Abstract:
Methods and Systems for controlling boiler systems are disclosed. In one illustrative embodiment, a derivative action control is used reduce the likelihood of overshoot from a newly activated boiler. When a newly activated boiler becomes active, the boiler is held at a low firing rate for a predetermined period of time. The predetermined period of time may be cut short or even entirely eliminated under certain conditions. The methods and devices are further adapted for use in multi-stage boiler systems. In one embodiment, only the first stage of a multi-stage boiler system that becomes active is held at the low firing rate.

Description:
FIELD 
     The present invention is related to the field of heating and/or hot water boilers, and more particularly, to methods and devices for controlling multi-stage boiler systems. 
     BACKGROUND 
     Recent innovations in boiler technologies have led to the introduction of modular boiler systems making use of several small boilers for applications where, in the past, a single, larger boiler may have been used. Such modular systems are often adaptable for changing uses over time when, for example, an addition may necessitate greater boiler capacity than that originally needed in a building. 
     One of the challenges with multi-stage systems such as modular systems is the need to meet changing heat loads over time in a stable and efficient manner. In a given system, different times of the day may require different amounts of heat production. For example, given a relatively simple example of a three boiler system, during setback periods (e.g. night), only one of the boilers may be needed to satisfy the heat load of the building. During a warmup period (e.g. early morning) following a setback period, all three boilers may be needed, while during ordinary operation (e.g. late morning), only two boilers may be needed, and during light ordinary operation (e.g. mid-afternoon) a single boiler may be sufficient. 
     Control of such boiler systems can be further complicated by the relative efficiencies of certain boilers. For example,  FIG. 1  is an illustrative graph of efficiency data for an example commercial boiler. It can be seen that efficiency may improve as return water temperature drops, while efficiency may drop as the percentage of total output capacity increases. It should be noted that a minimum firing rate is also sometimes needed for stability and safety purposes. Meanwhile, difficulties can arise with return water temperatures at low firing rates, as explained by Pouchak, et al., in U.S. Pat. No. 6,694,927, which is incorporated herein by reference. 
     When a system operates with a relatively light heat load, the characteristics of the system and boilers can create difficulties or inefficiencies. Often, the built-in deadband of a system creates a delay between an increase in load and an increase in system capacity. For example, if all boilers are off and a call for heat occurs in a lightly loaded situation, the deadband typically causes the system to wait before turning on a first boiler. By the time the first boiler comes on, however, system temperatures may be relatively far from their setpoints, and the firing rate of the first boiler turned on will quickly ramp up. If the heat load is small, however, the load can be quickly met and the boiler turned off. This cycle is inefficient and may create undesired system temperature variations. 
     SUMMARY 
     The present invention, in an illustrative embodiment, includes methods of controlling boilers newly added to the operating set of a multi-boiler system. In a first embodiment, a derivative action control is used reduce the likelihood of overshoot. In another embodiment, when a boiler is brought online, a newly active boiler is held at a low firing rate for a predetermined period of time. In a yet another illustrative embodiment, the predetermined period of time may be cut short under certain conditions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a graph of efficiency data for an example commercial boiler; 
         FIG. 2  is a schematic diagram of an illustrative high efficiency condensing boiler; 
         FIG. 3  is a schematic diagram of an illustrative boiler system incorporating more than one boiler; 
         FIGS. 4A-4B  show a block diagram for an illustrative boiler control system; 
         FIG. 5  is a block diagram for an illustrative firing rate control for a boiler; 
         FIG. 6  shows in block form an illustrative stage control; and 
         FIGS. 7-8  show illustrative configurations of system/stage control for a boiler system. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description should be read with reference to the drawings. The drawings, which are not necessarily to scale, depict illustrative embodiments and are not intended to limit the scope of the invention. 
     Copending U.S. patent application Ser. No. 10/809,115, entitled MULTI-STAGE BOILER STAGING AND MODULATION CONTROL METHODS AND CONTROLLERS, published as US 2005-0230490 A1, is incorporated herein by reference. Copending U.S. patent application Ser. No. 10/809,116, entitled COLD WATER BYPASS AND FIRING RATE CONTROL, now U.S. Pat. No. 6,904,874, is also incorporated herein by reference. These patent applications illustrate some interface and control methods for individual and multi-stage boiler systems. U.S. Pat. Nos. 6,536,678 and 6,694,927 illustrate several example boiler control systems and methods, and are also incorporated herein by reference. 
       FIG. 1  is a graph of efficiency data for an example commercial high efficiency condensing boiler. It can be seen that as input water temperatures drop, efficiency generally improves. Further, as the firing rate drops, efficiency also improves. Due to exhaust/ventilation needs, however, the boiler may have a minimum firing rate level. For the commercial boiler of  FIG. 1 , a minimum firing rate occurs at about 25% of heat output capacity. Reducing blower output, fan speeds, and gas feed below this level can create potential hazards including the poor venting and accumulation of fumes resulting from spent fuel. A minimum firing rate is typically set for the boiler such that there is a minimum heat output for the boiler. 
       FIG. 2  is a schematic diagram of a high efficiency condensing boiler. The system includes a pump  12  for moving water through a primary heat exchanger  14 . Flue gasses  17  from the primary heat exchanger  14  are directed to a secondary heat exchanger  16  placed in the return water path for the system. Firing rate and flue gas flows are controlled using a combustion/purge blower  18 . The firing rate may also be controlled using various valves for controlling fuel flow, fuel/air mixers, etc. (not shown). 
     A bypass valve  20  is used to direct output water back to the return flow before it enters the primary heat exchanger  14 , as needed, to keep the temperature of fluid entering the primary heat exchanger  14  above a predefined set point. Several sensors  22 ,  24 ,  26  may be provided to relay signals to a controller  28 . The controller  28  may control several system components including the pump  12 , blower  18 , and bypass valve  20 , and other boiler components such as gas valves, ignition controls, fuel/air mixers, etc. 
     The combustion/purge blower  18  may be a variable frequency drive unit that can provide a variable firing rate for the boiler. When the variable frequency drive of the blower  18  operates at an increased level, the blower  18  increases the heating output of the boiler and forces a greater amount of flue gasses  17  to the secondary heat exchanger  16 . The blower  18  works in conjunction, typically, with other devices for mixing air and fuel, controlling ventilation, etc. 
     During operation, the secondary heat exchanger  16  typically warms inlet water before it enters the primary heat exchanger  14  by heat exchange with flue gasses  17 . As the flue gasses  17  pass through the secondary heat exchanger  16 , condensate can form inside the secondary heat exchanger  16 . The secondary heat exchanger  16  is usually designed or adapted to handle condensation without becoming damaged. 
     For the purposes herein, the sensor that senses the fluid temperature prior to the secondary heat exchanger  16  may be referred to as the inlet sensor  24 . The sensor that senses the fluid temperature of the mix of circulating fluid coming from the secondary heat exchanger  16  and fluid passed back through the bypass valve before it enters the primary heat exchanger  14  may be referred to as the bypass sensor  26 . The sensor that senses the fluid temperature after the primary heat exchanger  14  may be referred to as the output sensor  22 . 
     If the temperature sensed by the bypass temperature sensor  26  is too low, condensation may occur in the primary heat exchanger  14 . Such condensation may occur when the bypass temperature falls below one-hundred thirty degrees Fahrenheit (fifty-five degrees Celsius), and some times blow around one-hundred twenty degrees Fahrenheit (forty-nine degrees Celsius). Such condensation in the primary heat exchanger can, in some cases, damage the primary heat exchanger  14 . As such, when the bypass temperature sensor  26  senses a temperature that is approaching a predefined bypass temperature limit, the bypass valve  20  may open to a greater degree to allow heated output water to feed back to the return flow and warm the temperature sensed at the bypass temperature sensor  26 . If the bypass valve  20  is already fully open, the firing rate of the boiler may also be adjusted to increase the heat output of the boiler, and thus the temperature of the water that is fed back to the return flow. Copending U.S. patent application Ser. No. 10/809,116, entitled COLD WATER BYPASS AND FIRING RATE CONTROL, now U.S. Pat. No. 6,904,874, which is expressly incorporated herein by reference, illustrates various methods for overcoming low bypass temperatures and avoiding condensation in the primary heat exchanger  14 . 
       FIG. 3  is a schematic diagram for a boiler system incorporating more than one boiler. The illustrative system  100  includes a number of modular boilers  102  which may, in terms of the system  100 , be referred to as “stages” of the system  100 . In the illustrative embodiment, each boiler  102  includes a boiler control  104 , which is coupled to a corresponding temperature sensor  106  for monitoring fluid temperatures to and from the corresponding boiler  102 . In the illustrative embodiment, the boiler controls  104  are connected via a bus (E-bus) to a boiler system controller  108 . A pump  110  may be provided separate of the internal pumps of the modular boilers  102  to help provide consistent fluid flow regardless of how may of the boiler stages  102  are currently active. If desired, the boiler system controller  108  may also be coupled to and help control the pump  110 , if desired. 
     In the illustrative embodiment, the boiler system controller  108  is coupled to a system supply temperature sensor  112  and a system return temperature sensor  114 . The outputs of either or both of the sensors  112 ,  114  may be used by the boiler system controller  108  to determine the present load conditions. Various other sensors may also be used independently or in conjunction with sensors  112 ,  114 . Typically, the system return temperature sensor  114  provides a return temperature signal that can be compared to a desired return temperature set point, and the operation of the system  100  may be adjusted by the boiler system controller  108  to meet the desired return temperature set point. 
     In the illustrative embodiment, the boiler system controller  108  sends both enable/disable (or active/inactive) control signals as well as an output capacity or firing rate signal to each of the boiler controllers  104  via the E-bus. When the boiler system controller  108  calls for a selected boiler  102  to be active, the corresponding boiler controller  104  activates the selected boiler  102 . Once activated, the circulating fluid enters the selected boiler  102 , is heated, and is pumped out by the pump  110 . While a parallel configuration is illustrated in  FIG. 3 , any of a number of parallel, series, or combination parallel/series boiler configurations may be used and still be within the scope of the present invention. 
       FIGS. 4A-4B  show a block diagram of an illustrative boiler control system. The illustrative boiler control system is divided into two main blocks, a system control block  150  and a stage control block  152 . In the system control block  150 , a system sensor block  154  provides a sensed temperature to temperature control block  156 , which may be configured to calculate an error signal, which may relate, for example, a proportional and integral derivative error. Alternatively, or in addition, the temperature control block may simply look up an error signal via a look-up table or the like. 
     In the illustrative embodiment, the temperature control block  156  sends a percent error or demand signal to an analog stage control block  158  and a stager block  160 , as shown. The analog stage control block  158  generates a percent demand signal that is sent to the stager block  160  as well as a system derivative action block  162 . The stager block  160  determines how many stages should be called, and determines whether to add or remove stages in response to the error signal. Some illustrative staging methods are shown in copending U.S. patent application Ser. No. 10/809,115, entitled MULTI-STAGE BOILER STAGING AND MODULATION CONTROL METHODS AND CONTROLLERS, published as US 2005-0230490 A1. 
     The illustrative embodiment also shows that the stager block  160  may create a signal indicating whether a new stage should be called, and may provide that signal to the system derivative action block  162 . In one illustrative embodiment, the stager block  160  only provides this signal if a transition from no active stages to one active stage occurs. If the system derivative action block  162  is enabled, and determines that limited firing rates may be appropriate, the percent control signal received from the analog stage control block  158  may be modified before it is sent to the network interface block  164 . Illustrative methods for making such a determination are further explained below with reference to  FIGS. 5 and 6 . 
     In one illustrative configuration of the system, the system derivative action block  162  may generate a signal to reduce the percent control signal received from the analog stage control block  158  to a level that is selected to limit the firing rate of a newly activated stage. In another illustrative configuration, the system derivative action block  162  may not be enabled, and the control signal received from the analog stage control block  158  is passed unchanged to the network interface block  164 . 
     If a runtime equalization setting is desired, a sequencer block  166  may identify which stages to call/activate/enable to equalize runtime among the stages. The sequencer block  166  also may send a signal to the network interface block  164  indicating how many and which stages to activate/enable. If a other settings are desired the sequencer block  166  may enable such settings, for example to identify which stages to activate or de-activate to create first-on/first-off or first-on/last-off sequencing. 
     The network interface block  164  of the system block  150  may be coupled to a network interface block  168  in each boiler stage  152 . In some embodiments, there may be several boiler stage blocks  152  connected to a single system block  150 . The connections may be serial or parallel connections, as desired. One illustrative system makes use of an Echelon Lonworks™ compatible bus, which is a relatively simple two wire bus system. 
     In the illustrative embodiment, the network interface block  168  in boiler stage  152  is coupled to a stage on/off network mode block  170  and a modulation rate control block  172 . The stage on/off network mode block  170  controls whether the boiler stage  152  is on or off. The stage on/off network mode block  170  also provides a signal to the flame safety control block  174 , which in turn operates and monitors an ignition source  176  and a gas valve  178 . 
     The stage on/off network mode block  170  may also provide a signal to a stage derivative action block  180 . In one illustrative embodiment, the stage derivative action block  180  is disabled and, the system derivative action block  162  of system control block  150  may control stage firing rates by, for example, artificially lowering the analog stage control signal as desired or when needed. 
     In some cases, however, the derivative action block  180  may be enabled. In such cases, signals may be taken from the stage on/off network mode block  170  to determine whether the boiler stage  152  is newly called and, optionally, whether the stage  152  is a first-called stage. If the derivative action block  180  determines that derivative action is proper under the circumstances (again, examples of the decisions made in the derivative action block  180  are explained below with reference to  FIGS. 5 and 6 ), then the derivative action block  180  may, for example, check a stage configuration signal to determine whether the timer  186  is enabled, as well as determine the settings for derivative action since these may also be configured by a user. The derivative action block  180  may also receive signals from a temperature sensor  184  to determine if derivative action block  180  should be disabled due to temperature variations in the circulating/working fluid of the boiler system. Also, the derivative action block  180  may check a timer  186  to determine if the period for action has expired. 
     Some example conditions where derivative action is enabled include: operation within a set time period after a first stage is added along with operation while temperatures of the circulating/working fluid of the boiler system are not dropping quickly. If the conditions for action are all met, then the derivative action block  180  may send a signal to logic block  188  indicating that the firing rate should remain low. This signal is used to override the signal sent to logic block  188  from modulation rate control block  172  to provide a desired heat output level or firing rate. 
     In one embodiment, the boiler stage  152  may further include a bypass control system  190  that provides one or more signals to both a cold water draw block  192  and the stage on/off network mode block  170 . The bypass control system  190  is further illustrated in  FIG. 4B , which is further described below. If desired, the stage on/off network mode block  170  may disable the derivative action block  180  if commanded by the bypass control system  190 . Alternatively, the bypass control system  190  may signal to logic block  188 , via the cold water draw block  192 , that the derivative action block  180  may be overridden. Logic block  188  makes the determination of what the variable firing rate demand  194  should be to control the firing rate of the stage  152 . 
     As noted, a first source for the logic block  188  may be the modulation rate control block  172 , which provides a heat load signal related to the load signal generated by the analog stage control  158  as modified (potentially) by the system derivative action  162 . The signal from block  172  may be overridden or modified by either the bypass control system  190  or the derivative action block  180 , when enabled. In one embodiment, the bypass control system  190  receives highest priority because it is responsible for protecting the boiler or stage  152  from damage caused by, for example, condensation in the primary heat exchanger. 
       FIG. 4B  shows several features of an illustrative bypass temperature control block  190 . In the illustrative embodiment, a sensed bypass temperature  200  is compared to a set point (not shown) and/or sent to a lookup table  202  (this may include calculating a rate or integral error on the bypass temperature  200 , if desired), and the resulting signal or signals are sent to a bypass temperature control block  204  which is adapted to determine the desired position of the bypass valve  208 . The bypass temperature control  204  receives configuration settings  206  that may include the current position of the bypass valve  208 , system operation history analysis, system settings, user preferences, and/or any other settings, parameters and/or characteristics, as desired. 
     If the bypass temperature  200  is in a range where adjustment of the bypass valve  208  is desired, the bypass temperature control  204  may send a signal to change the position of the bypass valve  208 . The signal for changing the position of the bypass valve  208  may also be received and recorded by a position accumulator  210 . The position accumulator  210  may provide a signal that indicates a current position of the bypass valve  208 . For example, the position accumulator  210  may indicate that the bypass valve  208  is open by a certain percentage, or by some other scale as desired. 
     If the bypass temperature control  204  determines that the bypass temperature  200  is well above a temperature where condensation in the heat exchanger is likely to occur, the bypass temperature control  208  may call for the bypass valve  208  to partly or fully close. On the other hand, if the bypass temperature  200  is or has been dropping into a range where a higher bypass temperature would be desirable to help prevent condensation in the primary heat exchanger, the bypass temperature control  204  may call for opening the bypass valve  208  further from its current position. If the bypass valve  208  is already fully open, the bypass temperature control  204  may call for an increase in the stage firing rate to increase the temperature of the water that is fed back to the water return. 
     In one embodiment, as explained in co-pending U.S. patent application Ser. No. 10/809,116, entitled COLD WATER BYPASS AND FIRING RATE CONTROL, now U.S. Pat. No. 6,904,874, the bypass temperature control may include or operate in parallel with an inlet temperature sensor. In such a method, if the inlet temperature drops below a predefined level, then the firing rate may be raised right away, rather than waiting for the bypass valve to open completely. The call for increased firing rate, as further explained above and below, may override a derivative action call for reduced firing rate. 
       FIG. 5  is a flow diagram showing an illustrative method for performing firing rate control for a boiler that is adapted to limit the firing rate of the boiler during a time period after a stage is first activated. The control method is illustrated as a number of steps, and may be implemented in several ways by allocating control steps to different parts of a boiler system including a system controller and/or a stage controller. Two illustrative allocations of control using both a system controller and a stage controller are illustrated in  FIGS. 7-8 . 
     The method shown in  FIG. 5  is repetitively executed at a predetermined interval, each time beginning at start block  250 . The predetermined interval may vary as desired. In some embodiments, the predetermined interval is chosen to be shorter than a staging/modulation control interval at which overall staging and modulation are calculated and performed. For example, in one embodiment, the predetermined interval may be about five seconds, while the interval for the overall staging/modulation control may be about fifteen seconds. Alternatively, the overall staging may have a first interval, modulation may have a second interval, and the firing rate control may have a third interval shorter than the first and/or second intervals. In an illustrative embodiment, the first interval is about fifteen seconds, the second interval is about five seconds, and the third interval is about five seconds or less. Other values may be used, as desired. As the intervals decrease in time, the system accuracy may increase, however, efficiency may be reduced due to accelerated cycling. 
     Control is passed from the start block  250  to a first decision block  252 , which checks whether derivative action is enabled. If derivative action is not enabled (DAEnable=0), control is passed to block  254  which allows the normal firing rate to be used. Control is passed from block  254  to end block  256 , and the method is terminated until the beginning of the next interval. 
     Referring back to decision block  252 , if derivative action is enabled, the stage request is read as shown at  258 . Next, control is passed to block  260 , which determines if the system is transitioning from no active stages to one active stage. If so, control is passed to block  262 , which sets a timer to a timer setpoint. The timer setpoint may represent a delay that must occur before the firing rate may be increased for a newly activated first stage. In one embodiment, the timer setpoint may be set to about two minutes, while greater and lesser values may be used as desired. 
     Having set the timer at  262 , the method continues by setting the error as the difference between a desired set point and a measured temperature value. In some embodiments, the measured temperature value correspond to the system return temperature or the system supply temperature (e.g. sensors  114 ,  112 , respectively, in  FIG. 3 ). 
     Returning to block  260 , if there is no transition from zero active stages to one active stage during this interval, the method checks to see whether the timer is less than or equal to zero, as shown at  266 . If the timer is not less than or equal to zero, the timer has not yet expired, and control is passed to block  262 . If the timer is less than or equal to zero, the timer is set to zero as shown at  268  (avoiding possible overflow errors), and control is passed to block  254  where the normal firing rate is used as shown at  254 , and control is passed to end block  256 . 
     Referring back to the set error block  264 , the next step is to check whether the timer is disabled by comparing TEnable to zero, as shown at  270 . If the timer is disabled (TEnable=zero), the derivative action is disabled at block  274 , and continues using the normal firing rate as shown at block  254 . If the timer is not disabled (TEnable is not equal to zero), the method compares the error to a predetermined error value. In the illustrative embodiment, the predetermined error value is twenty-five degrees Fahrenheit, though other error values may also be used, as desired. If the error is too great, the derivative action is disabled by setting DAEnable=zero at block  274 , and then control is passed to block  254  where the normal firing rate is used. 
     If the error is less than the predetermined error value (e.g. 25 degrees F.), the method determines whether the stage is a standalone boiler as shown at  276  (i.e. whether the system is not a multi-boiler or multi-stage system). If the stage is a stand alone boiler, the firing rate is set to a minimum value at block  278 . In some cases, the minimum value may correspond to a minimum firing rate for the boiler. Several commercial boilers have a minimum firing rate of about 25% of their maximum firing rate, but other minimum values may also be used. If the stage is not a standalone boiler, the method sets the firing rate for the boiler stage (stage X) that has been just activated to its minimum value (FR_StgX=MIN), and sets other stages to OFF, as shown at  280 . The other stages are set to off because at decision block  260 , it was determined that the system is transitioning from no active stages to one active stage. 
     Coming from either block  278  or block  280 , the timer is decremented as shown at block  282 . Then, the method comes to end block at shown at  256  and waits for the beginning of the next interval. As noted above, the various steps may be performed by controllers, sensors, and/or logic at either a system control level or a stage control level, or a combination thereof, as desired.  FIGS. 7 and 8  illustrate two example configurations. 
       FIG. 6  shows in block form an illustrative stage control method for disabling derivative control. The method  300  begins with an error calculation  302  which takes the output of a temperature sensor  304  and compares it to a desired set point  306 . An old measured value  308  is also read in. The error is set to the difference between the set point  306  and the measured value, and a rate value is set as the difference between the measured value and the old measured value  308 , as shown at  302 . 
     A comparison step  310  comes next. In the illustrative embodiment, if the error is greater than twenty-five degrees Fahrenheit, and the temperature is dropping, then the derivative action is disabled as shown at  312 . If the output of the comparison step  310  is a “no”, or after the disable step  312 , control is passed to the derivative action block  314 . 
     The derivative action block  314  receives signals from a stager on/off block  316  and a timer  318 . The stager on/off block  316  provides a signal indicating if derivative action  314  is otherwise appropriate, such as if the current stage is being activated and, in one embodiment, if the current stage is the first stage activated. The timer  318  is used by the derivative action  314  to determine how long derivative action should continue. Using these signals to indicate whether derivative action is enabled, appropriate, and should continue, the derivative action block  314  sends a signal to logic  320 . 
     In the illustrative embodiment, logic  320  receives signals from each of the derivative action block  314 , cold water draw control  322 , and modulation control  324 , and uses these signals to determine a firing rate demand to send to the VFD  326 . As noted above, the cold water draw logic  322  may generate a call for increased firing rate to avoid condensation in a boiler. The modulation control  324  may be controlled in various ways. Some illustrative modulation control methods are shown in copending U.S. patent application Ser. No. 10/809,115 entitled MULTI-STAGE BOILER CONTROL STAGING AND MODULATION METHODS AND CONTROLLERS, published as US 2005-0230490 A1, which is incorporated herein by reference. 
       FIGS. 7-8  illustrate configurations for system and stage control for a multi-stage boiler system. As shown in  FIG. 7 , the system controller  350  and stage controller  352  interact in several illustrative fashions. Commands, status and information are exchanged between system configuration  354  and stage configuration  356 . The example shown illustrates that the system controller  352  may send a stage command to the stage controller  352  indicating whether a boiler stage should be on or off. Also, the system controller  350  may send a stage modulation signal that is used in the stage controller  352  by firing rate control logic to control the firing rate of the stage. The stage controller  352  may send status indications to a status memory in the system controller  350 . 
     For the example of  FIG. 7 , the system controller  350  and stage controller  352  are configured to allocate derivative action control to the level of the system controller  350 . In particular, the system controller  350  is configured with derivative action control to keep the heat demand signal relatively low for a predefined time period, with certain exceptions, as shown at  358 . Some illustrative exceptions including: a continuing drop in circulating fluid temperatures after a boiler stage is activated; whether or not a stage newly activated is the first stage activated; and user overrides. 
     Further, because the system controller  350  may receive calls for heat from multiple sensors, certain calls may be of a type indicating that derivative action control is either not needed or undesirable. For example, a system may receive a call for heat from a small area (single office heat) and a large area (gymnasium heat). The system may elect to use derivative action only when the small area calls for heat. 
     Meanwhile, the stage controller  352  may be optionally configured with a cold water bypass control override as shown at  360 , and may not have a derivative action block enabled, though such functionality may be programmed (or hardwired, as desired) into the stage controller  352 . 
     In contrast to  FIG. 7 , the configuration of  FIG. 8  operates differently. In particular, the system controller  370 , while having a set of stage and status commands and memory shown at  374 , interacts with the stage controller  372 , also having certain command receiving and status indicators. In the illustrative example of  FIG. 8 , the system controller  370  simply indicates whether the stage controller  372  is the first stage activated or not, as shown at  378 . The stage controller  372 , as shown at  380 , has the derivative action control which is enabled if the stage is the first stage called, and in the illustrative embodiment, also retains the optional cold water bypass control override. 
     In some cases, the controllers  350 ,  352 ,  370 ,  372  may be readily adjusted for either configuration. When so provided, this may allow the individual components to be used with a variety of systems. For example, the system controller  350  may be operated in conjunction with a boiler/stage controller lacking the derivative action control. In some cases, this may allow the system controller  350  to be provided as a retrofit controller onto an existing system. 
     Those skilled in the art will recognize that the present invention may be manifested in a variety of forms other than the specific embodiments described and contemplated herein. Accordingly, departures in form and detail may be made without departing from the scope and spirit of the present invention as described in the appended claims.