Abstract:
A method is disclosed for operating a gas-fired heater to maintain temperature within a zone. The gas-fired heater is modulated between a higher firing rate and a lower firing rate within a pseudo steady-state mode until a current firing rate exceeds a predetermined maximum time period t trans . The gas-fired heater is then modulated between an updated higher firing rate and an updated lower firing rate within a transient mode until an updated current firing rate exceeds a predetermined maximum time period t diag . Finally, the higher firing rate and the lower firing rate are redefined in a diagnostic mode until the gas-fired heater returns to the pseudo steady-state mode.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to heat modulation of a gas-fired heater, particularly a heater suitable for installation outdoors. This invention relates to digital heat modulation to incrementally modulate a heat input to a gas-fired heater by independently controlling and operating at least one solenoid valve to activate or deactivate a corresponding burner, such as an in-shot burner. The digital heat modulation method and apparatus of this invention can be easily adapted to receive one or more input data signals from a conventional single-stage or two-stage thermostat, so that a control algorithm of a modulator can provide an output signal to digitally control heat modulation.  
           [0003]    2. Description of Related Art  
           [0004]    Conventional outdoor or rooftop heating units are sized to a building heating design load. According to heating, ventilation and air-conditioning (HVAC) design practice, a heating unit preferably has a maximum capacity greater than the building heating design load. Generally, a rooftop heating unit is oversized 1.2 times to 1.7 times the building heating design load. An oversized heating unit responds quickly to a thermostat set point from a much lower set point condition, such as those associated with operation during evenings, weekends, and other unoccupied times.  
           [0005]    A building heating design load includes an amount of heat needed to warm outside air that is mixed with return air, to ventilate the building. Increasing requirements and expectations for indoor air quality may require an HVAC system to introduce more outside air to a building. The amount of outside air introduced to a rooftop heating unit can range from about 20% to about 35% of the total air flow through the rooftop heating unit.  
           [0006]    Many conventional rooftop heating units have a constant volume operation for controlling air flow to satisfy indoor air quality requirements. In a constant volume operation, a supply blower runs continuously in an on mode, regardless of whether the rooftop heating unit burners are firing.  
           [0007]    As a result of the percentage of outside air introduced into the rooftop heating unit and constant volume operation, vent outlet air temperatures may drop quickly during off-cycle periods and may discomfort many occupants. To prevent these temperature fluctuations that may discomfort occupants, the heat input of conventional rooftop heating units is modulated.  
           [0008]    In many conventional rooftop heating units, the heat input is adjusted by modulating a main gas valve. Thus, all burners of the rooftop heating unit are modulated simultaneously. This modulation approach limits turndown to about 3:1. With a turndown of about 3:1, excess combustion air is significantly increased and thus decreases the rooftop heating unit efficiency. To achieve a turndown of about 3:1 and to maintain efficiency these approaches require a multi-speed inducer fan to control excess combustion air. Further, if excess combustion air is controlled to maintain a constant air-to-fuel ratio, as the rooftop heating unit is turned down, the combustion products may condense in the heat exchanger or may condense in unintended portions of the heat exchanger. To avoid this condensation of combustion products and the subsequent corrosion damage to the heat exchanger requires a multi-speed indoor air blower to control condensation.  
           [0009]    To provide some degree of heat modulation many conventional rooftop units use a two-stage main gas valve and are controlled by either a single-stage or two-stage thermostat. Conventional rooftop units equipped with a two-stage main gas valve can operate the burners at a full firing rate, at approximately 70% of the full firing rate and in an off condition, to maintain set points and to provide more continuous heat input to the rooftop heating unit while satisfying thermostat set points.  
           [0010]    However, recognizing that for most operating hours of a unit the building load is less than 50% of the full firing rate, a rooftop heating unit with a two-stage main gas valve, which can only reduce the unit firing rate to about 70% of the full firing rate, will often provide heat input well above the heat load requirement. Therefore, to meet the heating load requirements, a rooftop heating unit will cycle between the on mode and the off mode, with the off-cycle periods increasing as the heating load decreases. As a result, many conventional rooftop heating units with a two-stage main gas valve do not improve the comfort level of the air circulated through the conditioned space of the building.  
           [0011]    There is an apparent need for an outdoor or rooftop heating unit that reduces fluctuations in the supply air temperature to improve the comfort level of the air circulated through the conditioned building space.  
           [0012]    It is also apparent that there is a need for a heat modulation method that incrementally modulates the heat input to a gas-fired heater for better control of the supply air temperature.  
         SUMMARY OF THE INVENTION  
         [0013]    It is an object of this invention to provide a gas-fired heater having a heat modulation device that independently controls the activation of in-shot burners to modulate a heat input to a gas-fired heater over a wide range of overall firing rates.  
           [0014]    It is another object of this invention to provide a heat modulation device that incrementally modulates a heat input to a gas-fired heater by independently operating solenoid valves to activate and deactivate corresponding in-shot burners.  
           [0015]    It is another object of this invention to provide a heat modulation device that controls the activation or deactivation of a plurality of in-shot burners based only on feedback from a single-stage thermostat.  
           [0016]    It is another object of this invention to provide a heat modulation device that manages the feedback from a single-stage thermostat, the initiation of the electronic ignition system of a gas-fired heater, the activation or deactivation of the main gas or combination gas valve of a gas-fired heater, and the activation or deactivation of independently operating solenoid valves.  
           [0017]    It is another object of this invention to independently and/or sequentially control activation of a plurality of in-shot burners and to control a firing rate of at least one in-shot burner.  
           [0018]    It is yet another object of this invention to control the amount of excess air in the gas-fired heater with a multi-speed inducer fan or with another flow restriction device.  
           [0019]    The above and other objects of this invention are accomplished with a gas-fired heater, for example an outdoor or rooftop heater, having a plurality of burners, for example in-shot burners, each corresponding to a discrete section of a heat exchanger. The burners can have either approximately equal firing rates or different firing rates. In one embodiment of this invention, at least one burner has a variable firing rate.  
           [0020]    Each burner is in fluidic communication with a fuel supply which furnishes a fuel to each burner. Within the burner the fuel is mixed with some portion of the air needed for complete combustion. Flames issue from the burners, mix with at least the remaining portion of air needed for complete combustion, and enter into the heat exchanger sections releasing heat and combustion products into the heat exchanger sections.  
           [0021]    An induced draft fan, activated by a modulation controller, is preferably mounted to communicate with the combustion heat exchanger. The induced draft fan draws the combustion products through the heat exchanger and discharges the combustion products to the atmosphere.  
           [0022]    A pressure switch mounted upstream of an induced draft fan or a centrifugal switch attached to the induced draft fan is responsive to a pressure or a rotational speed, respectively, within a range of normal operation. A pressure or rotational speed within a range of normal operation causes a pressure switch or centrifugal switch to electrically energize an electronic ignition system.  
           [0023]    Once energized, an electronic ignition system electrically communicates with an ignition source or sources near one or more of the burners or near a pilot burner, the main gas valve or combination gas valve including a pilot valve section and a flame sensing device. An electronic ignition system safely and reliably lights the burners and any pilot burner.  
           [0024]    The gas-fired heater has a supply blower which draws air from both the conditioned space of the building and the outside air. The blower moves the air over the heat exchanger. The heat exchanger transfers heat by convection and/or conduction to the air. The heated air is forced through a conduit, a duct system for example, and circulated throughout the conditioned space of a building.  
           [0025]    At least one valve, such as a solenoid valve is positioned with respect to a corresponding burner. Each valve is independently controlled and/or moved between an open position and a closed position, to control fuel flow from the fuel supply to the corresponding burner.  
           [0026]    A modulator electrically communicates with each valve and emits a signal that is used to control movement, if any, of each valve, such as between an open position and a closed position. The modulator of this invention incrementally modulates the heat input rate to the gas-fired heater by independently moving at least one valve to the open position or the closed position.  
           [0027]    A single-stage or two-stage thermostat, preferably a single-stage thermostat, electrically communicates with the modulator to provide feedback on the heat input rate by closing the thermostat circuit to signal that the heating load is not met or by opening the thermostat circuit to signal that the heating load is met.  
           [0028]    In a method for modulating the heat input to the gas-fired heater, the modulator emits a control signal, preferably but not necessarily a dedicated signal, to each solenoid valve to independently operate or control each solenoid valve, such as between the open position and the closed position. With the solenoid valve in the open position, the fuel flows from the fuel supply to the corresponding burner. The modulator can also activate any burner by emitting a control signal to ignite and combust or burn the fuel. Additional solenoid valves can be independently or collectively operated or controlled to move from the closed position, which prevents or restricts fluidic communication between the fuel supply and the corresponding burner, to an open position allowing fluidic communication between the fuel supply and the corresponding burner. The dedicated signal selectively activates the corresponding burner. Thus, the heat input to the gas-fired heater can be incrementally modulated.  
           [0029]    The modulator of this invention uses a control algorithm that can receive a signal emitted from a conventional single-stage or two-stage thermostat and in response emit one or more control signals to one or more of the burners and to an electronic ignition system, to digitally control modulation. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]    The drawings show different features of a gas-fired heater having a modulation device for controlling a heat input to the gas-fired heater, according to different embodiments of this invention, wherein:  
         [0031]    [0031]FIG. 1 is a schematic view of a gas-fired heater, according to one preferred embodiment of this invention;  
         [0032]    [0032]FIG. 2 is a schematic diagram of a gas-fired heater with control valves in parallel, according to one preferred embodiment of this invention;  
         [0033]    [0033]FIG. 3 is a graphical representation of a firing input as a function of time, for the gas-fired heater shown in FIG. 2;  
         [0034]    [0034]FIG. 4 is a schematic diagram of a gas-fired heater having control valves in series, according to another preferred embodiment of this invention;  
         [0035]    [0035]FIG. 5 is a graphical representation of a firing input as a function of time, for the gas-fired heater as shown in FIG. 4;  
         [0036]    [0036]FIG. 6 is a schematic diagram of a gas-fired heater with control valves in parallel and with an intermittent tube pilot, according to another preferred embodiment of this invention;  
         [0037]    [0037]FIG. 7 is a graphical representation of a firing input as a function of time, for the gas-fired heater shown in FIG. 6;  
         [0038]    [0038]FIG. 8 is a flow diagram of a main control loop of an algorithm for a modulator, according to one preferred embodiment of this invention;  
         [0039]    [0039]FIG. 9 is a flow diagram of a pseudo-steady-state mode of an algorithm for a modulator, according to one preferred embodiment of this invention;  
         [0040]    [0040]FIG. 10 is a flow diagram of a transient mode of an algorithm for a modulator, according to one preferred embodiment of this invention; and  
         [0041]    [0041]FIG. 11 is a flow diagram of a diagnostic routine of an algorithm for a modulator, according to one preferred embodiment of this invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0042]    A gas-fired heater  10 , for example an outdoor or rooftop heater as shown in FIG. 1, comprises a plurality of burners  15 , such as in-shot burners. As used throughout this specification and in the claims, the term burner is intended to relate to an in-shot burner and/or any other suitable burner for a gas-heater, as known to those skilled in the art of furnace design. In one preferred embodiment of this invention, burners  15  have approximately equal firing rates. For example, gas-fired heater  10  may have an overall or total firing rate of about 100,000 Btu/hr with four burners  15  each having a firing rate of about 25,000 Btu/hr. In another preferred embodiment of this invention, burners  15  may have different firing rates. For example, one burner  15  may have a firing rate of about 20,000 Btu/hr and another burner  15  may have a firing rate of about 30,000 Btu/hr, without effecting the total firing rate of gas-fired heater  10 . In one preferred embodiment of this invention, at least one burner  15  has a variable firing rate. According to this invention, a variable firing rate of each burner  15  can be adjusted or controlled periodically to operate at different firing rates.  
         [0043]    As shown in FIG. 1, each burner  15  is in fluidic communication with and receives fuel from a fuel supply  20 . Fuel supply  20  provides a fuel, preferably but not necessarily natural gas or propane, to each burner  15  wherein the fuel is mixed with a portion of the air needed for complete combustion. Flames issue from each burner  15 , mix with at least a remaining portion of the air needed for complete combustion, and enter into heat exchanger  37  releasing heat and combustion products into heat exchanger  37 .  
         [0044]    In one preferred embodiment of this invention, heat exchanger  37  comprises a plurality of heat exchange tubes  38 . Preferably, but not necessarily, each heat exchange tube  38  has a generally circular cross-section. Heat exchange tube  38  may have any suitable shape and/or cross-section known to those skilled in the art. Preferably, but not necessarily, each heat exchange tube  38  is bent along a longitudinal axis of heat exchange tube  38 , for example to form an S-shape. In one preferred embodiment of this invention, each heat exchange tube  38  is dedicated to a corresponding burner  15 , wherein each heat exchange tube  38  is positioned with respect to and in communication with the corresponding in-shot burner  15  to transfer heat from the corresponding in-shot burner  15 . Preferably, but not necessarily, a manifold  40  is in communication with an output end portion of each heat exchange tube  38 .  
         [0045]    An induced draft fan  42  draws combustion products through each heat exchange tube  38  and manifold  40 . Induced draft fan  42  discharges the combustion products to the atmosphere or to any suitable environmental system or apparatus. In one preferred embodiment of this invention, in response to a demand signal from a thermostat or other control device, modulator  30  emits a signal to activate induced draft fan  42 . A sensor switch  43  that is responsive to some physical characteristic indicative of normal operation of induced draft fan  42 , such as pressure in manifold  40  or rotational speed of induced draft fan  42 , energizes an electronic ignition system  50 .  
         [0046]    Once energized, an electronic ignition system  50  electrically communicates with an ignition source  46 , a main gas valve  45 , which preferably includes a valve section to directly and independently supply pilot burner  18 , and a flame detector  48 . An electronic ignition system  50  activates an ignition source  46  located near the outlet of one of the burners  15  or pilot burner  18  and then activates main gas valve  45  to release gas to burners  15  or pilot burner  18 . The gas, mixed with some portion of the air needed for complete combustion, issues from each of burners  15  or pilot burner  18  and is ignited by ignition source  46 . Electronic ignition system  50  monitors flame detector  48 , which is positioned in at or near the flame issuing from burners  15  or pilot burner  18  to ensure that a flame is established at burners  15  or pilot burner  18 . For the case in which electronic ignition system  50  first activates main gas valve  45  to release gas to pilot burner  18  and then monitors flame detector  48  to ensure that a flame is established at pilot burner  18 , electronic ignition system  50  then activates main gas valve  45  to release gas to burners  15 . Electronic ignition system  50  will keep main gas valve  45  activated to release gas to burners  15  or pilot burner  18  as long as flame detector  48  emits and acceptable signal.  
         [0047]    Gas-fired heater  10  further comprises a supply blower  35 . Preferably, but not necessarily, supply blower  35  draws air from within a conditioned space of the building and the atmosphere and moves return air over or across heat exchanger  37 . As the air moves across heat exchanger  37 , heat is transferred from heat exchanger  37  by convection and/or conduction. Heated air  36  is forced through a duct system, for example, and circulated throughout the conditioned space of the building.  
         [0048]    In one preferred embodiment of this invention, gas-fired heater  10  further comprises at least one control valve  25 , such as a solenoid valve, as shown in FIG. 1. As used throughout this specification and in the claims, the term valve is intended to be interchangeable with the terms control valve, solenoid valve or any other type of valve that can be controlled, as known to those skilled in the art. Each valve  25  controls at least one corresponding burner  15 . Preferably, each valve  25  is positioned upstream from the corresponding burner  15 . Valve  25  is moveable between a fully open position, a partially open position and a closed position to control fuel flow from main gas valve  45  to the corresponding burner  15 . In the open position, valve  25  allows fuel flow from main gas valve  45  and the corresponding burner  15 . In the closed position, valve  25  prevents or restricts fluidic communication between main gas valve  45  and the corresponding burner  15  and thus prevents the corresponding burner  15  from firing or reduces the firing rate of burner  15 .  
         [0049]    In one preferred embodiment of this invention, one burner  15 ′ has no corresponding valve  25  positioned upstream, as shown in FIG. 1. As a result, this particular burner  15 ′ continuously fires when gas valve  45  is open and fuel flows to burner  15 ′. In one preferred embodiment of this invention, at least two burners  15 ′ have no valve  25  positioned upstream to control fuel flow to burner  15 ′.  
         [0050]    As shown in FIG. 1, gas-fired heater  10  further comprises a modulator or modulating device  30 . Preferably, but not necessarily, modulator  30  is a digital modulator or is digitally operated. Modulator  30  is in electrical communication with and can receive a signal, such as a temperature indication signal, from a thermostat  60  and/or any other suitable temperature feedback mechanism known to those skilled in the art. Modulator  30  is in electrical communication with induced draft fan  42  to activate or deactivate induced draft fan  42 . Modulator  30  is in electrical communication with each valve  25  to electronically control and/or operate movement of each valve  25  independently between the open position, the partially open position and the closed position. Modulator  30  of this invention incrementally modulates the heat input rate of gas-fired heater  10  by independently operating at least one valve  25  to move to the open position, the partially open position or the closed position.  
         [0051]    The term incrementally modulate as used throughout this specification and in the claims refers to modulating the heat input of gas-fired heater  10  by either opening or closing one or more valves  25  in response to a demand signal from the thermostat or other temperature feedback mechanism or control device. As valves  25  are opened or closed to maintain the set point, the corresponding burners  15  are activated or deactivated, respectively. The incremental modulation of the heat input rate of gas-fired heater  10  may occur in positive increments or negative increments. The number of increments depends upon the number of independently controllable valves  25  of gas-fired heater  10  and the desired firing rates of corresponding burners  15 .  
         [0052]    In one preferred embodiment of this invention, modulator  30  comprises a control logic and/or algorithm having adaptive controls and/or parameters related to thermostatic operations. In a first mode, modulator  30  receives feedback or the demand signal from a thermostat, such as either a single stage, a multi-stage, or a zone temperature sensor, which is processed to adaptively control the heat input of gas-fired heater  10 . In a second mode, modulator  30  receives information from the thermostat or the zone temperature sensor and information from an on board temperature sensor and/or sensors internal to gas-fired heater  10 , which is processed by modulator  30 , for example to calculate a rate of temperature change within a conditioned space. The control logic and/or algorithm interprets the feedback information to toggle or increment between in-shot burners  15  firing to control heat input. Modulator  30  then adaptively controls the heat input of gas-fired heater  10  to the conditioned space, accordingly.  
         [0053]    In one preferred embodiment of this invention, a control algorithm provides digital modulation control as a function of one or more demand signals received from a conventional single-stage thermostat. The control algorithm of this invention can adapt to both microelectronic and electromechanical thermostats. In another embodiment, a control algorithm operates using a signal from a two-stage thermostat. Both control algorithms of this invention provide digital control as a function of relatively recent historical information of the operation of gas-fired heater  10 . FIG. 8 shows a basic flow diagram for such control algorithms.  
         [0054]    A conventional single-stage thermostat or any other conventional temperature feedback mechanism sends a signal to a conventional rooftop unit. An operator sets thermostat  60  to a particular set point in order to maintain a defined zone at a desired temperature. If the zone temperature is above a first temperature, then thermostat  60  emits an off signal. If the zone temperature is below a second temperature which is lower than the first temperature, then thermostat  60  emits an on signal. A hysteresis band, usually a few degrees Fahrenheit, is established between the first temperature and the second temperature. With microelectronic thermostats, the hysteresis band varies as a function of time. With electromechanical thermostats, an anticipator can be used to alter the effect of the hysteresis band, for example to minimize overshoot.  
         [0055]    In one embodiment of this invention, as shown in FIG. 8, the control algorithm includes a main control loop with three different modes of operation: a pseudo-steady-state mode  100 , as shown in detail in FIG. 9; a transient mode  200 , as shown in detail in FIG. 10; and a diagnostic mode  300 , as shown in detail in FIG. 11.  
         [0056]    [0056]FIG. 9 shows a flow diagram for pseudo-steady-state mode  100 , according to one embodiment of this invention. During usual operating hours, a digitally modulating rooftop unit will operate in pseudo-steady-state mode  100 . Pseudo steady state refers to a smooth operation and interaction between gas-fired heater  10  and the zone, for example when the zone has no significant load changes. In pseudo-steady-state mode  100 , modulator  30  can operate burners  15  in a relatively constant fashion, periodically and repetitively operating one burner  15  or a same or different group of burners  15  on and off as dictated by thermostat  60  positioned in the zone, thereby satisfying the zone load.  
         [0057]    In pseudo-steady-state mode  100 , a certain number of burners  15  are constantly on during an entire on/off cycle. This particular firing rate is called a lower firing rate and these particular burners  15  fire when thermostat  60  calling for no heat. Under conditions of low heating load the lower firing rate may be zero and no burners  15  fire when thermostat  60  is calling for no heat.  
         [0058]    When the zone temperature falls below a set point, thermostat  60  emits a demand signal to modulator  30  calling for heat. Modulator  30  then steps up the firing rate to a higher firing rate by turning on an additional burner  15  or an additional set of burners  15 . As thermostat  60  cycles between a demand signal for heat and a demand signal for no heat, modulator  30  toggles between the higher firing rate and the lower firing rate, respectively.  
         [0059]    For some applications, especially those with an electromechanical thermostat  60 , a step between the lower firing rate and the higher firing rate may include several firing rate increments to provide better control. The step number refers to the number of firing rate increments between the lower firing rate and the higher firing rate.  
         [0060]    [0060]FIG. 10 shows a flow diagram for transient mode  200  of operation. In transient mode  200 , the control algorithm of this invention handles relatively large changes in zone load or set point, which pseudo-steady-state mode  100  cannot follow. When operating in pseudo-steady-state mode  100 , if modulator  30  senses no change in the demand signal within a prescribed time period t trans , then modulator  30  enters transient mode  200 . A value for a t trans  can be set at any suitable time period, for example at 15 minutes.  
         [0061]    Once in transient mode  200 , modulator  30  follows one of two routines, depending on the higher firing rate or the lower firing rate.  
         [0062]    If modulator  30  operates at the higher firing rate, modulator  30  presumes that the zone receives insufficient heat. Modulator  30  attempts to correct by increasing to a next higher firing rate, as shown in step  210  of FIG. 10.  
         [0063]    Modulator  30  then waits for another prescribed time period t diag , during which if thermostat  60  is satisfied, as shown in step  220  of FIG. 10, modulator  30  defines the higher firing rate and the lower firing rate as one increment higher than the previous values. Modulator  30  then returns to pseudo-steady-state mode  100 , as shown in FIG. 10, and resumes toggling between the new lower firing rate and the higher firing rate. However, if during time period t diag  thermostat  60  is not satisfied, modulator  30  assumes that relatively larger load changes have occurred over a relatively short time period and modulator  30  then proceeds to diagnostic mode  30 .  
         [0064]    If modulator  30  operates at the lower firing rate, modulator  30  presumes that the zone is receiving too much heat. As shown in step  260  of FIG. 10, modulator  30  attempts to correct by decreasing to the next step of the firing rate. Modulator  30  then waits for another time period t diag , during which if thermostat  60  is not satisfied, as shown in step  270  of FIG. 10, modulator  30  redefines the higher firing rate and the lower firing rate as one increment lower than the previous values. Modulator  30  then returns to pseudo-steady-state mode  100 , as shown in FIG. 10, and resumes toggling between the new lower firing rate and the higher firing rate. However, if during time period t diag  thermostat  60  remains satisfied, modulator  30  presumes that relatively larger load changes have occurred over a relatively short time period and modulator  30  enters diagnostic mode  300 .  
         [0065]    [0065]FIG. 11 shows a flow diagram for a diagnostic routine of the control algorithm according to one embodiment of this invention. Diagnostic mode  300  responds to relatively larger and relatively faster changes in load requirements, as compared to transient mode  200 . While operating in transient mode  200 , if modulator  30  senses no change in the demand signal from thermostat  60  within a second time period t diag , then modulator  30  enters diagnostic mode  300 .  
         [0066]    In diagnostic mode  300 , modulator  30  drives the firing rate over many increments, such as from a full firing rate to an off condition, and then estimates a new higher firing rate and lower firing rate that roughly bracket a new zone load. Modulator  30  returns to pseudo-steady-state mode  100  with the new higher firing rate and the new lower firing rate. Once returned to pseudo-steady-state mode  100 , the system dynamics will tune modulator  30  to the load.  
         [0067]    Once in diagnostic mode  300 , from transient mode  200 , modulator  30  follows one of two routines, each which depends upon recent history of operation of gas-fired heater  10 . If modulator  30  operates at the higher firing rate, then the zone is not heated enough. Modulator  30  meets the higher load requirement as quickly as possible by activating all burner states or firing at a full rate until thermostat  60  is satisfied. For each present thermostat cycle modulator  30  records a duration of each half of the thermostat cycle. As shown in step  370  of FIG. 11, modulator  30  then returns to the last higher firing rate until thermostat  60  again emits a signal calling for heat. When thermostat  60  calls for heat, modulator  30  activates all burner states. Once thermostat  60  is satisfied at the end of such cycle, modulator  30  calculates a time-weighted average of the firing rate for this cycle.  
         [0068]    Modulator  30  uses an average firing rate to select a burner state associated with the next greater firing rate. Modulator  30  then resets the higher firing rate to this particular burner state and resets the lower firing rate to a step below this particular burner state. Modulator  30  then returns to pseudo-steady-state  100  mode and resumes toggling between the new lower rate and the new higher rate.  
         [0069]    If modulator  30  is operating at the lower firing rate, the zone is overheated and modulator  30  meets the lower load as quickly as possible by deactivating all valves  25  or by going to a full off condition, until thermostat  60  again calls for heat, as shown by step  310  in FIG. 11.  
         [0070]    For the present thermostat cycle, modulator  30  will record a duration of each half of the thermostat cycle. Modulator  30  then returns to the last lower firing rate until thermostat  60  is satisfied. Once thermostat  60  is satisfied, modulator  30  deactivates all valves  25 . When thermostat  60  calls for heat at the end of this cycle, modulator  30  calculates a time-weighted average of the firing rate for this particular cycle. Modulator  30  uses this average firing rate to select a burner state associated with the next lesser firing rate. Modulator  30  resets the lower firing rate to this particular burner state and resets the higher firing rate to a step above this particular burner state. Modulator  30  then returns to pseudo-steady-state mode  100  and resumes toggling between the new lower firing rate and the new higher firing rate.  
         [0071]    As shown in FIG. 11, diagnostic mode  300  also has a startup calibration routine  390 . Modulator  30  can go into startup calibration routine  390  if a substantial time period has passed since the system has been in a heating mode or after a particular event, such as a power failure. Startup calibration routine  390  can satisfy the load quickly and provide a reasonable starting place for pseudo-steady-state mode  100 .  
         [0072]    Startup calibration routine  390  can adapt a digital modulating system to its application, which is advantageous because a thermostat sensitivity and response to operation of gas-fired heater  10  may differ from one application to another. Some factors affecting thermostat sensitivity and system response include thermostat position, thermostat type, zone size, zone height, and the number of digital states. The adaptation is achieved by varying the number of steps between the higher firing rate and the lower firing rate. Regarding diagnostic mode  300  and transient mode  200 , one step in the firing rate is assumed to be between the higher firing rate and the lower firing rate.  
         [0073]    As shown in FIGS. 2 and 4, carry-over wings  62  positioned between parallel burners  15  can be used to ensure cross-lighting of adjacent burners  15 , particularly in-shot burners. An electronic ignition system can be used with flame sensor  48  located at burners  15 ′, the gas flow to which is controlled only by main combination gas valve  45 , and ignition source  46  located at an opposite end of burners  15 . Through a process referred to as the ignition detection mode, the burner control valves  25  and the main combustion gas valve  45  are controlled so that for every change in the burner state, the entire burner system is shut down. Then, as soon as possible, the ignition and proof of flame sequence is started, the flame is proven at a fill fire rate, and then modulator  30  can deactivate one or more valves  25  or burners  15  to achieve the desired burner state. FIG. 3 shows a graphical representation of a firing input as a function of time, with a 65% load.  
         [0074]    [0074]FIG. 4 shows burners  15  arranged in series and having carry-over wings  62  to ensure cross-lighting of adjacent burners  15 . Electronic ignition system  50  is used with a flame sensor  48  located near burners  15 ′, the gas flow to which is controlled only by main combination gas valve  45 , and ignition source  46  located at an opposite end of burners  15 . Through a process referred to as ignition detection mode, burner control valves  25  and main combination gas valve  45  are controlled, so that for every increase in the burner state, the entire burner system is shut down. Then, as soon as possible, the ignition and proof of flame sequence is started, the flame is proven at full fire, and then modulator  30  can deactivate one or more burners  15 , to achieve a desired burner state. FIG. 5 shows a graphical representation of a firing input as a function of time, assuming a 65% load.  
         [0075]    In a preferred embodiment for the ignition system arrangement, FIG. 6 shows burners  15  arranged in parallel, which can be used with or without carry-over wings  62 . An intermittent tube pilot burner system is used with flame sensor  48  and ignition source  46  which are located at opposite ends of a tube pilot burner  18 . In this configuration burner control valves  25  can be controlled independently of main gas valve  45  so that changes in the burner state can be made without shutting down the entire burner system.  
         [0076]    Referring to FIG. 1, in a method for modulating the heat input to gas-fired heater  10 , modulator  30  preferably but not necessarily emits a dedicated control signal to each valve  25 . The dedicated control signal or signals emitted from modulator  30  independently operates or controls each valve  25  to move at least one valve  25  between the open position, the partially open position and/or the closed position. With valve  25  in the open position, fuel from fuel supply  20  flows to corresponding in-shot burner  15 . Additional valves  25  can be independently operated or controlled, for example in response to the demand signal, to move from a closed position, which prevents or restricts fuel flow between fuel supply  20  and the corresponding burner  15 , to an open position allowing fuel flow between fuel supply  20  and the corresponding burner  15 . The dedicated signal selectively activates the corresponding burner  15  to produce heat and combustion products. Thus, the heat input of gas-fired heater  10  can be incrementally modulated.  
         [0077]    For example, gas-fired heater  10  as shown in FIG. 1 has five burners  15  that are activated to fire at approximately equal firing rates for allowing gas-fired heater  10  to operate at a total firing rate of 100%. Preferably, but not necessarily, one burner  15 ′ is not controlled by a corresponding valve  25  and thus fires at a constant firing rate of about 20% of the total firing rate. Modulator  30  selectively deactivates one burner  15  by operating corresponding solenoid valve  25  to move corresponding valve  25  to the closed position, preventing fluidic communication between fuel supply  20  and one burner  15 . With one burner  15  deactivated, gas-fired heater  10  operates at about 80% of the total firing rate of gas-fired heater  10 . Similarly, an additional burner  15  can be selectively deactivated. As a result, gas-fired heater  10  operates at about 60% of the total firing rate of gas-fired heater  10 . Selectively deactivating an additional burner  10  reduces the firing rate of gas-fired heater  10  to about 40% of the total firing rate. An additional burner  15  may be deactivated to operate gas-fired heater  10 , for example with only in-shot burner  15 ′, at about 20% of the total firing rate.  
         [0078]    In one preferred embodiment of this invention, a flame carry over mechanism is positioned between each of burners  15 , to ensure that each corresponding burner  15  ignites when valve  25  is open. In one preferred embodiment of this invention, burners  15  are activated in a specific sequence to ensure proper carry over. However, this sequential activation does not inhibit the ability to modulate the heat input over a wide range.  
         [0079]    In another preferred embodiment of this invention, the activated burners  15  have different firing rates. In yet another preferred embodiment of this invention, at least one burner  15  has a firing rate that varies over a time period. Thus, the heat input of gas-fired heater  10  can be incrementally modulated more precisely or at a larger number of increments.  
         [0080]    While in the foregoing specification this invention has been described in relation to certain preferred embodiments, and many details are set forth for purpose of illustration, it will be apparent to those skilled in the art that this invention is susceptible to additional embodiments and that certain of the details described in this specification and in the claims can be varied considerably without departing from the basic principles of this invention.