Patent Publication Number: US-2021180800-A1

Title: Hot-water supply device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Japan Patent Application No. 2019-224401, filed on Dec. 12, 2019. The entirety of each of the above-mentioned patent applications is hereby incorporated by reference herein and made a part of this specification. 
     BACKGROUND 
     Technical Field 
     The disclosure relates to control of a hot-water supply device, and more particularly relates to control of a hot-water supply device having an instant hot-water function. 
     Related Art 
     Regarding a hot-water supply device having a so-called instant hot-water function that eliminates waiting for hot-water supply, for example, Japanese Patent No. 2555840 (patent literature 1) discloses “a hot-water supplier control method that improves hot-water discharge characteristics at the time of hot-water discharge start or hot-water re-discharge under various conditions, such as a length of a hot-water discharge stop time or the like, and has an instant hot-water discharge function” (see paragraph [0005]). 
     SUMMARY 
     In a hot-water supply device which has a heating flow path for supplying water to a heat exchanger to supply warm water after heating performed by the heat exchanger and a bypass flow path for bypassing the heat exchanger, when hot-water supply is stopped, an amount of water sent to the bypass flow path is adjusted to be larger than an amount of water sent to the heating flow path. When the amount of the water sent to the heating flow path decreases, if the hot-water supply is restarted when the hot-water supply device includes, in the heating flow path, a water amount sensor that detects an amount of water to be heated, there is a possibility that the water amount sensor cannot detect the amount of water in the heating flow path. As a result, stable hot-water supply may not be performed. 
     In addition, in an area where waterworks infrastructure is not developed, only clean water with a water pressure even lower than an assumed low water pressure may be supplied. In that case, in a hot-water supply device including a total water amount servo and a bypass water amount servo, a stepping motor of the bypass water amount servo is moved to a position close to a fully-open position in order that an amount of supply to the bypass flow path increases when a set temperature for hot-water supply is low. As a result, the amount of water supplied to the heat exchanger is reduced. Considering this point, a method of setting a standby position of the stepping motor of the total water amount servo may also be considered. However, there is the possibility that the amount of the water supplied to the heat exchanger increases when the set temperature for the hot-water supply is high, and low-temperature water flows out from the heat exchanger before the water is sufficiently heated, or there is also a possibility that the amount of the water to the heat exchanger is small in the case of low water entry temperature, the amount of the water cannot be detected, and combustion does not occur. 
     Thus, there is a need for a technique that enables stable hot-water supply. The disclosure is completed in view of the background as described above, and an object in an aspect is to provide a technique in which a hot-water supply device having an instant hot-water function can stably supply hot water. 
     A hot-water supply device according to an embodiment includes: a heat exchanger; a bypass flow path which is connected to a water entry path to the heat exchanger and a hot-water discharge path from the heat exchanger; a bypass water amount adjustment portion which has an opening/closing portion and adjusts each amount of water flowing to the water entry path and the bypass flow path by adjusting an opening/closing degree of the opening/closing portion; a hot-water supply water amount adjustment portion which adjusts an amount of water after merging between the hot-water discharge path and the bypass flow path; a storage portion which stores a set temperature of hot-water supply; and a control device which controls an action of the hot-water supply device. The control device adjusts, when the hot-water supply is stopped, an opening degree of the hot-water supply water amount adjustment portion according to an opening degree specified to be proportional to a change in the set temperature. 
     A hot-water supply device according to another embodiment includes: a heat exchanger; a bypass flow path which is connected to a water entry path to the heat exchanger and a hot-water discharge path from the heat exchanger; a bypass water amount adjustment portion which has an opening/closing portion and adjusts each amount of water flowing to the water entry path and the bypass flow path by adjusting an opening/closing degree of the opening/closing portion; a hot-water supply water amount adjustment portion which adjusts an amount of water after merging between the hot-water discharge path and the bypass flow path; and a control device which controls an action of the hot-water supply device. The control device adjusts, when the hot-water supply is stopped, an opening degree of the hot-water supply water amount adjustment portion according to a ratio of a flow amount to the bypass flow path relative to a flow amount to the water entry path. 
     In one aspect, the hot-water supply water amount adjustment portion is a stepping motor. The adjustment of the opening degree includes changing the number of steps of the stepping motor. 
     According to another embodiment, a control method of a hot-water supply device including a heat exchanger is provided. The control method includes: a step of detecting that hot-water supply performed by the hot-water supply device is stopped; a step of accessing a set temperature of the hot-water supply; and a step of adjusting, in response to the detection of the stop of the hot-water supply, each amount of water flowing to a water entry path toward the heat exchanger and a bypass flow path connected to a hot-water discharge path from the heat exchanger according to an opening degree specified to be proportional to a change in the set temperature. 
     According to another embodiment, a control method of a hot-water supply device including a heat exchanger includes: a step of detecting that hot-water supply performed by the hot-water supply device is stopped; and a step of adjusting, in response to the detection of the stop of the hot-water supply, an amount of water after merging between a hot-water discharge path and a bypass flow path according to a ratio of a flow amount to the bypass flow path connected to the hot-water discharge path from the heat exchanger relative to a flow amount to a water entry path toward the heat exchanger. 
     In one aspect, the step of adjustment includes changing the number of steps of a stepping motor arranged in the hot-water discharge path. 
     According to still another embodiment, a program is provided that causes a computer to execute any one of the above methods. 
     According to an embodiment, stable hot-water supply can be realized. 
     The above and other objects, features, aspects and advantages of the present invention are apparent from the following detailed description of the present invention which is understood in association with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing an example of a hardware configuration of a hot-water supply device  100 . 
         FIG. 2  is a block diagram showing an example of a hardware configuration of a control device  110 . 
         FIG. 3  is a diagram showing state transitions of the hot-water supply device  100  according to an embodiment. 
         FIG. 4  is a diagram showing a relationship between a set temperature of hot-water supply performed by the hot-water supply device  100  and a position of a stepping motor which configures a total water amount servo  130 . 
         FIG. 5  is a flowchart showing a part of a process executed by the control device  110  of the hot-water supply device  100 . 
         FIG. 6  is a diagram conceptually showing a configuration of flow paths of the hot-water supply device  100 . 
         FIG. 7  is a diagram showing a relationship between a bypass ratio and a standby position of the stepping motor. 
         FIG. 8  is a flowchart showing a part of a process executed by a control device  110  of a hot-water supply device  700 . 
         FIG. 9  is a diagram showing an example of a hardware configuration of a hot-water supply device  900  according to still another aspect. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Embodiments of the present invention are described below with reference to the drawings. In the following description, the same components are designated by the same signs. Names and functions of these components are also the same. Thus, detailed description thereof is not repeated. 
     [Hardware Configuration of Hot-Water Supply Device] 
     First, a configuration of a hot-water supply device  100  according to the embodiment is described with reference to  FIGS. 1 and 2 .  FIG. 1  is a block diagram showing an example of a hardware configuration of the hot-water supply device  100 . As shown in  FIG. 1 , the hot-water supply device  100  includes a control device  110 , a bypass water amount servo (also referred to as “bypass servo”)  122 , a can body  124 , a heat exchanger  126 , a combustion mechanism  128 , a total water amount servo  130 , a water amount sensor  131 , and temperature sensors  141 ,  142 , and  143 . The bypass water amount servo  122  and the total water amount servo  130  respectively include a stepping motor (not shown). 
     The heat exchanger  126  is connected to a water entry path  150  and a hot-water discharge path  152 . The bypass water amount servo  122  and the hot-water discharge path  152  are connected by a bypass flow path  151 . A water entry side of the bypass water amount servo  122  and a hot-water discharge side of the total water amount servo  130  are connected by a flow path  153 . More specifically, the flow path  153  connects a water entry portion  10  and a hot-water discharge portion  20 . When so-called instant hot-water circulation running is performed, warm water flows through the flow path  153 . At least one hot-water supply tap  21  is connected to the flow path  153 . 
     The hot-water supply device  100  receives supply of clean water from the water entry portion  10  and supplies warm water (hot water) from one or more faucets or hot-water supply taps via hot-water discharge portion  20 . When the hot-water supply device  100  does not perform circulation running, the hot-water supply device  100  receives the supply of the clean water from the water entry portion  10 . The hot-water supply device  100  is electrically connected to a remote controller  30  and a notification device  40 . The action of the hot-water supply device  100  is controlled according to an operation on the remote controller  30 . The notification device  40  notifies a state of the hot-water supply device  100  based on a signal sent from the hot-water supply device  100 . 
     The control device  110  respectively receives input of a signal output from the water amount sensor  131 , input of signals output from the temperature sensors  141 ,  142 , and  143 , and input of a signal transmitted from the remote controller  30 . The control device  110  controls the action of the hot-water supply device  100  based on the input signals and setting data specified in advance. More specifically, the control device  110  controls combustion in the hot-water supply device  100 , stopping of the combustion, an amount of water supplied to the heat exchanger  126 , and the like. 
     The bypass water amount servo  122  adjusts (distributes) water supplied from the water entry portion  10  to the water supplied to the heat exchanger  126  and water flowing into the bypass flow path  151 . The bypass water amount servo  122  can adjust a temperature of warm water from the heat exchanger  126  by adjusting the amount of the water supplied to the heat exchanger  126 . 
     The stepping motor of the bypass water amount servo  122  moves to a position derived from a relational expression consisting of a set temperature of the hot-water supply, a temperature of the heat exchanger  126 , and a water entry temperature. For example, when the temperature of the heat exchanger  126  is used as a variable and the temperature of the heat exchanger  126  decreases during hot-water discharge standby, a standby position in consideration of the decrease is set as a standby position of the stepping motor, and the control device  110  outputs, to the bypass water amount servo  122 , a command of moving the stepping motor to the standby position. 
     Water flowing into the heat exchanger  126  from the water entry path  150  flows out to the hot-water discharge path  152 . The heat exchanger  126  is heated by the combustion mechanism  128 . In one aspect, the combustion mechanism  128  is configured by a burner that generates heat by combustion of gas, oil, or the like. The heat exchanger  126  uses the heat generated by the combustion mechanism  128  to raise a temperature of the water introduced by the water entry path  150 . Thus, the heat exchanger  126  and the combustion mechanism  128  configure an example of a “heating mechanism”. 
     The water (hot water) whose temperature is raised by the heat exchanger  126  flows into the total water amount servo  130  through the hot-water discharge path  152 . The bypass flow path  151  is connected to the hot-water discharge path  152 . The high-temperature water output from the heat exchanger  126  is mixed with water (low-temperature water) supplied from the bypass water amount servo  122  through the bypass flow path  151 , and the temperature of the high-temperature water may be adjusted to a temperature instructed by the controller  110 . 
     The total water amount servo  130  changes an opening/closing degree of a valve (not shown) based on a signal output from the control device  110 , and thereby, such an amount of water that a temperature of the warm water supplied by the hot-water supply device  100  to the flow path  153  reaches the set temperature is adjusted according to a hot-water discharge capacity. Warm water flowing out from the total water amount servo  130  can be supplied from the hot-water supply tap  21  via the hot-water discharge portion  20 . Moreover, part of the warm water flowing out from the total water amount servo  130  is returned to the water entry portion  10  via the flow path  153 . When the hot-water supply tap  21  is closed and the warm water flowing out from the total water amount servo  130  is not supplied to the outside of the hot-water supply device  100  through the hot-water discharge portion  20 , the hot-water supply device  100  performs instant hot-water circulation running through an instant hot-water circulation flow path composed of the flow path  153 , the water entry path  150 , and the hot-water discharge path  152 . By this instant hot-water circulation running, the hot-water supply device  100  according to one embodiment can supply high-temperature water immediately after opening the hot-water supply tap  21 . 
     In addition, during standby for the hot-water supply, the stepping motor of the total water amount servo  130  moves to a standby position specified according to the set temperature. For example, when the stepping motor is set at a position where a flow amount from the bypass water amount servo  122  to the heat exchanger  126  is small, the stepping motor of the total water amount servo  130  moves to a position where the flow amount increases. 
     The remote controller  30  receives an operation of a user and transmits a signal corresponding to the operation to the hot-water supply device  100 . For example, the remote controller  30  receives input of settings for specifying running and stopping of the hot-water supply device  100 , a set temperature of warm water to be supplied, and other actions of the hot-water supply device  100 . The remote controller  30  is connected to the hot-water supply device  100  in a wired or wireless manner. 
     The notification device  40  notifies a state of the hot-water supply device  100  based on the signal output from control device  110 . In one aspect, the notification device  40  is realized by display, sound, and the like, and outputs information indicating a state of the hot-water supply device  100 . A notification form includes voice, image or text, light, and the like. In still another aspect, the notification device  40  can also be realized as a mobile terminal in which a program (app) for realizing notification of the hot-water supply device  100  is installed. 
     The water amount sensor  131  detects the amount of the water flowing into the heat exchanger  126 . The temperature sensor  141  detects the temperature of the water flowing into the heat exchanger  126 . The temperature sensor  142  detects the temperature of the warm water flowing out from the can body  124 . The temperature sensor  143  detects the temperature of the warm water supplied from the total water amount servo  130 . 
     The hot-water supply device  100  according to the embodiment can control a flow amount to the bypass flow path  151  during the instant hot-water circulation running, and can control the temperature of the hot water flowing in the flow path  153  during the instant hot-water circulation running and the hot-water supply running. 
     [Hardware Configuration of Control Device] 
       FIG. 2  is a block diagram showing an example of a hardware configuration of the control device  110 . The control device  110  is typically configured by a microcomputer. The control device  110  includes a central processing unit (CPU)  210 , a memory  220 , an input/output circuit  230 , and an electronic circuit  240 . The CPU  210 , the memory  220 , and the input/output circuit  230  can exchange signals with each other via a bus  250 . The electronic circuit  240  is configured to execute a preset arithmetic process by dedicated hardware. The electronic circuit  240  can exchange signals between the CPU  210  and the input/output circuit  230 . 
     The CPU  210  respectively receives input of output signals (detection values) from the respective sensors including the temperature sensors  141 ,  142 , and  143  and the water amount sensor  131  through input/output circuit  230 . Furthermore, the CPU  210  receives input of a signal indicating an operation instruction given to the remote controller  30  through the input/output circuit  230 . The operation instruction includes, for example, an on/off operation of a running switch of the hot-water supply device  100 , the hot-water supply set temperature, and various time reservation settings (also referred to as “timer setting”). The CPU  210  controls an action of each component including the combustion mechanism  128  and the total water amount servo  130  in order that the hot-water supply device  100  operates in accordance with the operation instruction. 
     The CPU  210  can output information that can be visually or acoustically recognized by controlling the notification device  40 . For example, the notification device  40  can output the information by displaying visually recognizable information such as texts, figures, and the like. In this case, the notification device  40  can be configured by a display screen of a monitor arranged in the remote controller  30 . Alternatively, the notification device  40  may be configured by a speaker and may use voice, melody, or the like to output the information. 
     The memory  220  holds data for specifying the action of the hot-water supply device  100 . In one aspect, the data includes a set temperature  221  and position determination data  222 . The set temperature  221  is set by the user of the hot-water supply device  100  via the remote controller  30  as the temperature of the warm water supplied by the hot-water supply device  100 . The position determination data  222  is data for determining the standby position of the stepping motor of the total water amount servo  130 . In one aspect, the position determination data  222  is a standby position (the number of steps of the stepping motor) derived in advance according to the set temperature. In another aspect, the position determination data  222  may be a standby position derived in advance according to a bypass flow amount ratio. 
     [State Transitions of Hot-Water Supply Device] 
     Action modes of the hot-water supply device  100  are described with reference to  FIG. 3 .  FIG. 3  is a diagram showing state transitions of the hot-water supply device  100  according to one embodiment. 
     As shown in  FIG. 3 , the action modes of the hot-water supply device  100  include a combustion function prohibiting mode  310 , a non-freezing prevention pump mode  311 , a hot-water supply mode  313 , and an instant hot-water mode  316 . The hot-water supply mode  313  includes a hot-water supply standby mode  314  and a hot-water supply combustion mode  315 . The instant hot-water mode  316  includes an instant hot-water standby mode  317  and an instant hot-water circulation mode  318 . 
     (Combustion function prohibiting mode) In one aspect, when power of the hot-water supply device  100  is turned on, an action mode of the hot-water supply device  100  is switched to the combustion function prohibiting mode  310  (step S 320 ). In the combustion function prohibiting mode  310 , the combustion mechanism is forcibly stopped and combustion is not performed. A command to the bypass water amount servo  122  instructs a stop at a preset position, and the bypass water amount servo  122  maintains the stopped state at the position. Similarly to the command to the bypass water amount servo  122 , a command to the total water amount servo  130  also instructs a stop at a preset position, and the total water amount servo  130  maintains the stop state at the position. Thereafter, when a preset combustion function is confirmed for the hot-water supply device  100  and it is confirmed that there is no abnormality, the action mode is switched from the combustion function prohibiting mode  310  to the hot-water supply standby mode  314  (step S 330 ). 
     (Hot-water supply standby mode) In the hot-water supply standby mode  314 , the hot-water supply device  100  is normally stopped. More specifically, each command from the control device  110  to the bypass water amount servo  122  and the total water amount servo  130  indicates “hot-water discharge standby”. 
     In one aspect, when the hot-water supply tap  21  is opened for hot-water supply, water is introduced into the water entry passage by supply pressure of the water supplied from the water entry portion  10 . When the water amount sensor  131  detects an amount of water that exceeds a minimum operation quantity (MOQ), the control device  110  operates the combustion mechanism  128 . That is, the hot-water supply device  100  is switched from the hot-water supply standby mode  314  to the hot-water supply combustion mode  315  (step S 331 ). 
     (Hot-water supply combustion mode) When the action mode becomes the hot-water supply combustion mode  315 , the control device  110  sends a command for combustion start to the combustion mechanism  128 . In response to the command, the combustion mechanism  128  starts the combustion. The control device  110  respectively outputs commands for controlling a hot-water discharge amount to the bypass water amount servo  122  and the total water amount servo  130 . The bypass water amount servo  122  and the total water amount servo  130  respectively adjust an opening degree of a valve (not shown) in accordance with the respectively input commands in order that designated hot water is supplied. 
     In one aspect, when the hot-water supply tap  21  is closed and the hot-water supply ends, the water amount sensor  131  thereafter detects a flow amount below the MOQ. In response to the detection, the control device  110  outputs a command for stopping combustion to the combustion mechanism  128 . In response to the command, the combustion mechanism  128  ends the combustion action. Furthermore, the control device  110  outputs a “hot-water discharge standby” command as each command to the bypass water amount servo  122  and the total water amount servo  130 . The bypass water amount servo  122  and the total water amount servo  130  are switched to a preset state as a hot-water discharge standby state. Thereby, the action mode of the hot-water supply device  100  is switched from the hot-water supply combustion mode  315  to the hot-water supply standby mode  314  (step S 332 ). 
     (Instant hot-water standby mode) In the hot-water supply standby mode  314 , when a post-purge (exhaust action) ends in a case where there is an instant hot-water request or a freezing prevention request, the action mode is switched to the instant hot-water standby mode  317  (step S 340 ). In the embodiment, the instant hot-water request means an instruction that instant water heating is performed only once (also simply referred to as “one instant water heating”) at the arrival of a pre-reserved instant hot-water time or within a predetermined time (for example, 30 minutes). In the instant hot-water standby mode  317 , when the control device  110  does not detect the instant hot-water request and the freezing prevention request, the action mode is switched to the hot-water supply standby mode  314  (step S 341 ). Moreover, a state of the hot-water supply device  100  in the hot-water supply standby mode  314  and a state of the hot-water supply device  100  in the instant hot-water standby mode  317  are the same. 
     In one aspect, when the temperature of the temperature sensor  143  that measures the temperature of the warm water flowing out from the heat exchanger  126  is equal to or higher than a temperature specified as a temperature for starting the instant hot-water circulation, the action mode of the hot-water supply device  100  is switched from the instant hot-water standby mode  317  to the instant hot-water circulation mode  318  (step S 342 ). 
     In another aspect, if the controller  110  detects an amount of water exceeding the MOQ, the action mode is switched from the instant hot-water standby mode  317  to the hot-water supply combustion mode  315  (step S 343 ). Moreover, in another aspect, instead of the measurement value of the temperature sensor  142 , a measurement value of the temperature sensor  141  that measures the temperature of the water flowing into the heat exchanger  126  may be used. 
     (Instant Hot-Water Circulation Mode) 
     In the instant hot-water circulation mode  318 , the control device  110  outputs a command for combustion start to the combustion mechanism  128 . In response to the command, the combustion mechanism  128  starts combustion. The control device  110  sends a command for hot-water discharge control to the bypass water amount servo  122 . In response to the command, the bypass water amount servo  122  adjusts the opening degree in order to maintain the temperature of the warm water during the instant hot-water circulation at a preset temperature. The control device  110  outputs a fully-open command to the total water amount servo  130 . In response to the fully-open command, the total water amount servo  130  fully opens an adjustment valve. 
     When the temperature of the water flowing into the heat exchanger  126  or the temperature of the water flowing out from the heat exchanger  126  is equal to or higher than a temperature preset for stopping the instant hot-water circulation, or when the use of the hot-water supply tap  21  is detected (so-called another tap interruption is detected) during the running of the instant hot-water circulation, the action mode is switched from the instant hot-water circulation mode  318  to the hot-water supply combustion mode  315  (step S 350 ). That is, the control device  110  sends a command for hot-water discharge control to the total water amount servo  130  in order to also maintain the preset temperature while the warm water is supplied from the hot-water supply device  100 . The total water amount servo  130  adjusts an opening degree of the adjustment valve in response to the command. 
     In one aspect, an upper limit of the set temperature in the instant hot-water mode may be set to an upper limit temperature of the instant water heating. When the reserved running or the one instant water heating is completed and the action mode shifts to the hot-water supply standby mode  314 , the upper limit of the hot-water supply set temperature returns to an original value. 
     [Relationship Between Set Temperature and Standby Position] 
     A relationship between the set temperature and the standby position of the total water amount servo  130  is described with reference to  FIG. 4 .  FIG. 4  is a diagram showing the relationship between the set temperature of the hot-water supply performed by the hot-water supply device  100  and the position of the stepping motor that configures the total water amount servo  130 . In one aspect, the set temperature and the position of the stepping motor may be specified by a linear graph  410 . 
     For example, the graph  410  is specified by linearly interpolating each point obtained by plotting each position (the number of steps) of the stepping motor in the case of the set temperatures of 32° C., 45° C. and 60° C. More specifically, the stepping motor is respectively standby at a 1,450-step site at 32° C., at a 1,550-step site at 45° C., and at a 1,750-step site at 60° C. 
     [Running conditions] As an example, the water entry temperature is 20° C. As a water pressure condition, water pressures at rest are 50 kPa, 80 kPa, and 200 kPa. In this case, water pressures during action are 20 kPa, 50 kPa, and 100 kPa. A time for hot-water re-discharge is 1 minute. 
     An evaluation reference of an ignition performance is that there is no ignition delay, for example, that a time to ignition is less than 1.5 seconds. If the MOQ does not exceed a predetermined amount, the hot-water supply device  100  does not enter the hot-water supply combustion mode  315 . Therefore, until the water amount sensor  131  detects an amount of water exceeding the MOQ, the control device  110  increases an opening degree of the total water amount servo  130  so as to increase the amount of the water flowing into the heat exchanger  126  from the total water amount servo  130 . 
     [Set temperature=32° C.] For example, if the set temperature of the hot-water supply is 32° C. and the water pressure at rest is 50 kPa, when the standby position of the stepping motor is 1,450-step, the time to ignition is 0.8 second. In this case, the ignition performance satisfies the evaluation reference. When the standby position becomes 1,500-step (=when the opening degree of the stepping motor decreases so as to reduce the amount of the water flowing into the heat exchanger  126 ) at the same water pressure, the time to ignition is 1.5 seconds, and the evaluation reference is not satisfied. 
     [Set temperature=45° C.] If the set temperature of the hot-water supply is 45° C. and the water pressure at rest is 50 kPa, when the standby position of the stepping motor is 1,450-step, the time to ignition is 0.7 second. When the standby position is 1500-step, the time to ignition is 0.9 second. When the standby position is 1550-step, the time to ignition is 1.2 seconds. In these cases, the ignition performance satisfies the evaluation reference. 
     [Set temperature=60° C.] If the set temperature of the hot-water supply is 60° C. and the water pressure at rest is 50 kPa, when the standby position of the stepping motor is 1750-step, the time to ignition is 0.9 second, and the ignition performance satisfies the evaluation reference. 
     Based on the above results, the standby position of the stepping motor of the total water amount servo  130  at each set temperature is as follows. 
                                             Set temperature   standby position                                                    32° C.   1450           45° C.   1550           60° C.   1750                        
Therefore, by plotting these values and performing linear interpolation, the following relationship is derived as shown by the graph  410  in  FIG. 4 .
 
         y= 10.781 x+ 1091 
     Here, x represents the set temperature, and y represents the standby position (steps) of the stepping motor of the total water amount servo  130 . 
     [Control Structure] 
     A control structure of the hot-water supply device  100  is described with reference to  FIG. 5 .  FIG. 5  is a flowchart showing a part of a process executed by the control device  110  of the hot-water supply device  100 . 
     In step S 510 , the control device  110  detects that the hot-water supply performed by the hot-water supply device  100  is stopped. 
     In step S 520 , the control device  110  reads the set temperature  221  for the hot-water supply from the memory  220 . 
     In step S 530 , the control device  110  calculates a target opening degree of the total water amount servo  130  based on the read set temperature  221 . In one aspect, the target opening degree is calculated as the number of steps for specifying the standby position of the stepping motor. For example, the standby position (steps) y can be specified by the following equation using the set temperature x and the reference number of steps. 
       Standby position  y =α·set temperature  x +reference number of steps
 
     Here, the coefficient α is determined by an experiment and is 10.781 in the example shown in  FIG. 4 .
 
The reference number of steps corresponds to an initial position of the stepping motor as in a conventional hot-water supply device. In the example shown in  FIG. 4 , the reference number of steps is 1091.
 
     In step S 540 , the control device  110  sends, to the total water amount servo  130 , a command for setting the opening degree of the passage through which water flows into the heat exchanger  126  from the total water amount servo  130  to the target opening degree. 
     In step S 550 , the stepping motor of the total water amount servo  130  moves in response to the command. Thereafter, within a time when the predetermined evaluation reference is satisfied, the heat exchanger  126  ignites, and heating of the water flowing into the heat exchanger  126  is started. 
     [Bypass Ratio] 
     Distribution (a bypass ratio) to the bypass flow path  151  in the hot-water supply device  100  according to one aspect is described with reference to  FIG. 6 .  FIG. 6  is a diagram conceptually showing a configuration of the flow paths of the hot-water supply device  100 . 
     In  FIG. 6 , a water entry temperature Tc represents the temperature of the water flowing out from the bypass water amount servo  122 . According to the configuration of each flow path shown in  FIG. 6 , a temperature of the water flowing through the water entry path  150  and a temperature of the water flowing through the bypass flow path  151  are the same. A heat exchange side temperature Tk represents the temperature of the water (hot water) flowing out from the heat exchanger  126 . A hot-water discharge temperature Ts represents the temperature of the warm water flowing into the total water amount servo  130 , that is, the temperature of the warm water supplied from the hot-water supply device  100 . 
     In one aspect, a ratio (hereinafter also referred to as “distribution ratio” or “bypass ratio”) X of an amount of water supplied from the bypass water amount servo  122  to the bypass flow path  151  (hereinafter also referred to as “bypass flow amount Q2”) relative to an amount of water supplied from the bypass water amount servo  122  to the water entry path  150  (hereinafter also referred to as “heat exchange side flow amount Q1”) can be calculated as follows. 
     Water having the heat exchange side flow amount Q1 (liter/minute) flows into the heat exchanger  126 . Water having the bypass flow amount Q2 (liter/minute) flows into the bypass flow path  151  from the bypass water amount servo  122 . Thus, a total flow amount Qt flowing into the total water amount servo  130  is Q1+Q2. The ratio of the bypass flow amount Q2 to the heat exchange side flow amount Q1 is defined as the distribution ratio X, and X=Q2/Q1. In this case, if there is no heat loss in the hot-water supply device  100 , the following relationship is satisfied. 
         TsQt=TkQ 1+ TcQ 2  (1)
 
     Equation (1) is transformed as follows. 
         Ts ( Q 1+ Q 2)= TkQ 1+ TcQ 2  (2)
 
     Because X=Q2/Q1 changes to Q2=Q1X, if Q2 is replaced with Q1X, Equation (2) is shown in the form of Equation (3). 
         Ts ( Q 1+ Q 1 X )= TkQ 1+ Tc ( Q 1 X )  (3)
 
     Equation (3) is transformed into Equation (4), and furthermore, Equation (5) is derived. 
       ( Ts+TsX ) Q 1=( Tk+TcX ) Q 1  (4)
 
         Ts+TsX=Tk+TcX   (5)
 
     If Equation (5) is transformed as follows, the distribution ratio X specified using the flow amount is shown as Equation (6) using the heat exchange side temperature Tk, the water entry temperature Tc, and the hot-water discharge temperature Ts. 
     
       
      
       TsX−TcX=Tk−Ts  
      
     
       ( Ts−Tc ) X=Tk−Ts    
         X =( Tk−Ts )/( Ts−Tc )  (6)
 
     In another aspect, the distribution ratio X can also be calculated from a total flow amount ratio. First, the relationship of Equation (2) is satisfied in the same manner as above. 
         Ts ( Q 1+ Q 2)= TkQ 1+ TcQ 2  (2)
 
     When the total flow amount is 1, if the ratio of the bypass flow amount Q2 to the total flow amount Qt is α and the ratio of the heat exchange side flow amount Q1 to the total flow amount Qt is β, the following relationship is satisfied. 
       1=α+β
 
         Q 1=β Qt =β( Q 1+ Q 2)=β Q 1+β Q 2  (7)
 
     If Equation (7) is transformed, Q1 is derived in the form of Equation (8). 
         Q 1−β Q 1=β Q 2
 
         Q 1(1−β)=β Q 2
 
         Q 1=β Q 2/(1−β)  (8)
 
     If Equation (8) is used, Equation (2) is shown as follows. 
         Ts{βQ 2/(1−β)+ Q 2}= Tk{βQ 2/(1−β)}+ TcQ 2  (9)
 
     Furthermore, Equation (9) is transformed as follows, and Equation (10) is derived. 
         Ts {β/(1−β)+1}= Tk{β /(1−β)}+ Tc  
 
         Tsβ+Ts (1−β)= Tkβ+Tc (1−β)
 
     
       
      
       Ts=Tkβ+Tc−Tcβ 
      
     
         Ts−Tc=Tkβ−Tc β=( Tk−Tc )β
 
       β=( Ts−Tc )/( Tk−Tc )  (10)
 
     Here, Equation (10) represents a distribution ratio with respect to the bypass flow path, that is, a bypass flow amount with respect to the total flow amount. 
     In addition, because β=1−α, if Equation (10) is transformed, Equation (11) is derived. 
       1−α=( Ts−Tc )/( Tk−Tc )
 
       α=1−( Ts−Tc )/( Tk−Tc )=( Tk−Ts )/( Tk−Tc )  (11)
 
     If α and β are used, the distribution ratio X is X=α/β, and thus Equation (12) is derived as follows. 
         X ={( Tk−Ts )/( Tk−Tc )}/{( Ts−Tc )/( Tk−Tc )} 
         X =( Tk−Ts )/( Ts−Tc )  (12)
 
     When Equation (6) and Equation (12) are compared, it can be seen that the distribution ratio X is calculated in the form of (Tk−Ts)/(Ts−Tc) in both cases. 
     [Another Aspect] 
     Another aspect is described with reference to  FIG. 7 .  FIG. 7  is a diagram showing the relationship between the bypass ratio and the standby position of the stepping motor. 
     Although the standby position of the stepping motor of the total water amount servo  130  is specified as a function of the set temperature in the above description, the standby position can also be specified as a function using a variable other than the set temperature. In another aspect, a hot-water supply device  700  can use a position specified as a graph  710  as the position of the stepping motor. More specifically, the hot-water supply device  700  can use a standby position according to the bypass ratio as the standby position of the stepping motor. 
     A control structure of the hot-water supply device  700  according to another aspect is described with reference to  FIG. 8 .  FIG. 8  is a flowchart showing a part of a process executed by the control device  110  of the hot-water supply device  700 . Moreover, processes the same as those described above are designated by the same step numbers. Thus, description of the same processes is not repeated. 
     In step S 810 , the control device  110  detects the heat exchange side temperature Tk and the water entry temperature Tc. In step S 820 , the control device  110  detects the hot-water discharge temperature Ts. 
     In step S 830 , the control device  110  calculates the bypass ratio from the heat exchange side temperature Tk, the water entry temperature Tc, and the hot-water discharge temperature Ts. 
     In step S 840 , the control device  110  derives the target opening degree specified in advance according to the bypass ratio. For example, the control device  110  uses the relationship shown in  FIG. 7  to calculate the target opening degree. 
     Moreover, the order in which steps S 810  and S 820  are performed is not limited to the above order and may be reversed. 
     Still another aspect is described with reference to  FIG. 9 .  FIG. 9  is a diagram showing an example of a hardware configuration of a hot-water supply device  900  according to still another aspect. The hot-water supply device  900  differs from the hot-water supply device  100  in terms of not having the bypass water amount servo  122  included in the hot-water supply device  100 . According to the configuration shown in  FIG. 9 , the water delivered from the water entry portion  10  flows into the heat exchanger  126  or the bypass flow path  151 . The hot-water supply device  900  having this configuration can also realize stable hot-water supply even at low water pressure. 
     As described above, according to the hot-water supply devices  100  and  900  of the embodiment, the standby position of the stepping motor of the total water amount servo  130  changes depending on the set temperature, and the amount of the water supplied from the total water amount servo  130  also differs depending on the set temperature. According to this configuration, even when clean water is supplied at low water pressure, the hot-water supply devices  100  and  900  can operate by the MOQ without deteriorating the hot-water discharge characteristic, and stable hot-water supply can be realized. 
     As a result, in countries or areas where waterworks infrastructure is not developed, even if clean water having a lower pressure than an assumed low water pressure is supplied, the hot-water supply devices  100  and  900  can operate by the MOQ. 
     Conventionally, when the set temperature of the hot-water supply device is low, the stepping motor of the bypass water amount servo  122  is moved to a position close to a fully-open position in order that the supply amount to the bypass flow path  151  becomes the maximum, and thus the amount of the water supplied to the heat exchanger  126  is greatly reduced. Because the MOQ is judged based on the amount of the water flowing to the heat exchanger  126 , the effect of the standby position of the stepping motor of the bypass water amount servo  122  is great. When the set temperature of the hot-water supply is low, or when the temperature of the water flowing into the heat exchanger  126  is high, the stepping motor of the bypass water amount servo  122  is moved to a position where the flow amount of the water to the bypass flow path  151  increases, and conversely, the flow amount of the water to the heat exchanger  126  decreases. At this time, when the clean water is supplied only at a water pressure lower than the assumed low water pressure, the amount of the water to the heat exchanger  126  does not exceed the MOQ, and as a result, the water heater  100  may not start combustion. 
     On the other hand, according to the embodiment, the standby position of the stepping motor of the total water amount servo  130  differs depending on the set temperature. As a result, even if the hot-water supply tap  21  is opened in a state where the low temperature is set, an amount of water exceeding the MOQ is supplied to the heat exchanger  126  and combustion is started. Thereby, the stable hot-water supply can be realized. 
     The embodiments disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is shown not by the above description but by the claims, and is intended to include meanings equivalent to the claims and all modifications within the scope.