GB2537539A

GB2537539A - Heat-pump water heater system

Info

Publication number: GB2537539A
Application number: GB1610804.5A
Authority: GB
Inventors: Otsubo Yusuke
Original assignee: Mitsubishi Electric Corp
Current assignee: Mitsubishi Electric Corp
Priority date: 2014-02-09
Filing date: 2014-02-09
Publication date: 2016-10-19
Anticipated expiration: 2034-02-09
Also published as: JP6138285B2; WO2015118674A1; GB201610804D0; JPWO2015118674A1; GB2537539B

Abstract

　Provided are: a heat-pump water heater (100); a hot water storage tank (101) for supplying water to the heat-pump water heater (100) via a heat-retention circuit (107); and a water-receiving tank (102) for supplying water to the heat-pump water heater (100) via a water-supply circuit (106) connected indoors to the heat-retention circuit (107).

Description

DESCRIPTION Title of Invention

HEAT PUMP WATER HEATING SYSTEM

Technical Field

[0001] The present invention relates to a heat pump water heating system. Background Art [0002] There have hitherto been heat pump water heating systems using open hot water tanks, the heat pump water heating system mainly including three water pipes in total, namely, a water supply circuit supplying low-temperature water from a water tank to a heat pump water heater, a hot water supply circuit changing cold water to high-temperature water in the heat pump water heater, and supplying the high-temperature water to an open hot water tank, and a circulation circuit supplying the high-temperature water from the open hot water tank to the heat pump water heater (for example, see Non Patent Literature 1).

Citation List Non Patent Literature [0003] Non Patent Literature 1: Mitsubishi Electric Catalogue, Eco Cute (Heat Pump Water Heater) for Commercial Applications, page 8 (published in May 2013)

Summary of Invention

Technical Problem [0004] The heat pump water heating system as described above has a hot water storage operation mode of storing hot water in the hot water tank, and a heat retention operation mode of retaining a temperature in the hot water tank at a certain level.

[0005] When the heat pump water heating system starts the hot water storage operation mode, low-temperature water flowing out of the water tank is supplied to the heat pump water heater through the water supply circuit. The low-temperature water supplied to the heat pump water heater exchanges heat with refrigerant in the heat pump water heater to increase its temperature, and then is supplied to the hot water tank through the hot water supply circuit. When the hot water storage operation mode is being performed, the circulation circuit is filled with the high-temperature water having flowed out of the hot water tank and the high-temperature water in the circulation circuit does not flow.

[0006] When the heat pump water heating system performs the hot water storage operation mode and determines that an amount of the high-temperature water stored in the hot water tank reaches a set level, the heat pump water heating system stops the hot water storage operation mode. Further, when the heat pump water heating system determines that a temperature of the high-temperature water stored in the hot water tank decreases to reach a set temperature or less, the heat pump water heating system starts the heat retention operation mode for increasing the water temperature in the hot water tank.

[0007] When the heat pump water heating system starts the heat retention operation mode, high-temperature water flowing out of the hot water tank is supplied to the heat pump water heater through the circulation circuit. The high-temperature water supplied to the heat pump water heater exchanges heat with the refrigerant in the heat pump water heater to increase its temperature, and is then supplied to the hot water tank through the hot water supply circuit. When the heat retention operation mode is being performed, the water supply circuit is filled with the low-temperature water having flowed out of the water tank and the low-temperature water in the water supply circuit does not flow.

[0008] Here, heat pump water heating systems having the two operation modes described above are designed to store hot water in hot water tanks with the use of electric power in the middle of the night. Thus, the heat pump water heating system performs the hot water storage operation mode during the night, and performs the heat retention operation mode from the morning, after the hot water storage operation is ended, to the early evening in many cases.

[0009] Thus, for example, the heat pump water heating system sometimes performs the hot water storage operation mode during the night in winter when an outside air temperature around the heat pump water heating system is the lowest. Even in Japan, depending on local weather conditions, the outside air temperature may decrease to about -20 degrees C, and the heat retention circuit through which no water flows during the hot water storage operation mode is exposed to the outside air at low temperature. In such a case, a heater is provided to the heat retention circuit in the unit to prevent water in the heat retention circuit from decreasing in temperature and freezing.

[0010] However, although description of manuals or other instructions for heat retention circuits that are constructed on-site calls attention to installation of a heater during on-site water pipe construction to prevent a water circuit from freezing, whether or not a heater is installed or how a heater is installed is up to the on-site constructors.

[0011] Under such a situation, there is a problem in that it is difficult to locate which part of the water circuits starts to be frozen when water on the heat retention circuit side is frozen in the hot water storage operation mode in a low-temperature outside air environment. Further, there is another problem in that antifreeze methods for the water circuits of the related-art heat pump water heating system depends on on-site determinations, and hence the water circuits may be frozen depending on on-site construction conditions, for example.

[0012] The present invention has been made in the context of the problems described above, and has an object to obtain a heat pump water heater preventing water circuits from freezing in an environment at low outside air temperature with a cost lower than that in the related art.

Solution to Problem [0013] A heat pump water heating system according to the present invention includes a heat pump water heater, a hot water tank supplying water to the heat pump water heater through a first circuit, and a water tank supplying water to the heat pump water heater through a second circuit connected to the first circuit indoors.

Advantageous Effects of Invention [0014] According to the present invention, a connection between the first circuit connecting the hot water tank and the heat pump water heater, and the second circuit through which water flowing out of the water tank flows is made indoors. Thus, it is possible to prevent water circuits from freezing in an environment at low outside air temperature with a cost lower than that in the related art.

Brief Description of Drawings

[0015] [Fig. 1] Fig. 1 is a diagram for schematically illustrating a heat pump water heating system 200 according to Embodiment 1 of the present invention.

[Fig. 2] Fig. 2 is a diagram for illustrating a refrigerant circuit and a water circuit of a heat pump water heater 100 of the heat pump water heating system 200 according to Embodiment 1 of the present invention.

[Fig. 3] Fig. 3 is a diagram for schematically illustrating a heat pump water heating system 400 according to Comparative Example.

[Fig. 4] Fig. 4 is a diagram for illustrating a refrigerant circuit and a water circuit of a heat pump water heater 100 of a heat pump water heating system 200 according to Embodiment 2 of the present invention.

[Fig. 5] Fig. 5 is a diagram for schematically illustrating a heat pump water heating system 200 according to Embodiment 3 of the present invention.

[Fig. 6] Fig. 6 is a diagram for schematically illustrating a heat pump water heating system 200 according to Embodiment 4 of the present invention.

[Fig. 7] Fig. 7 is a diagram for illustrating a refrigerant circuit and a water circuit of a heat pump water heater 100 of the heat pump water heating system 200 according to Embodiment 4 of the present invention.

[Fig. 8] Fig. 8 is a correlation table of outside air temperature, hot water tank temperature, and heat retention circuit water temperature, for estimating threshold values for opening or closing a motor valve 13 of a water pipe kit 103 of the heat pump water heating system 200 according to Embodiment 4 of the present invention.

Description of Embodiments

[0016] Embodiment 1 Fig. 1 is a diagram for schematically illustrating a heat pump water heating system 200 according to Embodiment 1 of the present invention. As illustrated in Fig. 1, the heat pump water heating system 200 includes a heat pump water heater 100, a hot water tank 101, a water tank 102, a water pipe kit 103, and a pump 108. [0017] The heat pump water heater 100 is installed outdoors, for example. The details of the heat pump water heater 100 are described later. The hot water tank 101 is an open tank storing hot water with a temperature increased in the heat pump water heater 100, and is installed indoors, for example. The water tank 102 is a tank storing cold water that is supplied to the heat pump water heater 100, and is installed indoors, for example.

[0018] The heat pump water heater 100 and the hot water tank 101 are connected to each other by a water inlet circuit 104, a hot water supply circuit 105, and a heat retention circuit 107. The hot water tank 101 and the water tank 102 are connected to each other by a water supply circuit 106 and the heat retention circuit 107. The water tank 102 and the heat pump water heater 100 are connected to each other by the water inlet circuit 104 and the water supply circuit 106. The pump 108 is arranged on the water supply circuit 106.

[0019] The water pipe kit 103 includes a motor valve 10, check valves 12a and 12b, a part of the water supply circuit 106, and a part of the heat retention circuit 107. Note that, in the following description, the check valves 12a and 12b may be collectively referred to as a check valve 12. The water pipe kit 103 is configured so that the water supply circuit 106 and the heat retention circuit 107 are joined together That is, the portions of the water pipes where the water supply circuit 106 and the heat retention circuit 107 are joined together are arranged indoors as a pipe kit separately from the heat pump water heater 100. Note that, it is desired that the water pipe kit 103 be installed indoors close to the hot water tank 101 to prevent the water pipe kit 103 from being affected by low-temperature outside air [0020] The motor valve 10 is a valve arranged to the water supply circuit 106. The motor valve 10 passes therethrough or blocks water flowing from the water tank 102 toward the water inlet circuit 104 side.

[0021] The check valve 12a is a valve arranged to the heat retention circuit 107. The check valve 12a functions to allow water in the hot water tank 101 to flow into the heat pump water heater 100, and prevent water in the heat pump water heater 100 and water in the water tank 102 from flowing into the hot water tank 101.

[0022] The check valve 12b is a valve arranged to the water supply circuit 106. The check valve 12b functions to allow water in the water tank 102 to flow into the heat pump water heater 100, and prevent water in the heat pump water heater 100 and water in the hot water tank 101 from flowing into the water tank 102.

[0023] Inside the water pipe kit 103, a joint portion 9 is arranged at a position which the water supply circuit 106 and the heat retention circuit 107 are joined together The joint portion 9 is located indoors. With this configuration, water flowing out of the hot water tank 101 to reach the joint portion 9 and water flowing out of the water tank 102 to reach the joint portion 9 are joined together to flow into the heat pump water heater 100 through the water inlet circuit 104.

[0024] Fig. 2 is a diagram for illustrating a refrigerant circuit and a water circuit of the heat pump water heater 100 of the heat pump water heating system 200 according to Embodiment 1 of the present invention. As illustrated in Fig. 2, the heat pump water heater 100 includes a compressor 1, a radiator 2, a pressure reducing device 3, an evaporator 4, and a control unit 8.

[0025] The compressor 1 compresses refrigerant flowing through a refrigeration cycle into high-temperature and high-pressure refrigerant, and discharges the high-temperature and high-pressure refrigerant. The radiator 2 is a heat exchanger exchanging heat between refrigerant flowing through the refrigerant circuit and water supplied from the water supply circuit 106. An outlet pipe of a water circuit of the radiator 2 is connected to the hot water supply circuit 105. The pressure reducing device 3 reduces a pressure of refrigerant. The evaporator 4 is a heat exchanger exchanging heat between refrigerant flowing through the refrigerant circuit and outside air.

[0026] The control unit 8 adjusts a frequency of the compressor 1 and an opening degree of the pressure reducing device 3. Further, the control unit 8 opens or closes the motor valve 10. The control unit 8 opens the motor valve 10 in the hot water storage operation mode. The control unit 8 closes the motor valve 10 in the heat retention operation mode. The control unit 8 includes, for example, hardware such as a circuit device configured to realize the above-mentioned functions, or software to be executed on an arithmetic unit such as a microcontroller and a CPU.

[0027] Next, the outline of heat exchange in the heat pump water heater 100 is described.

Refrigerant flowing through a refrigerant pipe in the heat pump water heater 100 is compressed in the compressor 1 to be high-temperature and high-pressure refrigerant, and then flows into the radiator 2. The refrigerant flowing into the radiator 2 exchanges heat with water flowing into the heat pump water heating system 200 through the water inlet circuit 104 to decrease its temperature. The refrigerant with a temperature decreasing in this way is decompressed in the pressure reducing device 3. The decompressed refrigerant flows into the evaporator 4, exchanges heat with outside air, and flows into the compressor 1 again The same cycle is subsequently repeated. On the other hand, water flowing into the heat pump water heater 100 exchanges heat with refrigerant in the refrigerant circuit to increase its temperature, and then flows out to the hot water supply circuit 105.

[0028] Next, operations of the hot water storage operation mode and the heat retention operation mode of the heat pump water heating system 200 are described.

Note that, the hot water storage operation mode is an operation mode of storing hot water in the hot water tank 101. Further, the heat retention operation mode is an operation mode of retaining a temperature in the hot water tank 101 at a certain level. [0029] (1) Hot Water Storage Operation Mode In the hot water storage operation mode, the control unit 8 opens the motor valve 10. Thus, water flowing out of the water tank 102 flows into the water pipe kit 103 through the water supply circuit 106, sequentially passes through the motor valve 10 and the check valve 12b, and is joined into water flowing through the heat retention circuit 107 at the joint portion 9. Note that, water flowing out of the hot water tank 101 to flow into the heat retention circuit 107 does not reach the joint portion 9, and is thus not supplied to the heat pump water heater 100.

[0030] In this manner, water flowing out of the water tank 102 is joined into water flowing through the heat retention circuit 107 at the joint portion 9 to be supplied to the heat pump water heater 100 through the water inlet circuit 104. The water flowing into the heat pump water heater 100 exchanges heat with refrigerant, flows out of the heat pump water heater 100 as high-temperature water, and flows into the hot water tank 101 through the hot water supply circuit 105.

[0031] Here, the check valve 12a is arranged to the heat retention circuit 107. Thus, the water flowing out of the water tank 102 does not flow into the heat retention circuit 107, and the high-temperature water is stored in the hot water tank 101. The hot water storage operation mode is continued until a water level of the high-temperature water stored in the hot water tank 101 reaches a set water level.

[0032] Then, when the control unit 8 determines that the water level of the hot water in the hot water tank 101 reaches the set water level, the control unit 8 ends the hot water storage operation mode. After the hot water storage operation mode is ended, the temperature of the water stored in the hot water tank 101 gradually decreases due to the influence of indoor temperature, and may fall below a temperature required for hot water supply. Thus, when the control unit 8 determines that the temperature of the water in the hot water tank 101 falls below a preset temperature, the control unit 8 performs the heat retention operation mode.

[0033] (2) Heat Retention Operation Mode In the heat retention operation mode, the control unit 8 closes the motor valve 10. Thus, water flowing out of the water tank 102 is accumulated in the water supply circuit 106, but does not flow, and is thus not supplied to the heat retention circuit 107.

As a result, only water flowing out of the hot water tank 101 is supplied to the heat pump water heater 100 through the heat retention circuit 107 and the water inlet circuit 104.

[0034] The water supplied to the heat pump water heater 100 exchanges heat with refrigerant in the heat pump water heater 100 to increase its temperature, and then flows into the hot water tank 101 through the hot water supply circuit 105. The control unit 8 continues the heat retention operation mode until a temperature of the hot water in the hot water tank 101 reaches a set value or more. In this manner, in the heat retention operation mode, the hot water in the hot water tank 101 increases its temperature to become high-temperature water again, thereby being capable of maintaining the water temperature in the hot water tank 101.

[0035] Fig. 3 is a diagram for schematically illustrating a heat pump water heating system 400 according to Comparative Example.

As illustrated in Fig. 3, the heat pump water heating system 400 includes a heat pump water heater 300, a hot water tank 301, a water tank 302, a hot water supply circuit 305, a water supply circuit 306, a heat retention circuit 307, and a pump 308. A joint portion (not shown) between the water supply circuit 306 and the heat retention circuit 307 is arranged outdoors.

[0036] Thus, the hot water supply circuit 305, the water supply circuit 306, and the heat retention circuit 307 are constructed outdoors, and water circuit portions through which no water flows in each operation mode are exposed to a low-temperature outside air environment. Then, there arises a problem in that pipes located outdoors may be frozen. Further, there arises another problem in that it is necessary to provide a heating unit such as a heater to prevent the pipes located outdoors from freezing, resulting in an increase in manufacturing cost.

[0037] Meanwhile, the heat pump water heating system 200 according to Embodiment 1 includes the heat pump water heater 100, the hot water tank 101 supplying water to the heat pump water heater 100 through the heat retention circuit 107, and the water tank 102 supplying water to the heat pump water heater 100 through the water supply circuit 106 connected to the heat retention circuit 107 indoors. That is, a connection between the heat retention circuit 107 connecting the hot water tank 101 and the heat pump water heater 100 to each other, and the water supply circuit 106 through which water flowing out of the water tank 102 flows is made indoors. Thus, the water circuits can be prevented from freezing without using any heater. It is thus possible to prevent the water circuits from freezing in an environment at low outside air temperature with a cost lower than that in the related art.

[0038] Further, even when heaters are provided to prevent the pipes installed outdoors from freezing, the number of pipes can be reduced and the number of heaters can thus be reduced. Hence, on-site water pipe construction can be simplified.

[0039] Further, the water pipe kit 103 is installed indoors. Thus, the water circuits installed outdoors are two, namely, a part of the hot water supply circuit 105 and the water inlet circuit 104. As a result, there is no part in the water pipe portions installed outdoors through which no water flows in each operation mode, and hence the water circuits can be prevented from freezing during the heat pump operation.

[0040] Further, the water pipe kit 103 configured so that the water supply circuit 106 and the heat retention circuit 107 are joined together is installed indoors. Thus, the on-site construction can be simplified.

[0041] Embodiment 2 Embodiment 2 of the present invention differs from Embodiment 1 in that a heat pump water heating system 200 includes an inlet pipe temperature sensor 5 and an outlet pipe temperature sensor 6.

[0042] Fig. 4 is a diagram for illustrating a refrigerant circuit and a water circuit of a heat pump water heater 100 of the heat pump water heating system 200 according to Embodiment 2 of the present invention.

As illustrated in Fig. 4, the heat pump water heater 100 includes the inlet pipe temperature sensor 5 and the outlet pipe temperature sensor 6. The inlet pipe temperature sensor 5 is a temperature detection unit arranged on the inlet side of the radiator 2. The outlet pipe temperature sensor 6 is a temperature detection unit arranged on the outlet side of the radiator 2.

[0043] In the hot water storage operation mode, the inlet pipe temperature sensor 5 detects a temperature of water supplied from the water tank 102 to the heat pump water heater 100 through the water supply circuit 106 and the water inlet circuit 104. In the heat retention operation mode, the inlet pipe temperature sensor 5 detects a temperature of water supplied from the hot water tank 101 to the heat pump water heater 100 through the heat retention circuit 107 and the water inlet circuit 104.

[0044] In the hot water storage operation mode, the outlet pipe temperature sensor 6 detects a temperature of water having exchanged heat with refrigerant after the water is supplied from the water tank 102 to the heat pump water heater 100 through the water supply circuit 106 and the water inlet circuit 104. In the heat retention operation mode, the outlet pipe temperature sensor 6 detects a temperature of water having exchanged heat with refrigerant after the water is supplied from the hot water tank 101 to the heat pump water heater 100 through the heat retention circuit 107 and the water inlet circuit 104.

[0045] Next, operation of the heat pump water heating system 200 according to Embodiment 2 is described. When the control unit 8 determines that at least one of detected values of the inlet pipe temperature sensor 5 and the outlet pipe temperature sensor 6 is a threshold temperature (for example, 3 degrees C) or less, the control unit 8 starts operation of the compressor 1. Further, when the control unit 8 performs operation of the compressor 1 and determines that a detected value of the inlet pipe temperature sensor 5 or a detected value of the outlet pipe temperature sensor 6 is a threshold temperature (for example, 10 degrees C) or more, the control unit 8 stops operation of the compressor 1.

[0046] As described above, in the heat pump water heating system 200 according to Embodiment 2, when the control unit 8 determines that at least one of detected values of the inlet pipe temperature sensor 5 and the outlet pipe temperature sensor 6 is a threshold temperature or less, the control unit 8 performs operation of the compressor 1. When operation of the compressor 1 is performed in this way, high-pressure and high-temperature refrigerant flows to the radiator 2 and a temperature of the entire radiator 2 becomes high due to heat conduction, to thereby increase a temperature of the water circuit in the heat pump water heater 100. Thus, even when the heat pump water heater 100 stops its operation and water in the water pipe in the heat pump water heater 100 does not flow, and the water circuit is thus exposed to a low-temperature outside air environment, the water circuit can be prevented from freezing without using any heater. It is thus possible to prevent the water circuits from freezing in an environment at low outside air temperature with a cost lower than that in the related art.

[0047] Embodiment 3 Embodiment 3 of the present invention differs from Embodiment 1 in that a water pipe kit 103 is installed outdoors close to the heat pump water heater 100 or installed in the heat pump water heater 100.

[0048] Fig. 5 is a diagram for schematically illustrating a heat pump water heating system 200 according to Embodiment 3 of the present invention.

As illustrated in Fig. 5, inside the water pipe kit 103, a connection port between the hot water supply circuit 105 and the heat retention circuit 107 is provided.

Further, inside the water pipe kit 103, a bypass circuit 20 bypassing the hot water supply circuit 105 and the heat retention circuit 107 is arranged. A motor valve 13 is arranged to the bypass circuit 20. A temperature sensor 14 is arranged to an inlet of the heat retention circuit 107 inside the water pipe kit 103. The temperature sensor 14 is a temperature detection unit configured to detect a temperature of the heat retention circuit 107.

[0049] The control unit 8 adjusts the opening degree of the motor valve 13 based on a detected value of the temperature sensor 14. For example, when the control unit 8 determines that a detected value of the temperature sensor 14 is a threshold temperature (for example, 3 degrees C) or less, the control unit 8 opens the motor valve 13. Thus, high-temperature water flowing out of the heat pump water heater 100 is supplied to the heat retention circuit 107 through the hot water supply circuit 105 and the bypass circuit 20. Further, for example, when the control unit 8 determines that a detected value of the temperature sensor 14 is a threshold temperature (for example, 5 degrees C) or more, the control unit 8 closes the motor valve 13. Thus, the operation of preventing the water circuits from freezing is ended. [0050] Here, as described above, while the hot water storage operation mode is being performed, water flowing out of the hot water tank 101 does not flow to the heat retention circuit 107. Thus, water in the pipes may be frozen in a low-temperature outside air environment, and hence heaters are installed in the related art to prevent the water circuits from freezing, for example.

[0051] Meanwhile, the heat pump water heating system 200 according to Embodiment 3 includes the bypass circuit 20 bypassing the hot water supply circuit 105 and the heat retention circuit 107, and the motor valve 13 is arranged on the bypass circuit 20. Thus, high-temperature water flowing out of the heat pump water heater 100 can be supplied to the heat retention circuit 107 through the hot water supply circuit 105 and the bypass circuit 20, and the heat retention circuit 107 can be prevented from freezing even when the heat retention circuit 107 is installed outdoors. As a result, it is possible to efficiently increase a temperature of the heat retention circuit 107 without providing any heating unit such as a heater, to thereby prevent the water circuit from freezing. Further, even when the water pipe kit 103 cannot be installed indoors depending on situations such as on-site installation conditions, the water circuit can be prevented from freezing.

[0052] Note that, when the opening degree of the motor valve 13 is large, an amount of water flowing to the hot water tank 101 is reduced and the performance may be deteriorated. Thus, it is desired that the control unit 8 set the opening degree of the motor valve 13 to be as small as possible, and then gradually increase the opening degree of the motor valve 13 while checking an increase in detected values of the temperature sensor 14. In this manner, the water circuit can be prevented from freezing while an influence on the hot water storage performance is reduced to be the minimum.

[0053] Embodiment 4 Embodiment 4 of the present invention differs from Embodiment 1 in that a heat pump water heating system 200 includes a hot water tank temperature sensor 101a, an inlet pipe temperature sensor 5, and an outside-air temperature sensor 7.

[0054] Fig. 6 is a diagram for schematically illustrating the heat pump water heating system 200 according to Embodiment 4 of the present invention. Fig. 7 is a diagram for illustrating a refrigerant circuit and a water circuit of a heat pump water heater 100 of the heat pump water heating system 200 according to Embodiment 4 of the present invention. Fig. 8 is a correlation table of outside air temperature, hot water tank temperature, and heat retention circuit water temperature, for estimating threshold values for opening or closing the motor valve 13 of the water pipe kit 103 of the heat pump water heating system 200 according to Embodiment 4 of the present invention.

[0055] As illustrated in Fig. 6, inside the hot water tank 101, the hot water tank temperature sensor 101a detecting a temperature of the inside of the hot water tank 101 is arranged. As illustrated in Fig. 7, inside the heat pump water heater 100, the inlet pipe temperature sensor 5 arranged on the inlet side of the radiator 2 and the outside-air temperature sensor 7 detecting outside air temperature are arranged.

[0056] Here, when the hot water storage operation mode is being performed, the motor valve 10 is opened, and water flowing out of the hot water tank 101 is not supplied to the heat pump water heater 100. Thus, the inlet pipe temperature sensor 5 detects a temperature of the water supply circuit 106 through which water flowing out of the water tank 102 flows. That is, in the hot water storage operation mode, a water temperature on the heat retention circuit 107 side where the water flowing out of the hot water tank 101 does not flow and thus tends to be frozen cannot be measured.

[0057] On the other hand, when the heat pump operation is stopped, the motor valve 10 is closed, and water flowing out of the water tank 102 to the water supply circuit 106 is not supplied to the heat pump water heater 100. Thus, the inlet pipe temperature sensor 5 detects a temperature of the heat retention circuit 107 through which water flowing out of the hot water tank 101 flows. In this case, when the heat pump operation is stopped, the control unit 8 stores correlations among a detected value of the outside-air temperature sensor 7, a detected value of the hot water tank temperature sensor 101a, and a detected value of the inlet pipe temperature sensor 5. Specifically, the control unit 8 has a built-in storage unit configured to store correlations as shown in Fig. 8. Note that, the correlations shown in Fig 8 are as described in (1-1) to (3-5) below.

[0058] (1-1) When the outside air temperature is 5 degrees C and the hot water tank temperature is 70 degrees C, the water inlet temperature is 10 degrees C. (1-2) When the outside air temperature is 5 degrees C and the hot water tank temperature is 65 degrees C, the water inlet temperature is 9 degrees C. (1-3) When the outside air temperature is 5 degrees C and the hot water tank temperature is 60 degrees C, the water inlet temperature is 8 degrees C. (1-4) When the outside air temperature is 5 degrees C and the hot water tank temperature is 55 degrees C, the water inlet temperature is 7 degrees C. (1-5) When the outside air temperature is 5 degrees C and the hot water tank temperature is 50 degrees C, the water inlet temperature is 5 degrees C. [0059] (2-1) When the outside air temperature is 0 degrees C and the hot water tank temperature is 70 degrees C, the water inlet temperature is 8 degrees C. (2-2) When the outside air temperature is 0 degrees C and the hot water tank temperature is 65 degrees C, the water inlet temperature is 6 degrees C. (2-3) When the outside air temperature is 0 degrees C and the hot water tank temperature is 60 degrees C, the water inlet temperature is 5 degrees C. (2-4) When the outside air temperature is 0 degrees C and the hot water tank temperature is 55 degrees C, the water inlet temperature is 3 degrees C. (2-5) When the outside air temperature is 0 degrees C and the hot water tank temperature is 50 degrees C, the water inlet temperature is 1 degree C. [0060] (3-1) When the outside air temperature is -5 degrees C and the hot water tank temperature is 70 degrees C, the water inlet temperature is 5 degrees C. (3-2) When the outside air temperature is -5 degrees C and the hot water tank temperature is 65 degrees C, the water inlet temperature is 4 degrees C. (3-3) When the outside air temperature is -5 degrees C and the hot water tank temperature is 60 degrees C, the water inlet temperature is 3 degrees C. (3-4) When the outside air temperature is -5 degrees C and the hot water tank temperature is 55 degrees C, the water inlet temperature is 2 degrees C. (3-5) When the outside air temperature is -5 degrees C and the hot water tank temperature is 50 degrees C, the water inlet temperature is 0 degrees C. [0061] During the hot water storage operation mode, the control unit 8 calculates (estimates) a temperature of the heat retention circuit 107 based on a detected value of the outside-air temperature sensor 7, a detected value of the hot water tank temperature sensor 101a, and the stored contents about the correlations described above. [0062] When the calculated temperature of the heat retention circuit 107 is a threshold temperature (for example, 3 degrees C) or less, the control unit 8 opens the motor valve 13. Thus, high-temperature water flowing out of the heat pump water heater 100 is supplied to the heat retention circuit 107 through the hot water supply circuit 105 and the bypass circuit 20, and hence the water circuit can be prevented from freezing. Note that, it is desired that the control unit 8 continuously open the motor valve 13 with such a small opening degree that does not affect the performance of the hot water storage operation mode.

[0063] As described above, in the heat pump water heating system 200 according to Embodiment 4, the control unit 8 opens the motor valve 13 when a value calculated based on a detected value of the outside-air temperature sensor 7, a detected value of the hot water tank temperature sensor 101a, and data stored in the storage unit of the control unit 8 is a threshold temperature or less. Thus, the water circuit can be prevented from freezing even when the temperature sensor 14 detecting a temperature of the heat retention circuit 107 is not arranged.

[0064] Note that, an example in which the control unit 8 includes the storage unit configured to store the correlations as shown in Fig. 8 is described, but the present invention is not limited thereto. A storage unit configured to store the correlations as shown in Fig. 8 may be arranged separately from the control unit 8.

[0065] Note that, the heat retention circuit 107 corresponds to a first circuit of the present invention.

Further, the water supply circuit 106 corresponds to a second circuit of the present invention.

Further, the hot water supply circuit 105 corresponds to a third circuit of the present invention.

Further, the water inlet circuit 104 corresponds to a fourth circuit of the present invention.

Reference Signs List [0066] 1 compressor 2 radiator 3 pressure reducing device 4 evaporator inlet pipe temperature sensor 6 outlet pipe temperature sensor 7 outside-air temperature sensor 8 control unit 9 joint portion 10 motor valve 12, 12a, 12b check valve 13 motor valve 14 temperature sensor 20 bypass circuit 100 heat pump water heater 101 hot water tank 101a hot water tank temperature sensor 102 water tank 103 water pipe kit 104 water inlet circuit 105 hot water supply circuit 106 water supply circuit 107 heat retention circuit 108 pump 200 heat pump water heating system 300 heat pump water heater 301 hot water tank 302 water tank 305 hot water supply circuit 306 water supply circuit 307 heat retention circuit 308 pump 400 heat pump water heating system