Patent Publication Number: US-11639813-B2

Title: Water heating apparatus and water heating system

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present disclosure relates to a water heating apparatus and a water heating system and more particularly to a water heating apparatus with an immediate hot water supply function and a water heating system. 
     Description of the Background Art 
     A water heating apparatus of one form is equipped with what is called an immediate hot water supply function for outputting hot water at an appropriate temperature immediately after start of hot water supply even after hot water supply has been off for a long period of time. Normally, in order to achieve the immediate hot water supply function, a mode in which a circulation path that goes through a heat source also while hot water supply is off is formed (an “immediate hot water supply operation mode” below) should be provided. 
     Japanese Patent Laying-Open No. 6-249507 discloses a configuration of a temperature-maintained circulation water heating apparatus that detects a flow rate in temperature-maintained circulation and a flow rate in hot water output with a single flow rate sensor and reliably detects use of a hot water supply faucet even in output of a small amount of hot water. 
     U.S. Pat. No. 6,536,464 discloses a configuration for forming a circulation path for the immediate hot water supply function by externally connecting a bypass valve (which is also referred to as a “crossover valve” below) for thermostatic control using a wax thermostatic element. The immediate hot water supply function can thus be achieved by simplified attachment works without adding a function to control the crossover valve on a side of the water heating apparatus. 
     SUMMARY OF THE INVENTION 
     According to Japanese Patent Laying-Open No. 6-249507, a flow rate value on which determination as hot water supply use is based (a flow rate in hot water supply use) is different between an active state and an inactive state of a circulation pump. This publication describes registration in advance of a circulation flow rate at the time when a length of disposed hot water supply path and return path is shortest as a provisional flow rate for the flow rate in hot water supply use in the active state of the circulation pump, detection thereafter of the circulation flow rate in a temperature-maintained circulation operation, and update of the circulation flow rate based on an actually detected circulation flow rate. 
     According to the configuration in Japanese Patent Laying-Open No. 6-249507, however, it is a concern that accuracy in detection of use of a hot water supply faucet is lowered when a condition of the circulation flow path formed while the circulation pump is active changes over time. In particular, it is a concern in an example where the circulation flow path is formed by connection of a crossover valve as described in U.S. Pat. No. 6,536,464 that above-described change over time tends to occur. 
     The present disclosure was made to solve such problems, and an object of the present disclosure is to improve accuracy in detection of use of a hot water supply faucet in an immediate hot water supply operation mode. 
     According to one aspect of the present disclosure, a water heating apparatus that outputs hot water to a hot water supply faucet includes a water entry port to which low-temperature water is introduced, a heating mechanism, a hot water output port for output of high-temperature water heated by the heating mechanism, a water entry path, a hot water output path, a flow rate detector, and a controller. The water entry path is formed between the water entry port and the heating mechanism. The hot water output path is formed between the heating mechanism and the hot water output port. In an immediate hot water supply operation mode in which a circulation pump is activated while the hot water supply faucet is closed, the water heating apparatus is configured to form an immediate hot water supply circulation path through which fluid passes through the heating mechanism by an inner path and an outer path as being combined, the inner path including at least a part of the water entry path, the heating mechanism, and the hot water output path, the outer path bypassing the hot water supply faucet on the outside of the water heating apparatus. The flow rate detector detects a flow rate in the immediate hot water supply circulation path. The controller gives an instruction to activate and deactivate the heating mechanism and the circulation pump. The controller stores for each immediate hot water supply operation mode, a flow rate detection value obtained by the flow rate detector at predetermined timing in the immediate hot water supply operation mode, and calculates a flow rate learning value based on a plurality of stored flow rate detection values. When the flow rate detection value becomes larger than a criterion value set in accordance with the flow rate learning value in the immediate hot water supply operation mode, the controller detects use of the hot water supply faucet and deactivates the circulation pump. 
     According to another aspect of the present disclosure, a water heating system includes a water heating apparatus including a water entry port and a hot water output port, a low-temperature water pipe, a high-temperature water pipe, and a circulation pump. The low-temperature water pipe introduces low-temperature water to a water entry port of the water heating apparatus. The high-temperature water pipe connects the hot water output port of the water heating apparatus and the hot water supply faucet to each other. The circulation pump is arranged inside or outside the water heating apparatus. The water heating apparatus includes a heating mechanism, a water entry path formed between the water entry port and the heating mechanism, a hot water output path formed between the heating mechanism and the hot water output port, a flow rate detector, and a controller that gives an instruction to activate and deactivate the heating mechanism and the circulation pump. In an immediate hot water supply operation mode in which the circulation pump is activated while the hot water supply faucet is closed, the water heating apparatus is configured to form an immediate hot water supply circulation path through which fluid passes through the heating mechanism by an inner path and an outer path as being combined, the inner path including at least a part of the water entry path, the heating mechanism, and the hot water output path, the outer path bypassing the hot water supply faucet on the outside of the water heating apparatus. The flow rate detector detects a flow rate in the immediate hot water supply circulation path. The controller stores for each immediate hot water supply operation mode, a flow rate detection value obtained by the flow rate detector at predetermined timing in the immediate hot water supply operation mode, and calculates a flow rate learning value based on a plurality of stored flow rate detection values. When the flow rate detection value becomes larger than a criterion value set in accordance with the flow rate learning value in the immediate hot water supply operation mode, the controller detects use of the hot water supply faucet and deactivates the circulation pump. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a block diagram illustrating a configuration of a water heating system including a water heating apparatus according to the present embodiment. 
         FIG.  2    is a block diagram illustrating an exemplary hardware configuration of a controller shown in  FIG.  1   . 
         FIG.  3    shows a chart illustrating switching between flow paths by means of a crossover valve shown in  FIG.  1   . 
         FIG.  4    is a flowchart illustrating control processing in an immediate hot water supply operation mode by the water heating apparatus according to the present embodiment. 
         FIG.  5    shows a conceptual waveform diagram of a flow rate detection value in the immediate hot water supply operation mode. 
         FIG.  6    is a flowchart illustrating processing for learning a flow rate detection value. 
         FIG.  7    shows a conceptual waveform diagram illustrating an example in which learning of a flow rate value is not carried out due to detection of hot water supply interrupt. 
         FIG.  8    shows a conceptual waveform diagram illustrating an example in which learning of a flow rate value is not carried out because variation in flow rate is great. 
         FIG.  9    is a conceptual diagram illustrating learning of a flow rate value in a circulation operation mode. 
         FIG.  10    is a flowchart illustrating diagnosis of an abnormal condition in an immediate hot water supply circulation path in the water heating system according to the present embodiment. 
         FIG.  11    is a block diagram illustrating a first modification of the configuration of the water heating system according to the present embodiment. 
         FIG.  12    is a block diagram illustrating a second modification of the configuration of the water heating system according to the present embodiment. 
         FIG.  13    is a block diagram illustrating a third modification of the configuration of the water heating system according to the present embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the present disclosure will be described in detail below with reference to the drawings. The same or corresponding elements in the drawings below have the same reference characters allotted and description thereof will not be repeated in principle. 
       FIG.  1    is a block diagram illustrating a configuration of a water heating system  1 A including a water heating apparatus according to the present embodiment. 
     Referring to  FIG.  1   , water heating system  1 A includes a water heating apparatus  100 , a low-temperature water pipe  110 , a high-temperature water pipe  120 , and a crossover valve  200 . Water heating apparatus  100  includes a water entry port  11 , a hot water output port  12 , and a circulation port  13 . 
     Low-temperature water pipe  110  is supplied with low-temperature water through a check valve  112 . Low-temperature water is representatively supplied from a not-shown water supply pipe. Low-temperature water pipe  110  is connected to water entry port  11  and circulation port  13 . 
     Water heating apparatus  100  includes a controller  10 , a water entry path  20 , a hot water output path  25 , a circulation path  28 , a bypass path  29 , a combustion mechanism  30 , a heat exchanger  40 , a circulation pump  80 , and a flow rate regulation valve  90 . 
     Water entry path  20  is formed between water entry port  11  and an input side (upstream side) of heat exchanger  40  with a check valve  21  being interposed. Combustion mechanism  30  is representatively implemented by a burner that generates a quantity of heat by combustion of fuel such as gas or petroleum or the like. 
     Heat exchanger  40  increases a temperature of low-temperature water (fluid) introduced through water entry path  20  by using the quantity of heat generated by combustion mechanism  30 . Therefore, combustion mechanism  30  and heat exchanger  40  can implement an embodiment of the “heating mechanism.” Alternatively, the “heating mechanism” can also be implemented by a heat pump or exhaust heat during power generation. 
     Hot water output path  25  is formed between an output side (downstream side) of heat exchanger  40  and hot water output port  12 . Bypass path  29  connects water entry path  20  and hot water output path  25  to each other without heat exchanger  40  being interposed. Under the control of flow rate regulation valve  90  by controller  10 , a ratio of a flow rate in bypass path  29  (a bypass flow rate ratio) to a total flow rate (the sum of a flow rate in heat exchanger  40  and a flow rate in bypass path  29 ) can be regulated. 
     According to such a bypass configuration, some of low-temperature water bypasses heat exchanger  40  and is mixed without being heated, in a portion downstream from heat exchanger  40 , and thus high-temperature water is supplied from hot water output port  12 . Since a temperature of output from heat exchanger  40  (heating mechanism) can thus be high, drainage water generated by cooling of exhaust from combustion mechanism  30  at a surface of heat exchanger  40  is advantageously suppressed. 
     A flow rate sensor  81  that outputs a value of a flow rate of low-temperature water is arranged in water entry path  20  and a flow rate sensor  82  is arranged in circulation path  28 . Flow rate sensor  81  is arranged to be included in an immediate hot water supply circulation path which will be described later. Detection values from flow rate sensors  81  and  82  are input to controller  10 . 
     A temperature sensor  71  is arranged in hot water output path  25  and a temperature sensor  73  is arranged in water entry path  20 . A temperature sensor  72  is arranged in circulation path  28 . Fluid temperatures detected by temperature sensors  71  to  73  are input to controller  10 . 
       FIG.  2    is a block diagram illustrating an exemplary hardware configuration of controller  10 . 
     Referring to  FIG.  2   , controller  10  is representatively implemented by a microcomputer. Controller  10  includes a central processing unit (CPU)  15 , a memory  16 , an input and output (I/O) circuit  17 , and an electronic circuit  18 . CPU  15 , memory  16 , and I/O circuit  17  can transmit and receive signals to one another through a bus  19 . Electronic circuit  18  is configured to perform prescribed operation processing with dedicated hardware. Electronic circuit  18  can transmit and receive signals to and from CPU  15  and I/O circuit  17 . 
     CPU  15  receives output signals (detection values) from sensors including temperature sensors  71  to  73  and flow rate sensors  81  and  82  through I/O circuit  17 . CPU  15  further receives a signal indicating an operation instruction input to a remote controller  92  through I/O circuit  17 . The operation instruction includes, for example, an operation to switch on and off an operation switch of water heating apparatus  100 , a set hot water supply temperature, and various types of programmed time setting (which is also referred to as “timer setting”). CPU  15  controls operations by constituent apparatuses including combustion mechanism  30  and circulation pump  80  such that water heating apparatus  100  operates in accordance with the operation instruction. 
     CPU  15  can output visually or aurally recognizable information by controlling a notification apparatus  95 . For example, notification apparatus  95  can output information by showing visually recognizable information such as characters and graphics on a screen. In this case, notification apparatus  95  can be implemented by a display screen provided in remote controller  92 . Alternatively, notification apparatus  95  may be implemented by a speaker so that information can also be output by voice and sound or melodies. 
     Operations by water heating apparatus  100  will be described with reference to  FIG.  1    again. 
     In use for hot water supply in which a hot water supply faucet  330  is open, low-temperature water is introduced into water entry path  20  by a supply pressure of low-temperature water. When flow rate sensor  81  detects a flow rate exceeding a minimum operating quantity (MOQ) of working water while the operation switch of water heating apparatus  100  is on, controller  10  activates combustion mechanism  30 . 
     Consequently, high-temperature water heated by combustion mechanism  30  and heat exchanger  40  is mixed with low-temperature water that passes through bypass path  29  and thereafter output to high-temperature water pipe  120  through hot water output port  12 . 
     During a normal hot water supply operation, controller  10  deactivates circulation pump  80  and controls a temperature of fluid (hot water output temperature Th) detected by temperature sensor  71  to a set hot water supply temperature Tr input to remote controller  92 . Specifically, a temperature of hot water output can be controlled based on combination of control of a quantity of heating (a quantity of generated heat) by combustion mechanism  30  (heating mechanism) and control of the bypass flow rate ratio by means of flow rate regulation valve  90 . 
     Circulation path  28  is formed between circulation port  13  and water entry path  20  (a connection point  22 ). Circulation pump  80  is connected to circulation path  28 . Alternatively, circulation pump  80  may be connected to circulation port  13  on the outside of water heating apparatus  100 . Activation and deactivation of circulation pump  80  are controlled by controller  10 . 
     While the hot water supply operation is off, a temperature of fluid that remains in hot water output path  25  and high-temperature water pipe  120  is lowered. Therefore, there is a concern about a long time period required until supply of high-temperature water to hot water supply faucet  330  after start of the next hot water supply operation. Therefore, water heating apparatus  100  is provided with an immediate hot water supply function for promptly supplying high-temperature water after start of the hot water supply operation. The immediate hot water supply function is performed by forming an immediate hot water supply circulation path including combustion mechanism  30  and heat exchanger  40  by activation of circulation pump  80  while the faucet is closed, that is, while hot water supply faucet  330  is closed. 
     For example, a user can designate by timer setting, a period for which the immediate hot water supply operation is to be performed. Timer setting can be input, for example, by operating remote controller  92 . Alternatively, the period for which the immediate hot water supply operation is to be performed may automatically be set based on learning of a history of use by the user in the past. Alternatively, the period for which the immediate hot water supply operation is performed can also be started or ended directly in response to a switch operation by the user. 
     In water heating system  1 A, the immediate hot water supply operation mode with activation of circulation pump  80  can be executed by using crossover valve  200 . Crossover valve  200  is configured similarly to the thermostatically controlled bypass valve described in U.S. Pat. No. 6,536,464 and includes ports  201  to  204  and a wax thermostatic element  210 . Ports  201  and  203  internally communicate with each other and ports  202  and  204  internally communicate with each other. Wax thermostatic element  210  is connected between ports  201  and  203  and ports  202  and  204 . 
     Wax thermostatic element  210  forms a thermal bypass path between ports  201  and  203  and ports  202  and  204  in a low-temperature state. Wax thermostatic element  210  closes the thermal bypass path owing to thermal expansion force in a high-temperature state. A switching temperature at which switching between formation and closing of the thermal bypass path is made is designed in advance depending on a material and a configuration of wax thermostatic element  210 . A state that a fluid temperature in crossover valve  200  is higher than the switching temperature is also referred to as a high-temperature state and a state that the fluid temperature is lower than the switching temperature is also referred to as a low-temperature state below. 
     Crossover valve  200  thus corresponds to an embodiment of the “thermal water stop bypass valve.” A pressure loss in the thermal bypass path is designed to be higher than a pressure loss in each of a path through which ports  201  and  203  communicate with each other and a path through which ports  202  and  204  communicate with each other. 
     Port  201  is connected to high-temperature water pipe  120  and port  202  is connected to low-temperature water pipe  110 . Ports  203  and  204  are connected to hot water supply faucet  330 . Hot water supply faucet  330  is provided as a combination faucet in which high-temperature water from port  203  and low-temperature water from port  204  are mixed. Valves  331  and  332  for adjustment of a ratio of mixing between high-temperature water and low-temperature water can be provided between port  204  and hot water supply faucet  330  and between port  203  and hot water supply faucet  330 , respectively. 
       FIG.  3    shows a chart illustrating switching between flow paths by means of crossover valve  200  shown in  FIG.  1   . 
     Referring to  FIGS.  3  and  1   , while the faucet is open, that is, while paths from ports  203  and  204  to hot water supply faucet  330  are formed, due to the pressure loss described above, in each of the high-temperature state and the low-temperature state, a flow path Pa between high-temperature water pipe  120  and hot water supply faucet  330  and a flow path Pb between low-temperature water pipe  110  and hot water supply faucet  330  are formed. 
     While the faucet is closed, that is, while the paths from ports  203  and  204  to hot water supply faucet  330  are cut off, the flow path is switched between the low-temperature state and the high-temperature state. In the low-temperature state, a thermal bypass path Pc is formed between ports  201  and  202 , that is, between high-temperature water pipe  120  and low-temperature water pipe  110 , through a thermal bypass path formed in wax thermostatic element  210 . In the high-temperature state, the thermal bypass path is closed so that the flow path between high-temperature water pipe  120  and low-temperature water pipe  110  is cut off. 
     In the hot water supply operation, in water heating system  1 A, high-temperature water is obtained by heating of low-temperature water introduced into water entry port  11  through low-temperature water pipe  110  by combustion mechanism  30  and heat exchanger  40  (heating mechanism). High-temperature water is output from hot water supply faucet  330  through hot water output port  12  and high-temperature water pipe  120  as well as crossover valve  200  (flow path Pa). 
     In the immediate hot water supply operation mode, as circulation pump  80  is activated, a fluid path (outer path) from hot water output port  12  through high-temperature water pipe  120 , crossover valve  200  (thermal bypass path Pc), and low-temperature water pipe  110  to circulation port  13  can be formed on the outside of water heating apparatus  100 . In addition, in the inside of water heating apparatus  100 , a fluid path (an inner path) including circulation port  13 , circulation path  28 , water entry path  20  (on the downstream side of connection point  22 ), heat exchanger  40  (heating mechanism), hot water output path  25 , and hot water output port  12  can be formed. By forming the immediate hot water supply circulation path by the inner path and the outer path as such, high-temperature water flows through the immediate hot water supply circulation path also while the faucet is closed, so that high-temperature water can be supplied to hot water supply faucet  330  from immediately after the faucet is opened. 
     In the configuration in which water heating apparatus  100  includes the bypass configuration (bypass path  29  and flow rate regulation valve  90 ), the bypass flow rate ratio in the immediate hot water supply operation mode is preferably fixed to a predetermined identical value. In particular, a pressure loss in the thermal bypass path formed by wax thermostatic element  210  is high. Therefore, in consideration of a low flow rate in the immediate hot water supply circulation path including crossover valve  200 , in the immediate hot water supply operation mode, flow rate regulation valve  90  is preferably controlled to maintain the bypass flow rate ratio to a minimum value (including a value when the valve is fully closed). 
     In the present embodiment, the description proceeds below assuming that a bypass ratio r (0≤r&lt;1.0) in water heating apparatus  100  in the immediate hot water supply operation mode is controlled to r=0 by fully closing flow rate regulation valve  90 . In this case, a flow rate in the immediate hot water supply circulation path is equal to a flow rate detection value obtained by flow rate sensor  81 . When bypass ratio r is not equal to 0 (r≠0) as well, by correcting a flow rate detection value Q obtained by flow rate sensor  81  by a factor of 1/(1−r) by using a bypass ratio in accordance with opening of flow rate regulation valve  90  at that time, control processing as will be described later can be applied. 
     When hot water supply faucet  330  is used in the immediate hot water supply operation mode, circulation pump  80  is preferably deactivated. As described above, in the normal hot water supply operation, circulation pump  80  is inactive. Therefore, when hot water is supplied while circulation pump  80  is maintained active, the supply pressure of low-temperature water through flow path Pb ( FIG.  1   ) is lower than in the normal hot water supply operation. Consequently, when balance between the pressure of high-temperature water and the pressure of low-temperature water is varied in hot water supply faucet  330  as compared with balance in the normal hot water supply operation, a temperature of output from hot water supply faucet  330  changes due to change in balance of mixing between high-temperature water and low-temperature water, which leads to a concern about lowering in usability by a user. Therefore, it is required to accurately detect start of use of hot water supply faucet  330  (which is also referred to as “hot water supply interrupt” below) in the immediate hot water supply operation. 
     Referring again to  FIG.  1   , in general, in the configuration in which circulation path  28  is provided, in the immediate hot water supply operation mode, a difference between a flow rate detected by flow rate sensor  82  and a flow rate detected by flow rate sensor  81  changes in response to activation of circulation pump  80 , and the difference is different between before and after opening of hot water supply faucet  330 . Therefore, hot water supply interrupt in the immediate hot water supply operation mode can be detected based on a difference in flow rate detected by flow rate sensors  81  and  82 . 
     In the configuration in which crossover valve  200  is connected, however, a pressure loss in the thermal bypass path formed by wax thermostatic element  210  is high as described above and hence the flow rate detected by flow rate sensor  82  in the immediate hot water supply operation mode is low. Therefore, the difference in flow rate detected by flow rate sensors  81  and  82  is not much different between before and after opening of hot water supply faucet  330 . Accordingly, it is difficult to accurately detect hot water supply interrupt based on a difference in flow rate detected by flow rate sensors  81  and  82 . 
     In consideration of such an aspect, in the present embodiment, use of hot water supply faucet  330  in the immediate hot water supply operation mode, that is, hot water supply interrupt, is detected as below. 
       FIG.  4    is a flowchart illustrating control processing in the immediate hot water supply operation mode by the water heating apparatus according to the present embodiment. Control processing shown in  FIG.  4    is repeatedly performed by controller  10  during a period provided by timer setting or the like for which the immediate hot water supply operation is performed. 
     Referring to  FIG.  4   , controller  10  determines in a step (which is simply also denoted as “S” below)  100 , whether or not a condition for starting the immediate hot water supply operation mode has been satisfied. For example, the start condition is satisfied when a temperature detected by temperature sensor  71  is lower than a predetermined temperature while the hot water supply operation is off (while the faucet is closed). 
     When the start condition has been satisfied (determination as YES in S 100 ), controller  10  starts the immediate hot water supply operation mode by starting up processing in S 110  or later. When the start condition has not been satisfied (determination as NO in S 100 ), processing in S 110  or later is not started up. 
     When controller  10  activates circulation pump  80  in S 130 , the immediate hot water supply circulation path described above is formed in water heating system  1 A. Combustion mechanism  30  is ready for activation in the immediate hot water supply operation mode, and it is activated and generates a quantity of heat while flow rate sensor  81  detects a flow rate exceeding a minimum operating quantity (MOQ) of working water. 
     When circulation pump  80  is activated (S 130 ), in S 110 , controller  10  reads a flow rate learning value Qln in the immediate hot water supply operation mode, and in S 120 , controller  10  sets a criterion value Qth for detection of hot water supply interrupt in accordance with read flow rate learning value Qln. 
     In the immediate hot water supply operation mode in which circulation pump  80  is active, in S 140 , controller  10  determines whether or not hot water supply interrupt is occurring based on comparison between a flow rate detection value Q obtained by flow rate sensor  81  and criterion value Qth set in S 120 . 
     While flow rate detection value Q does not exceed criterion value Qth (determination as NO in S 140 ), the immediate hot water supply operation mode is continued in S 150 . While the immediate hot water supply operation mode is continued, controller  10  determines in S 160  whether or not a condition for learning of the flow rate has been satisfied. When the learning condition has been satisfied (determination as YES in S 160 ), in S 170 , processing for updating the flow rate learning value which will be described later is performed, and thereafter the process returns to S 140 . When the learning condition has not been satisfied (determination as NO in S 160 ), S 170  is skipped and the process returns to S 140 . Thus, in the immediate hot water supply operation mode, determination as to detection of hot water supply interrupt in S 140  is repeatedly made. 
     When flow rate detection value Q exceeds criterion value Qth continuously for a certain time period (for example, approximately 0.3 second), controller  10  makes determination as YES in S 140 , and detects hot water supply interrupt in S 180 . Furthermore, controller  10  deactivates circulation pump  80  in S 190 . Consequently, the immediate hot water supply operation mode is once quitted and the hot water supply operation is started. In this case, the process returns to S 100 . When the hot water supply operation is stopped and the temperature detected by temperature sensor  71  becomes lower than a predetermined temperature while the immediate hot water supply operation is being performed, the immediate hot water supply operation mode is started again in response to determination as YES in S 100 . 
     When the temperature detected by temperature sensor  71  increases while the immediate hot water supply operation mode is continued (S 150 ) as well, the process proceeds to S 190  as shown with a dotted line in the figure, and the immediate hot water supply operation mode is once quitted by deactivating circulation pump  80 . In this case as well, as in detection of hot water supply interrupt, the process returns to S 100 . 
       FIG.  5    shows a conceptual waveform diagram of a flow rate detection value in the immediate hot water supply operation mode. The ordinate in  FIG.  5    represents flow rate detection value Q obtained by flow rate sensor  81 . 
     Referring to  FIG.  5   , at time t 0 , determination as YES is made in S 100  ( FIG.  4   ) and the immediate hot water supply operation mode is started. Since a temperature of retained fluid is low at the time point of start of the immediate hot water supply operation mode, crossover valve  200  is in such a state that the thermal bypass path has been formed by wax thermostatic element  210 . Therefore, from time t 0 , the flow rate in the immediate hot water supply circulation path increases in response to activation of circulation pump  80  and flow rate detection value Q increases. During a period until a temperature of wax thermostatic element  210  increases to close the thermal bypass path, the flow rate in the immediate hot water supply circulation path (flow rate detection value Q) is substantially constant. Therefore, in order to learn flow rate detection value Q during that period, at timing (time tx) after lapse of a predetermined time period Ta (for example, approximately five seconds) since time t 0 , learning processing shown in  FIG.  6    is started up. In the example in  FIG.  5   , after time tx, flow rate detection value Q exceeds criterion value Qth set in S 120  in  FIG.  4   , and thus hot water supply interrupt is detected at time t 1 . 
       FIG.  6    is a flowchart illustrating processing for learning a flow rate detection value. The flowchart shown in  FIG.  6    is started up at time tx. 
     Referring to  FIG.  6   , in S 210 , controller  10  stores flow rate detection value Q at time tx as an actual flow rate value Qx. Furthermore, controller  10  determines whether or not the learning condition has been satisfied in S 220  to S 240 . 
     In S 220 , checking of actual flow rate value Qx against an upper limit and a lower limit is performed. For example, when relation of Qxmin&lt;Qx&lt;Qxmax is satisfied based on comparison of a predetermined upper limit value Qxmax and a predetermined lower limit value Qxmin with actual flow rate value Qx (S 210 ), determination as YES is made in S 220 , and otherwise, determination as NO is made in S 220 . When actual flow rate value Qx is out of the range between the upper limit and the lower limit (determination as NO in S 220 ), in S 260 , learning using actual flow rate value Qx in S 210  is not carried out. 
     In S 230 , by monitoring flow rate detection value Q at time tx or later, whether or not hot water supply interrupt is absent until lapse of a predetermined time period Tb (Tb&gt;Ta, Tb being, for example, approximately ten seconds) since time t 0  is determined. In the example in  FIG.  5   , since time t 1  comes after lapse of prescribed time period Tb since time t 0 , determination as YES is made in S 230 . 
     On the other hand, when Q is larger than Qth (Q&gt;Qth) and hot water supply interrupt is detected before lapse of prescribed time period Tb since to as in the example in  FIG.  7   , determination as NO is made in S 230 . 
     In S 240 , whether or not change in flow rate detection value Q at time tx or later is equal to or smaller than a prescribed value is determined. 
     For example, as shown in  FIG.  8   , whether or not flow rate detection value Q at each timing is within a range of Qx-β&lt;Q&lt;Qx+β until lapse of a predetermined time period Tc (for example, approximately four seconds) since time tx is determined by using a prescribed reference value β. When relation of Qx-β&lt;Q&lt;Qx+β is maintained until lapse of Tc since time t 0 , determination as YES is made in S 240 . 
     When Q is smaller than Qx-β (Q&lt;Qx−β) at time ty before lapse of Tc since time tx as in the example in  FIG.  8   , on the other hand, determination as NO is made in S 240 . 
     Referring again to  FIG.  6   , when determination as YES is made in all of S 220  to S 240 , it is determined in S 250  that the learning condition has been satisfied and determination as YES is made in S 160  ( FIG.  4   ). Consequently, in S 170  in  FIG.  4   , flow rate learning value Qln is updated by using actual flow rate value Qx (S 210 ) stored in the present immediate hot water supply operation mode. Flow rate learning value Qln read in S 110  in the next immediate hot water supply operation mode is thus updated. After S 170  is performed, determination as NO is maintained in S 160  until the immediate hot water supply operation mode is quitted. 
     When determination as NO is made in at least any one of S 220  to S 240  in  FIG.  6   , the process proceeds to S 260  and determination as “No” is made in S 160 . When the immediate hot water supply operation mode ends without determination as YES in S 160 , learning using actual flow rate value Qx in S 210  in the immediate hot water supply operation mode is not carried out. Flow rate learning value Qln read in S 110  in the next immediate hot water supply operation mode does not change from the value read in S 110  in the present immediate hot water supply operation mode. 
       FIG.  9    shows a conceptual diagram illustrating learning of a flow rate value in a circulation operation mode. 
     Referring to  FIG.  9   , during a period set by the timer or the like for which the immediate hot water supply operation is performed, the immediate hot water supply operation mode is intermittently provided in such a manner as being started each time determination as YES is made in S 100  and quitted by deactivation of circulation pump  80  in S 190 . In the example in  FIG.  9   , within periods T 1  and T 2  for which the immediate hot water supply operation is performed, the immediate hot water supply operation mode is provided for periods P 1  to P 4 . 
     During each of periods P 1  to P 4 , at timing corresponding to time tx in  FIG.  5   , actual flow rate value Qx is read. Thereafter, in accordance with determination in S 220  to S 240  in  FIG.  6   , for example, the flow rate learning value is updated (S 170 ) during periods P 1 , P 2 , and P 4 , whereas flow rate learning value Qln is not updated during period P 3  because determination as YES is not made in all of S 220  to S 240 . 
     Flow rate learning value Qln is calculated based on a plurality of actual flow rate values Qx including actual flow rate value Qx in the immediate hot water supply operation mode in which processing for updating the learning value is performed and actual flow rate value Qx in the immediate hot water supply operation mode in the past. Preferably, flow rate learning value Qln can be calculated as an exponential moving average value in accordance with an expression (1) below:
 
 Qln *=( N×Qln+Qx )/( N+ 1)  (1)
 
where Qln* represents an updated flow rate learning value, Qln represents a current (yet-to-be-updated) flow rate learning value, and Qx represents an actual flow rate value stored in the immediate hot water supply operation mode in which processing for updating the learning value is performed. N (N&gt;0) represents a smoothing factor. As N is greater, a speed of reflection of a new actual flow rate value Qx on a flow rate learning value (learning speed) is lower.
 
     An initial value for learning value Qln can be set by writing a standard value into memory  16  of controller  10  at the time of shipment from the factory. Alternatively, an initial value can be set also by writing a standard value adapted to crossover valve  200  into memory  16  by performing a predetermined specific operation onto remote controller  92  at the time of works for attachment of crossover valve  200 . 
     Updated flow rate learning value Qln* is preferably checked against upper and lower limits. For example, in S 170 , in checking against a predetermined upper limit value Qlnmax and a predetermined lower limit value Qlnmin, when Qln* calculated in accordance with the expression (1) is larger than upper limit value Qlnmax (Qln*&gt;Qlnmax), Qln* is corrected to Qln*=Qlnmax. Similarly, when Qln* calculated in accordance with the expression (1) is smaller than lower limit value Qlnmin (Qln*&lt;Qlnmin), Qln* is corrected to Qln*=Qlnmin. 
     As described above, in water heating system  1 A described with reference to  FIG.  1   , even when a flow rate in the immediate hot water supply circulation path formed to include the thermal bypass path formed by wax thermostatic element  210  of crossover valve  200  changes over time, such change in flow rate can appropriately be reflected on a criterion value for detection of hot water supply interrupt through learning of the flow rate value. Therefore, accuracy in detection of use of the hot water supply faucet in the immediate hot water supply operation in water heating system  1 A can be improved. 
     Determination as to hot water supply interrupt based on a flow rate learning value can be made only based on the flow rate detection value obtained by flow rate sensor  81  without using a flow rate detection value obtained by flow rate sensor  82  arranged in circulation path  28 . Consequently, flow rate sensor  82  unnecessary in the hot water supply operation does not have to be arranged. 
     In S 120  in  FIG.  4   , criterion value Qth (S 120 ) is set preferably to a value larger than flow rate learning value Qln (S 110 ), such as Qth=Qln+α. As described above, in the immediate hot water supply operation mode, flow rate regulation valve  90  is controlled to minimize the bypass flow rate ratio. Therefore, when transition to the hot water supply operation is made while the flow rate is low, the flow rate detection value obtained by flow rate sensor  81  may be equal to or smaller than the minimum operating quantity (MOQ) of working water and combustion mechanism  30  may not be activated. Therefore, by setting criterion value Qth beyond which transition is made from the immediate hot water supply operation mode to the hot water supply operation to be high to some extent, combustion mechanism  30  can reliably be activated immediately after detection of hot water supply interrupt. 
     By taking into account variation in flow rate for a factor different from a factor for change in flow rate in the immediate hot water supply circulation path by using S 220  to S 240  in the learning processing in  FIG.  6   , incorrect learning of flow rate learning value Qln can be suppressed. 
     In water heating system  1 A according to the present embodiment, an abnormal condition of the immediate hot water supply circulation path can also be diagnosed based on the flow rate learning value described above. 
       FIG.  10    is a flowchart illustrating diagnosis of an abnormal condition in the immediate hot water supply circulation path in the water heating system according to the present embodiment. 
     Referring to  FIG.  10   , when the flow rate learning value is updated in S 170  ( FIG.  4   ), controller  10  makes determination as YES in S 310 , and makes abnormal condition diagnosis in S 320  or later. Controller  10  determines in step S 320  whether or not updated flow rate learning value Qln is within a predetermined normal range (Ql to Qh). 
     When the bypass flow path is clogged in crossover valve  200 , the flow rate in the immediate hot water supply circulation path becomes lower than the normal range. When breakage occurs in crossover valve  200 , on the other hand, the flow rate in the immediate hot water supply circulation path becomes higher than the normal range. 
     Therefore, when a condition of Qln&lt;Ql or Qln&gt;Qh is satisfied (determination as NO in S 320 ), controller  10  senses an abnormal condition of the immediate hot water supply circulation path in S 340 . In S 340 , a user is preferably notified of sensing of the abnormal condition through notification apparatus  95 . In this case, different information can be given between the condition of Qln&lt;Ql and the condition of Qln&gt;Qh. 
     When a condition of Ql≤Qln≤Qh is satisfied (determination as YES in S 320 ), controller  10  does not sense an abnormal condition of the immediate hot water supply circulation path in S 330 . Lower limit value Ql and upper limit value Qh of the normal range may be common to lower limit value Qlnmin and upper limit value Qlnmax in checking of the flow rate learning value against the upper limit and the lower limit described above, respectively, or may separately be set. 
     Thus, in the water heating system according to the present embodiment, an abnormal condition in the immediate hot water supply circulation path can be diagnosed based on the flow rate learning value in the immediate hot water supply operation mode. In particular, by making determination based on the flow rate learning value, abnormal condition diagnosis that achieves suppressed erroneous detection of the abnormal condition at the time of detection of a sporadic abnormal value due to temporary malfunction of crossover valve  200  can be realized. 
     A modification of the configuration of the water heating system to which detection of hot water supply interrupt in the immediate hot water supply operation mode can be applied according to the present embodiment will now further be described. 
       FIG.  11    shows a block diagram illustrating a first modification of the configuration of the water heating system according to the present embodiment. 
     Referring to  FIG.  11   , a water heating system  1 B includes a water heating apparatus  100 X, low-temperature water pipe  110 , high-temperature water pipe  120 , and crossover valve  200 . Water heating apparatus  100 X includes water entry port  11  and hot water output port  12  without including circulation port  13 . Therefore, unlike water heating apparatus  100  in  FIG.  1   , no circulation path  28  is provided in the inside of water heating apparatus  100 X. 
     Low-temperature water pipe  110  supplied with low-temperature water through check valve  112  has a first end connected to water entry port  11  of water heating apparatus  100 X and a second end connected to port  202  of crossover valve  200 . Connection of crossover valve  200  to low-temperature water pipe  110 , high-temperature water pipe  120 , and hot water supply faucet  330  is the same as in water heating system  1 A shown in  FIG.  1   . Circulation pump  80  is connected to water entry port  11 . 
     In water heating system  1 B, during the hot water supply operation, at least some of low-temperature water introduced from low-temperature water pipe  110  into water entry port  11  is heated by the heating mechanism (combustion mechanism  30  and heat exchanger  40 ). High-temperature water obtained by heating is output from hot water supply faucet  330  through hot water output port  12  and high-temperature water pipe  120  as well as crossover valve  200  (flow path Pa) as in water heating system  1 A. Water heating apparatus  100 X can thus perform the hot water supply operation similarly to water heating apparatus  100 . 
     In the immediate hot water supply operation mode, as circulation pump  80  is activated while the faucet is closed, a fluid path (outer path) from hot water output port  12  through high-temperature water pipe  120 , crossover valve  200  (thermal bypass path Pc), and low-temperature water pipe  110  to water entry port  11  can be formed on the outside of water heating apparatus  100 X. In addition, an inner path that passes through water entry port  11 , water entry path  20 , heat exchanger  40  (heating mechanism), hot water output path  25 , and hot water output port  12  can be formed in the inside of water heating apparatus  100 X as in  FIG.  1   . The immediate hot water supply circulation path can be formed by the inner path and the outer path also in water heating system  1 B. In the immediate hot water supply operation mode, flow rate sensor  81  can detect a flow rate in the immediate hot water supply circulation path and temperature sensor  73  can detect a temperature of fluid in the immediate hot water supply circulation path. 
     In water heating system  1 B as well, a behavior of the flow rate detection value obtained by flow rate sensor  81  is similar to the behavior in water heating system  1 A. Therefore, hot water supply interrupt during the immediate hot water supply operation can be detected in accordance with the control processing in  FIGS.  4  and  6   . Furthermore, abnormal condition diagnosis based on the flow rate learning value can also be made in accordance with the control processing in  FIG.  10    as in water heating system  1 A. 
     Crossover valve  200  described in U.S. Pat. No. 6,536,464 and shown in the present embodiment is merely an exemplary “thermal water stop bypass valve” and a valve containing a thermal bypass path of which formation and closing are switched depending on a temperature could be employed instead of crossover valve  200  in the present embodiment. 
     Detection of hot water supply interrupt in the immediate hot water supply operation mode according to the present embodiment can be applied also to a water heating system configured such that the immediate hot water supply circulation path is disposed by disposing a circulation pipe without crossover valve  200  (that is, the “thermal water stop bypass valve”). 
       FIG.  12    shows a block diagram illustrating a second modification of the configuration of the water heating system according to the present embodiment. 
     Referring to  FIG.  12   , a water heating system  2 A includes water heating apparatus  100  as in  FIG.  1   , low-temperature water pipe  110 , high-temperature water pipe  120 , and circulation pipe  130 . Crossover valve  200  shown in  FIG.  1    is not externally connected to water heating apparatus  100 . 
     As in  FIG.  1   , low-temperature water pipe  110  supplied with low-temperature water through check valve  112  is connected to water entry port  11  and high-temperature water pipe  120  connects hot water output port  12  and hot water supply faucet  330  to each other. Circulation pipe  130  connects high-temperature water pipe  120  and circulation port  13  to each other. 
     By activating circulation pump  80  while the faucet is closed also in water heating system  2 A, a fluid path (inner path) as in water heating system  1 A can be formed in the inside of water heating apparatus  100 . In addition, a fluid path (outer path) that includes hot water output port  12 , high-temperature water pipe  120 , circulation pipe  130 , and circulation port  13  and bypasses hot water supply faucet  330  can be formed on the outside of water heating apparatus  100 . Consequently, the immediate hot water supply circulation path can be formed by the inner path and the outer path, and hence the immediate hot water supply operation mode as in water heating system  1 A can be executed. 
     In water heating system  2 A as well, hot water supply interrupt in the immediate hot water supply operation mode can be detected by learning the flow rate detection value obtained by flow rate sensor  81  in the immediate hot water supply operation mode in accordance with the control processing in  FIGS.  4  and  6   . Thus, change over time in the immediate hot water supply circulation path can be reflected and accuracy in detection of use of the hot water supply faucet in the immediate hot water supply operation can be improved without flow rate sensor  82  in circulation path  28 . An abnormal condition in the immediate hot water supply circulation path can also be diagnosed based on the flow rate learning value in the immediate hot water supply operation mode. 
       FIG.  13    shows a block diagram illustrating a third modification of the configuration of the water heating system according to the present embodiment. 
     Referring to  FIG.  13   , a water heating system  2 B includes water heating apparatus  100 X as in  FIG.  11   , low-temperature water pipe  110 , high-temperature water pipe  120 , and circulation pipe  130 . Crossover valve  200  shown in  FIG.  11    is not externally connected to water heating apparatus  100 X. 
     As in  FIG.  11   , low-temperature water pipe  110  supplied with low-temperature water through check valve  112  is connected to water entry port  11  of water heating apparatus  100 X and high-temperature water pipe  120  connects hot water output port  12  of water heating apparatus  100 X and hot water supply faucet  330  to each other. Circulation pipe  130  connects high-temperature water pipe  120  and low-temperature water pipe  110  to each other. 
     Circulation pump  80  can be connected to circulation pipe  130 . During the hot water supply operation in which circulation pump  80  is deactivated, as hot water supply faucet  330  is opened, at least some of low-temperature water introduced from low-temperature water pipe  110  into water entry port  11  is heated by the heating mechanism (combustion mechanism  30  and heat exchanger  40 ). High-temperature water obtained by heating is output from hot water supply faucet  330  through hot water output port  12  and high-temperature water pipe  120 . Water heating system  2 B can thus also perform the hot water supply operation by water heating apparatus  100 X. 
     By activating circulation pump  80  while the faucet is closed also in water heating system  2 B, a fluid path (inner path) as in water heating system  1 B can be formed in the inside of water heating apparatus  100 X. In addition, a fluid path (outer path) that extends from hot water output port  12  through high-temperature water pipe  120 , circulation pipe  130 , and low-temperature water pipe  110  to water entry port  11  and bypasses hot water supply faucet  330  can be formed on the outside of water heating apparatus  100 X. Consequently, the immediate hot water supply circulation path can be formed also in water heating system  2 B. By forming the immediate hot water supply circulation path by the inner path and the outer path, the immediate hot water supply operation mode the same as described in connection with water heating system  1 A can be executed. 
     In water heating system  2 B as well, hot water supply interrupt in the immediate hot water supply operation mode can be detected by learning the flow rate detection value obtained by flow rate sensor  81  in the immediate hot water supply operation mode in accordance with the control processing in  FIGS.  4  and  6   . Thus, change over time in the immediate hot water supply circulation path can be reflected and accuracy in detection of use of the hot water supply faucet during the immediate hot water supply operation can be improved without flow rate sensor  82  in circulation path  28 . An abnormal condition in the immediate hot water supply circulation path can also be diagnosed based on the flow rate learning value in the immediate hot water supply operation mode. 
     In water heating systems  1 A,  1 B,  2 A, and  2 B, so long as the immediate hot water supply circulation path as above can be formed, circulation pump  80  can be arranged at any position on the outside or in the inside of water heating apparatus  100  without being limited to the configuration in the illustration in  FIGS.  1  and  11  to  13   . Even in such a configuration that circulation pump  80  is not contained in water heating apparatus  100 , the immediate hot water supply operation mode described in the present embodiment can be realized by including controller  10  that controls deactivation and activation of circulation pump  80 . 
     Though an example in which water heating apparatuses  100  and  100 X each include a bypass configuration (bypass path  29  and flow rate regulation valve  90 ) is described in the present embodiment, detection of hot water supply interrupt and diagnosis of an abnormal condition of the immediate hot water supply circulation path based on the flow rate learning value detected by flow rate sensor  81  in the immediate hot water supply operation mode described in the present embodiment can be applied also to the configuration of water heating apparatuses  100  and  100 X from which the bypass configuration is excluded. In this case, the flow rate detection value obtained by flow rate sensor  81  is always equal to the flow rate in the immediate hot water supply circulation path. 
     Though embodiments of the present invention have been described, it should be understood that the embodiments disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.