Abstract:
A fluid heating system including a fluid supply subsystem having a fluid heating device, a fluid output subsystem, and an intermediary fluid device. The fluid heating system also includes a control device for the fluid supply subsystem, a first temperature sensor, a second temperature sensor, and a control circuit coupled to the control device. The control device is configured to control one selected from a group consisting of the fluid heating device and an amount of water input to the intermediary fluid device. The first and second temperature sensors are configured to output first and second temperature signals, respectively. The control circuit is configured to generate a first control signal based on the second temperature signal, determine a multiplier, generate a second control signal based on the first temperature signal, and send a main control signal to the control device based on the first and second control signals.

Description:
RELATED APPLICATIONS 
       [0001]    The present application claims priority to U.S. Provisional Patent Application No. 62/336,138, filed on May 13, 2016, the entire contents of which are hereby incorporated. 
     
    
     FIELD 
       [0002]    Embodiments relate to water heaters. 
       SUMMARY 
       [0003]    Tankless, or instantaneous, water heaters may include a heat exchanger to heat water for consumer use. Regulating the temperature of the water provided to the consumer includes regulating the amount of water from a heating loop entering the heat exchanger. Providing an appropriate amount of water from the heating loop to the heat exchanger may be difficult when the temperature of a cold water inlet varies with, for example, outdoor temperature. 
         [0004]    In one embodiment, the application provides a fluid heating system including a fluid supply subsystem having a fluid heating device, a fluid output subsystem, and an intermediary fluid device. The intermediary fluid device is coupled to the fluid supply subsystem and the fluid output subsystem. The intermediary fluid device includes a first input configured to receive fluid from the fluid output subsystem, a first output configured to output fluid to the fluid output subsystem, a second input configured to receive fluid from the fluid supply subsystem, and a second output configured to output fluid to the fluid output subsystem. The fluid heating system also includes a control device for the fluid supply subsystem, a first temperature sensor, a second temperature sensor, and a control circuit coupled to the control device. The control device is configured to control one selected from a group consisting of the fluid heating device and an amount of water input to the intermediary fluid device. The first temperature sensor is configured to output a first temperature signal indicative of an input temperature at the first input of the intermediary fluid device, and the second temperature sensor is configured to output a second temperature signal indicative of an output temperature at the first output of the intermediary fluid device. The control circuit is coupled to the control device, the first temperature sensor, and the second temperature sensor. The control circuit is configured to generate a first control signal based on the second temperature signal, determine a multiplier based on the second temperature signal, generate a second control signal, separate from the first control signal, based on the multiplier and the first temperature signal, and send a main control signal to the control device based on the first control signal and the second control signal. The control device is configured to receive the main control signal, and change operation of the control device according to the main control signal. 
         [0005]    In another embodiment, the application provides a method of controlling a fluid heating system. The method includes receiving, fluid from a fluid output subsystem at a first input of an intermediary fluid device, receiving fluid from a fluid supply subsystem at a second input of the intermediary fluid device, the fluid supply subsystem including a fluid heating device, outputting fluid to the fluid output subsystem at a first output of the intermediary fluid device, and outputting fluid to the fluid supply subsystem at a second output of the intermediary fluid device. The method also includes receiving, at a control circuit, a first temperature signal from a first temperature sensor, receiving, at the control circuit, a second temperature signal from the second temperature sensor. The first temperature signal is indicative of an input temperature at the first input of the intermediary fluid device. Analogously, the second temperature signal is indicative of an output temperature at the first output of the intermediary fluid device. The method further includes generating, with the control circuit, a first control signal based on the second temperature signal, determining, with the control circuit, a multiplier based on the second temperature signal, and generating, with the control circuit, a second control signal, separate from the first control signal, based on the multiplier and the first temperature signal. The method also includes sending a main control signal to a control device for the fluid supply subsystem based on the first control signal and the second control signal, and changing operation of the control device in response to receiving the main control signal at the control device. The control device controls one selected from a group consisting of the fluid heating device and an amount of water input to the intermediary fluid device. 
         [0006]    Other aspects of the application will become apparent by consideration of the detailed description and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic diagram of a water heating system according to some embodiments of the application. 
           [0008]      FIGS. 2A-2C  are diagrams of a three-way valve of the water heating system of  FIG. 1  in different positions. 
           [0009]      FIG. 3  is a block diagram of a control circuit of the water heating system of  FIG. 1 . 
           [0010]      FIG. 4  is a flowchart illustrating a method of operating the water heating system of  FIG. 1  according to some embodiments of the application. 
           [0011]      FIG. 5  is a flowchart illustrating a method of determining a multiplier value for the water heating system of  FIG. 1  according to some embodiments of the application. 
           [0012]      FIG. 6  is a flowchart illustrating a method of operating a mixing valve of the water heating system of  FIG. 1  based on a modified multiplier signal according to some embodiments of the application. 
           [0013]      FIG. 7  is a block diagram of an implementation of the control circuit of  FIG. 3  using an electronic processor. 
           [0014]      FIG. 8  is a schematic diagram of another water heating system according to another embodiment of the application. 
           [0015]      FIG. 9  is a block diagram of a control circuit of the water heating system of  FIG. 8 . 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Before any embodiments of the application are explained in detail, it is to be understood that the application is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawing. The application is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings. 
         [0017]      FIG. 1  is a diagram of a water heating system  100  according to some embodiments of the application. The water heating system  100  includes an intermediary device  102 , a water supply subsystem  103 , and a water output subsystem  104 . In the illustrated embodiment, the intermediary device  102  corresponds to a heat exchanger  105 , the water supply subsystem  103  corresponds to a heating loop  110 , and the water output subsystem  104  corresponds to an output loop  115 . In the illustrated embodiment, the water heating system  100  may be, for example, a commercial or domestic tankless hot water heater. The heat exchanger  105  includes a first portion  120  and a second portion  125 . The first portion  120  receives water from the heating loop  110 , while the second portion  125  receives water from the output loop  115 . The first portion  120  includes a first inlet  122  and a first outlet  124 . Water from the heating loop  110  is received at the first inlet  122  and output at the first outlet  124  back into the heating loop  110 . The second portion  125  includes a second inlet  127  and a second outlet  129 . Cold inlet water is received at the second inlet  127  and hot water, for use by a consumer, is output from the second outlet  129 . The heat exchanger  105  transfers heat from the water of the heating loop  110  to the water of the output loop  115  to provide hot water to a consumer. 
         [0018]    The heating loop  110  includes a mixing valve  130 , a heating system  135  (for example, or heating device), and a pump  140 . In some instances, the mixing valve  130  may also be referred to as a control device for the heating loop  110 . In the illustrated embodiment, the mixing valve  130  is a three-way valve that controls how much water from the heating loop  110  enters the heat exchanger  105 . Controlling the amount of water that enters the heat exchanger  105  helps maintain the water of the output loop  115  at a setpoint temperature. The mixing valve  130  includes a first valve inlet  145 , a second valve inlet  150 , and a valve outlet  155 . The first valve inlet  145  is coupled to the first outlet  124  of the heat exchanger  105  and thus receives the water from the heating loop  110  that has been circulated through the heat exchanger  105 . The second valve inlet  150  is coupled between the pump  140  and the first inlet  122  of the heat exchanger  105  and thus receives water that is diverted from entering the heat exchanger  105 , and is instead recirculated through the heating loop  110 . The valve outlet  155  of the mixing valve  130  is coupled to the heating system  135  and circulates the water received from the first outlet  124  of the heat exchanger  105  and/or the water diverted from the first inlet  122  of the heat exchanger  105  toward the heating system  135 . The mixing valve  130  is movable between positions to change the amount of water that is diverted from the first inlet  122  of the heat exchanger  105  and thereby controls how much water from the heating loop  110  enters the heat exchanger  105 . 
         [0019]    The heating system  135  includes components that heat the water in the heating loop  110 . The heating system  135  may include, for example, boilers, heat pumps, electric water heaters, and the like. The heating system  135  receives the water from the mixing valve  130 , heats the water, and outputs the hot water to the pump  140 . The pump  140  circulates the heating loop water toward the heat exchanger  105  continuously. As discussed above, the water propelled by the pump  140  may enter the heat exchanger  105  through the first inlet  122  of the heat exchanger  105 , or may be diverted away from the heat exchanger  105  toward the second valve inlet  150  of the mixing valve  130 . 
         [0020]      FIGS. 2A-2C  illustrate diagrams of different positions of the mixing valve  130 . For example,  FIG. 2A  illustrates a first position of the mixing valve  130  in which the second valve inlet  150  of the mixing valve  130  is closed. In the first position, the mixing valve  130  receives water only through the first outlet  124  of the heat exchanger  105 . When the mixing valve  130  is in the first position, all of the water from the heating loop  110  is directed to the heat exchanger  105 , processed by the heat exchanger  105 , and released by the heat exchanger  105  to the mixing valve  130 . The mixing valve  130  may be in the first position when, for example, there is a high demand for hot water and thus more heat is necessary at the first portion  120  of the heat exchanger  105 .  FIG. 2B , on the other hand, illustrates a second position of the mixing valve  130  in which the first valve inlet  145  of the mixing valve  130  is closed. In the second position, the mixing valve  130  receives only the water that is diverted from the first inlet  122  of the heat exchanger  105 . When the mixing valve  130  is in the second position, the water from the heating loop  110  does not enter the heat exchanger  105 , and the heat exchanger  105  does not receive heat at the first portion  120 . The mixing valve  130  may be in the second position when, for example, there is no demand for hot water and no heat is necessary at the heat exchanger  105  to maintain the domestic water at the setpoint temperature.  FIG. 2C  illustrates a third position of the mixing valve  130  in which both the first valve inlet  145  and the second valve inlet  150  are open. In the third position, the mixing valve  130  receives water from the first outlet  124  of the heat exchanger  105  and water that is diverted from the first inlet  122  of the heat exchanger  105 . The mixing valve  130  may change between more than the three positions illustrated by  FIGS. 2A-C . For example, the first valve inlet  145  and/or the second valve inlet  150  may be partially opened, and do not need to be fully opened or fully closed. The first valve inlet  145  and/or the second valve inlet  150  may change between different positions while remaining partially opened. Such movement of the mixing valve  130  provides a gradual change in the mixing valve  130  and provides better control of the amount of water from the heating loop  110  entering the heat exchanger  105 . The valve outlet  155  of the mixing valve  130  directs the water toward the heating system  135 . 
         [0021]    In the illustrated embodiment, the output loop  115 , also referred to as the domestic water loop  115 , provides cold inlet water to the heat exchanger  105  and provides hot water to a consumer. As shown in  FIG. 1 , the output loop  115  includes a cold water inlet  170 , a hot water outlet  175 , a circulation pump  180 , a first sensor  185 , and a second sensor  190 . The cold water inlet  170  provides cold water to the output loop  115  from, for example, a cold water reservoir such as a well or a city water system. The cold water then enters the heat exchanger  105  at the second inlet  127  of the heat exchanger  105 , and exits the heat exchanger  105  as hot water at the second outlet  129  of the heat exchanger  105 . The hot water outlet  175  provides hot water to the consumer. 
         [0022]    The circulation pump  180  circulates the water from the output loop  115  continuously. The circulation pump  180  is coupled between the cold water inlet  170  and the hot water outlet  175 , and circulates the water from the hot water outlet  175  back to the heat exchanger  105 . When there is no draw of hot water at the hot water outlet  175 , the water in the output loop  115  continues to loop through the heat exchanger  105  without the need to add water from the cold water inlet  170  to the water directed to the heat exchanger  105 . Therefore, when there is no draw of hot water at the hot water outlet  175 , the temperature of the water at the second inlet  127  of the heat exchanger  105  is approximately the same as the temperature of the water at the hot water outlet  175  (since it is the same water from the hot water outlet  175  going into the second inlet  127  of the heat exchanger  105 ). When, however, there is a water draw at the hot water outlet  175 , some of the water from the cold water inlet  170  is directed to the second inlet  127  of the heat exchanger  105 . The higher the water draw at the hot water outlet  175 , the more cold water from the cold water inlet  170  that is directed to the heat exchanger  105 . 
         [0023]    The first sensor  185  is positioned between the circulation pump  180  and the second inlet  127  of the heat exchanger  105 . The first sensor  185  includes a temperature sensor and provides an indication of the sensed water temperature at the second inlet  127  of the heat exchanger  105 . That is, the first sensor  185  outputs a temperature signal indicative of an input temperature at the second inlet  127  of the heat exchanger  105 . The temperature sensor may be any variety of temperature sensors, including but not limited to, resistance temperature detectors, thermocouples, thermistors, thermostats, and the like. As discussed above, cold water enters the heat exchanger  105  at the second inlet  127  when there is a water draw at the hot water outlet  175 . Since the first sensor  185  measures a water temperature at the second inlet  127  of the heat exchanger  105 , the first sensor  185  provides an approximate measure of the water draw at the hot water outlet  175 . The second sensor  190  also includes a temperature sensor. In some embodiments, the temperature sensor of the second sensor  190  is substantially similar to the temperature sensor of the first sensor  185 . The second sensor  190  is positioned between the second outlet  129  of the heat exchanger  105  and the circulation pump  180 . In this position, the second sensor  190  provides an indication of the sensed water temperature at the second outlet  129  of the heat exchanger  105 . That is, the second sensor  190  outputs a temperature signal indicative of an output temperature at the second outlet  129  of the heat exchanger  105 . As discussed above, the water temperature at the hot water outlet  175  is ideally maintained at the user-defined setpoint. Since the second sensor  190  measures a water temperature at the second outlet  129  of the heat exchanger  105 , the second sensor  190  provides an indication of whether the water at the hot water outlet  175  is at the setpoint. 
         [0024]    The first and second sensors  185 ,  190  are coupled to a control circuit  200  shown in  FIG. 3 . The control circuit  200  is coupled to the mixing valve  130  to control the position of the mixing valve  130  such that the temperature of the water at the hot water outlet  175  is maintained at the setpoint. The control circuit  200  of the illustrated embodiment includes a feed-forward loop  205 , a feedback loop  210 , and a multiplying factor determining circuit  215 . The feed-forward loop  205  determines when a water draw occurs at the hot water outlet  175 , and sends a signal to the mixing valve  130  to change position before there is a change in water temperature at the hot water outlet  175 . The feed-forward loop  205  includes the first sensor  185 , a differentiator  220 , a multiplier  225 , and a first adder  230 . The first sensor  185  is coupled to the differentiator  220 . The differentiator  220  generates a difference signal  235  based on the temperature signal from the first sensor  185 . The difference signal  235  corresponds, or is based on, the difference between the temperature signal from the first sensor  185  and the setpoint temperature. The difference between the temperature signal from the first sensor  185  and the setpoint temperature indicates how much heat may be needed to compensate for the hot water draw. This difference signal is therefore used as the basis to control the position of the mixing valve  130 . The differentiator  220  is coupled to the first sensor  185  and the multiplier  225 . The differentiator  220  sends the difference signal  235  to the multiplier  225 . The multiplier  225  is coupled to the differentiator  220 , the multiplying factor determining circuit  215 , and the first adder  230 . The multiplier  225  receives a multiplying factor  240  from the multiplying factor determining circuit  215 , and generates a primary control signal  245 . The primary control signal  245  includes a product of the multiplying factor  240  and the difference signal  235 . The multiplier  225  then sends the primary control signal  245  to the first adder  230 . The first adder  230  is coupled to the multiplier  225  and to the feedback loop  210 . The first adder  230  generates a control signal  250  based at least on the primary control signal  245 . The mixing valve  130  receives the control signal  250  and changes its position based on the control signal  250 . 
         [0025]    The feedback loop  210  includes the second sensor  190 , a first PID (proportional, integral, derivative) controller  255 , and the first adder  230 . The second sensor  190  is coupled to the first PID controller  255  and provides the first PID controller  255  with a sensed water temperature at the second outlet  129  of the heat exchanger  105 . The first PID controller  255  generates a secondary control signal  260  based on a comparison of the setpoint temperature and the sensed temperature at the second outlet  129  of the heat exchanger  105 . The first PID controller  255  then sends the secondary control signal  260  to the first adder  230 . As discussed above, the first adder  230  generates the control signal  250  based on the primary control signal  245  and the secondary control signal  260 . 
         [0026]    The multiplying factor determining circuit  215  determines (e.g., calculates) the multiplying factor  240  used by the multiplier  225  of the feed-forward loop  205 . The multiplier determining circuit  215  is coupled between the feedback loop  210  and the feed-forward loop  205 , and more specifically, between the feedback loop  210  and the multiplier  225 . In the illustrated embodiment, the multiplying factor determining circuit  215  includes a second PID controller  265  and a second adder  270 . The second PID controller  265  receives the secondary control signal  260  from the first PID controller  255 , and generates an error signal  275 . The second PID controller  265  is coupled to the second adder  270  and sends the error signal  275  to the second adder  270 . The second adder  270  is coupled to the second PID controller  265  and the multiplier  225 . The second adder  270  generates the multiplying factor  240  based on the secondary control signal  260  and an adjustable variable (further discussed below), and sends the multiplying factor  240  to the multiplier  225 . 
         [0027]      FIG. 4  is a flowchart illustrating a method  300  of operation of the control circuit  200  to change a position of the mixing valve  130 . First, the differentiator  220  receives a first temperature from the first sensor  185  (block  305 ). The first temperature corresponds to a sensed water temperature at the second inlet  127  of the heat exchanger  105  from the first sensor  185 . The differentiator  220  also receives a setpoint (block  307 ). As discussed above, in some embodiments, the setpoint may be a user-defined setpoint. In such embodiments, the water heating system  100  may include a user interface (e.g., physical and/or virtual actuators) to receive an indication of the setpoint. The control circuit  200 , and more specifically, the first PID controller  255  also receives a second temperature from the second sensor  190  (block  310 ). The second temperature corresponds to a sensed water temperature at the second outlet  129  of the heat exchanger  105 . The differentiator  220  then generates the difference signal  235  between the first temperature and the setpoint (block  315 ). Monitoring the difference between the first temperature and the setpoint allows the control circuit  200  to detect when a water draw begins to occur. Using the difference signal  235  to control the position of the mixing valve  130  enables the control circuit  200  to change the position of the mixing valve  130  before the water temperature at the hot water outlet  175  decreases due to the water draw. 
         [0028]    After generating the difference signal  235 , the multiplying factor determining circuit  215  determines the multiplying factor  240  based on the second temperature (block  320 ). The multiplier  225  then generates the primary control signal  245  (block  325 ). The multiplier  225  generates the primary control signal  245  by multiplying the difference signal  235  with the multiplying factor  240 . Multiplying the difference signal  235  and the multiplying factor  240  allows the control circuit to more accurately change the position of the mixing valve  130  based on the difference signal  235 . The multiplying factor  240  provides a scaling factor to determine how much change in position of the mixing valve  130  corresponds to the difference signal  235 . The control circuit  200  then operates the mixing valve  130  (e.g., changes the position of the mixing valve  130 ) based on the modified multiplier signal (block  330 ). 
         [0029]      FIG. 5  is a flowchart illustrating a method  400  of determining the multiplying factor  240 . First, the first PID controller  255  generates the secondary control signal  260  between the second temperature and the setpoint (block  405 ). The first PID controller  255  determines when the water temperature at the hot water outlet  175  is below or above the user-defined setpoint. The second PID controller  265  then generates the error signal  275  between the first error signal and an error threshold (block  410 ). The error threshold corresponds to the allowable variation in the water temperature at the hot water outlet  175  with respect to the setpoint. In the embodiment shown in  FIG. 3 , the error threshold corresponds to zero. In other words, the water temperature at the hot water outlet  175  is expected to be at the setpoint. Therefore, the error signal  275  indicates how different the water temperature at the hot water outlet  175  is from the setpoint. When the multiplying factor  240  is ideal, the feed-forward loop  205  anticipates the position change necessary at the mixing valve  130  to maintain the water temperature at the hot water outlet  175  at the setpoint. In these instances the secondary control signal  260  is approximately zero, and thus the second PID controller  265  determines no difference between the secondary control signal  260  and the zero error threshold. 
         [0030]    The second adder  270  then aggregates (e.g., adds) the error signal and an adjustable variable to generate the multiplying factor  240  (block  415 ). The adjustable variable is a variable that changes according to the setpoint. In other words, the adjustable variable is a function of the setpoint. In one embodiment, the adjustable variable is calculated by the following equation: 
         [0000]    
       
         
           
             
               Adjustable 
                
               
                   
               
                
               Variable 
             
             = 
             
               
                 ( 
                 
                   210 
                    
                   ° 
                    
                   
                       
                   
                    
                   
                     F 
                     . 
                     
                       - 
                       Setpoint 
                     
                   
                 
                 ) 
               
               Setpoint 
             
           
         
       
     
         [0000]    However, in other embodiments, the adjustable variable may be calculated in a different manner, for example but not limited to, using a second equation shown below: 
         [0000]    
       
         
           
             
               Adjustable 
                
               
                   
               
                
               Variable 
             
             = 
             
               
                 ( 
                 
                   240 
                   
                     Setpoint 
                     + 
                     25 
                   
                 
                 ) 
               
               - 
               1 
             
           
         
       
     
         [0000]    Still in other embodiments, the adjustable variable may be determined using different methods. In some embodiments, the equation used to calculate the adjustable variable is determined empirically by testing different setpoints, multipliers, and equations. 
         [0031]      FIG. 6  is a flowchart illustrating a method  500  of operating the mixing valve  130  based on the primary control signal  245 . First, the first adder  230  receives the primary control signal  245  (block  505 ). The first adder  230  also receives the secondary control signal  260  from the feedback loop  210  (block  510 ). The first adder  230  then aggregates (e.g., adds) the primary control signal  245  and the secondary control signal  260  to generate the control signal  250  (block  515 ). The mixing valve  130  then receives the control signal  250  from the first adder  230  (block  520 ) and changes its position according to the control signal  250  (block  525 ). In other words, the mixing valve  130  changes its operation in response to receiving the control signal  250  and based on the control signal  250 . Therefore, the mixing valve  130  changes its position based on both the primary control signal  245  and the secondary control signal  260 . Taking into account both the water temperature at the second inlet  127  of the heat exchanger  105  and the water temperature at the second outlet  129  of the heat exchanger  105  provides a more precise and accurate control of the position of the mixing valve. 
         [0032]    Although the steps for the flowcharts above have been described as being performed serially, in some embodiments, the steps may be performed in a different order and two or more steps may be carried out in parallel to, for example, expedite the control process. Additionally, although the control circuit  200  is shown in  FIG. 3  as including two PID controllers  255 ,  265 , two adder circuits  230 ,  270 , and other components, in some embodiments, the control circuit  200  may be implemented using an electronic processor.  FIG. 7  illustrates an example of the implementation  600  of the control circuit  200  with an electronic processor. In the illustrated example, the implementation  600  includes an electronic processor  605 , a memory  610 , the first sensor  185 , the second sensor  190 , and the mixing valve  130 . The electronic processor  605  of the illustrated example, implements the functionality of the first PID controller  255 , the second PID controller  265 , the first adder  230 , the second adder  270 , the multiplier  225 , and the differentiator  220 . To implement such functionality, the electronic processor  605  may execute instructions from software. As shown in  FIG. 7 , the electronic processor  605  is coupled to the first sensor  185  to receive an indication of the water temperature at the second inlet  127  of the heat exchanger  105 . The electronic processor  605  is also coupled to the second sensor  190  to receive an indication of the water temperature at the second outlet  129  of the heat exchanger  105 . Additionally, the electronic processor  605  receives an indication of the user-defined setpoint  615 . When the control circuit  200  is implemented with the electronic processor  605 , the electronic processor  605  executes the methods described with respect to  FIGS. 4-6 . Additionally, the electronic processor  605  may access the memory  610  to retrieve specific set points or formulas for calculating a specific variable, such as the adjustable variable used by the second adder  270 . 
         [0033]      FIG. 8  illustrates another exemplary embodiment of a water heating system  800 . As shown in  FIG. 8 , the water heating system  800  includes a water supply subsystem  805 , an intermediary water device  810 , and a water output subsystem  815 . The water supply subsystem  805  includes a heating device  820  including a control device  825  for the water supply subsystem, and a pump  821 . The pump  821  operates similar to the pump  140 , and directs water to the intermediary device  810 . In the illustrated embodiment, the control device  825  includes an electronic processor included as part of the heating device  820  and controls operation of the heating device  820 . The heating device  820  may include, for example, a commercial or residential water heater. The heating device  820  receives water from the water supply subsystem  805 , heats the water, and sends heated water to the intermediary device  810 . 
         [0034]    In the illustrated embodiment, the intermediary device  810  includes a buffer water tank. The buffer water tank  810  receives heated water from the water supply subsystem  805  and maintains the heater water near a desired setpoint (for example, a setpoint received from a user input). Similar to the heat exchanger  105  of  FIG. 1 , the intermediary device  810  includes a first input  822  to receive water from the water supply subsystem  805 , a first output  824  to return water back to the water supply subsystem  805 , a second input  827  to receive return water from the water output subsystem  815 , and a second output  829  to output heated water to the water output subsystem  815 . The water output system  815  includes a first temperature sensor  830 , a second temperature sensor  835 , and a recirculation pump  840 . The water output subsystem  815  receives heated water from the intermediary device  810  via the second output  829  and returns unused water to the intermediary device  810  at the second input  827 . 
         [0035]    The water heating system  800  may operate similar to the water heating system  100  described with reference to  FIGS. 1-7 . In particular, the water heating system  800  may also includes a control circuit  900  similar to the control circuit  200  shown in  FIG. 3 .  FIG. 9  illustrates the control circuit  900  according to some embodiments. The control circuit  900  may include similar components as the control circuit  200  of  FIG. 3  and similar elements have been given the same reference numbers plus  700 . As shown in  FIG. 9 , the main control signal  950  instead of being directed to a mixing valve  130  as shown in  FIG. 3  is directed to the control device  825 . That is, the main control signal  950  is sent to the electronic processor  825  of the heating device  820 . In some embodiments, an intermediary control device is positioned between the control circuit  900  and the electronic processor  825  of the heating device  820  to translate the main control signal to a control signal expected by the electronic processor  825  of the heating device  820 . The electronic processor  825  then changes operation of the heating device  820  based on the received main control signal  950 . 
         [0036]    In some embodiments, the electronic processor  825  activates and/or deactivates the heating elements of the heating device  820  (for example, when the heating device  820  is an electric water heater) in response to receiving the main control signal  950  (and in accordance with the main control signal  950 ). For example, the electronic processor  825  sends an activation signal to one or more heating elements when the main control signal  950  indicates that water in the water output subsystem  815  has fallen (or is falling) below the desired setpoint. Analogously, the electronic processor  825  may activate and/or deactivate a burner when the heating device is a gas-fired heating device  820 . In some embodiments, the heating device  820  may include, for example, a condensing water heater for which a firing rate may be regulated. For example, the electronic processor  825  may regulate a firing rate of the heating device  820  to match the current demand for heated water. In some embodiments, the electronic processor  825  may regulate the firing rate between approximately 10% to a maximum of approximately 100%. In such embodiments, the electronic processor  825  receives the main control signal  950  from the control circuit  900  and adjusts the firing rate of the heating device  820  based on the main control signal  950 . That is, the electronic processor  825  may increase the firing rate of the heating device  820  and/or reduce the firing rate of the heating device  820 .