- 1 Description Title of the Invention HEAT-PUMP-TYPE HOT WATER SUPPLY DEVICE Technical Field The present invention relates to a heat pump-type hot water supply device which heats water to be supplied as hot water by a heat pump circuit and stores the heated water in a hot water storage tank. Background art Conventionally, a heat pump-type hot water supply device of this type includes: a hot water storage tank which stores water to be supplied as hot water; a hot water supply circuit which extracts by a pump the water to be supplied as hot water inside the hot water storage tank from a lower part of the hot water storage tank, and circulates the water to be supplied as hot water through a water heat exchanger before feeding it to an upper part of the hot water storage tank; a heat pump circuit which heats by a refrigerant the water to be supplied as hot water circulating through the water heat exchanger of the hot water supply circuit; and an inflow water temperature sensor which detects the temperature of the water to be supplied as hot water flowing from the lower part of the hot water storage tank into the water heat exchanger of the hot water supply circuit (e.g., see Patent Document 1) . Another heat pump-type hot water supply device is provided with multiple stored hot water temperature sensors, which detect the temperature of the water to be supplied as hot water inside the hot water storage tank, at multiple positions in the 6431864_1 (GHMatters) P96613.AU TERUNI - 2 vertical direction of the hot water storage tank (e.g., see Patent Document 2). This hot water supply device detects an amount of remaining hot water inside the hot water storage tank on the basis of the temperature detected by the stored hot water temperature sensors, and when the amount of the water to be supplied as hot water inside the hot water storage tank has decreased to or below a preset amount of remaining hot water, performs boiling-up operation of heating the water to be supplied as hot water until the temperature detected by the inflow water temperature sensor has increased to or above a predetermined inflow water temperature, thereby replenishing the hot water inside the hot water storage tank. In this case, the temperature of the water to be supplied as hot water inside the hot water storage tank is detected by the stored hot water temperature sensors at multiple positions in the vertical direction. Accordingly, shortage of supply hot water is prevented, for example, by setting the amount of water to be boiled up based on what is learned from a use history of the user, and performing the boiling-up operation on the basis of the temperature detected by the stored hot water temperature sensors so as to meet the set amount of remaining hot water. The above hot water supply device further includes: a heating unit where the heat pump circuit, the water heat exchanger, and the inflow water temperature sensor are disposed; and a tank unit where the hot water storage tank and the pump are disposed, and the heating unit and the tank unit are connected through a pipe of the hot water supply circuit. Citation List 6431864_1 (GHMatters) P96613.AU TERUNI -3 Patent Document Patent Document 1: Japanese Patent Publication 2010-25493 Patent Document 2: Japanese Patent Publication 2007-178063 Disclosure of the invention Problems to be solved by the invention The heating unit of the above hot water supply device is provided with a control unit for controlling the heat pump circuit and the pump. Since the multiple stored hot water temperature sensors are provided in the hot water storage tank, connecting the stored hot water temperature sensors directly to the control unit of the heating unit requires a large number of wires. For this reason, another control unit to which the stored hot water temperature sensors are connected is provided in the tank unit, and the control unit of the heating unit and the control unit of the tank unit are connected with each other. However, as the control unit is required for each of the heating unit and the tank unit, the hot water supply device becomes structurally complicated and expensive to manufacture. The need for providing the control unit in the tank unit is eliminated if the stored hot water temperature sensors are directly connected to the control unit of the heating unit by reducing the number of the stored hot water temperature sensors and reducing the number of wires. In this case, however, the amount of water to be boiled up cannot be set appropriately due to the smaller number of positions at which the stored hot water temperature sensors detect the amount of remaining hot water, which makes shortage of supply hot water likely to occur when a large amount of hot water is used. 6431864_1 (GHMatters) P96613.AU TERUNI The present invention has been made in view of the above problems, and an advantage thereof is to provide a heat pump-type hot water supply device which can realize a simpler structure while preventing shortage of supply hot water. Means for solving the problems In order to achieve the above advantage, the present invention provides a heat pump-type hot water supply device comprising: a hot water storage tank which stores water to be supplied as hot water; a hot water supply circuit which extracts by a pump the water to be supplied as hot water inside the hot water storage tank from a lower part of the hot water storage tank, and circulates the water to be supplied as hot water through a water heat exchanger before feeding it to an upper part of the hot water storage tank; a heat pump circuit which heats by a refrigerant the water to be supplied as hot water circulating through the water heat exchanger of the hot water supply circuit; and an inflow water temperature sensor which detects the temperature of the water to be supplied as hot water flowing from the lower part of the hot water storage tank into the water heat exchanger of the hot water supply circuit. The heat pump-type hot water supply device performs boiling-up operation of heating the water to be supplied as hot water until the temperature detected by the inflow water temperature sensor has increased to or above a predetermined inflow water temperature by activating the pump of the hot water supply circuit and the heat pump circuit, wherein the hot water supply device includes: a stored hot water temperature sensor which is provided at one position in the vertical direction of the hot water storage tank and detects the temperature of the water to be supplied as hot water inside the 6431864_1 (GHMatters) P96613.AU TERUNI - 5 hot water storage tank; and control means which performs the boiling-up operation when the temperature detected by the stored hot water temperature sensor has decreased to or below a predetermined stored hot water temperature, and even when the temperature detected by the stored hot water temperature sensor is above the predetermined stored hot water temperature, performs the boiling-up operation at intervals of a predetermined time. Thus, the number of wires for the stored hot water temperature sensor is reduced, since the stored hot water temperature sensor is provided only at one position in the vertical direction of the hot water storage tank. In this case, the boiling-up operation is performed when the temperature detected by the stored hot water temperature sensor has decreased to or below a predetermined stored hot water temperature, and even when the temperature detected by the stored hot water temperature sensor is above the predetermined stored hot water temperature, the boiling-up operation is performed at intervals of a predetermined time. Thus, the hot water supply device can prevent shortage of supply hot water without having to detect the amount of remaining hot water by providing the stored hot water temperature sensor at multiple positions in the vertical direction of the hot water storage tank. Advantages of the invention According to the present invention, the hot water supply device can be made structurally simpler and less expensive, since the number of wires for the stored hot water temperature sensor can be reduced and no other control unit is required for connecting the stored hot water temperature sensor. In this case, a sufficient amount of supply hot water can always be secured, 6431864_1 (GHMatters) P96613.AU TERUNI - 6 since the hot water supply device can prevent shortage of supply hot water without having to detect the amount of remaining hot water by providing the stored hot water temperature sensor at multiple positions in the vertical direction of the hot water storage tank. Brief description of the drawings Figure 1 is a configuration view of a heat pump-type hot water supply device showing one embodiment of the present invention. Figure 2 is a block diagram showing a control system. Figure 3 is a partially-exploded cross-sectional side view of a hot water storage tank showing another embodiment of the present invention. Figure 4 is a partial cross-sectional side view of the hot water storage tank. Mode for carrying out the invention Figure 1 through Figure 4 show one embodiment of the present invention, and show a heat pump-type hot water supply device which heats water to be supplied as hot water by a heat pump circuit and stores the heated water in a hot water storage tank. The hot water supply device in this embodiment includes: a hot water storage tank 10 for storing the water to be supplied as hot water; a hot water supply circuit 20 for circulating the water to be supplied as hot water of the hot water storage tank 10; a heat pump circuit 30 for circulating a refrigerant; a water heat exchanger 40 for exchanging heat between the water to be supplied as hot water of the hot water supply circuit 20 and the refrigerant of the heat pump circuit 30, and the hot water supply 6431864_1 (GHMatters) P96613.AU TERUNI device heats the water to be supplied as hot water circulating through the hot water supply circuit 20 by the refrigerant of the heat pump circuit 30. The hot water storage tank 10 is constituted of a vertically long watertight vessel, and a water supply pipe 11 for supplying tap water to the hot water storage tank 10 is connected at its lower part. The water supply pipe 11 is provided with a pressure reducing valve 12. A hot water supply pipe 13 for supplying the hot water in the hot water storage tank 10 to a destination of the supply hot water such as the inside of a residence is connected to an upper part of the hot water storage tank 10, and the hot water supply pipe 13 and the water supply pipe 11 are connected through a bypass pipe 14. A mixing valve 15 is provided between the hot water supply pipe 13 and the bypass pipe 14, and the mixing valve 15 mixes the hot water of the hot water storage tank 10 and the water of the water supply pipe 11 to circulate the mixture through the hot water supply pipe 13. The hot water storage tank 10 is provided with one stored hot water temperature sensor 16 which detects the temperature of the water to be supplied as hot water inside the hot water storage tank 10, and the stored hot water temperature sensor 16 is provided only at one position roughly in the center in the vertical direction of the hot water storage tank 10. The hot water supply circuit 20 is constituted of: an inflow pipe 21 connected with the lower part of the hot water storage tank 10 and an inflow side of the water heat exchanger 40; an outflow pipe 22 connected with an outflow side of the water heat exchanger 40 and the upper part of the hot water storage tank 10; and a pump 23 provided in the inflow pipe 21, and the inflow pipe 21 is provided with an inflow water temperature sensor 24 for 6431864_1 (GHMatters) P96613.AU TERUNI - 8 detecting the temperature of the water to be supplied as hot water. The outflow pipe 22 is provided with a safety valve 25 for regulating the pressure inside the hot water storage tank 10 to or below a predetermined pressure. The heat pump circuit 30 is constituted of a compressor 31, an evaporator 32, an expansion valve 33, and the water heat exchanger 40 being connected together, and, as indicated by the solid arrows in the figure, circulates a refrigerant through the compressor 31, the water heat exchanger 40, the expansion valve 33, the evaporator 32, and the compressor 31, in this order. The refrigerant used in this heat pump circuit 30 is a natural refrigerant, for example, carbon dioxide. The water heat exchanger 40 is constituted of a double pipe type heat exchanger having a refrigerant passage 41 and a water passage 42, and the refrigerant passage 41 is connected with the heat pump circuit 30, while the water passage 42 is connected with the inflow pipe 21 and the outflow pipe 22 of the hot water supply circuit 20. The hot water supply device includes: a heating unit 50 where the pump 23, the heat pump circuit 30, the water heat exchanger 40, and the inflow water temperature sensor 24 are disposed; and a tank unit 60 where the hot water storage tank 10 is disposed, and the heating unit 50 and the tank unit 60 are connected through the inflow pipe 21 and the outflow pipe 22 of the hot water supply circuit 20. The hot water supply device is provided with a control unit 70 which is connected with the stored hot water temperature sensor 16, the inflow water temperature sensor 24, the compressor 31, and the pump 23, and the control unit 70 is disposed in the heating unit 50. The control unit 70 includes a CPU which stores a 6431864_1 (GHMatters) P96613.AU TERUNI - 9 program as control means, and controls the compressor 31 and the pump 23 on the basis of the temperatures detected by the stored hot water temperature sensor 16 and the inflow water temperature sensor 24. The control unit 70 further includes a timer (not shown) . In the hot water supply device configured as described above, as the pump 23 of the hot water supply circuit 20 is activated, the water to be supplied as hot water inside the hot water storage tank 10 flows from the lower part of the hot water storage tank 10 via the inflow pipe 21 through the water passage 42 of the water heat exchanger 40, and thereafter flows into the upper part of the hot water storage tank 10 via the outflow pipe 22. On the other hand, as the compressor 31 of the heat pump circuit 30 is activated, the high-temperature refrigerant discharged from the compressor 31 flows through the refrigerant passage 41 of the water heat exchanger 40, and heat is exchanged between the high temperature refrigerant of the refrigerant passage 41 and the water to be supplied as hot water of the water passage 42. Thus, the water to be supplied as hot water of the hot water supply circuit 20 is heated in the water heat exchanger 40. When the hot water is used at the destination of the supply hot water, the water to be supplied as hot water in the upper part of the hot water storage tank 10 flows out to the hot water supply pipe 13, and the water corresponding to the amount of the outflow is supplied to the lower part of the hot water storage tank 10 from the water supply pipe 11. That is, as the hot water is used, the high-temperature water to be supplied as hot water inside the hot water storage tank 10 decreases, while the low-temperature water to be supplied as hot water increases from the lower part of 6431864_1 (GHMatters) P96613.AU TERUNI - 10 the hot water storage tank 10, gradually lowering the temperature detected by the stored hot water temperature sensor 16. Now, the actions through the program of the control unit 70 will be described with reference to the flow chart of Figure 2. First, the timer is reset (Si), and the timer starts time measurement at the same time (S2) . Next, if a predetermined time t (e.g., 24 hours) has not elapsed (S3), and a temperature Ti detected by the stored hot water temperature sensor 16 is above a predetermined stored hot water temperature Tsi (e.g., 45 0 C) (S4), the process returns to the preceding step S3 and repeats the actions of steps S3 and S4. Here, if the high-temperature water to be supplied as hot water inside the hot water storage tank 10 decreases due to a large amount of hot water used, etc., and the temperature Ti detected by the stored hot water temperature sensor 16 has decreased to or below the stored hot water temperature Tsi in step S4, the compressor 31 and the pump 23 are activated to start the boiling-up operation (S5). After that, when a temperature T2 detected by the inflow water temperature sensor 24 has increased to or above a predetermined inflow water temperature Ts2 (e.g., 50 0 C) (S6), the boiling-up operation is ended (S7) and the process returns to step S3. Even if the temperature Ti detected by the stored hot water temperature sensor 16 has not decreased to or below the stored hot water temperature Tsi in step S4, when the predetermined time t has elapsed in step S3, the compressor 31 and the pump 23 are activated to start the boiling up operation (S8) . After that, when the temperature T2 detected by the inflow water temperature sensor 24 has increased to or above the predetermined inflow water temperature Ts2 (S9), the boiling-up operation is ended (Si0) and the process returns to step Si. 6431864_1 (GHMatters) P96613.AU TERUNI - 11 According to this embodiment, the number of wires for the stored hot water temperature sensor 16 can be reduced, since the stored hot water temperature sensor 16 is provided only at one position in the vertical direction of the hot water storage tank 10. Thus, it is not necessary to provide another control unit for connecting the stored hot water temperature sensor 16 in the tank unit 60, and the hot water supply device can be made structurally simpler and less expensive. In this case, the boiling-up operation is performed when the temperature Ti detected by the stored hot water temperature sensor 16 has decreased to or below the predetermined stored hot water temperature Ts1, and even when the temperature detected by the stored hot water temperature sensor 16 is above the predetermined stored hot water temperature Ts1, the boiling-up operation is performed at intervals of the predetermined time t. Thus, the hot water supply device can prevent shortage of supply hot water and always secure a sufficient amount of supply hot water without having to detect the amount of remaining hot water by providing the stored hot water temperature sensor 16 at multiple positions in the vertical direction of the hot water storage tank 10. That is, while the amount of hot water used is larger in the morning and evening (at night) in ordinary households, the stored hot water temperature can decrease to a temperature near the stored hot water temperature Ts1 even when the detected temperature Ti is above the stored hot water temperature Ts1. Even in such a case, a sufficient amount of hot water can be secured for the hours of a larger amount of hot water used, since the boiling-up operation is forcibly performed at intervals of the predetermined time t in this embodiment. In this case, setting the predetermined time t to 12 hours or more but 24 hours or less 6431864_1 (GHMatters) P96613.AU TERUNI - 12 is highly effective for suppressing bacterial proliferation without unnecessarily increasing the frequency of the forced boiling-up operation. Moreover, the tank unit 60 can be made structurally simpler and the tank unit 60 can be easily designed and manufactured, since the hot water supply device includes the heating unit 50, where the pump 23, the water heat exchanger 40, the heat pump circuit 30, and the inflow water temperature sensor 24 are disposed, and the control unit 70 with which the pump 23, the heat pump circuit 30, the inflow water temperature sensor 24, and the stored hot water temperature sensor 16 are connected is disposed in the heating unit 50. While the stored hot water temperature sensor 16 shown in the above-described embodiment is disposed at a predetermined position in the vertical direction of the hot water storage tank 10, as shown in another embodiment in Figure 3 and Figure 4, a sensor mounting portion 10a to which the stored hot water temperature sensor 16 can be mounted may be provided at multiple positions in the vertical direction of the hot water storage tank 10 and the stored hot water temperature sensor 16 may be mounted at any one of these sensor mounting portions 10a. The sensor mounting portions 10a are each constituted of a screw hole provided in a side surface of the hot water storage tank 10, and disposed at intervals from one another in the vertical direction. The stored hot water temperature sensor 16 includes a bolt-like screw part 16a, a temperature detection part 16b to be disposed inside the hot water storage tank 10, and a lead wire 16c connected with the temperature detection part 16b, and the stored hot water temperature sensor 16 is mounted to the sensor mounting portion 10a by its screw part 16a being screwed 6431864_1 (GHMatters) P96613.AU TERUNI - 13 into the sensor mounting portion 10a. In this case, a gap between the screw part 16a and the sensor mounting portion 10a is sealed by inserting a packing (not shown) between the screw part 16a and the hot water storage tank 10. On the other hand, the other sensor mounting portions 10a to which the stored hot water temperature sensor 16 is not mounted are closed by closing members 17. Similarly to the stored hot water temperature sensor 16, the closing member 17 has a bolt-like screw part 17a and is mounted on the sensor mounting portion 10a by its screw part 17a being screwed into the sensor mounting portion 10a. In this case, as with the stored hot water temperature sensor 16, a gap between the screw part 17a and the sensor mounting portion 10a is sealed by inserting a packing (not shown) between the screw part 17a and the hot water storage tank 10. This embodiment can improve the versatility of the hot water supply device, since it allows different types of specifications with different positions in the vertical direction of the stored hot water temperature sensors 16 to be met by mounting the stored hot water temperature sensor 16 to any one of the sensor mounting portions 10a during the manufacture of the hot water supply device. That is, since different types of specifications with different positions in the vertical direction of the stored hot water temperature sensor 16 is met, the amount of stored hot water can be adjusted according to the situation of where the hot water supply device is installed, so that no unnecessary water is boiled, which is economically advantageous for the users. It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country. 6431864_1 (GHMatters) P96613.AU TERUNI - 14 In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention. Description of symbols 10.. .Hot water storage tank 10a... Sensor mounting portion 16.. .Stored hot water temperature sensor 20.. .Hot water supply circuit 23.. .Pump 24.. .Inflow water temperature sensor 30.. .Heat pump 40.. .Water heat exchanger 50.. .Heating unit 70.. .Control unit 6431864_1 (GHMatters) P96613.AU TERUNI