Patent Publication Number: US-2018038616-A1

Title: Control unit, continuous-flow heater and method for controlling a continuous-flow heater

Description:
FIELD OF THE INVENTION 
     The present invention relates to a control unit, a continuous-flow heater and a method relating thereto. 
     BACKGROUND INFORMATION 
     Continuous-flow heaters are believed to be understood which heat fresh water to produce hot water. The hot water is provided at a constant temperature to a faucet. Normally, the continuous-flow heater heats up to 60 degrees Celsius, in order to provide sufficiently hot water for both cleaning dishes and for showering. In order to achieve a sufficiently comfortable temperature for showering, the hot water is mixed with cold water at the faucet. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an improved control unit, an improved continuous-flow heater and an improved method. 
     This object is achieved with the aid of a control unit according to the description herein. Advantageous specific embodiments are specified in the further descriptions herein. 
     According to the present invention, it is believed that an improved control unit may be provided in that the control unit includes an interface, a control device and a memory. The control device is connected to the interface and the memory. A predefined characteristic is stored in the memory. The interface is connectable to a flow rate sensor of a continuous-flow heater. The interface is configured to detect a flow rate signal of the flow rate sensor and to provide it to the control device. The control device is configured to ascertain a flow rate characteristic on the basis of the flow rate signal over a period of time and in a comparison to compare the flow rate characteristic with the predefined characteristic. The control device is configured to provide a control signal at the interface for controlling a heat output of the continuous-flow heater as a function of the result of the comparison. 
     In this way, with a faucet present fitted with a thermostatic valve, which is usually situated in the bathroom, it may be detected that the user requires cooler warm water than that for cleaning dishes in the kitchen. As a result, the continuous-flow heater may be operated with greater efficiency in one operation. 
     In another specific embodiment, the predefined characteristic corresponds to a valve flow-through characteristic of a faucet. As a result, the utilization of this faucet may be detected and its operational behavior may be adapted as a function of the utilization of the faucet of the continuous-flow heater. 
     In another specific embodiment, the predefined characteristic includes a first time-delimited segment, a second time-delimited segment, and a third time-delimited segment. The second segment follows chronologically after the first segment and the third segment follows chronologically after the second segment. In the first segment, a predefined value is essentially constant over time. In the second segment, the predefined value essentially decreases over time. In the third segment, the predefined value is essentially constant over time and is lower than in the first segment. 
     In another specific embodiment, a tolerance range of the predefined characteristic is stored in the memory, the control device being configured to take the tolerance range into account in the comparison of the predefined characteristic with the ascertained flow rate characteristic. 
     In another specific embodiment, the interface is connectable to a temperature sensor and is configured to detect a temperature signal of the temperature sensor and provide it to the control device, the control device being configured to take the temperature signal into account when ascertaining the control signal. 
     The object is also achieved, however, by a continuous-flow heater according to the further descriptions herein. Advantageous specific embodiments are specified in the further descriptions herein. 
     According to the present invention, it is believed that an improved continuous-flow heater for providing hot water in a hot water system may be provided in that the continuous-flow heater includes a heat source, a flow rate sensor and a control unit. The control unit is configured as described above. The interface is connected to the flow rate sensor and to the heat source. The flow rate sensor is configured to detect a flow rate of hot water through the heat source and to provide a flow rate signal correlating with the flow rate through the heat source. The heat source is configured to detect the control signal provided at the interface and to adapt the heat output based on the detected control signal for heating the control unit. 
     In another specific embodiment, the control signal correlates with a first heat output of the heat source when the ascertained flow rate characteristic deviates from the predefined characteristic. When the ascertained flow rate characteristic coincides with the predefined characteristic, the control signal correlates with a second heat output of the heat source. The second heat output in this case is lower than the first heat output. 
     In another specific embodiment, at least one heat exchanger is provided. The heat source is configured as a burner, the heat exchanger including a first heat exchanger module having a first primary side, the first primary side being coupled to the heat source. The heat source is configured to combust a fuel for providing the heat output, an waste gas forming during the combustion of the fuel being guided to the primary side of the first heat exchanger module, the second heat output being selected in such a way that at least one component of the waste gas condenses at least partially on the first primary side. In this way, a condensation energy, in addition to the heat energy of the waste gas, may be guided into the secondary side of the heat exchanger to heat the hot water, so that the continuous-flow heater may operate particularly energy-efficiently. 
     In another specific embodiment, the first heat exchanger module includes a first secondary side, the first secondary side being connectable on the input side to a fresh water supply and on the output side to at least one faucet. The first heat exchanger module is configured on its secondary side to heat fresh water coming from the fresh water supply to produce hot water. A temperature sensor is also provided, the temperature sensor being situated on the output side of the first secondary side and being connected to the interface, the temperature sensor being configured to detect a temperature of the hot water on the output side of the heat exchanger and to provide a temperature signal correlating with the detected temperature to the interface. The control device is configured to control the heat output of the heat source as a function of the detected temperature and the detected flow rate. 
     In another specific embodiment, the heat exchanger includes a second heat exchanger module having a second primary side and a second secondary side. The first heat exchanger module includes a first secondary side, the first secondary side being thermally coupled to the second primary side of the second heat exchanger module, the second secondary side being connectable on the input side to a fresh water supply and on the output side to at least one faucet. The second heat exchanger module is configured on its secondary side to heat fresh water coming from the fresh water supply to produce hot water. A temperature sensor is also provided. The temperature sensor is situated on the output side of the second secondary side of the second heat exchanger module, and is connected to the interface, the temperature sensor being configured to detect a temperature of the hot water on the output side of the second heat exchanger module and to provide a temperature signal correlating with the detected temperature to the interface. The control device is configured to control the heat output of the heat source as a function of the detected temperature and of the detected flow rate. 
     The object is also achieved, however, by a method according to the descriptions herein. Advantageous specific embodiments are specified in the further descriptions herein. 
     According to the present invention, it is believed that an improved method for controlling a continuous-flow heater may be provided in that a flow rate of hot water through a continuous-flow heater is detected, a flow rate characteristic being ascertained on the basis of the detected flow rate over a period of time, in a comparison, the ascertained flow rate characteristic being compared with a predefined characteristic, a heat output of the continuous-flow heater being controlled as a function of the result of the comparison. 
     In another specific embodiment, a control signal correlating with a first heat output of the heat source is ascertained when the ascertained flow rate characteristic deviates from the predefined characteristic. When the ascertained flow rate characteristic coincides with the predefined characteristic, the control signal correlating with a second heat output of the heat source is ascertained. The second heat output in this case is lower than the first heat output. 
     The present invention is explained in greater detail below with reference to the figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically shows a depiction of a hot water system. 
         FIG. 2  schematically shows a depiction of a continuous-flow heater of the hot water system shown in  FIG. 1 . 
         FIG. 3  schematically shows a depiction of a faucet. 
         FIG. 4  shows a diagram of a predefined characteristic. 
         FIG. 5  shows a diagram with multiple variables plotted over a period of time. 
         FIG. 6  shows a diagram of a flow rate plotted over time. 
         FIG. 7  shows a flow chart of a method for controlling the hot water system. 
         FIG. 8  schematically shows a depiction of a hot water system according to another specific embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  schematically shows a depiction of a hot water system  10  in a building  15 . Hot water system  10  includes a continuous-flow heater  20 , a first faucet  25  and a second faucet  30 . First faucet  25  is situated, for example, in a bathroom  35  of building  15 . Second faucet  30  is situated, for example, in a kitchen  40  of building  15 . Additional faucets may, of course, also be provided. 
     Continuous-flow heater  20  includes an input side  41  and an output side  42 . Input side  41  is connected via a first line  45  to a fresh water supply  50 . Fresh water supply  50  provides fresh water  55 . Fresh water  55  in this case has a low temperature, for example, in the range of 12 degrees, and is referred to below as cold water  56 . 
     Output side  42  of continuous-flow heater  20  is connected via a line  60  to first faucet  25  and to second faucet  30 . Furthermore, first faucet  25  is connected to fresh water supply  50  via a third line  65 . Second faucet  30  is also connected to fresh water supply  50  via third line  65 . 
       FIG. 2  schematically shows a depiction of continuous-flow heater  20  of hot water system  10  shown in  FIG. 1 . Continuous-flow heater  20  includes a control unit  70 , a heat source  75 , a heat exchanger  80 , a flow rate sensor  85 , and a temperature sensor  90 . Heat exchanger  80  includes a heat exchanger module  81  having a primary side  95  and a secondary side  100 . Primary side  95  is connected to heat source  75 . Secondary side  100  is connected to input side  41  as well as to output side  42 . Heat source  75  in the specific embodiment is configured as a burner, in particular, as a gas burner. Heat source  75  in this case is further connected to a fuel supply  105 . Fuel supply  105  in this case provides a fuel  110 . Fuel  110  in this case is combusted with atmospheric oxygen  112  in heat source  75  during the operation of continuous-flow heater  20 . A waste gas  111  forming during the combustion of fuel  110  is guided to primary side  95  of heat exchanger module  81 . A heat transfer of the heat from waste gas  111  takes place in heat exchanger module  81  from primary side  95  to secondary side  100 . Waste gas  111 , after flowing through primary side  95 , is guided via a flue  115  of continuous-flow heater  20  out of continuous-flow heater  20 . 
     Control unit  70  includes a control device  120 , an interface  125  and a memory  130 . Interface  125  is connected to control device  120  via a first connection  135 . Memory  130  is connected to control device  120  via a second connection  140 . Interface  125  is connected to heat source  75  via a third connection  145  and to flow rate sensor  85  via a fourth connection  150 . Interface  125  is connected to temperature sensor  90  via a fifth connection  155 . Temperature sensor  90  is configured to ascertain a temperature of fresh water  55  flowing from heat exchanger module  81 . Temperature sensor  90  provides a correlating temperature signal corresponding to the detected temperature to interface  125  via fifth connection  155 . Interface  125  relays the temperature signal to control device  120  via first connection  135 . Flow rate sensor  85  detects flow rate f of fresh water  55  on the output side of heat exchanger module  81  in second line  60 . Flow rate sensor  85  provides a flow rate signal corresponding to detected flow rate f. The flow rate signal is conducted via fourth connection  150  to interface  125 , which provides the flow rate signal to control device  120  via first connection  135 . 
     A predefined characteristic, a predefined first temperature threshold value T s1 , a predefined second temperature threshold value T s2 , as well as a first predefined flow rate threshold value f s1  and a second predefined flow rate threshold value f s2  are stored in memory  130 . In this case, second flow rate threshold value f s2  is greater than first predefined flow rate threshold value f s1 . First temperature threshold value T s1  is selected to be lower than second temperature threshold value T s2 . First temperature threshold value T s1  may be 50° C., for example. Second temperature threshold value T s2  may be 60° C., for example. Also stored in memory  130  are a first setpoint value, for example, 60° C., and a second setpoint value, for example, 45° C. 
     A control parameter is also stored in memory  130  of control unit  70 . The control parameter in this case includes a classification of a heat output as a function of a setpoint temperature and of ascertained flow rate f. The control parameter in this case may be configured as a tabular classification, as a characteristic diagram or as a mathematical formula. Furthermore, the control parameter may be expanded to the extent that the control parameter is configured as a control algorithm, which takes temperature T ascertained on the output side into account when ascertaining the heat output. Control device  120  ascertains a control signal formed corresponding to the heat output as a function of the ascertained heat output. 
     The provision of hot water to faucets  25 ,  30  will be only broadly described below, since it will be discussed in detail in the subsequent method. Pressurized fresh water  55  is provided in continuous-flow heater  20  via first line  45 . If one of the two faucets  25 ,  30  is opened and hot water is required, then heat source  75  of continuous-flow heater  20  is activated. Fresh water  55  is heated in secondary side  100  and flows as fresh water  55  at a temperature greater than the temperature of cold water  56  as hot water  57  from secondary side  100  via output side  42  into second line  60 . 
       FIG. 3  schematically shows a depiction of first faucet  25 . First faucet  25  includes a first connection  160  and a second connection  165 . First faucet  25  is connected via first connection  160  to second line  60 . First faucet  25  is connected via second connection  165  to third line  65 . First faucet  25  further includes a third connection  170 . A shower hose  175 , for example, may be attached to third connection  170 . It is also conceivable that, in addition or alternatively to a third connection  170 , an outlet is provided for filling a bathtub or a sink or for connecting to a household appliance, for example, a washing machine or a dishwasher. 
     First faucet  25  in the specific embodiment includes a, for example, cylindrically configured housing  176 . Housing  176  includes an inner chamber  117 . Inner chamber  177  is fluidically connected to second connection  165 . 
     First faucet  25  includes a temperature control device  180 . Temperature control device  180  includes a temperature valve  185 , a temperature valve actuation element  190  and a temperature pre-selection element  195 . Temperature pre-selection element  195  is situated in the specific embodiment on the left side on housing  176  and is coupled to temperature valve  185 . Temperature valve  185  is fluidically situated between inner chamber  177  and first connection  160 . First faucet  25  further includes an opening valve  200 . Opening valve  200  is situated in the specific embodiment on the right side of housing  176  and is fluidically situated between inner chamber  177  and third connection  170 . 
     Second faucet  30  may be configured as a conventional mixing faucet, for example, as a single lever mixer. These are normally used in the kitchen, since on the one hand a higher flow velocity for washing dishes is advantageous for the user and, on the other hand, these are particularly easy to operate and to quickly open and close. 
     First faucet  25  as well as second faucet  30  are used to tap fresh water  55  having a different temperature. Here, the user in bathroom  35 , in particular, in the shower, is far more sensitive to temperature than when dishes are rinsed. Furthermore, when washing dishes in kitchen  40 , fresh water  55  having a temperature higher than fresh water  55  tapped via first faucet  25  for showering/bathing/washing, is normally used in order to easily remove residue, such as grease, from kitchenware. Furthermore, hot water  57  having a particularly high temperature, for example, 60 degrees, is tapped via second faucet  30  in order to clean floors of building  15 . 
     Fresh water  55  tapped from first faucet  25  should normally have a constant temperature, which is lower than fresh water  55  tapped at second faucet  30 . Based on experience, fresh water  55  having a temperature of 36° C. to 39° C. is tapped at first faucet  25 . 
     To tap fresh water  55  from first faucet  25 , a desired tapping temperature, for example, 38° C., is set by the user on temperature pre-selection element  195 . In addition, the user opens first faucet  25  with the aid of opening valve  200 , so that fresh water  55  flows from first faucet  25  via third connection  170 . 
     At the start of a temperature control operation by temperature control device  180 , temperature valve  185  is in the wide opened state. As a result, cold fresh water  55  flows via first connection  160  from second line  60  and cold water  56  flows via second connection  165  into inner chamber  177 . Fresh water  55  initially flowing in from second line  60  normally has a lower temperature than hot water  57  flowing out of continuous-flow heater  20 . In inner chamber  177 , cold water  56  is mixed with fresh water  55  originating from second line  60  to produce warm water  178 . Depending on the temperature of warm water  178 , temperature valve actuation element  190  shifts temperature valve  185  as a function of the desired temperature set by the user with the aid of temperature pre-selection element  195 , in order to provide warm water  178  at the desired temperature at third connection  170 . 
     Warm water  178  tappable at faucet  25 ,  30  is produced at faucets  25 ,  30  by mixing hot water  57  supplied via second line  60  with cold water  56  supplied via third line  65 . Continuous-flow heater  20  is activated if warm water  178  is required. If warm water  178  is no longer required at faucet  25 ,  30 , then faucet  25 ,  30  is closed and continuous-flow heater is deactivated. 
       FIG. 4  shows a diagram of a predefined characteristic stored in memory  130 . The predefined characteristic corresponds to a valve flow rate characteristic of first faucet  25 . 
     The predefined characteristic in the specific embodiment includes, for example, three graphs  300 ,  305 ,  310 . Here, a first graph  300  correlates to a flow rate f of fresh water  55  through continuous-flow heater  20 , plotted over a time t from the start of the tapping of fresh water  55  at first faucet  25  with a constant tapping of warm water  178  at first faucet  25  for example, for first graph  300  of 10.2 l/min. A second graph  305  correlates with a second flow rate f from the start of the tapping of fresh water  55  at first faucet  25  with a constant tapping of warm water  178  at first faucet  25 , for example, for second graph  305  of 8 l/min. A third graph  310  correlates with a third flow rate f from the start of the tapping of fresh water  55  at first faucet  25  with a constant tapping of warm water  178  at first faucet  25 , for example, for third graph  310  of 7 l/min. It is also conceivable, of course, that the predefined characteristic includes additional graphs. It is also conceivable that the predefined characteristic is stored in memory  130 , not as a graph, but rather as a mathematical function or parameterized. 
       FIG. 5  shows a diagram with multiple variables plotted over time t. A fourth graph  350 , a fifth graph  355 , a sixth graph  360  and a seventh graph  365  are depicted in the diagram. Fourth graph  350  shows the temperature of hot water  57  on output side  42  of continuous-flow heater  20  in decimal degrees Celsius (dC). Fifth graph  355  shows a temperature curve of warm water  178  at third connection  170  in decimal degrees Celsius. Sixth graph  360  corresponds to first graph  300  shown in  FIG. 4  and corresponds to a flow rate f of hot water  57  through continuous-flow heater  20  in deciliters at a tapping of, for example, 10.2 l/min of warm water  178  at first faucet  25 . Seventh graph  365  shows an output P delivered by continuous-flow heater  20  in percentage relative to a maximum output of continuous-flow heater  20 . 
     First graph  300  is explained below by way of example for additional graphs  305 ,  310 . First graph  300  correlates with a control behavior of temperature control device  180  of first faucet  25 . First graph  300  of the predefined characteristic includes a first time-delimited segment  315 , a second time-delimited segment  320  and a third time-delimited segment  325 . First segment  315  is delimited initially by a start  330  of the tapping. An end of first segment  315  is delimited by second segment  320 . Third segment  325  is delimited initially by an end of second segment  320 . Third segment  325  may theoretically be infinitely long chronologically, however, the characteristic in the specific embodiment has a predefined duration which, in the specific embodiment, is 35 seconds, for example. In first segment  315 , first graph  300  has a predefined value, which is essentially constant over time t. In second segment  320 , the predefined value decreases from the value in first segment  315 . In third segment  325 , the predefined value is essentially constant over time t. Here, the predefined value in third segment  325  is lower than in first segment  315 . 
     The control behavior of first faucet  25  corresponding to the predefined characteristic in individual segments  315 ,  320 ,  325  is explained below. 
     At the start of a tapping of fresh water  55  from first faucet  25 , temperature valve  185  is completely opened. The tapping from first faucet  25  starts with the opening of opening valve  200 . In the process (cf. first segment  315 ), fresh water  55  at a low temperature, which is cooled in second line  60  over time t prior to the tapping, flows from second line  60  into inner chamber  177 , where it is mixed with cold water  56  coming from third line  65 . The admixed water has a temperature, which is lower than the set desired temperature, so that in first segment  315 , flow rate f is constant over time t. The temperature of cold water  56  is essentially constant over the tapping. 
     As explained above, continuous-flow heater  20  is activated with the tapping of fresh water  55  from second line  60 . In second segment  320 , fresh water  55  subsequently flowing via second line  60  has a higher temperature with increasing time t until fresh water  55  reaches first faucet  25  as hot water  57 . The increasingly warming fresh water  55  is mixed in inner chamber  177  with cold water  56  to produce warm water  178 . Warm water  178  heats temperature valve actuation element  190 , which then actuates temperature valve  185  and reduces the inflow of hot water  57  via first connection  160  over time t. As a result, flow rate f in second segment  320  decreases over time t. The reduction of flow rate f also causes a temperature increase in hot water  57  (cf. fourth graph  350 ). As a result, temperature valve actuation element  190  closes temperature valve  185  further over time t, so that flow rate f through heat exchanger module  81  decreases further until an equilibrium is established between flow rate f and the temperature of hot water  57  in third segment  325  following second segment  320  and flow rate f is constant over time t. 
       FIG. 6  shows a diagram of flow rate f plotted over time t as fresh water  55  is tapped via second faucet  30 . The curve of flow rate f over time t is not, as explained in  FIG. 5 , due to the control behavior of temperature control device  180 , but rather is arbitrary and a function of how the user operates second faucet  30 . Thus, the tapping of fresh water  55  via second faucet  30  does not have the characteristic shown in  FIG. 4 . 
       FIG. 7  shows a flow chart of a method for operating hot water system  10  described in  FIGS. 1 through 3 . 
     In a first method step  400 , control device  120  checks whether continuous-flow heater  20  is in the stand-by mode. If this is the case, control device  120  continues with a second method step  405 . If this is not the case, control device  120  waits until continuous-flow heater  20  is activated. 
     In a second method step  405 , control device  120  checks whether heat source  75  is activatable. If this is the case, control device  120  continues with a third method step  410 . If this is not the case, control device  120  waits to see whether heat source  75  is activatable. 
     In a third method step  410 , control device  120  detects the temperature signal and the flow rate signal. 
     Control device  120  compares detected temperature T on the output side of heat exchanger module  81  with first temperature threshold value T s1  in a first comparison. If temperature T exceeds first temperature threshold value T s1 , then control device  120  continues with a fourth method step  415 . If detected temperature T falls below first temperature threshold value T s1 , then control device  120  waits until temperature T exceed first temperature threshold value T s1 . 
     In fourth method step  415 , control device  120  compares in a second comparison detected flow rate f with first flow rate threshold value f s1  and with second flow rate threshold value f s2 . 
     If ascertained flow rate f exceeds first flow rate threshold value f s1  and ascertained flow rate f falls below second flow rate threshold value f s2 , then control device  120  continues with a fifth method step  420 . If ascertained flow rate f falls below first flow rate threshold value f s1  or ascertained flow rate f exceeds second flow rate threshold value f s2 , then control device  120  continues with a sixth method step  425 . 
     In fifth method step  420 , control device  120  compares in a third comparison ascertained temperature T with second temperature threshold value T s2 . If ascertained temperature T falls below second temperature threshold value T s2 , then control device  120  waits until ascertained temperature T is greater than or equal to second temperature threshold value T s2 . If ascertained temperature T exceeds second temperature threshold value T s2 , then control device  120  continues with a seventh method step  430 . 
     In a sixth method step  425 , control device  120  selects the first setpoint value as the setpoint temperature for ascertaining the control signal for controlling the heat output of heat source  75 . As a function of the first setpoint value, control device  120  ascertains a first control signal on the basis of the control parameter, which correlates with a first heat output P 1  and provides the first control signal to heat source  75  via interface  125 . Heat source  75  detects the first control signal. Heat source  75  is controlled with the aid of the first control signal in such a way that the heat source delivers first heat output P 1  and fresh water  55  flowing out on the output side from heat exchanger module  81  exhibits essentially the temperature of the first setpoint value. 
     In a seventh method step  430 , control device  120  assigns detected flow rate f time t from the start of the tapping and stores the detected value of flow rate f with assigned time t in memory  130 . Control device  120  ascertains a flow rate characteristic of flow rate f over time t on the basis of the values for flow rate f stored in memory  130 . In a fourth comparison, control device  120  compares the ascertained flow rate characteristic with the predefined characteristic. Thus, for example, the ascertained flow rate characteristic may coincide with first graph  300 , with second graph  305  or with third graph  310 , depending on how wide opening valve  200  is opened. 
     A tolerance range may also be stored in memory  130 , which control unit  120  takes into account during the fourth comparison of the ascertained flow rate characteristic with the predefined characteristic, so that a deviation of the ascertained flow rate characteristic is assignable by control device  120  to the corresponding predefined characteristic. In this way, a tapping of fresh water  55  from first faucet  25  may be reliably detected. 
     If fresh water  55  is tapped from first faucet  25 , then the flow rate characteristic ascertained, for example, by control device  120  corresponds to the curve of flow rate f over time t shown in  FIG. 5 , but not to the predefined characteristic. 
     If the ascertained flow rate characteristic coincides with the predefined characteristic, then control device  120  continues with eighth method step  435 . If the ascertained flow rate characteristic does not coincide with the predefined characteristic, then control device  120  continues with sixth method step  425 . 
     In eighth method step  435 , control device  120  selects the second setpoint value as the second setpoint temperature which, in the specific embodiment, is 45° C. In addition, control device  120  may, if the setpoint temperature value was the first setpoint value during a previous run-through of the described method, continuously lower this first setpoint value in eighth method step  435  on the basis of a predefined lowering parameter. Thus, it is conceivable, for example, to lower the setpoint temperature value over time t from the first setpoint value down to the second setpoint value by 1° C. every 100 milliseconds. As a function of the second setpoint value, control device  120  ascertains on the basis of the control parameter a second control signal, which correlates with a second heat output P 2 , and provides the second control signal to heat source  75  via interface  125 . Heat source  75  detects the second control signal. Heat source  75  is controlled with the aid of the second control signal in such a way that the heat source delivers second heat output P 2  and fresh water  55  flowing out on the output side from heat exchanger module  81  exhibits essentially the temperature of the second setpoint value. 
     If control device  120  establishes the second setpoint value as the setpoint temperature value, this means then that when second heat output P 2  is delivered by heat source  75 , the waste gas  111  forming during combustion of fuel  110  when guided through primary side  95  condenses in heat exchanger module  81  at least partially on primary side  95 . This has the advantage that, in addition to the heat energy, a condensation energy may be utilized to heat fresh water  55  in secondary side  100  of heat exchanger module  81 . In this way, the efficiency of continuous-flow heater  20  may be further increased. 
     In a ninth method step  440  following eighth method step  435 , control device  120  compares in a fifth comparison flow rate f with second predefined flow rate threshold value f s2 . If flow rate f exceeds predefined second flow rate threshold value f s2 , the method is continued by a tenth method step  445  by control device  120 . If flow rate f falls below predefined second flow rate threshold value f s2 , the method is continued by an eleventh method step  450 . 
     In a tenth method step  445 , the first setpoint value is established as the setpoint temperature value, so that fresh water  55  flowing through heat exchanger module  81  is heated more intensively and may be tapped via second faucet  30  at a temperature of 60° C. In addition, control device  120  may, if the setpoint temperature value was the second setpoint value during a previous run-through of the described method, continuously raise this second setpoint value in tenth method step  445  on the basis of a predefined raising parameter. Thus, it is conceivable, for example, to raise the setpoint temperature value over time t from the second setpoint value up to the first setpoint value by 5° C. every 100 milliseconds. 
     In the eleventh method step  450 , the second setpoint value is established as the setpoint temperature value. 
     Following eleventh method step  450  is a twelfth method step  455 , in which it is checked whether flow rate f equals zero. If this is not the case, control device  120  continues with ninth method step  440 . If this is the case, control device  120  continues with a thirteenth method step  460 . 
     In thirteenth method step  460 , the setpoint temperature value is set to the first setpoint value and thus, for example, at 60 degrees in the specific embodiment. Thirteenth method step  460  is followed by first method step  400 . 
     Following tenth method step  445  is a fourteenth method step  465 . In fourteenth method step  465 , control device  120  compares in a sixth comparison whether detected temperature T, like the setpoint temperature value, corresponds with the first setpoint value. If this is the case, control device  120  continues with a fifteenth method step  470 . If this is not the case, the control device repeats tenth method step  445 . 
     In fifteenth method step  470 , control device  120  checks whether flow rate f equals zero. If this is the case, control device  120  continues with thirteenth method step  460 . If this is not the case, fifteenth method step  470  is repeated. 
     It is noted that additional method steps may, of course, be provided and/or the above described method steps may be carried out in a different sequence. 
       FIG. 8  schematically shows a depiction of a hot water system  10  according to another specific embodiment. 
     Hot water system  10  is configured similarly to hot water system  10  shown in the preceding figures. In contrast to the latter, heat exchanger  80  is constructed in multiple parts and includes a first heat exchanger module  499  and a second heat exchanger module  500 . First heat exchanger module  499  is configured essentially identical to heat exchanger module  81  described in  FIGS. 1 through 7 . First heat exchanger module  499  includes a first primary side  501  and a first secondary side  502 . First primary side  501  corresponds to primary side  95  of heat exchanger module  81  described in  FIGS. 1 through 7 . 
     Second heat exchanger module  500  includes a second primary side  505  and a second secondary side  510 . Second heat exchanger module  500  in the specific embodiment is configured as a counterflow heat exchanger. Other embodiments of the second heat exchanger module are, of course, also conceivable such as, for example, as a cross-flow heat exchanger or as a parallel flow heat exchanger  500 . 
     In contrast to  FIGS. 1 through 7 , first secondary side  502  of first heat exchanger module  499  is fluidically connected on the output side by a fourth line  515  to second primary side  505  of second heat exchanger module  500 . On the input side, first secondary side  502  of first heat exchanger module  499  is fluidically connected to second primary side  505  of second heat exchanger module  500  via a fifth line  520 . Fourth line  515 , fifth line  520  and second primary side  505  of second heat exchanger module  500  and, in contrast to the preceding figures—first secondary side  502  of first heat exchanger module  499  are filled with a heat transfer medium  525 , which may include water, for example. As a result, first secondary side  502  is thermally coupled with second primary side  505  of second heat exchanger module  500 . 
     In contrast to  FIGS. 1 through 7 , second secondary side  510  of second heat exchanger module  500  is connected on the input side to input side  41  of continuous-flow heater  20  and, therefore, to fresh water supply  50  via first line  45 . On the output side, second secondary side  510  of second heat exchanger module  500  is connected to output side  42  of continuous-flow heater  20  and, therefore, to second line  60 . In this case, flow rate sensor  85  and temperature sensor  90  are situated on the output side of second secondary side  510 , temperature sensor  90  being connected to interface  125 , temperature sensor  90  being configured to detect a temperature T of hot water  57  on the output side of second heat exchanger module  500  and to provide a temperature signal to interface  125  correlating with detected temperature T. Flow rate sensor  85  detects the flow rate of cold fresh water  55  and/or fresh water  55  heated to produce hot water  57  through second secondary side  50  of second heat exchanger module  500  and provides the flow rate signal correlating with flow rate f to interface  125 . 
     Furthermore, it is also conceivable, as shown by way of example in  FIG. 8 , that fourth line  515  and fifth line  520  are connected to a heating circuit  530  for heating a building  15 . For this purpose, a valve  535  may also be provided in fifth line  520 , in order to separate heating circuit  530  fluidically from fifth line  520 . A feed pump  540  is further provided, for example, in fifth line  520  for feeding heat transfer medium  525 . Feed pump  540  may, of course, alternatively also be situated in fourth line  515 . 
     The operation of continuous-flow heater  20  is similar to the method described in  FIGS. 1 through 7 . In contrast thereto, heat transfer medium  525  and not fresh water  55 , as described in  FIGS. 1 through 7 , is heated in first heat exchanger module  81 . Heated heat transfer medium  525  is conveyed by feed pump  540  via fourth line  515  to second primary side  505  of second heat exchanger module  500 . In second heat exchanger  500 , heat transfer medium  525  dissipates at least a part of its heat for heating fresh water  55  present in second secondary side  510  to hot water  57 . Cooled heat transfer medium  525  flows via fifth line  520  back to first secondary side  502  of first heat exchanger module  499 . Control device  120  controls heat output P of heat source  75  as described above as a function of the temperature signal and of the flow rate signal. In this case, control device  120  may also provide an additional control signal for activating feed pump  540  when detecting a tapping of hot water  57  via at least one of the two faucets. 
     The embodiment of continuous-flow heater  20  described in  FIG. 8  has the advantage that heat source  75 , in addition to heating fresh water  55  to produce hot water  57 , may further be utilized to heat heating circuit  530 . Heat source  75  may further be situated spatially separated from second heat exchanger module  500 , so that the installation space requirement of continuous-flow heater  20  is adaptable. 
     It is further conceivable that second primary side  505  of second heat exchanger module  500  is connected to an additional heat source (not depicted). The additional heat source in this case may be configured, for example, as a thermal solar collector. Here, too, it is advantageous that heat source  75  in conjunction with second heat exchanger module  500  may be operated with less output P when detecting the tapping of warm water  178  at first faucet  25 , so that the efficiency of continuous-flow heater  20  is increased.