HEAT MEDIUM CIRCULATION DEVICE

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation Application of PCT Application No. PCT/JP2023/022574, Filed Jun. 19, 2023, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates generally to a heat medium circulation device circulating a heat medium having a temperature adjusted, between a heat source unit and a utilization-side equipment by a pipe and pump.

Description of the Related Art

A heat medium circulation device is known in which a heat medium (for example, water) heated by a heat source unit in outdoor is sent to a utilization-side equipment (for example, a fan coil, a floor heating panel, a hot water tank, or the like) in indoor via a pipe and a pump and the heat medium having passed through the utilization-side equipment is returned to the heat source unit via a pipe.

In such a heat medium circulation device, the heat medium may freeze due to a drop in the outside air temperature while the device is stopped, and as the freezing progresses, the expansion of the heat medium may cause damage to the pipe. To prevent this freezing, in the heat medium circulation device, a temperature sensor for detecting the temperature of the heat medium is attached to the pipe and, when the temperature detected by the temperature sensor falls below a predetermined threshold value while the heat source unit is stopped, freeze prevention control to drive the pump to circulate the heat medium is executed.

The pipe between the heat source unit in outside and the utilization-side equipment in inside has a part which is exposed to the cold outside air and a part which is contained within a casing or building of the heat source unit and is not in contact with the outside air. If the temperature sensor is installed in a place which is not in contact with the outside air, the drop in temperature of the heat medium in the part exposed to the cold outside air cannot be detected properly. For this reason, there is a possibility that the heat medium will freeze before the temperature detected by the temperature sensor falls below the threshold value and the pump operation starts.

One possible solution is to uniformly increase the threshold value of the heat medium temperature for pump operation determination, but this may result in the pump being operated unnecessarily even in a situation where there is no risk of freezing, which is undesirable in terms of saving energy. In addition, a method of detecting the outside air temperature without using the heat medium temperature and, if the detected outside air temperature is low, driving (operating) the pump can also be conceived. Even in this method, however, if the outside air temperature is low, the pump may be operated as soon as the heat source unit is stopped, and the pump may be operated unnecessarily even when the temperature of the heat medium in the pipe is high enough that there is no risk of freezing.

BRIEF SUMMARY OF THE INVENTION

According to one embodiment, a heat medium circulation device circulates a heat medium between a heat source unit in outdoor and a utilization-side equipment in indoor by a pipe and a pump, and includes an outside air temperature sensor which detects a temperature of the outdoor air, a heat medium temperature sensor which detects a temperature of the heat medium, and a controller executing freeze prevention control to drive the pump when the temperature detected by the heat medium temperature sensor falls below a threshold value while the heat source unit is stopped, and changes the threshold value in accordance with the temperature detected by the outdoor air temperature sensor.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiment given below, serve to explain the principles of the invention.

FIG. 1 is a block diagram showing an entire configuration of an embodiment.

FIG. 2 is a flowchart showing control of the embodiment.

FIG. 3 is a flowchart continuing from FIG. 2.

FIG. 4 is a chart showing a flow rate change and a temperature change of a heat medium in the embodiment.

FIG. 5 is a graph showing a relationship between an outside air temperature and a maximum change amount of the heat medium temperature, and a relationship between the outside air temperature and a corrected value in the embodiment.

FIG. 6 is a graph showing a relationship between the outside air temperature and a threshold value in the embodiment.

FIG. 7 is a block diagram showing a modified example of an entire configuration of the embodiment.

DETAILED DESCRIPTION OF THE INVENTION

A heat medium circulation device according to one embodiment will be described hereinafter with reference to the accompanying drawings.

As shown in FIG. 1, a utilization-side equipment (for example, a fan coil, a floor heating panel, a hot water tank, or the like) 9 is connected to a heat source unit (also referred to as an outdoor unit) A installed outdoors via pipes 6a and 6b and an indoor unit B. Then, a signal line C for data transmission and control is connected between the heat source unit A and the indoor unit B.

The heat source unit A includes a compressor 1 which sucks in, compresses, and discharges a refrigerant, a gas side pipe 2a which sends the gas refrigerant discharged from a discharge port of the compressor 1 to the refrigerant flow path 3a of the heat medium heat exchanger (also referred to as a water heat exchanger) 3, a liquid side pipe 2b which leads the liquid refrigerant condensed by heat exchange with the heat medium in the refrigerant flow path 3a to a flow control valve (pressure reducer) 4, a liquid side pipe 2c which leads the liquid refrigerant decompressed by the flow control valve 4 to the outdoor heat exchanger 5, a gas side pipe 2d which returns the gas refrigerant evaporated by heat exchange with the outside air in the outdoor heat exchanger 5 to the suction port of the compressor 1, a part of a heat medium pipe (water pipe) 6a which leads the heat medium returning from the utilization-side equipment 9 to a heat medium flow path (water flow path) 3b of the heat medium heat exchanger 3, and a part of a heat medium pipe 6b which leads the water that is subjected to heat exchange with the refrigerant in the heat medium flow path 3b and whose temperature is raised to the indoor unit B. Various refrigerants such as R410A, R32, and R290 can be used as refrigerants for the refrigeration cycle. In addition, various fluids can also be used as the heat medium and, in this embodiment, a heat medium that freezes in a low-temperature environment, such as “water”, is used. A plate-type heat exchanger or a multi-tube heat exchanger can be used as the heat medium heat exchanger 3 which exchanges heat between the refrigerant and the heat medium. An outdoor fan 30 is provided to be opposite to the outdoor heat exchanger 5 which is composed of a fin-tube type heat exchanger or the like. The heat exchange between the refrigerant and the outside air in the outdoor heat exchanger 5 is promoted by outside air blow which is executed by the operation of the outdoor fan 30.

A heat pump type refrigeration cycle which pumps up heat from the outside air and provides the pumped heat to the heat medium via the heat medium flow path 3b of the heat medium heat exchanger 3 is constituted by a refrigerant circuit which passes from the compressor 1 through the gas side pipe 2a, the refrigerant flow path 3a of the heat medium heat exchanger 3, the liquid side pipe 2b, the flow control valve 4, the liquid side pipe 2c, the outdoor heat exchanger 5, and the gas side pipe 2d in sequence and returns to the compressor 1. The heat pumped by this heat pump type refrigeration cycle is sent to the utilization-side equipment 9 for heating or hot water supply by the heat medium that passes through the heat medium heat exchanger 3.

Furthermore, the heat source unit A includes an outdoor controller 10 which is composed of a microcomputer and its peripheral circuits, an outdoor air temperature sensor 11, a heat medium temperature sensor 12, and an inverter 13 which are connected to the outdoor controller 10. The outdoor air temperature sensor 11 is installed on the windward side of the outdoor heat exchanger 5 and detects an outdoor air temperature To which is the temperature of the outdoor air flowing into the outdoor heat exchanger 5. The heat medium temperature sensor 12 is attached to a part of the heat medium pipe 6a, which is accommodated in a housing of the heat source unit A and detects the temperature of the heat medium flowing into the heat medium flow path 3b of the heat medium heat exchanger 3, i.e., a return temperature Twi, which is the temperature of the heat medium returning to the heat source unit A after releasing heat in the utilization-side equipment 9. The inverter 13 converts the AC voltage of the AC power source 14 into a DC voltage, converts the DC voltage into an AC voltage of a frequency commanded by the outdoor controller 10, and outputs the AC voltage as driving power for a motor 1M which drives a compression mechanism unit of the compressor 1. The rotation speed of the motor 1M changes according to a frequency (output frequency) of the AC voltage output from the inverter 13, and the capability of the compressor 1 changes according to the change in the rotation speed.

The outdoor controller 10 includes a memory 10m for storing data, and transmits the above-mentioned outdoor air temperature To and return temperature Twi to the indoor controller 20 to be described below via a signal line C, and controls the operation of the compressor 1 (output frequency of the inverter 13) and the opening of the flow control valve 4 in response to instructions from the indoor controller 20 via the signal line C. The data on the outdoor air temperature To detected by the outdoor air temperature sensor 11 is used for normal operation control and various protection controls of the compressor 1 in the heat source unit A, as well as for freeze prevention control to be described below.

The indoor unit B has a housing attached to an indoor wall or the like, accommodates an electric heater 7, a pump 8, an indoor controller 20 composed of a microcomputer and its peripheral circuits, a heat medium temperature sensor 21 connected to the indoor controller 20 and a heat medium temperature sensor 22, and comprises a remote control type operation device 23 that can be operated by the user on a front surface of the housing. The electric heater 7 is provided in the heat medium pipe 6b and, when the temperature of the heat medium cannot be sufficiently increased by the heating of the heat pump type refrigeration cycle alone, at an extremely low outdoor temperature or the like, supplementarily heats the heat medium flowing from the heat source unit A based on instructions of the indoor controller 20. The pump 8 is provided in the heat medium pipe 6c extending from the heat medium pipe 6b to the utilization-side equipment 9, and sends the heat medium that has passed through the electric heater 7 to the utilization-side equipment 9. When the pump 8 operates, the heat medium circulates through the heat medium pipe 6c, the utilization-side equipment 9, the heat medium pipe 6a, the heat medium heat exchanger 3, the heat medium pipe 6b, and the electric heater 7.

The heat medium temperature sensor 21 is attached to a part of the heat medium pipe 6b, which is accommodated in the housing of the indoor unit B, and which is upstream of the position of the electric heater 7, and detects a temperature of the heat medium sent out from the heat source unit A and flowing into the electric heater 7, so-called supply temperature Two. The heat medium temperature sensor 22 is attached to a part of the heat medium pipe 6c, which is accommodated in the housing of the indoor unit B and which is upstream of the position of the pump 8, and detects a temperature of the heat medium flowing through the electric heater 7 to the utilization-side equipment 9, so-called operating temperature Twh. Incidentally, the operating temperature Twh is equal to the supply temperature Two when the electric heater 7 does not operate.

The operation device 23 located on the front surface of the housing of the indoor unit B has a display 23a and is connected to the indoor controller 20 via a signal line, sets various operation conditions such as start and stop of operations of the heat source unit A, the operation time of the heat source unit A, and the target temperature of the heat medium in response to user operations, and displays the settings and data sent from the outdoor controller 10 in text on the display 23a. Incidentally, the indoor controller 20 may be installed separately in a different location outside the housing of the indoor unit B.

The indoor controller 20 includes a memory 20m for storing data, and controls the electric heater 7 and the pump 8 according to the settings of the operation device 23, the supply temperature Two, which is the temperature detected by the heat medium temperature sensor 21, the operating temperature Twh, which is the temperature detected by the heat medium temperature sensor 22, and also according to the data sent from the outdoor controller 10, such as the return temperature Twi, which is the temperature detected by the heat medium temperature sensor 12, and the outdoor air temperature To. Then, the indoor controller 20 functions as a parent device that controls the entire heat medium circulation device including the heat source unit A via the outdoor controller 10, and the outdoor controller 10 functions as a child device that controls the heat source unit A according to instructions from the indoor controller 20. In this embodiment, the indoor controller 20, the outdoor controller 10, and the operation device 23 work together to function as controllers for the heat medium circulation device.

The indoor controller 20 executes normal operation control to supply heat drawn from the outside air by the operation of the heat source unit A to the utilization-side equipment 9 by operating the pump 8, and also executes freeze prevention control to drive the pump 8 when the return temperature Twi falls below a threshold value while the heat source unit A is stopped. Then, when executing the freeze prevention control, the indoor controller 20 changes the above-mentioned threshold value according to the outside air temperature To. More specifically, the indoor controller changes the threshold value in a direction of being higher if the outside air temperature To falls. Incidentally, in normal operation control, the pump 8 of the indoor unit B is stopped in synchronization with the stop of the heat source unit A, i.e., the stop of the compressor 1, or with a slight time delay from the stop of the compressor 1. Therefore, when the compressor 1 is stopped, the pump 8 is also stopped. The operation of the heat source unit A is stopped when supplying heat to the utilization-side equipment 9 becomes unnecessary, in a so-called thermo-off state, and when the user operates the operation device 23 to stop the operation of the heat medium circulation device (operation stop).

When the pump 8 stops, the heat medium remaining in the parts of the heat medium pipes 6a and 6b exposed to the outside air is cooled by the outside air. At this time, if the heat medium is water, the heat medium in the heat medium pipes 6a and 6b begins to freeze when the temperature of the parts of the heat medium pipes 6a and 6b exposed to the outside air falls below 0° C.

As specific functions of freeze prevention control for preventing such freezing of the heat medium, the indoor controller 20 includes a first control section 20a to a fourth control section 20d.

If the heat source unit A is stopped and the outside air temperature To is higher than or equal to a predetermined reference value Tos, the first control section 20a executes a first freeze prevention control to drive the pump 8 when the return temperature Twi falls below a threshold value T1 (first threshold value; for example, 4° C.) and to stop the pump 8 when the return temperature Twi exceeds the threshold value T1. As regards the reference value Tos, a temperature higher than the freezing temperature of the heat medium, i.e. a temperature higher than 0° C., which is the freezing temperature of water in this embodiment, is set as the reference value Tos (for example, 5° C.). Incidentally, if the outside air temperature To is higher than 5° C., there is no risk of freezing. For this reason, the first freeze prevention control is not executed.

If the outdoor fan 30 operates and the outside air flows into the outdoor heat exchanger 5, the outdoor air temperature To can be detected with high accuracy by the outdoor air temperature sensor 11. However, when the heat source unit A is stopped, the outdoor fan 30 also stops as the compressor 1 stops. When the outdoor fan 30 stops and outside air no longer flows into the outdoor heat exchanger 5, the accuracy of detection of the outdoor air temperature To by the outdoor air temperature sensor 11 decreases. In addition, if the heat source unit A is installed in a sunny location, the outdoor air temperature sensor 11 detects a temperature higher than the actual outside air temperature due to the influence of solar radiation. In anticipation of these situations, a temperature higher than the freezing temperature of the heat medium is set as the reference value Tos as described above to reliably prevent the heat medium in the heat medium pipes 6a and 6b from freezing.

If the heat source unit A is stopped and the outside air temperature To is below the reference value Tos, the second control section 20b executes a second freeze prevention control to drive the pump 8 when the return temperature Twi falls below a threshold value T2 higher than the threshold value 1 (second threshold value; for example, 7° C.) and to stop the pump 8 when the return temperature Twi exceeds the threshold value T2.

The above-mentioned threshold value T1 is set to a value that can reliably prevent freezing even if the heat medium pipes 6a, 6b, and 6c are at their longest according to their specifications when the outside air temperature To is higher than or equal to the reference value Tos. Similarly, the threshold value T2 is set with a margin as a value at which freezing can be reliably prevented even if the heat medium pipes 6a, 6b, and 6c are at their longest according to their specifications in a situation where the outside air temperature To is below the reference value Tos.

Thus, by preparing the threshold value T1 (for example, 4° C.) and the threshold value T2 (for example, 7° C.) higher than the threshold value T1 as the reference values of the heat medium temperature for determining whether or not to operate the pump 8 to prevent freezing, and selecting and using the threshold value T1 or T2 according to the outside air temperature To while the heat source unit A is stopped, it is possible to reduce opportunities to operate the pump 8 due to unnecessary freeze prevention control intervention, thereby enabling the freeze prevention control with excellent energy saving. In this case, the threshold value T2 is set to a value higher than the threshold value T1 (7° C.>4° C.), and the outside air temperature To is changed based on the reference value Tos used as a reference. In other words, if the outside air temperature To drops, the threshold value is changed to a higher value.

Furthermore, since the threshold value T2 is set to a high value with a sufficient margin as described above, the threshold value can be adjusted to an appropriate value according to the installation conditions of the heat medium circulation device. In order to suppress unnecessary operation of the pump 8 by adjusting this threshold value T2 and further improve energy saving, a third control section 20c and a fourth control section 20d are further provided.

After the heat source unit A is stopped, the third control section 20c stores (updates and stores) in the memory 20m the outside air temperature To at the time when the pump 8 starts to operate and a maximum downward change amount ΔTwi of the return temperature Twi within a certain time (for example, 10 seconds) from the start operation of the pump 8, as threshold setting data for setting a threshold value T2 (new threshold value T2′) of the next second freeze prevention control, at each execution of the first (initial) second freeze prevention control and the second-time and subsequent freeze prevention controls. The maximum downward change amount ΔTwi of the return temperature Twi means a difference between the return temperature Twi(s) at or immediately before the start of operation of the pump 8 and a minimum value Twi(min) of the return temperature Twi which occurs within a certain time (for example, 10 seconds) from the start of operation of the pump 8. ΔTwi=Twi(s)−Twi(min). Maximum change amounts ΔTwi1 to ΔTwi4 to be described later also have the same meaning.

The fourth control section 20d sequentially sets a new threshold value T2′ suitable for each second freeze prevention control each time the second and subsequent freeze prevention controls are executed after the heat source unit A is stopped, based on the current outside air temperature To, the reference value Tos, and the above-mentioned stored threshold value setting data (the outside air temperature To and the maximum change amount ΔTwi at the time when the previous second freeze prevention control is executed). More specifically, a ratio of [difference between the current outside air temperature To(n) and the reference value Tos (=To(n)−Tos)] to [difference between the outside air temperature To(n−1) at the start of the previous second freeze prevention control and the reference value Tos (=To(n−1)−Tos)] is multiplied by the maximum change amount ΔTwi(n−1) of the return temperature Twi caused by the previous second freeze prevention control, and the multiplication result is added to the threshold value T1 as a correction value α, thereby setting a new threshold value T2′ (=T1+α) for the second and subsequent second freeze prevention controls.

α = [ ( T ⁢ o ⁡ ( n ) - Tos ) / ( To ⁡ ( n - 1 ) - Tos ) ] × Δ ⁢ Twi ⁡ ( n - 1 )

In this case, “n” refers to the number of times of execution of the second freeze prevention control, and is an integer greater than or equal to “2”.

By the processing of the first to fourth control sections 20a to 20d, the indoor controller 20 executes appropriate freeze prevention control that reflects the installation status of the heat medium circulating device, including the external environment in which the heat source unit A is installed, and, in particular, the lengths and installation positions of the heat medium pipe 6b between the heat source unit A and the indoor unit B, the heat medium pipe 6c between the indoor unit B and the utilization-side equipment 9, and the heat medium pipe 6a between the utilization-side equipment 9 and the heat source unit A.

Next, the control executed by the indoor controller 20 while the heat source unit A is stopped will be described with reference to the flowchart in FIG. 2 and the flowchart in FIG. 3 following FIG. 2.

[First Freeze Prevention Control]

After the operation of the heat source unit A is stopped, if an execution flag f of the second freeze prevention control is “0” (YES in S1), the indoor controller 20 compares the outdoor air temperature To with the reference value Tos (=5° C.), based on determination that the second freeze prevention control is not yet executed (S2).

If the outside air temperature To is higher than or equal to the reference value Tos (=5° C.) (NO in S2), the indoor controller 20 compares the return temperature Twi with the threshold value T1 (=4° C.) (S3). If the return temperature Twi is higher than or equal to the threshold value T1 (NO in S3), the indoor controller 20 returns to the above-described determination in S1, based on the determination that there is no risk of the heat medium freezing.

If the return temperature Twi is lower than the threshold value T1 (YES in S3), the indoor controller 20 drives the pump 8, based on the determination that there is a risk of the heat medium freezing (S4). By driving the pump 8, the heat medium in the heat medium pipes 6a, 6b, and 6c circulates through the heat medium heat exchanger 3 and the utilization-side equipment 9. Freezing of the heat medium is suppressed by this circulation, and the effect of suppressing freezing is enhanced by warming the circulating heat medium by the residual heat of the heat medium heat exchanger 3 and the utilization-side equipment 9 (start of first freeze prevention control).

Solid lines in FIG. 4 indicate a state in which a flow rate F of the heat medium in the heat medium pipes 6a, 6b, and 6c changes in an increasing direction by the execution of this first freeze prevention control, and a state in which the return temperature Twi and the supply temperature Two change in a decreasing direction once and then change in an increasing direction as the flow rate F increases. The phenomenon in which the return temperature Twi and the supply temperature Two change in a decreasing direction once occurs when the low-temperature heat medium remaining in the parts of the heat medium pipes 6a, 6b, and 6c, which are exposed to the outside air, flows all at once to the installation positions of the heat medium temperature sensors 12 and 21, and the heat of the low-temperature heat medium is transferred to the heat medium temperature sensors 12 and 21.

After the operation of the pump 8 is started, when the return temperature Twi rises to a value exceeding the threshold value T1 (T1+hysteresis, for example 1° C.) (YES in S5), the indoor controller 20 stops the pump 8 (S6) and returns to the above-described determination in S1 (end of first freeze prevention control). Incidentally, it is possible to eliminate the above-described temperature addition of the hysteresis by providing a delay time in the determination.

[First (Initial) Second Freeze Prevention Control]

After the operation of the heat source unit A is stopped, if an execution flag f of the second freeze prevention control is “0” (YES in S1), the indoor controller 20 compares the outdoor air temperature To with the reference value Tos (=5° C.) (S2).

If the outdoor air temperature To is less than the reference value Tos (YES in S2), the indoor controller 20 compares the return temperature Twi with a threshold value T2 (7° C.) higher than the threshold value T1 (S7). If the return temperature Twi is higher than or equal to the threshold value T2 (NO in S7), the indoor controller 20 returns to the above-described determination in S1, based on the determination that there is no risk of the heat medium freezing.

If the return temperature Twi is lower than the threshold value T2 (YES in S7), the indoor controller 20 drives the pump 8, based on the determination that there is a risk of the heat medium freezing (S8). By driving the pump 8, freezing of the heat medium is prevented (start of initial second freeze prevention control).

If the outside air temperature To is lower than the reference value Tos, the threshold value T2 higher than the normal threshold value T1 is used as a criterion of the determination, and the timing for driving the pump 8 to prevent freezing therefore becomes earlier than that in a case where the threshold value T1 is used. Accordingly, as indicated by a dashed line in FIG. 4, the timing at which the return temperature Twi drops and then starts to rise also becomes earlier than that in a case where the threshold value T1 is used.

Therefore, it is possible to reliably prevent the heat medium, which are in the parts of heat medium pipes 6a, 6b, and 6c as exposed to the outside air and are in a low temperature state, from freezing. Moreover, since the threshold value T2 is used only when the outside air temperature To is lower than the reference value Tos, the pump 8 is not unnecessarily driven in a situation where there is no possibility of freezing.

Then, the indoor controller 20 stores the outdoor air temperature To when the pump 8 starts to operate as “To1” in the memory 20m (S9), and detects the maximum downward change ΔTwi in the return temperature Twi within a certain time (=for example, 10 seconds) from the start of operation of the pump 8 as “ΔTwi1” (NO in S10 and S11). After a certain time has elapsed (YES in S10), the indoor controller 20 stores the above-described stored outdoor air temperature To1 and the above-described detected maximum change ΔTwi1 in the memory 20m as threshold setting data (To1, ΔTwi1) for setting a new threshold value T2′ for a next second freeze prevention control (S12).

Then, when the return temperature Twi rises to a value exceeding the threshold value T2 (T2+, for example, 1° C. as a hysteresis part) (YES in S13), the indoor controller 20 stops the pump 8 (S14) and sets a flag f to “1” to indicate that the second freeze prevention control has been executed (S15), and returns to the above-described determination in S1 (end of the initial second freeze prevention control). Incidentally, the above-described temperature addition for the hysteresis may be eliminated by setting a delay time in the determination.

The certain time (=10 seconds) for monitoring the maximum change amount ΔTwi (n) detected in the first and second and subsequent second freeze prevention controls is the time during which the heat medium existing in the parts of the heat medium pipes 6a, 6b, and 6c, which are exposed to the outside air, flows up to the position of the heat medium temperature sensor 12 that detects the return temperature Twi, including the response period until the time when the heat medium temperature sensor 12 can accurately detect the minimum temperature value Twi (min) of the flowing heat medium, and is set as short as possible within that time. By shortening this time, the operation time of the pump 8 can be reduced, and the power consumption of the pump 8 can be reduced.

[Second-Time Second Freeze Prevention Control]

After the heat source unit A is stopped, if the flag f is “1” (NO in S1), the indoor controller 20 multiplies the maximum downward change ΔTwi1 in the return temperature Twi under the previous second freeze prevention control by the ratio of [difference between the current outdoor air temperature To2 and the reference value Tos (To2−Tos)] to [difference between the outdoor air temperature To1 at the start of the previous second freeze prevention control and the reference value Tos (To1−Tos)], based on the determination that the second freeze prevention control has already been executed, and obtains the multiplication result as a correction value α for setting an optimal threshold value T2′ for the current (second-time) second freeze prevention control (S16).

α = [ ( T ⁢ o ⁢ 2 - T ⁢ os ) / ( To ⁢ 1 - Tos ) ] × Δ ⁢ Twi ⁢ 1

The relationship between the outside air temperature To and the maximum change amount ΔTwi, and the relationship between the outside air temperature To and the correction value α are shown in FIG. 5. The maximum change amount ΔTwi (=0) of the return temperature Twi at the time when the outside air temperature To is the reference value Tos is defined as “ΔTwi0”, and the figure shows that a linear straight line X1 extending from an intersection (Tos, ΔTwi0) of the reference value Tos and the change amount ΔTwi0 to an intersection (To1, ΔTwi1) of the outside air temperature To1 captured in the previous second freeze prevention control and the maximum change amount ΔTwi1 is the correction value α to be added to the threshold value T1 to find the threshold value T2′ optimal for the current second-time second freeze prevention control.

Then, the indoor controller 20 sets the threshold value T2′ (=T1+α) optical for the second-time second freeze prevention control and stores this in the memory 20m by adding the obtained correction value α to the threshold value T1 (S17). A relationship between the outside air temperature To and the threshold values T1, T2, and T2′ is shown in FIG. 6. The larger the maximum change amount ΔTwi for the outside air temperature To during the previous second freeze prevention control, i.e., the greater the decrease in the return temperature Twi, the larger the correction value α. Furthermore, the lower the current outside air temperature To, the larger the correction value α.

Then, if the outdoor air temperature To is less than the reference value Tos (YES in S18), the indoor controller 20 compares the return temperature Twi with the threshold value T2′ (S19). If the return temperature Twi is higher than or equal to the threshold value T2′ (NO in S19), the indoor controller 20 returns to the above-described determination in S1, based on the determination that there is no risk of the heat medium freezing.

If the return temperature Twi is lower than the threshold value T2′ (YES in S19), the indoor controller 20 drives the pump 8, based on the determination that there is a possibility of the heat medium freezing (S20). By driving the pump 8, the heat medium in the heat medium pipes 6a, 6b, and 6c circulates through the heat medium heat exchanger 3 and the utilization-side equipment 9, preventing the heat medium from freezing (start of the second-time second freeze prevention control).

The indoor controller 20 stores the outdoor air temperature To when the pump 8 starts to operate as “To2” in the memory 20m (S21), and detects the maximum downward change ΔTwi in the return temperature Twi within a certain time (=10 seconds) from the start of operation of the pump 8 as “ΔTwi2” (NO in S22 and S23). After a certain time has elapsed (YES in S22), the indoor controller 20 stores (updates and stores) the above-described stored outdoor air temperature To2 and the above-described detected maximum change ΔTwi2 in the memory 20m as threshold setting data (To2, ΔTwi2) for setting a new threshold value T2′ for a next second freeze prevention control (S24).

Next, when the return temperature Twi rises to a value exceeding the threshold value T2′ (T2′+, for example, 1° C. as a hysteresis amount) (YES in S25), the indoor controller 20 stops the pump 8 (S26) and returns to the above-described determination in S1 (end of the second-time second freeze prevention control).

As described above, a new threshold value T2′ is set based on the current outside air temperature To, the reference value Tos, and the threshold setting data (To1, ΔTwi1) captured in the previous second freeze prevention control. Therefore, even if the heat medium temperature sensor 12 is installed inside the housing of the heat source unit A, the temperature drop that leads to the heat medium freezing can be properly captured. Therefore, freezing of the heat medium can be prevented reliably, and the reliability of the freeze prevention control is improved.

Incidentally, if the outdoor air temperature To is higher than or equal to the reference value Tos in the above-described determination in S18 (NO in S18), the indoor controller 20 executes the above-described first freeze prevention control based on a comparison between the return temperature Twi and the threshold value T1 (S3 to S6).

[Third-Time Second Freeze Prevention Control]

After the second-time second freeze prevention control is completed, since the execution flag f is “1” (YES in S1), the indoor controller 20 multiplies the maximum downward change ΔTwi2 in the return temperature Twi under the previous second freeze prevention control by the ratio of [difference between the current outside air temperature To3 and the reference value Tos “To3−Tos”] to [difference between the outside air temperature To2 at the start of the previous second freeze prevention control and the reference value Tos “To2−Tos”], based on the determination that the second freeze prevention control has already been executed, and obtains the multiplication result as a correction value α for setting the optimal threshold value T2′ for the current (third-time) second freeze prevention control (S16).

α = [ ( To ⁢ 3 - Tos ) / ( To ⁢ 2 - Tos ) ] × Δ ⁢ Twi ⁢ 2

FIG. 5 shows that a linear straight line X2 extending from the intersection (Tos, ΔTwi0) of the reference value Tos and the change amount ΔTwi to an intersection (To2, ΔTwi2) of the outside air temperature To2 captured in the previous second freeze prevention control and the maximum change amount ΔTwi2 is a correction value a to be added to the threshold value T1 to obtain the optimal threshold value T2′ in the current second-time second freeze prevention control.

Then, the indoor controller 20 sets the threshold value T2′ (=T1+α) optical for the third-time second freeze prevention control and stores this in the memory 20m by adding the obtained correction value α to the threshold value T1 (S17). Then, if the outdoor air temperature To is less than the reference value Tos (YES in S18), the indoor controller 20 compares the return temperature Twi with the threshold value T2′ (S19). If the return temperature Twi is higher than or equal to the threshold value T2′ (NO in S19), the indoor controller 20 returns to the above-described determination in S1, based on the determination that there is no risk of the heat medium freezing.

If the return temperature Twi is lower than the threshold value T2′ (YES in S19), the indoor controller 20 drives the pump 8, based on the determination that there is a risk of the heat medium freezing (S20). By driving the pump 8, the drop in the return temperature Twi is suppressed, and freezing of the heat medium is prevented (start of the third second freeze prevention control).

The indoor controller 20 stores the outdoor air temperature To when the pump 8 starts to operate as “To3” in the memory 20m (S21), and detects the maximum downward change ΔTwi in the return temperature Twi within a certain time (=10 seconds) from the start of operation of the pump 8 as “ΔTwi3” (NO in S22 and S23). After the certain period of time has elapsed (YES in S22), the indoor controller 20 stores (updates and stores) the above-described stored outdoor air temperature To3 and the above-described detected maximum change ΔTwi3 in the memory 20m as threshold setting data (To3, ΔTwi3) for setting the next second freeze prevention control threshold T2 (S24).

Next, when the return temperature Twi rises to a value exceeding the threshold value T2′ (T2′+, for example, 1° C.) (YES in S25), the indoor controller 20 stops the pump 8 (S26) and returns to the above-described determination in S1 (end of the third-time second freeze prevention control).

As described above, a new threshold value T2′ is set based on the current outside air temperature To, the reference value Tos, and the threshold setting data (To2, ΔTwi2) captured in the previous second freeze prevention control. Therefore, even if the heat medium temperature sensor 12 is installed inside the housing of the heat source unit A, the temperature drop that leads to the heat medium freezing can be properly captured.

[Fourth-Time Second Freeze Prevention Control]

After the third-time second freeze prevention control is completed, since the execution flag f is “1” (YES in S1), the indoor controller 20 multiplies the maximum downward change ΔTwi3 in the return temperature Twi under the previous second freeze prevention control by the ratio of [difference between the current outside air temperature To4 and the reference value Tos “To4-Tos”] to [difference between the outside air temperature To3 at the start of the previous second freeze prevention control and the reference value Tos “To3-Tos”], based on the determination that the second freeze prevention control has already been executed, and obtains the multiplication result as a correction value α for setting the optimal threshold value T2′ for the current (fourth-time) second freeze prevention control (S16).

α = [ ( To ⁢ 4 - Tos ) / ( To ⁢ 3 - Tos ) ] × Δ ⁢ Twi ⁢ 3

FIG. 5 shows that a linear straight line X3 extending from the intersection (Tos, ΔTwi0) of the reference value Tos and the change amount ΔTwi0 to an intersection (To3, ΔTwi3) of the outside air temperature To3 captured in the previous second freeze prevention control and the maximum change amount ΔTwi3 is a correction value α to be added to the threshold value T1 to obtain the optimal threshold value T2′ in the current second-time second freeze prevention control.

Then, the indoor controller 20 sets the threshold value T2′ (=T1+a) optical for the fourth-time second freeze prevention control and stores this in the memory 20m by adding the obtained correction value a to the threshold value T1 (S17). Then, if the outdoor air temperature To is less than the reference value Tos (YES in S18), the indoor controller 20 compares the return temperature Twi with the threshold value T2′ (S19). If the return temperature Twi is higher than or equal to the threshold value T2′ (NO in S19), the indoor controller 20 returns to the above-described determination in S1, based on the determination that there is no risk of the heat medium freezing.

If the return temperature Twi is less than the threshold value T2′ (YES in S19), the indoor controller 20 drives the pump 8, based on the determination that there is a risk of the heat medium freezing (S20). By driving the pump 8, the drop in the return temperature Twi is suppressed, and freezing of the heat medium is prevented (start of the fourth second freeze prevention control).

The indoor controller 20 stores the outdoor air temperature To when the pump 8 starts to operate as “To4” in the memory 20m (S21), and detects the maximum downward change ΔTwi in the return temperature Twi within a certain time (=10 seconds) from the start of operation of the pump 8 as “ΔTwi4” (NO in S22 and S23). After the certain period of time has elapsed (YES in S22), the indoor controller 20 stores (updates and stores) the above-described stored outdoor air temperature To4 and the above-described detected maximum change ΔTwi4 in the memory 20m as threshold setting data (To4, ΔTwi4) for setting the next second freeze prevention control threshold T2 (S24).

Next, when the return temperature Twi rises to a value exceeding the threshold value T2′ (T2′+, for example, 1° C.) (YES in S25), the indoor controller 20 stops the pump 8 (S26) and returns to the above-described determination in S1 (end of the fourth-time second freeze prevention control).

[Fifth-Time and Subsequent Second Freeze Prevention Controls]

In the fifth-time and subsequent second freeze prevention controls as well, the indoor controller 20 executes the same control as the fourth-time second freeze prevention control. Then, the indoor controller 20 executes the first freeze prevention control and the first to second freeze prevention controls in association with the next start and stop of operation of the heat source unit A.

As described above, in the second-time and subsequent second freeze prevention controls, the threshold value T2′ suitable for each second freeze prevention control is sequentially set using the maximum change amount ΔTwi, which is the data obtained from the most recent return temperature Twi, and the operation time of the pump 8 required to prevent the heat medium from freezing can be minimized. In other words, unnecessary operation of the pump 8 is suppressed, which can contribute to energy saving. This unnecessary operation means the operation of the pump 8 performed in a region between the threshold value T2 and the threshold value T2′ shown in FIG. 6. In other words, if a fixed value of only the threshold value T2 (for example, 7° C.) is set as the criterion for the start of operation of the pump 8, the pump 8 is also operated in the above-mentioned region. In contrast, by using a new threshold value T2′, the pump 8 is not operated in this region. In a specific example, when the threshold value T2 is used, the pump 8 is operated when the outside air temperature To is −10° C. and the return temperature Twi is 6° C. On the other hand, when the threshold value T2′ is used, the pump 8 is not operated even when the outside air temperature To is −10° C. and the return temperature Twi is 6° C.

Modified Example 1

In the above-described embodiment, the return temperature Twi has been used for the freeze prevention control, but the supply temperature Two or the operating temperature Twh, which has a value substantially equal to the return temperature Twi while the pump 8 is stopped, may be used for the freeze prevention control instead of the return temperature Twi. In other words, even if the “return temperature Twi” used for various determinations in the flowcharts of FIG. 2 and FIG. 3 is changed to the “supply temperature Two” or the “operating temperature Twh” to achieve substantially the same freeze prevention control. Incidentally, when the “return temperature Twi” is changed to the “supply temperature Two” or the “operating temperature Twh”, it takes time for the heat medium existing in the parts of the heat medium pipes 6a, 6b, and 6c exposed to the outside air to flow to the installation position of the heat medium temperature sensor after the operation of the pump 8 is started. Therefore, the certain time for monitoring the change amount used in the processes S10 and S22 in FIG. 2 and FIG. 3 needs to be extended to the time longer than 10 seconds (for example, 20 seconds). In addition, when the “supply temperature Two” or the “operating temperature Twh” is used instead of the “return temperature Twi”, the temperature of the heat medium which has passed through the heat medium heat exchanger 3 is detected. Therefore, since the heat medium existing in the parts of the heat medium pipes 6a, 6b, and 6c exposed to the outside air passes through the heat medium heat exchanger 3 and thus the temperature is exchanged in the heat medium heat exchanger 3, the accuracy of detection of the temperature of the heat medium held in the pipes is slightly lower than that of the return temperature Twi.

Modified Example 2

In the above-described embodiment, the heat medium temperature sensor 21 for detecting the supply temperature Two has been attached to the heat medium pipe 6b in the indoor unit B. Instead, the heat medium temperature sensor may be attached to the heat medium pipe 6b located at the outlet side of the heat medium flow path 3b of the heat medium heat exchanger 3 in the heat source unit A, similarly to the heat medium temperature sensor 21′ represented by a dashed line in FIG. 1.

Modified Example 3

In the above-described embodiment, the following formula (1) has been used as a formula for calculating the correction value α, but the following formula (2) for calculating the correction value α by multiplying the right-hand side of the following formula (1) by a coefficient β (approximately 0.5 to 1.5) may also be used.

α = [ ( T ⁢ o   ( n )   - T ⁢ o ⁢ s )   /   ( T ⁢ o ⁡ ( n - 1 ) - T ⁢ o ⁢ s ) ] × Δ ⁢ Twi ⁡ ( n - 1 ) ( 1 ) α = β × [ ( T ⁢ o   ( n )   - T ⁢ o ⁢ s )   /   ( T ⁢ o ⁡ ( n - 1 ) - T ⁢ o ⁢ s ) ] × Δ ⁢ Twi ⁡ ( n - 1 ) ( 2 )

Modified Example 4

In the above-described embodiment, the indoor unit B is provided between the heat source unit A and the utilization-side equipment 9, and the electric heater 7, the pump 8, the indoor controller 20, the heat medium temperature sensors 21 and 22, and the operation device 23 are accommodated in the indoor unit B. As shown in FIG. 7, however, the indoor unit B may be omitted, and the electric heater 7, the pump 8, and the heat medium temperature sensors 21 and 22 may be accommodated in the heat source unit A, and the control function of the indoor controller 20 (first to fourth control sections 20a to 20d) may be provided in the outdoor controller 10. Incidentally, the operation device 23 can also be accommodated in the heat source unit A, but is desirably installed indoors from the viewpoint of the ease of operation for the user.

In this configuration, the heat medium pipe 6a serving as the return pipe for the heat medium is similar to that in the embodiment of FIG. 1, but the heat medium piping 6c serving as the forward pipe for flowing the heat medium from the heat source unit A to the utilization-side equipment 9 is directly connected from the heat source unit A to the utilization-side equipment 9.

The embodiment and modifications described herein have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments and modification examples described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments or examples described herein may be made without departing from the spirit of the inventions. These embodiments and modifications thereof are included in the scope and gist of the invention, and are also included in the scope of the invention described in the claims and the equivalent scope thereof.