and is also used to exhaust products of combustion from the furnace  101 . The indoor circulation fan  130  and the inducer  140  are each operable in at least two speed settings corresponding to the at least two modes of operation (i.e., two stages of operation) of the furnace  101 . 
         [0018]    The low pressure switch  152  and the high pressure switch  154  measure combustion air pressure on the discharge side of the combustion air inducer  140 . One skilled in the art will understand the operation and configuration of these pressure switches and that pressure may also be measured at other points in the heat exchanger  120  or as a differential pressure across a flow limiting orifice in the heat train. 
         [0019]    The furnace  101  is a multi-stage or variable input furnace operable in at least two modes of operation (e.g., low fire and high fire modes). Assuming two stages or two modes of operation, the furnace  101  also includes the low fire gas valve  162  and the high fire gas valve  164 . In low fire operation, only the low fire gas valve  162  is opened to supply fuel to burner assembly  110 . In high fire operation, both the low fire gas valve  162  and the high fire gas valve  164  are open to supply more fuel to burner assembly  110 . One skilled in the art will understand that more gas valves and/or a different combination or arrangement of gas valves may be employed to supply fuel for multiple operation stages. 
         [0020]    The furnace controller  170  is configured to control the operation of the furnace  101  including the combustion air inducer  140  and the indoor circulation fan  130 , respectively. Additionally, furnace controller  170  controls operation of the low fire gas valve  162 , the high fire gas valve  164  including controlling the gas input rate to the burner  110 . In some embodiments, the furnace controller  170  may include a designated burner control board and an air blower control board for controlling the gas valves  162 ,  164 , the combustion air inducer  140  and the indoor circulation fan  130 . In other embodiments, the burner control board and the air blower control board may be physically separated from each other or the furnace controller  170  with the furnace controller  170  communicating therewith to control operation of the gas valves  162 ,  164 , the combustion air inducer  140 , and the indoor air circulation fan  130 . As such, the furnace controller  170  may be an integrated controller or a distributed controller that directs operation of the furnace  101 . 
         [0021]    The furnace controller  170  is configured to adjust the gas input rate of the furnace  101  to maintain a discharge air temperature of the furnace  101 . Thus, in one embodiment, the furnace controller  170  can be configured to adjust the gas input rate of the furnace  101  to hold the discharge air temperature at a designated temperature. As such, the furnace  101  can reduce overshooting a desired temperature in a designated zone of an conditioned space. 
         [0022]    The furnace controller  170  may include an interface to receive heating calls and a blower control signal, and a processor, such as a microprocessor, to direct the operation of the furnace  101 . In one embodiment, the furnace controller  170  is installed without a second heating call W2 connection but instead employs a single heating call to direct the operation of the furnace  101 . 
         [0023]    The furnace controller  170  may include a memory section having a series of operating instructions stored therein that direct the operation of the furnace controller  170  (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. For example, the algorithms can implement the method illustrated in  FIG. 3 . The furnace controller  170  also includes or communicates with a delay timer. The delay timer can be a conventional clock that can be reset and can be used to keep track of a designated amount of time that is used to allow settling of discharge air temperatures. In some embodiments, the designated amount of time is two minutes. As illustrated in  FIG. 1 , the furnace controller  170  is coupled to the zone controller  190 , the DAS  194 , the thermostats  196 ,  198  and components of the furnace  101 . In some embodiments, the connections therebetween are through a wired-connection. A conventional cable and contacts may be used to couple the furnace controller  170  to the various components of the furnace  100 . In some embodiments, a wireless connection may also be employed to provide at least some of the connections. 
         [0024]    The DAS  194  may be a conventional temperature sensor configured to determine the ambient temperature of the area where positioned and provide this temperature data to the furnace controller  170  to use in directing the operation of the furnace  100 . The DAS  194  is a temperature sensor that is designated and positioned to determine the discharge air temperature of the furnace  101 . In  FIG. 1 , the DAS  194  is located in the cabinet  180 . In other embodiments, the DAS  194  can be positioned in other locations to measure the discharge air temperature of the furnace  100 . For example, the DAS  194  can be positioned in a duct (not illustrated) between the cabinet and the conditioned space. In some embodiments, multiple temperature sensors can be used and an average discharge air temperature determined therefrom. The discharge air sensor  194  can be, for example, a 10 k Negative Temperature Coefficient (NTC) sensor. 
         [0025]    The thermostats  196 ,  198 , can be a conventional thermostats employed in HVAC systems that generate heating calls based on temperature settings. Each of the thermostats  196 ,  198 , is a user interface that allows a user to input a desired temperature for a designated area or zone of the conditioned space. Thermostat  196  is designated for Zone 1 and thermostat  198  is designated for Zone 2. In addition to being connected to the furnace controller  170 , the thermostats  196 ,  198 , are also connected to the zone controller  190  in a typical conventional configuration. 
         [0026]    The zone controller  190  is configured to manage conditioned air for designated zones of a conditioned space. A zone is a portion of a HVAC system that includes at least one demand unit, such as the furnace  101 , and includes at least one user interface, such as the thermostat  196  or  198 . The zone controller  190  operates electronic dampers (not illustrated) to control air flow to Zone 1 and Zone 2 of the conditioned space. The zone controller  190  generates a blower control signal to request a blower volume for the circulation fan  130 . In some embodiments, the zone controller  190  is configured to provide greater air flow to Zone 1 than Zone 2 to compensate for greater heating load or air flow requirements. As such, the blower control signal requests a greater blower volume with respect to a heating call for Zone 1 than for Zone 2. The zone controller  190  can be a conventional controller for delivering conditioned air to designated zones of a conditioned space. For example, the zone controller  190  can be a Harmony III™ Zone Controller. 
         [0027]      FIG. 2  illustrates a block diagram of an embodiment of a furnace controller  200  constructed according to the principles of the disclosure. The furnace controller  200  is configured to direct the operation of or at least part of the operation of a furnace, such as the furnace  101 . As such, the furnace controller  200  is configured to generate control signals that are transmitted to the various components to direct the operation thereof. The furnace controller  200  may generate the control signals in response to feedback data and/or operating data that is received from various sensors and/or components of a heating system, such as a DAS, thermostats, zone controllers or comfort sensors. For example, furnace controller  200  can generate a control signal to operate the gas valves of a furnace to control the gas input rate to a burner of the furnace. The furnace controller  200  includes an interface  210  that is configured to receive and transmit the feedback data, operating data and control signals. The operating data received by the interface  210  includes a discharge air temperature and a blower control signal. The interface  210  may be a conventional interface that is used to communicate (i.e., receive and transmit) data for a controller, such as a microcontroller. 
         [0028]    The furnace controller  200  also includes a processor  220  and a memory  230 . The memory  230  may be a conventional memory typically located within a controller, such as a microcontroller, that is constructed to store data and computer programs. The memory  230  may store operating instructions to direct the operation of the processor  220  when initiated thereby. The operating instructions may correspond to algorithms that provide the functionality of the operating schemes disclosed herein. For example, the operating instructions may correspond to the algorithm or algorithms that implement the method illustrated in  FIG. 3 . The processor  220  may be a conventional processor such as a microprocessor. The interface  210 , processor  220  and memory  230  may be coupled together via conventional means to communicate information. The furnace controller  200  may also include additional components typically included within a controller for a furnace, such as a power supply or power port. 
         [0029]    The memory  220  is configured to store gas input rates with respect to blower volumes represented by blower control signals. The gas input rates correspond to discharge air temperatures of the furnace. In one embodiment, the gas input rates correspond to a predefined temperature rise, such as a midpoint thereof, based on the blower volume requested per the blower control signal. The stored values can be pre-programmed in the memory  220  during manufacturing or installation and can be based on the model or type of furnace. A table or tables, such as a look-up table, may store the various gas input rates and corresponding blower volumes. 
         [0030]    The processor  230  is configured to operate the furnace by controlling the gas input rate to maintain a discharge air temperature for the furnace in response to the received heating calls and the blower control signal. In one embodiment, the processor  230  is configured to operate the furnace according to the method illustrated in  FIG. 3 . 
         [0031]      FIG. 3  illustrates a flow diagram of a method  300  of operating a furnace carried out according to the principles of the disclosure. The furnace controller  170  of  FIG. 1  or the furnace controller  200  of  FIG. 2  may be used to perform the method  300 . The method  300  includes adjusting the input rate of the furnace to maintain a discharge air temperature. The method  300  begins in a step  305 . 
         [0032]    In a step  305 , a heating call is received. The heating call can be a conventional request for a first stage of heat initiated by, for example, a thermostat of a HVAC system. Typically, the heating call for the first stage of heat is represented by W1. The thermostat can be associated with a zone controller of the HVAC system. 
         [0033]    A temperature stage flag for the HVAC system is set to a first stage in a step  310 . The temperature stage flag is set to the first stage to indicate a first discharge air temperature. In one embodiment, the temperature stage flag is set in response to the heating call. In one embodiment a furnace controller sets the temperature stage flag to zero for the first discharge air temperature. 
         [0034]    In a step  315 , the ignition sequence for the furnace is initiated. The ignition sequence for the furnace can be a conventional start-up sequence for a furnace. For example, considering a first stage (i.e., W1) heating call, circuits of the furnace associated with a first operating stage are activated. 
         [0035]    The circulation fan begins to ramp-up in a step  320  according to a received blower control signal. The blower control signal represents a requested blower volume, e.g., CFM, for the circulation fan. The blower control signal is typically an electrical signal such as a pulse width modulated signal generated by a zone controller. 
         [0036]    In a step  325 , a discharge air temperature is selected based on the blower control signal and a gas input rate for the furnace is selected that corresponds to the discharge air temperature. A particular gas input rate for a designated discharge air temperature can be determined during manufacturing of the furnace and calibrated during installation with respect to a particular discharge air sensor that is used to determine the discharge air temperature of the furnace. A table can be used to store the various gas input rates with related blower volumes and discharge air temperatures. The actual discharge air temperature is determined by a discharge air sensor associated with the furnace. The gas input rate can change to maintain or hold the discharge air temperature. This changing of the gas input rate corresponds to an inner control loop of cascaded control loops. The outer control loop of the cascaded control loops is selecting the discharge air temperature from the blower control signal (e.g., per the requested blower volume). 
         [0037]    In a first decisional step  330 , a determination is made if the blower control signal has changed by a predetermined amount. For example, a determination is made if the operating speed or blower volume of the circulation fan has changed by a predetermined amount. In one embodiment, determining a change is based on the duty cycle of the blower control signal. In some embodiments, a determination is made if the duty cycle has changed by a certain percentage, such as five percent as indicated in the illustrated embodiment. Thus, for example, the circulation fan may initially operate at a duty cycle of 60 percent in step  320  and in step  330 , a determination is made if the duty cycle of the circulation fan has changed to more than 65 percent. 
         [0038]    If the blower control signal has changed by the predetermined amount, e.g., greater than five percent of the duty cycle, the method  300  returns to step  325  wherein the gas input rate is set or adjusted to correspond to a discharge air temperature determined by the blower control signal that has changed (e.g., a new discharge air temperature per the outer control loop). The gas input rate is selected to maintain or hold the new discharge air temperature (i.e., per the inner control loop). For example, a new discharge air temperature is selected based on the blower control signal change greater than the predetermined amount and adjustment of the gas input rate is performed to maintain this new discharge air temperature per the discharge air sensor. 
         [0039]    If the blower control signal has not changed by the predetermined amount, a determination is made in a second decisional step  335  if a second stage heating call (W2) is present. If so, the method  300  continues to a third decisional step  340  where a determination is made if the gas input rate is set at its maximum value. As such, a determination is made if gas input rate is set at 100 percent of its determined rate. If so, the method  300  continues to step  330 . If not at 100 percent, the method  300  continues to a fourth decisional step  345  where a determination is made if the discharge air temperature (i.e., targeted discharge air temperature) has been increased to a second discharge air temperature. In one embodiment, this determination is made based on if the temperature stage flag is set to one as noted in step  345  of  FIG. 3 . In some embodiments, multiple iterations could occur wherein the gas input rate is increased. As such, the temperature stage flag could be ignored. 
         [0040]    If so, the method  300  continues to step  330 . If not, a determination is made in the fifth decisional step  350  if a delay timer has expired. 
         [0041]    If not, the method  300  continues to step  330 . If the delay timer has expired, the gas input rate is adjusted to an amount that corresponds to a predetermined increase of the discharge air temperature in a step  355 . In one embodiment, the predetermined increase of the discharge air temperature can be within a range of two to five degrees. For example, if the first discharge air temperature is seventy degrees Fahrenheit (e.g., temperature stage flag=0) and the predetermined temperature increase is 2 degrees Fahrenheit, the gas input rate can be increased to a rate that corresponds to a second discharge air temperature of seventy two degrees Fahrenheit (e.g., temperature stage flag=1). 
         [0042]    In a step  360 , the temperature stage flag is set to one to indicate the second discharge air temperature. The delay timer is then reset in a step  365 . In one embodiment, the delay timer is set to two minutes as indicated in step  365  of  FIG. 3 . The delay timer can be located internally with a furnace controller or associated therewith. From step  365 , the method  300  continues to step  330 . 
         [0043]    Returning now to the second decisional step  335 , if the second stage heating call W2 is not present, a determination is then made in a sixth decisional step  370  if the first stage heating call W1 is present. If not, the method  300  continues to step  375  where the call for heat and the method  300  ends. In furnace installations wherein the W2 heating call is not connected to the furnace controller, then the second heating call W2 will not be present at step  335 . As such, in these installations the method  300  would always continue to step  370  from step  335 . 
         [0044]    If the first stage heating call is present, a determination is made in the seventh decisional step  380  if the discharge air temperature is the first discharge air temperature. As illustrated in step  380  of  FIG. 3 , this determination is made in one embodiment by determining if the temperature stage flag is set to zero. 
         [0045]    If so, the method continues to step  330 . If not, the method  300  continues to an eighth decisional step  385  and a determination is made if the delay timer has expired. If not, the method  300  continues to step  330 . If so, the gas input rate is adjusted to correspond to a predetermined reduction of the discharge air temperature in step  390 . In some embodiments, the gas input rate is adjusted (e.g., reduced) to reduce the discharge air temperature by two to five degrees as represented by “X” in step  390  of  FIG. 3 . In some embodiments, the gas input rate can be adjusted as in step  390  when determining that the discharge air temperature is the first discharge air temperature in step  380 . 
         [0046]    The temperature stage flag is then set to indicate the first discharge air temperature in a step  395 . The method  300  then continues to step  365 . 
         [0047]    The above-described methods may be embodied in or performed by various conventional digital data processors, microprocessors or computing devices, wherein these devices are programmed or store executable programs of sequences of software instructions to perform one or more of the steps of the methods, e.g., steps of the method of  FIG. 3 . The software instructions of such programs may be encoded in machine-executable form on conventional digital data storage media that is non-transitory, e.g., magnetic or optical disks, random-access memory (RAM), magnetic hard disks, flash memories, and/or read-only memory (ROM), to enable various types of digital data processors or computing devices to perform one, multiple or all of the steps of one or more of the above-described methods, e.g., one or more of the steps of the method of  FIG. 3 . Additionally, an apparatus, such as dedicated furnace controller, may be designed to include the necessary circuitry or programming to perform each step of the method of  FIG. 3  and include a memory to store a data table for obtaining operating values such as gas input rates. 
         [0048]    Those skilled in the art to which this application relates will appreciate that other and further additions, deletions, substitutions and modifications may be made to the described embodiments.