JP2001065988A - Electric water heater - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress an electric bill to a low level and the prevent a lack of a hot water by limiting an unnecessary energization to a heater in a daytime zone having a high electric bill as much as possible. SOLUTION: The electric water heater comprises a heater 8 to be supplied with a daytime power or night power, water supply side piping 3 and hot water supply piping 4 connected to a hot water storage tank 2, a temperature sensor 9, and a flow sensor 7 for detecting a supply amount of a hot water to an exterior. In this case, whether a energization time zone is a time zone supplied with the daytime power or night power is discriminated. When the sensor 7 detects a predetermined or more hot water supply amount, a water temperature is raised to a boiling temperature, and a lapse time from a start of supplying daytime power is integrated, and if the lapse time arrives at a predetermined time, the power supply to the heater 8 is stopped.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to boiling water control of an electric water heater.

[0002]

2. Description of the Related Art In general, the primary purpose of an electric water heater is to supply hot water as required by a user, and it has been necessary to eliminate running out of hot water in order to achieve this purpose. The configuration of a conventional electric water heater is shown below. FIG. 7 is a system configuration diagram of a conventional electric water heater, and FIG. 8 is a control flow diagram of the conventional electric water heater.

In FIG. 7, a hot water storage tank 2 for storing hot water is disposed in the center of an electric water heater main body 1. The bottom of the hot water storage tank 2 is connected to a water supply pipe 3, and a pressure reducing valve 5 provided in the middle of the water supply pipe 3.
Thus, water is supplied to the hot water storage tank 2 at a constant pressure.
The upper portion of the hot water storage tank 2 is connected to a hot water supply side pipe 4 provided with a hot water supply valve (not shown).
The hot water heated inside is supplied to a bath or the like via the hot water supply pipe 4 by the action of the pressure on the water supply side by opening the hot water supply valve, and an amount of water equal to this hot water supply amount is supplied from the water supply side pipe 3. It is supplied into the hot water storage tank 2. At this time, a relief valve 6 is provided in the middle of the hot water supply pipe 4 in order to keep the inside of the hot water storage tank 2 at a constant pressure.

[0004] A heating element 8 for raising the temperature of the hot water in the tank 2 is provided at the inner bottom of the hot water storage tank 2, and the temperature of the hot water, the temperature of the supply water, and the temperature of the remaining water are provided outside the hot water storage tank 2. A temperature sensor 9 that can detect hot water temperature is attached. Further, in the electric water heater main body 1, a control unit 1 that detects a power supply time zone and controls the boiling of the hot water storage tank 2.
0 is set. The control unit 10 is electrically connected to the heating unit 8, the temperature sensor 9, and the operation unit 11 which is provided outside the electric water heater main body 1 and in which a user instructs boiling of hot water. That is, the control unit 10 is configured to control the energization of the heating element 8 based on the temperature detection value of the temperature sensor 8 and the input value from the operation unit 11.

Next, the operation of the conventional electric water heater having the above configuration will be described with reference to the control flow chart of FIG. When the power supply (not shown) of the electric water heater 1 is turned on, the electric water heater 1 starts energization control (step S1), and the control unit 10
Detects the current energization supply time zone (step S2).
When the power supply time zone is detected, the current daytime time zone (7
(From time till 11:00 p.m.) or night time (from 11:00 p.m. to 7:00 in the next morning) (step S3)
Is determined. Here, since the electricity bill is cheaper in the night time zone than in the daytime zone, there is an electric water heater that performs the heating control only in the night time zone.

If it is not the daytime zone, that is, if it is the nighttime zone (NO in step S3), the power supply to the heating element 8 is started (step S6), the temperature sensor 9 detects the boiling temperature, and the heating temperature is detected. Is heated to a predetermined T ° C. (90 ° C.) (step S7), the power supply to the heating element 8 is stopped (step S8), and the power supply control is terminated (step S9). On the other hand, if it is during the daytime (YES in step S3), control unit 10 determines the remaining amount in hot water storage tank 2 based on the detected temperature value of temperature sensor 9 on the side of hot water storage tank 2 (the mounting portion of temperature sensor 6). Calculate the amount of hot water (Step S
4). That is, when the temperature sensor 9 detects the temperature of the supply water, it means that the low-temperature water has been pushed up to the mounting position of the temperature sensor 9 of the hot water storage tank 2, so it is determined that there is no predetermined amount of remaining hot water. When the above is detected, it is determined that there is a predetermined remaining amount of hot water since hot water is secured at least at the mounting position of the temperature sensor 9 or more.

Therefore, when the remaining hot water amount is calculated by the control unit 10 and it is determined that the remaining hot water amount is less than the predetermined amount (step S
(YES in No. 5), since it is necessary to secure a predetermined amount of hot water in order to eliminate running out of hot water, energization of the heating element 8 is started (step S6), and the hot water in the hot water storage tank 2 is boiled to T ° C. (90 ° C.). After raising, the energization is stopped (step S8), and the energization control is ended. (Step S9). On the other hand, when the control unit 10 determines that the remaining hot water amount is equal to or more than the predetermined amount (NO in step S5), the boiling water control is not performed because the required hot water amount for preventing the running out of hot water is secured.

[0008]

The conventional electric water heater is
During the daytime, the temperature of the water in the temperature sensor mounting portion of the hot water storage tank is detected, and the presence or absence of a predetermined remaining hot water amount is determined based on whether or not hot water is pushed up to the temperature sensor mounting portion from the detected value. there were. Then, in order to prevent running out of hot water in the hot water storage tank, when the amount of remaining hot water becomes less than a predetermined remaining hot water amount, energization of the heating element is started to start boiling. Therefore, although the electricity bill is set higher during the daytime (from 7:00 am to 11:00 p.m.) than during the night time (from 11:00 p.m. to 7:00 in the next morning), hot water is consumed during the day. There was a problem that the heating element was most frequently energized and heated in the evening hours when the amount of used was large. In particular, this tendency was remarkable when the mounting position of the temperature sensor was below the hot water storage tank.

Also, in the daytime hours, the water is heated to the same temperature as the temperature during the nighttime hours, so that the water is mostly heated during the daytime hours when the electricity bill is high, and almost not during the nighttime hours when the electricity bill is low. There was a problem that the cost increased. Furthermore, even in summer, when the amount of hot water used is small, the same heating control as in winter is performed, so that there is a problem that the electricity bill is unnecessarily high in summer.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and it is possible to limit the supply of electricity to unnecessary heating elements during the daytime hours when electricity costs are high as much as possible and to raise the temperature during the daytime hours. The purpose of the present invention is to provide an electric water heater that can set the temperature lower than the boiling temperature in the nighttime hours, suppress the electricity bill cheaply, and achieve the prevention of running out of hot water, which is the purpose of the original electric water heater. I do.

[0011]

SUMMARY OF THE INVENTION The present invention provides a heating means to which daytime or nighttime power is supplied, a hot water storage tank for storing water heated by the heating means, a water supply side pipe connected to the hot water storage tank, and A hot water supply side pipe, a temperature sensor for detecting the temperature of water in the hot water storage tank, a flow rate sensor for detecting the amount of hot water to be supplied to the outside, and a power supply time period in which either daytime power or nighttime power is supplied. An electric water heater provided with control means for determining whether or not the water is in a zone, wherein when the flow rate sensor detects a hot water supply amount equal to or greater than a predetermined value, the water temperature is raised to the boiling temperature by the heating means, and during the daytime, The elapsed time from the start of power supply is integrated, and when the elapsed time reaches a predetermined time, the power supply to the heating means is stopped.

Further, the present invention changes the amount of hot water to start boiling by the heating means during a time period during which power is supplied during the daytime in accordance with a detected value of the temperature of the water supplied by the temperature sensor.

Further, the present invention changes the boiling temperature in a time period during which power is supplied during the daytime in accordance with the detected value of the feed water temperature of the temperature sensor.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1 FIG. 1 is a system configuration diagram of an electric water heater according to Embodiment 1 of the present invention, and FIG.
3 is a control block diagram of the electric water heater according to Embodiment 1 of the present invention, and FIG. 3 is a control flow diagram of the electric water heater according to Embodiment 1 of the present invention.

In FIGS. 1 and 2, a hot water storage tank 2 for storing hot water is disposed in the center of an electric water heater main body 1. The bottom of the hot water storage tank 2 is connected to a water supply side pipe 3, and water is supplied at a constant pressure by a pressure reducing valve 5 provided in the middle of the water supply side pipe 3. The upper portion of the hot water storage tank 2 is connected to a hot water supply pipe 4 provided with a hot water supply valve (not shown). Hot water heated in the hot water storage tank 2 passes through the hot water supply pipe 4 by opening the hot water supply valve. The water is supplied to a bath or the like, and the same amount of water is supplied from the water supply side pipe 3 into the hot water storage tank 2. At this time, a relief valve 6 for keeping the inside of the hot water storage tank 2 at a constant pressure.
And a flow rate sensor 7 for detecting the flow rate of hot water to be supplied are provided in the middle of the hot water supply pipe 4. Since the flow rate sensor 7 can detect the amount of outflow of hot water in an analog manner, the flow sensor 7 has higher accuracy than detecting the amount of remaining hot water using the temperature sensor 9, and the timing of starting boiling is determined by the temperature sensor 9. It does not depend on the mounting position.

A heating element 8 for raising the temperature of the hot water in the tank 2 is provided at the inner bottom of the hot water storage tank 2, and the temperature of the hot water, the temperature of the supply water, and A temperature sensor 9 for detecting hot water temperature is attached. Further, in the electric water heater main body 1, a control unit 10 that detects a power supply time zone and controls the boiling of the hot water storage tank 2 is installed. The control unit 10 is electrically connected to the flow sensor 7, the heating element 8, the temperature sensor 9, and the operation unit 11 which is provided outside the electric water heater main body 1 and in which a user instructs boiling of hot water. That is, the control unit 10 is configured to control the energization of the heating element 8 based on the temperature detection value of the temperature sensor 9, the detection value of the flow sensor 7, and the input value from the operation unit 11.

Next, the operation of the conventional electric water heater having the above configuration will be described with reference to the control flowchart of FIG. When the power supply (not shown) of the electric water heater 1 is turned on, the electric water heater 1 starts energization control (step S101), and the control unit 1
0 detects the current energization supply time zone (step S10).
2). When the power supply time zone is detected, the daytime time zone (from 7 am to 11 am) or the night time zone (11 am
From the time to 7:00 the next morning), it is determined which time zone it is (step S103).

If it is not the daytime zone, that is, if it is the nighttime zone (NO in step S103), the power supply to the heating element 8 is started (step S109), the hot water temperature is detected by the temperature sensor 9, and the hot water temperature is set to a predetermined value. Boiling temperature T1 (90
C.) (Step S110), the power supply to the heating element 8 is stopped (Step S111), and the power supply control ends (Step S112). On the other hand, if it is during the daytime (YES in step S103), the control unit 1
0 calculates the remaining hot water amount in the hot water storage tank 2 based on the hot water supply amount detected by the flow rate sensor 7 (step S104). That is, when the flow rate sensor 7 detects a hot water supply amount equal to or more than a predetermined amount, it is determined that the remaining hot water amount is less than a predetermined amount. Is determined. The closer the predetermined amount is to the limit amount for preventing running out of hot water, the more the number of times of heating by the heating element 9 can be reduced, and the cost of electricity can be reduced.

Therefore, when the control unit 10 determines that the remaining hot water amount is equal to or more than the predetermined amount (NO in step S105),
Since the amount of hot water necessary to prevent running out of hot water is secured, the boiling control is not performed. On the other hand, when it is determined that the remaining hot water amount is less than the predetermined amount (YE in step S105).
S), energization is started to eliminate the running out of hot water and to secure a predetermined amount of hot water (step S106). Next, when the daytime power supply is started, the elapsed time is counted at the same time, and whether or not a predetermined time (for example, 10 hours) has elapsed,
That is, it is determined whether it has passed 5:00 in the evening (step S107). If the elapsed time has passed 10 hours or more (YES in step S107), the power supply is stopped (step S111). On the other hand, when the elapsed time has not elapsed for 10 hours or more (NO in step S107), the temperature of the hot water in the hot water storage tank 2 is increased to the predetermined boiling temperature T2 (70).
(° C.) is determined (step S10).
8). If it exceeds, the energization is stopped to end the boiling control (step S111), and the energization control is ended (step S112). If not exceeded (NO in step S108), energization is continued.

That is, in the present embodiment, energization during the daytime period is performed for a predetermined time (10 hours) from the start of power supply during the daytime period.
And the temperature of the hot water in the hot water storage tank 2 is lower than the boiling temperature T1 in the night time zone.
When the temperature reaches (70 ° C.), the energization is stopped, and the energization control ends. Therefore, the amount of electric power in the daytime period when the electricity bill is high can be limited, and an electric water heater that can reduce the electricity bill can be provided.

In this embodiment, the heating element 8 is used as a heating device for raising the temperature of the hot water storage tank 2. However, the present invention is not limited to this, and a heat pump type heat exchanger may be used. May be. Further, in the present embodiment, only one sensor for measuring the boiling temperature, the feed water temperature, and the temperature of the temperature sensor mounting portion is provided. However, the present invention is not limited to this, and each sensor is provided using a separate sensor. Temperature can also be measured.

Further, in the present embodiment, the flow rate sensor 7 is provided in the middle of the hot water supply pipe 4 to detect the remaining hot water amount from the hot water supply amount. However, the present invention is not limited to this. A method of providing a flow sensor 7 on the way and detecting the remaining hot water amount from the supplied water amount may be used.

Embodiment 2 FIG. FIG. 4 is a control flow chart of the electric water heater according to Embodiment 2 of the present invention. Note that the basic configuration of the electric water heater is the same as that of the first embodiment, and a description thereof will be omitted. Further, the same or corresponding parts as those in the first embodiment are denoted by the same reference numerals, and description thereof is omitted.

Next, the operation of the electric water heater according to Embodiment 2 of the present invention will be described with reference to FIGS. The operation in the night time zone is the same as that of the first embodiment, and the different points from the first embodiment will be described below. FIG.
4 and FIG. 4, if it is a daytime zone (step S
203, YES), the temperature sensor 9 detects the water temperature in the hot water storage tank 2 (step S204). Even when the supplied cold water is pushed up to the mounting portion of the temperature sensor 9, the temperature of the water supplied from the water supply side pipe 3 is maintained at a predetermined temperature (15 ° C.).
° C) or more (YES in step S205), the amount of hot water is detected by the flow rate sensor 7 to calculate the amount of remaining hot water (step S206). If the hot water supply amount is 150 L or more, energization of the heating element 8 is started to prevent running out of hot water (step S210).

On the other hand, even when the water temperature is lower than the predetermined temperature (15 ° C.) (NO in step S205), the amount of hot water is detected by the flow rate sensor 7 (step S208). If the hot water supply amount is 100 L or more, energization of the heating element 8 is started to prevent running out of hot water (step S210). Here, the reason for providing the difference in the amount of hot water when the supplied water temperature is equal to or higher than the predetermined temperature (15 ° C.) is expected to be in summer when the supplied water temperature is equal to or higher than the predetermined temperature (15 ° C.) Since the amount of hot water used is not so large, there is little risk of running out of hot water. If the amount of hot water supplied is less than 150 L, it is considered that there is a sufficient amount of remaining hot water, and boiling is not performed. On the other hand, when the temperature of the supplied water is lower than the predetermined temperature (15 ° C.), it is expected that the temperature is in winter, etc., and the amount of hot water used is large. It is necessary to do it. Therefore, these controls can reduce unnecessary power consumption, especially in summer.

In the present embodiment, the difference between the hot water supply amounts at which the supply of water starts when the temperature of the supplied water is equal to or higher than the predetermined temperature (15 ° C.) is set to 150 L and 100 L, respectively. It is not limited to. In other words, when the temperature of the supplied water is equal to or higher than the predetermined temperature (15 ° C.) than when the temperature is lower than the predetermined temperature (15 ° C.), the predetermined hot water supply amount at which the energization is started is set to be smaller, so that the boiling in summer The purpose is to reduce the number of times, reduce the amount of electricity supplied to the heating element 9, and reduce the electricity cost.

Embodiment 3 FIG. 5 is a control flow chart of the electric water heater according to Embodiment 3 of the present invention. Note that the basic configuration of the electric water heater is the same as that of the first embodiment, and a description thereof will be omitted. Further, the same or corresponding parts as those in the first embodiment are denoted by the same reference numerals, and description thereof is omitted.

Next, the operation of the electric water heater according to Embodiment 3 of the present invention will be described with reference to FIGS. The operation in the night time zone is the same as that of the first embodiment, and the different points from the first embodiment will be described below. 1 and 5, when it is a daytime time zone (YES in step S303), the amount of hot water is detected by the flow rate sensor 7 to calculate the remaining hot water amount (step S304). When the remaining hot water amount is equal to or less than a predetermined amount for preventing running out of water (YES in step S305), the temperature sensor 9 detects the temperature of the water supply water (step S306), and starts energizing the heating element 8 (step S306). Step S307).

Here, when the temperature of the water supplied from the water supply side pipe 3 is equal to or higher than a predetermined temperature (15 ° C.) (step S308).
YES), the accumulated time from the start of daytime power supply is 5 hours or more (YES in step S311), or the temperature of hot water in the hot water storage tank 2 rises to T3 (50 ° C.) or more (YES in step S312). The energization of the heating element 8 is continued until one of the conditions is satisfied. On the other hand, if the temperature of the water supplied from the water supply side pipe 3 is lower than the predetermined temperature (15 ° C.) (NO in step S308), the integrated time from the start of the daytime power supply is 10 hours or more (YE in step S309).
S) Or, the energization of the heating element 8 is continued until the temperature of the hot water in the hot water storage tank 2 satisfies one of the conditions of the boiling temperature T2 (70 ° C.) or more (YES in step S310).

Therefore, in the present embodiment, control is performed in consideration of the temperature difference of the feedwater temperature due to the season, so that in summer when the feedwater temperature is high, the power supply time to the heating element 8 is shorter than in winter. By setting, the electricity bill is reduced. In addition, when the current supply time to the heating element 8 is not limited to the water temperature in the hot water storage tank 2, even if the accumulated time exceeds a predetermined time, the power supply is terminated to continue the power supply for a long time in the daytime when the electricity bill is high. To provide an electric water heater without the need.

Embodiment 4 FIG. FIG. 6 is a control flow chart of the electric water heater according to Embodiment 3 of the present invention. Note that the basic configuration of the electric water heater is the same as that of the first embodiment, and a description thereof will be omitted. Further, the same or corresponding parts as those in the first embodiment are denoted by the same reference numerals, and description thereof is omitted.

Next, the operation of the electric water heater according to Embodiment 4 of the present invention will be described with reference to FIGS. The operation in the night time zone is the same as that of the first embodiment, and the different points from the first embodiment will be described below. In FIGS. 1 and 6, when it is a daytime period (YES in step S403), the temperature sensor 9 detects the temperature of the water supply water (step S404), and determines whether the temperature of the water supply water is equal to or lower than a predetermined temperature (step S404). S405). As described in the second embodiment, when the water temperature is equal to or higher than the predetermined temperature (15 ° C.) (YES in step S405), if the hot water supply amount is equal to or higher than 150L, the heating element 8 is energized to prevent running out of hot water. Is started (step S410).

On the other hand, also when the water temperature is lower than the predetermined temperature (15 ° C.) (NO in step S405),
If it is 00L or more, the heating element 8
Is started (step S410).

After the start of energization, as in the third embodiment, when the water temperature is equal to or higher than the predetermined temperature (15 ° C.) (step S41).
1), when the accumulated time from the start of daytime power supply is 5 hours or more (YES in step S412), or when the temperature of the hot water in the hot water storage tank 2 is equal to or higher than the boiling temperature T3 (50 ° C.) (YES in step S415). The energization of the heating element 8 is continued until one of the conditions is satisfied. On the other hand, when the water temperature is lower than the predetermined temperature (15 ° C.) (step S41)
1), the integrated time from the start of daytime power supply is 10 hours or more (YES in step S413), or the hot water temperature in the hot water storage tank 2 is equal to or higher than the boiling temperature T2 (70 ° C.) (YES in step S414). The energization of the heating element 8 is continued until one of the conditions is satisfied.

Therefore, in the present embodiment, control is performed in consideration of the temperature difference of the feedwater temperature due to the difference in the seasons. Therefore, in the summer time when the feedwater temperature is high, the power supply time to the heating element 8 is shorter than in the winter. By setting, the electricity bill is reduced. In addition, when the current supply time to the heating element 8 is not limited to the water temperature in the hot water storage tank 2, even if the accumulated time exceeds a predetermined time, the power supply is terminated to continue the power supply for a long time in the daytime when the electricity bill is high. To provide an electric water heater without the need.

[0036]

As is apparent from the above invention, the electric water heater according to the present invention comprises a heating means to which daytime power or nighttime power is supplied, a hot water storage tank for storing water heated by the heating means, and a hot water storage tank. A water supply side pipe and a hot water supply side pipe connected to the tank, a temperature sensor for detecting a temperature of water in the hot water storage tank, a flow rate sensor for detecting a hot water supply amount to the outside, and a power supply time zone during daytime power or Control means for determining which of the nighttime electric power is supplied, and when the flow rate sensor detects a hot water supply amount equal to or more than a predetermined amount, the water temperature is raised by the heating means. The temperature is raised to the raised temperature, the elapsed time from the start of power supply during the daytime is integrated, and when the elapsed time reaches a predetermined time, the power supply to the heating means is stopped. As a result, electric water heaters that limit unnecessary electricity supply to unnecessary heating elements during the daytime hours when electricity costs are high and keep electricity costs inexpensive, and achieve the original purpose of electric water heaters are prevention of running out of hot water. Can be provided.

In the electric water heater according to the present invention, the amount of hot water to be started to be heated by the heating means during a time period during which daytime power is supplied is changed according to a detected value of the temperature of the supplied water by the temperature sensor. It is. As a result, electric power is supplied to unnecessary heating elements in the summertime during the daytime when electricity costs are high so that electricity can be reduced as much as possible, and electricity can be prevented from running out of water, which is the original purpose of electric water heaters. A water heater can be provided.

In the electric water heater according to the present invention, the boiling temperature in a time period during which daytime power is supplied is changed according to the detected value of the supply water temperature of the temperature sensor. As a result, electric power is supplied to unnecessary heating elements in the summertime during the daytime when electricity costs are high so that electricity can be reduced as much as possible, and electricity can be prevented from running out of water, which is the original purpose of electric water heaters. A water heater can be provided.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an electric water heater showing a configuration of Embodiment 1 of the present invention.

FIG. 2 is a control block diagram of the electric water heater showing the configuration of Embodiment 1 of the present invention.

FIG. 3 is a control flow chart of the electric water heater showing the configuration of the first embodiment of the present invention.

FIG. 4 is a control flow chart of an electric water heater showing a configuration of a second embodiment of the present invention.

FIG. 5 is a control flow chart of an electric water heater showing a configuration of Embodiment 3 of the present invention.

FIG. 6 is a control flow chart of an electric water heater showing a configuration of a fourth embodiment of the present invention.

FIG. 7 is a configuration diagram of a conventional electric water heater.

FIG. 8 is a control flowchart of a conventional electric water heater.

[Explanation of symbols]

1 Electric water heater main body, 2 Hot water storage tank, 3 Water supply side piping, 4 Hot water supply side piping, 5 Pressure reducing valve, 6 Relief valve, 7 Flow rate sensor, 8 Heating element, 9 Temperature sensor, 10 Control section, 11 Operation section.

Claims (3)

[Claims] 1. A heating means to which daytime or nighttime power is supplied, a hot water storage tank for storing water heated by the heating means, a water supply side pipe and a hot water supply side pipe connected to the hot water storage tank, and the hot water storage tank A temperature sensor for detecting the temperature of the water inside, a flow sensor for detecting the amount of hot water supplied to the outside,
A control unit that determines whether the energizing time period is a time period during which the daytime power or the nighttime power is supplied, and the flow rate sensor detects a predetermined amount or more of hot water supply. The water temperature is raised to the boiling temperature by the heating means, and the elapsed time from the start of power supply during daytime is integrated, and when the elapsed time reaches a predetermined time, the power supply to the heating means is stopped. And electric water heater. 2. The amount of hot water to start boiling by the heating means during a time period when power is supplied during the day is changed in accordance with a detected value of feed water temperature of the temperature sensor. Electric water heater. 3. The electric water heater according to claim 1, wherein the boiling temperature in a time period during which daytime power is supplied is changed in accordance with a detected value of a feed water temperature of the temperature sensor.