Patent Publication Number: US-2013233006-A1

Title: Air-conditioning apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a U.S. national stage application of International Patent Application No. PCT/JP2011/000411 filed on Jan. 26, 2011. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an air-conditioning apparatus applied to, for example, multi air-conditioners for buildings, and the like. 
     BACKGROUND 
     Some air-conditioning apparatuses, such as multi air-conditioners for buildings, have a heat source unit (an outdoor unit) provided outside a building and an indoor unit provided inside the building. The refrigerant circulating in a refrigerant circuit of such an air-conditioning apparatus transfers heat to (or removes heat from) air supplied to a heat exchanger of the indoor unit, thereby heating or cooling the air. Then, the heated or cooled air is sent to an air-conditioned space, and consequently, heating or cooling is performed. 
     Since a building usually has a plurality of indoor spaces, such an air-conditioning apparatus has a plurality of indoor units in accordance with the number of the indoor spaces. Moreover, when a building is large, a refrigerant pipe that connects an outdoor unit and an indoor unit may be as long as 100 m long. When the refrigerant pipe that connects the outdoor unit and the indoor unit is long, the amount of refrigerant used to fill the refrigerant circuit increases in accordance with the length. 
     If the refrigerant used to fill the refrigerant circuit leaks for some reason, the refrigerant that has leaked may flow into an indoor space. In this case, if the refrigerant is flammable, the refrigerant may lead to a fire. Moreover, if the refrigerant is poisonous, the refrigerant may harm the human body. 
     As a result, an existing air-conditioning apparatus has been proposed, in which when the refrigerant leaks, the operation of a compressor is stopped (for example, see Patent Literature 1). In a technology described in Patent Literature 1, carbon dioxide is used as the refrigerant and the compressor is stopped when the refrigerant, carbon dioxide, leaks. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2000-320936 (for example, see paragraphs [0024] to [0033] and FIGS. 2 to 4 in the specification) 
       
    
     TECHNICAL PROBLEM 
     In the technology described in Patent Literature 1, carbon dioxide is used as the refrigerant and the system is designed to stop in the case where the carbon dioxide refrigerant leaks in the same way that other existing refrigerants leak; however, the system takes no measures against leaking of the carbon dioxide refrigerant. That is, in the case where carbon dioxide is used as the refrigerant, it is regarded as a major prerequisite that the refrigerant does not harm the human body and it is necessary to take some measures for decreasing leakage of the refrigerant. Moreover, similarly, a flammable refrigerant is flammable, and thus it is necessary to have some type of safety apparatuses in order to obtain a high degree of safety, similarly to as in the case of carbon dioxide. 
     Recently, there has been a tendency to restrict use of HFC refrigerants that have a high global warming potential (for example, R410A and R404A, R407C, R134a, and the like). Air-conditioning apparatuses that use a refrigerant that has a low global warming potential (for example, HFO1234yf, R32, HC, carbon dioxide, and the like) in terms of global warming have been proposed. Also in the case where a flammable refrigerant (HFO1234yf, R32, and HC) or carbon dioxide is used as the refrigerant in multi air-conditioners for buildings, a large amount of refrigerant is necessary. Thus, some measures need to be taken against leaking of the refrigerant into an indoor space. 
     SUMMARY 
     An air-conditioning apparatus according to the present invention has been made in light of the above-described issues and aims to provide effective measures against leaking of a refrigerant in refrigeration cycle apparatuses. 
     An air-conditioning apparatus according to the present invention includes a refrigeration cycle in which a compressor, a heat-source side heat exchanger, an expansion unit, and a load-side heat exchanger are included and are connected with refrigerant pipes, and includes: a cutoff unit capable of flowing or cutting off a refrigerant circulating in the refrigeration cycle; a detection unit detecting leaking of the refrigerant on the basis of a resistance that changes with densities of plural kinds of leaked refrigerants; a concentration calculation unit calculating a concentration of the refrigerant that has leaked, on the basis of the resistance of the detection unit; a concentration detection unit outputting a calculation result of the concentration calculation unit in order that the calculation result is used for controlling the cutoff unit; and a cutoff control unit controlling the cutoff unit on the basis of an output of the concentration detection unit. At least R410A, R407C, R32, R404A, HFO1234yf, HFO1234ze, a refrigerant mixture including R32 and HFO1234yf, and all refrigerant mixtures including at least one of these refrigerants as a component are detectable by the single detection unit. 
     An air-conditioning apparatus according to the present invention has the above-described structure, and thus the degree of safety of the air-conditioning apparatus can be improved. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram illustrating an installation example of an air-conditioning apparatus according to Embodiment of the present invention. 
         FIG. 2  illustrates an example of the structure of a refrigerant circuit of the air-conditioning apparatus according to Embodiment of the present invention. 
         FIG. 3  is a diagram of a refrigerant circuit illustrating a flow of a refrigerant in the air-conditioning apparatus illustrated in  FIG. 2  in a cooling only operation mode. 
         FIG. 4  is a diagram of a refrigerant circuit illustrating a flow of the refrigerant in the air-conditioning apparatus illustrated in  FIG. 2  in a heating only operation mode. 
         FIG. 5  is a diagram of a refrigerant circuit illustrating a flow of the refrigerant in the air-conditioning apparatus illustrated in  FIG. 2  in a cooling main operation mode. 
         FIG. 6  is a diagram of a refrigerant circuit illustrating a flow of the refrigerant in the air-conditioning apparatus illustrated in  FIG. 2  in a heating main operation mode. 
         FIG. 7  illustrates relationships between the resistance of a detection member of a concentration detection unit and the concentration of the refrigerant according to types of refrigerants. 
         FIG. 8  is a diagram illustrating an example of the structure of another refrigerant circuit of the air-conditioning apparatus according to Embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the following, Embodiment according to the present invention will be described with reference to the drawings. 
     Embodiment 
       FIG. 1  is a schematic diagram illustrating an installation example of an air-conditioning apparatus  100  according to Embodiment of the present invention. With reference to  FIG. 1 , the installation example of the air-conditioning apparatus  100  will be described. 
     In the case where a refrigerant leaks, the air-conditioning apparatus  100  according to Embodiment has functions of detecting leaking and cutting off a flow of the refrigerant flowing out from an outdoor unit  1  (a heat source unit) and flowing into the outdoor unit  1 . As a result, an amount of the refrigerant that will leak from the air-conditioning apparatus  100  is decreased, thereby assuring users&#39; safety. 
     Moreover, the air-conditioning apparatus  100  according to Embodiment includes a refrigerant circuit A (see  FIG. 2 ), which is a refrigeration cycle which circulates a heat-source side refrigerant, and a heat medium circulation circuit B (see  FIG. 2 ), which circulates a heat medium. In each indoor unit, a cooling operation mode or a heating operation mode is selectable as an operation mode. 
     The air-conditioning apparatus  100  uses a method (an indirect method) in which the refrigerant (the heat-source side refrigerant) is indirectly utilized. That is, the air-conditioning apparatus  100  according to Embodiment transmits cooling energy or heating energy stored in the heat-source side refrigerant to a refrigerant (hereinafter referred to as a heat medium) different from the heat-source side refrigerant, and performs cooling or heating of an air-conditioned space by using the cooling energy or heating energy stored in the heat medium. 
     In  FIG. 1 , the air-conditioning apparatus  100  includes the outdoor unit  1 , which is a heat source unit, a plurality of indoor units  2 , and a heat medium relay unit  3  transmitting cooling energy or heating energy of the heat-source side refrigerant that flows in the outdoor unit  1  to the heat medium that flows in the indoor units  2 . The heat medium relay unit  3  is a unit that exchanges heat from the heat-source side refrigerant to the heat medium. The outdoor unit  1  and the heat medium relay unit  3  are connected with refrigerant pipes  4  for conducting the heat-source side refrigerant. The heat medium relay unit  3  and the indoor units  2  are connected with heat medium pipes  5  for conducting the heat medium. The cooling energy or heating energy generated by the outdoor unit  1  is transmitted to the heat medium of the heat medium relay unit  3  and is distributed to the indoor units  2 . The air-conditioning apparatus  100  according to Embodiment includes a first cutoff unit  37  and a second cutoff unit  38  (see  FIG. 2 ) inside the outdoor unit  1 , which will be described again with reference to  FIG. 2 . 
     The outdoor unit  1  is usually provided in outdoor space  6 , which is space (for example, on the roof and the like) outside a building  9  such as buildings, and supplies cooling energy or heating energy to the indoor units  2  through the heat medium relay unit  3 . The indoor units  2  are provided at positions so as to supply air for cooling or air for heating into a room space  7 , which is an internal space (for example, a room and the like) of the building  9 , and supply air for cooling or air for heating into the room space  7 , which is an air-conditioned space. The heat medium relay unit  3  is configured to be able to be provided as a separate unit from the outdoor unit  1  and the indoor units  2  at a position that is not in the outdoor space  6  or in the room space  7 . The heat medium relay unit  3  is connected to the outdoor unit  1  and the indoor units  2  by the refrigerant pipes  4  and the heat medium pipes  5 , respectively, and transmits cooling energy or heating energy supplied from the outdoor unit  1  to the indoor units  2 . 
     As illustrated in  FIG. 1 , in the air-conditioning apparatus  100  according to Embodiment, the outdoor unit  1  and the heat medium relay unit  3  are connected through the refrigerant pipes  4 , and the heat medium relay unit  3  and each of the indoor units  2  are connected through the respective heat medium pipes  5 . In this way, units (the outdoor unit  1 , the indoor units  2 , and the heat medium relay unit  3 ) are connected using the refrigerant pipes  4  and the heat medium pipes  5  in the air-conditioning apparatus  100 , and thus construction is easy. 
     Note that, in  FIG. 1 , a state in which the heat medium relay unit  3  is provided in a space different from the room space  7  but in the inside the building  9 , such a space above a ceiling (for example, a space such as a space above a ceiling in the building  9 , hereinafter referred to simply as a space  8 ) is illustrated as an example. In addition, the heat medium relay unit  3  may be provided, for example, in a common space such as elevators and the like. Moreover, in  FIG. 1 , a case in which the indoor units  2  are ceiling cassette units is illustrated as an example; however, the indoor units  2  are not limited thereto. The indoor units  2  may be ceiling concealed units, ceiling suspended units, or the like, and are not particularly limited as long as supplying air for cooling or air for heating into the room space  7  directly or through ducts and the like into the room space  7 . 
     Moreover, in  FIG. 1 , a case in which the outdoor unit  1  is provided in the outdoor space  6  is illustrated as an example; however, the outdoor unit  1  is not limited thereto. For example, the outdoor unit  1  may be provided in a closed space such as a machine room with a ventilating hole, and may also be provided inside the building  9  if waste heat may be exhausted to the outside the building  9  through an exhaust duct. Alternatively, in the case where a water-cooled outdoor unit  1  is used, the water-cooled outdoor unit  1  may also be provided inside the building  9 . 
     The heat medium relay unit  3  may be provided at a position near the outdoor unit  1  and away from the indoor units  2 . Note that if the distance from the heat medium relay unit  3  to each of the indoor units  2  is long, power (energy) needed to transfer the heat medium becomes quite large. Thus, the heat medium relay unit  3  is desirably provided under the consideration that the effect of energy conservation decreases. Furthermore, the number of outdoor units  1  to be connected, that of the indoor units  2  to be connected, and that of heat medium relay units  3  to be connected are not particularly predetermined, and the number of units is desirably set depending on the building  9 . 
       FIG. 2  illustrates an example of the structure of a refrigerant circuit of the air-conditioning apparatus  100  according to Embodiment 1 of the present invention. With reference to  FIG. 2 , the structure of the refrigerant circuit of the air-conditioning apparatus  100  will be described. As illustrated in  FIG. 2 , a heat exchanger related to heat medium  15   a  and a heat exchanger related to heat medium  15   b  that are included in the heat medium relay unit  3  and the outdoor unit  1  are connected through the refrigerant pipes  4  (a high-pressure side refrigerant pipe  4   a ( 2 ) and a low-pressure side refrigerant pipe  4   b ( 2 )). Moreover, the heat exchanger related to heat medium  15   a  and the heat exchanger related to heat medium  15   b  are connected to indoor units  2   a  to  2   d  (which may be simply referred to as the indoor unit  2 ) through the heat medium pipes  5 . 
     [Structure of Outdoor Unit  1 ] 
     In the outdoor unit  1 , a compressor  10 , a first refrigerant channel switch unit  11 , a heat-source side heat exchanger  12 , an accumulator  19 , the first cutoff unit  37 , and the second cutoff unit  38  are connected with the refrigerant pipes and provided. 
     The compressor  10  sucks and compresses the refrigerant and makes the refrigerant be in a high-temperature high-pressure state, and then transfers the refrigerant to the refrigerant circuit A. The discharge side of the compressor  10  is connected to the first refrigerant channel switch unit  11  and the suction side thereof is connected to the accumulator  19 . It is desirable that the compressor  10  be formed with, for example, an inverter compressor capable of controlling a capacity, or the like. 
     The first refrigerant channel switch unit  11  connects the discharge side of the compressor  10  and a check valve  13   b  and connects the heat-source side heat exchanger  12  and the suction side of the accumulator  19  in a heating only operation mode and in a heating main operation mode of a cooling and heating mixed operation mode. Moreover, the first refrigerant channel switch unit  11  connects the discharge side of the compressor  10  and the heat-source side heat exchanger  12  and connects a check valve  13   d  and the suction side of the accumulator  19  in a cooling only operation mode and in a cooling main operation mode of the cooling and heating mixed operation mode. It is desirable that the first refrigerant channel switch unit  11  be formed with, for example, a four-way valve and the like. 
     The heat-source side heat exchanger  12  functions as an evaporator when a heating operation is performed, functions as a radiator (a gas cooler) when a cooling operation is performed, and exchanges heat between air supplied from an air-sending device such as a fan, not illustrated in the drawings, and a refrigerant. In the heating operation mode, one side of the heat-source side heat exchanger  12  is connected to a check valve  13   c  and the other side thereof is connected to the suction side of the accumulator  19 . Moreover, in the cooling operation mode, one side of the heat-source side heat exchanger  12  is connected to the discharge side of the compressor  10  and the other side thereof is connected to a check valve  13   a . It is desirable that the heat-source side heat exchanger  12  be formed with a plate-fin-and-tube type heat exchanger, which may exchange heat, for example, between the refrigerant that flows in the refrigerant pipes and air that passes through the fan. 
     The accumulator  19  holds an excessive refrigerant resulting due to a difference between the heating operation mode and the cooling operation mode and an excessive refrigerant for a transitive change in operation (for example, a change in the number of operating indoor units  2 ). In the heating operation mode, the suction side of the accumulator  19  is connected to the heat-source side heat exchanger  12  and the discharge side thereof is connected to the suction side of the compressor  10 . Moreover, in the cooling operation mode, the suction side of the accumulator  19  is connected to the check valve  13   d  and the discharge side thereof is connected to the suction side of the compressor  10 . 
     Moreover, the outdoor unit  1  includes a first connection pipe  42   a , a second connection pipe  42   b , the check valve  13   a , the check valve  13   b , the check valve  13   c , and the check valve  13   d . By including the first connection pipe  42   a , the second connection pipe  42   b , the check valve  13   a , the check valve  13   b , the check valve  13   c , and the check valve  13   d , the heat-source side refrigerant that flows from the outdoor unit  1  into the heat medium relay unit  3  can keep in a certain direction regardless of whether in the heating operation mode or in the cooling operation mode. 
     That is, the refrigerant discharged from the compressor  10  flows into the high-pressure side refrigerant pipe  4   a ( 2 ). The refrigerant that has flown into the high-pressure side refrigerant pipe  4   a ( 2 ) flows into the heat medium relay unit  3 . The refrigerant that has flown in the heat medium relay unit  3  flows from the low-pressure side refrigerant pipe  4   b ( 2 ) into the outdoor unit  1 . 
     Here, a high-pressure side refrigerant pipe  4   a ( 1 ) is a pipe in the outdoor unit  1 , and is a pipe on a downstream side from a point P 2  illustrated in  FIG. 4 . The high-pressure side refrigerant pipe  4   a ( 2 ) is a pipe connected to the high-pressure side refrigerant pipe  4   a ( 1 ) among the refrigerant pipes  4 . 
     Moreover, a low-pressure side refrigerant pipe  4   b ( 1 ) is a pipe in the outdoor unit  1 , and is a pipe on an upstream side from a point P 3  illustrated in  FIG. 4 . The low-pressure side refrigerant pipe  4   b ( 2 ) is a pipe connected to the low-pressure side refrigerant pipe  4   b ( 1 ) among the refrigerant pipes  4 . 
     The first cutoff unit  37  and the second cutoff unit  38  are capable of flowing the refrigerant (in an open state) or cutting off the flow of the refrigerant (in a closed state) on the basis of operation of a cutoff control unit  40  to be described below. 
     It is preferable to provide the first cutoff unit  37  and the second cutoff unit  38  in the outdoor unit  1  or near the outdoor unit  1 . That is, it is desirable that the first cutoff unit  37  be connected to the high-pressure side refrigerant pipe  4   a ( 1 ) and the second cutoff unit  38  is connected to the low-pressure side refrigerant pipe  4   b ( 1 ). Moreover, it is also desirable that the first cutoff unit  37  be connected to the high-pressure side refrigerant pipe  4   a ( 2 ) and the second cutoff unit  38  is connected to the low-pressure side refrigerant pipe  4   b ( 2 ). 
     It is desirable that the first cutoff unit  37  and the second cutoff unit  38  be each formed with, for example, a two-way valve that includes a valve body, a sealing material that seals the valve body, a coil that opens and closes the valve body by electromagnetic force, and the like. In the following description, the first cutoff unit  37  and the second cutoff unit  38  are each assumed to be a two-way valve that includes a valve body, and a sealing material that seals the valve body, and a coil that opens and closes the valve body by electromagnetic force, and the description will be made. 
     [Concentration Detection Mechanism of Outdoor Unit  1 ] 
     This air-conditioning apparatus  100  includes a concentration detection unit  39  that has a detection member (not illustrated) whose resistance changes in accordance with the concentration of the refrigerant, a concentration calculation unit  41  that calculates the concentration of the refrigerant on the basis of the resistance of the detection member, and the cutoff control unit  40  that controls opening and closing of the first cutoff unit  37  and the second cutoff unit  38  on the basis of the calculated concentration of the refrigerant. 
     Moreover, the concentration detection unit  39  outputs a predetermined signal to the cutoff control unit  40  on the basis of a calculation result regarding the concentration of the refrigerant from the concentration calculation unit  41 . 
     Specifically, the concentration calculation unit  41  is designed not to output the predetermined signal to the cutoff control unit  40  in the case where the calculation result regarding the concentration of the refrigerant from the concentration calculation unit  41  is smaller than a predetermined concentration value. 
     On the other hand, the concentration calculation unit  41  is designed to output the predetermined signal to the cutoff control unit  40  in the case where the calculation result regarding the concentration of the refrigerant from the concentration calculation unit  41  is greater than or equal to the predetermined concentration value. 
     The concentration detection unit  39  is electrically connected to the concentration calculation unit  41  and the cutoff control unit  40 . An output to the cutoff control unit  40  is not particularly limited; however, for example, a direct-current voltage (5 V, 12 V, 24 V, and the like), an alternating-current voltage, a current, or other output is available. Note that, in the air-conditioning apparatus  100  according to Embodiment, a description will be made by considering the output to the cutoff control unit  40  to be DC 5 V. 
     Note that the predetermined concentration corresponds to a leak limiting concentration of or a lower explosive limit of the refrigerant used in the air-conditioning apparatus  100 . For example, in the case where carbon dioxide is used as the refrigerant, it is preferable to set the predetermined concentration to about 1/10 of the leak limiting concentration. Moreover, in the case where a flammable refrigerant (HFO1234yf, R32, or HC) is used, it is preferable to set the predetermined concentration to about 1/10 of the lower explosive limit. 
     The position at which the detection member of the concentration detection unit  39  is provided is not particularly limited; however, for example, it is desirable to arrange the detection member in the outdoor unit  1  as illustrated in  FIG. 1 , near the outdoor unit  1 , in the heat medium relay unit  3 , in the outdoor space  6 , in the room space  7 , in the space  8 , and the like. 
       FIG. 7  illustrates relationships between the resistance of the detection member of the concentration detection unit  39  and the concentration of the refrigerant according to different types of refrigerants. It is desirable that the detection member be formed with, for example, a stannic oxide semiconductor (SnO 2 ). As illustrated in  FIG. 7 , it is clear that as the gas concentration of the refrigerant increases, the resistance of the stannic oxide semiconductor (SnO 2 ) gradually decreases. That is, the concentration of the refrigerant can be uniquely obtained by calculating the resistance of the detection member. 
     Note that it is clear that in the case where the detection member is formed with the stannic oxide semiconductor (SnO 2 ), relationships between the resistance of major refrigerants (R410A, R407C, R32, and HFO1234yf) and the concentration of the refrigerant tend to be almost the same. That is, for the major refrigerants, the concentration of the refrigerant can be detected by using the same calibration curve. In other words, a single detection member makes it possible to detect densities of a plurality of refrigerants, and thus the concentration detection unit  39  can be standardized. 
     Moreover, in the case where the concentration detection unit  39  can be standardized in this way, there is no need to provide a plurality of detecting members for the kinds of refrigerant, thereby reducing the cost of the air-conditioning apparatus  100 . 
     HFO1234yf has the chemical formula CF 3 —CF═CH 2 . HFO1234ze, which is an isomer of HFO1234yf, has the chemical formula CHF 2 —CF═CHF. Since HFO1234ze has similar chemical characteristics to HFO1234yf, the characteristics of the electrical resistance of the detection unit of the concentration detection unit  39  according to the present invention show almost the same characteristics. Thus, the concentration detection unit  39  according to Embodiment can perform a detection operation. 
     Moreover, in the case where R32 and HFO1234yf are mixed, a non-azeotropic refrigerant mixture is obtained. In the case where such a refrigerant leaks, R32, which is a low boiling point component, has a greater leakage. That is, since R32 reaches the limiting concentration faster than HFO1234yf, leaking of the refrigerant can be detected in the safe side by detecting leaking of the refrigerant through measuring of leakage of R32. 
     Moreover, even in the case where other refrigerant mixtures are used, if any one among R410A, R407C, R32, HFO1234yf, and HFO1234ze is included as a component, the electrical resistance of the detection member changes and then the concentration detection unit  39  can perform the detection operation. That is, leaking of refrigerant mixtures containing HFC, HFO, and HFC and HFO can be detected by using the concentration detection unit  39  according to Embodiment. 
     On the other hand, in order to further improve the detection accuracy of the concentration detection unit  39 , such an improvement can be achieved by generating calibration curves varying from refrigerant to refrigerant instead of the calibration curve used in  FIG. 7 . In other words, it is desirable that the calibration curves varying from refrigerant to refrigerant be utilized instead of using single calibration curve. 
     The concentration calculation unit  41  calculates (computes) the concentration of the refrigerant on the basis of the resistance of the detection member of the concentration detection unit  39 . The concentration calculation unit  41  is electrically connected to the concentration detection unit  39 . 
     The relationship between the concentration of the refrigerant and the resistance of the detection member, as illustrated in  FIG. 7 , is stored in the concentration calculation unit  41 . As a result of this, the concentration of the refrigerant used in the air-conditioning apparatus  100  can be calculated on the basis of the resistance of the detection member of the concentration detection unit  39 . 
     Note that the position at which the concentration calculation unit  41  is provided is not particularly limited; however, it is desirable that the concentration calculation unit  41  be provided, for example, in the outdoor unit  1 , near the outdoor unit  1 , or the like. In the following, the air-conditioning apparatus  100  will be described with reference to  FIG. 2  again. 
     The cutoff control unit  40  controls opening and closing of the first cutoff unit  37  and the second cutoff unit  38  on the basis of an output (the predetermined signal) from the concentration detection unit  39 . 
     In the case where the concentration of the refrigerant is greater than or equal to the predetermined concentration value, the concentration detection unit  39  outputs the predetermined signal to the cutoff control unit  40 . As a result of this, the electrical connection between a voltage supply (not illustrated) and the first cutoff unit  37  and second cutoff unit  38  is cut off. That is, a voltage is not applied to the first cutoff unit  37  and the second cutoff unit  38  (feeding is not performed). Then, the coil of the first cutoff unit  37  and the second cutoff unit  38  are not excited, and thus the valve bodies close. That is, both the first cutoff unit  37  and the second cutoff unit  38  are in the closed state. 
     On the other hand, in the case where the concentration of the refrigerant is smaller than the predetermined concentration value, the predetermined signal is not output from the concentration detection unit  39  to the cutoff control unit  40 . As a result of this, the voltage supply (not illustrated) and the first cutoff unit  37  and second cutoff unit  38  are electrically connected. That is, a voltage is applied to the first cutoff unit  37  and the second cutoff unit  38  (feeding is performed). Then, the coils of the first cutoff unit  37  and the second cutoff unit  38  are excited, and thus the valve bodies open. That is, both the first cutoff unit  37  and the second cutoff unit  38  are in the open state. 
     Note that the position at which the cutoff control unit  40  is provided is not particularly limited; however, it is desirable that the cutoff control unit  40  be provided, for example, in the outdoor unit  1 , near the outdoor unit  1 , or the like. 
     As described above, the first cutoff unit  37  and the second cutoff unit  38  are two-way valves that each include a valve body, a sealing material that seals the valve body, and a coil that opens and closes the valve body by electromagnetic force. 
     The coil of each of the first cutoff unit  37  and the second cutoff unit  38  is connected to the voltage supply through the cutoff control unit  40 . When the air-conditioning apparatus  100  is in normal operation, the cutoff control unit  40  puts in a state (an energized state) in which the voltage supply is electrically connected to the first cutoff unit  37  and second cutoff unit  38 . As a result of this, the coil of each of the first cutoff unit  37  and the second cutoff unit  38  is applied with a voltage by the voltage supply and becomes an electromagnet. Then, each valve body is attracted by electromagnetic force, and the first cutoff unit  37  and the second cutoff unit  38  are in the open state. 
     On the other hand, when detecting leaking of the refrigerant of the air-conditioning apparatus  100 , the cutoff control unit  40  electrically cuts off the connection between the voltage supply and the first cutoff unit  37  and second cutoff unit  38  (a non-energized state). As a result of this, the coil of each of the first cutoff unit  37  and the second cutoff unit  38  is not applied with a voltage by the voltage supply. Then, each valve body is not attracted by the coil, and the first cutoff unit  37  and the second cutoff unit  38  are in the closed state. 
     Note that, when a mechanical electromagnetic relay is employed to perform switching between electrical connection and disconnection between the voltage supply and the first cutoff unit  37  and second cutoff unit  38 , in the case where a flammable refrigerant (HFO1234yf, R32, or HC) is used as the refrigerant, sparks may be emitted and may ignite the refrigerant. Thus, in the case where a flammable refrigerant is used, it is desirable to employ, as the cutoff control unit  40 , an SSR (solid state relay), which is a non-contact relay using a semiconductor element. As a result of this, there is no mechanical operation, and thus it is possible to reduce the possibility of sparking. As a result of this, even if a flammable refrigerant leaks into the outdoor unit  1 , switching between electrical connection and disconnection between the voltage supply and the first cutoff unit  37  and second cutoff unit  38  may be safely performed. 
     Moreover, it is desirable that the coils for opening and closing the valve bodies of the first cutoff unit  37  and the second cutoff unit  38  be excited by a direct current. It is because the lifetime of the coils can be sustained longer when excitation is performed with a direct-current voltage. Thus, a description will be made by considering that a direct-current voltage is applied to the first cutoff unit  37  and the second cutoff unit  38  as an operation voltage. Note that a description will be made by assuming that the operation voltage is DC 12 V; however, the operation voltage may be DC 24 V or the like and is not limited thereto. Moreover, the cutoff control unit  40  also includes a converter that can convert an output from a commercial power supply (an alternating-current voltage, AC 200 V in Embodiment) into a predetermined direct-current voltage (DC 12V in Embodiment). The above-described voltage supply corresponds to the commercial power supply and the converter. 
     Since the first cutoff unit  37  and the second cutoff unit  38  are provided in main pipes (the refrigerant pipes  4 ) of the refrigerant circuit, the main pipes need to have a large aperture. That is, the main pipes need to have a large value of a flow coefficient Cv. Thus, it is desirable to employ not a direct-acting valve but a pilot valve as the first cutoff unit  37  and the second cutoff unit  38 . 
     It is desirable that the value of Cv for the first cutoff unit  37  through which a high-pressure refrigerant flows be set to, for example, 1 or higher, and the value of Cv for the second cutoff unit  38  through which a low-pressure refrigerant flows be set to, for example, 5 or higher. 
     Even if the first cutoff unit  37  and the second cutoff unit  38  are provided in the refrigerant circuit, the values of Cv described above can make a pressure loss small, and make performance degradation small. That is, the amount of the refrigerant circulating in the refrigerant circuit A decreases in normal operation. 
     Moreover, it is desirable that the sealing material that seals the valve body be composed of rubber or PTFE (polytetrafluoroethylene). As a result of this, in case of emergency, the sealing material immediately conforms (closely adheres) to the valve body. That is, in the case where the first cutoff unit  37  and the second cutoff unit  38  are closed, the possibility of the refrigerant leak decreases. 
     Note that the first cutoff unit  37  and the second cutoff unit  38  do not open or close as often as a normal valve. Thus, it is not always necessary to employ a metal seal, which has a high durability, as the sealing material. 
     Moreover, it is desirable that the first cutoff unit  37  and the second cutoff unit  38  be designed in such a manner that leakage of the refrigerant in the closed state is set to, for example, 1.0×10 −6  [m 3 /s] or smaller. The reason will be described below. 
     When a large amount of a refrigerant leaks into a space, there is a danger of burning, an oxygen deficiency, and the like. For each kind of refrigerant, a corresponding limiting concentration is defined, which is the maximum concentration for leakage of the refrigerant so as to safely use the refrigerant. The limiting concentration is, for example, 0.44 [m 3 /kg] for R410A, 0.061 [m 3 /kg] for R32, 0.0578 [m 3 /kg] for HFO1234yf, and 0.008 [m 3 /kg] for propane. 
     When a refrigerant leaks into a room, leaking of the refrigerant is stopped by closing the first cutoff unit  37  and the second cutoff unit  38  provided in the refrigerant pipes. In this case, if a measure to stop leaking of the refrigerant is taken when the refrigerant reaches the limiting concentration, it is late. Thus, when the concentration of the refrigerant in a room reaches 95% of the limiting concentration, the first cutoff unit  37  and the second cutoff unit  38  are designed to be closed. That is, after the first cutoff unit  37  and the second cutoff unit  38  are closed and until the concentration of the refrigerant reaches the limiting concentration, the amount of the refrigerant that may further leak is 5%. 
     Here, in the case where a location in which the multi air-conditioners for buildings is expected to be installed is the smallest room, which is a single room of a hotel, it is assumed that the room has a capacity of 25 m 3 , the pressure difference between before and after the first cutoff unit  37  and the second cutoff unit  38  operate is 1.0 MPa, a substantial space capacity of the room except for the unit bath and other items is 0.5×25=12.5 m 3 . In this case, the amount of the refrigerant that may leak after the first cutoff unit  37  and the second cutoff unit  38  are closed is 12.5 m 3 ×0.05=0.625 m 3 . It is expected that users do not notice the refrigerant leaking while sleeping or the like and that the space is closed in a state in which windows are closed. Thus, leakage that does not reach the limiting concentration within 24 hours after the first cutoff unit  37  and the second cutoff unit  38  are operated is calculated to be 0.625/(24×60×60)=7.2×10 −6  [m 3 /s]. If leakage after the first cutoff unit  37  and the second cutoff unit  38  are closed is smaller than this value, the safety is considered to be kept. 
     Moreover, a position is unpredictable where the refrigerant leaks in a high-pressure pipe or in a fluid pipe. Thus, assuming that leaking occurs in a high-pressure pipe, leakage needs to be smaller than the limiting concentration described above, at a pressure difference of about 5 MPa. Leakage of the refrigerant is in proportion to the pressure difference to the power of 0.5 according to Bernoulli&#39;s theorem, which is commonly known in the field of fluid dynamics. Thus, leakage of the refrigerant is 7.2×10 −6  [m 3 /s]/(5/1) 0.5 =3.2×10 −6 . If leakage is smaller than this value, the safety is ensured. In order to achieve a higher degree of safety, leakage should be 1.0×10 −6  [m 3 /s] or smaller. 
     Note that it is desirable that the first cutoff unit  37  and the second cutoff unit  38  have a minimum operation pressure difference of about 0 (kgf/cm 2 ). 
     In the case where the heat-source side refrigerant leaks from a channel (the refrigerant circuit A) in which the heat-source side refrigerant circulates, the concentration of the refrigerant increases in an external portion (see the outdoor space  6 , the room space  7 , and the space  8  in  FIG. 1 ) affected by leaking. The resistance of the detecting member of the concentration detection unit  39  changes in accordance with the concentration of the refrigerant that has leaked. Then, the concentration calculation unit  41  calculates the concentration of the refrigerant that has leaked, on the basis of the resistance. 
     In the case where the calculation result from the concentration calculation unit  41  is smaller than a predetermined concentration value, which means that the refrigerant does not leak, the concentration detection unit  39  does not output the predetermined signal to the cutoff control unit  40 . Thus, since the predetermined signal is not output to the cutoff control unit  40 , the cutoff control unit  40  maintains an electrical connection between the voltage supply and the first cutoff unit  37  and second cutoff unit  38 . That is, a voltage is applied to the coil of each of the first cutoff unit  37  and the second cutoff unit  38  by the voltage supply. As a result of this, the first cutoff unit  37  and the second cutoff unit  38  stay in a state in which the valve bodies of the first cutoff unit  37  and the second cutoff unit  38  are opened. That is, both the first cutoff unit  37  and the second cutoff unit  38  are in the open state. 
     In the case where the calculation result from the concentration calculation unit  41  is greater than or equal to a predetermined concentration value, which means that the refrigerant leaks, the concentration detection unit  39  outputs the predetermined signal to the cutoff control unit  40 . The cutoff control unit  40  cuts off the electrical connection between the voltage supply and the first cutoff unit  37  and second cutoff unit  38  on the basis of this output (the predetermined signal). That is, no voltage is applied to the coil of each of the first cutoff unit  37  and the second cutoff unit  38  by the voltage supply. As a result of this, the valve bodies of the first cutoff unit  37  and the second cutoff unit  38  are closed. That is, both the first cutoff unit  37  and the second cutoff unit  38  are in the closed state. 
     As a result of this, the refrigerant flow that flows out from the outdoor unit  1  and the refrigerant flow that flows into the outdoor unit  1  are cut off. That is, the heat-source side refrigerant is prevented from circulating in the refrigerant circuit A. As a result of this, the amount of the refrigerant that will leak is decreased, thereby improving the degree of safety of the air-conditioning apparatus  100 . 
     [Indoor Unit  2 ] 
     The indoor unit  2  includes use-side heat exchangers  26   a  to  26   d  (also simply referred to as use-side heat exchangers  26 ). The use-side heat exchangers  26  are designed to be connected to heat medium flow control devices  25   a  to  25   d  (also simply referred to as heat medium flow control devices  25 ) through the second heat medium pipes  5  and to second heat medium channel switch units  23   a  to  23   d  (also simply referred to as second heat medium channel switch units  23 ) through the heat medium pipes  5 . The use-side heat exchangers  26  each exchange heat between air supplied from the air-sending device such as a fan, not illustrated, and the heat medium, and generate air for heating or air for cooling to be supplied to the room space  7 . 
       FIG. 2  illustrates an example in the case where four indoor units  2   a  to  2   d  are connected to the heat medium relay unit  3  through the heat medium pipes  5 . Moreover, the use-side heat exchangers  26  are the use-side heat exchanger  26   a , the use-side heat exchanger  26   b , the use-side heat exchanger  26   c , and the use-side heat exchanger  26   d  from the bottom of the sheet in accordance with the indoor units  2   a  to  2   d , respectively. Note that the number of the indoor units  2  to be connected is not limited to four. 
     [Heat Medium Relay Unit  3 ] 
     The heat medium relay unit  3  includes two heat exchangers related to heat medium  15   a  and  15   b  (also simply referred to as heat exchangers related to heat medium  15 ), two expansion units  16   a  and  16   b  (also simply referred to as expansion units  16 ), two on-off units  17   a  and  17   b  (also simply referred to as on-off units  17 ), two second refrigerant channel switch units  18   a  and  18   b  (also simply referred to as second refrigerant channel switch units  18 ), two pumps  21   a  and  21   b  (also simply referred to as pumps  21 ), four first heat medium channel switch units  22   a  to  22   d  (also simply referred to as first heat medium channel switch units  22 ), the four second heat medium channel switch units  23   a  to  23   d  (also simply referred to as second heat medium channel switch units  23 ), and the four heat medium flow control devices  25   a  to  25   d  (also simply referred to as heat medium flow control devices  25 ). 
     The heat exchangers related to heat medium  15  (load-side heat exchangers) function as condensers (radiators) or evaporators, and exchange heat between the heat-source side refrigerant and the heat medium. The heat exchangers related to heat medium  15  transmit cooling energy or heating energy generated by the outdoor unit  1  and stored in the heat-source side refrigerant, to the heat medium. The heat exchanger related to heat medium  15   a  is connected to a pipe that connects the expansion unit  16   a  and the second refrigerant channel switch unit  18   a  in the refrigerant circuit A illustrated in  FIG. 2 , and cools the heat medium in the cooling and heating mixed operation mode. The heat exchanger related to heat medium  15   b  is connected to a pipe that connects the expansion unit  16   b  and the second refrigerant channel switch unit  18   b  in the refrigerant circuit A illustrated in  FIG. 2 , and heats the heat medium in the cooling and heating mixed operation mode. 
     The expansion units  16  function as pressure reducing valves or expansion valves, and reduce pressure to expand the heat-source side refrigerant. The expansion unit  16   a  is provided on an upstream side of the heat exchanger related to heat medium  15   a  in the flow of the heat-source side refrigerant in the cooling only operation mode. The expansion unit  16   b  is provided on an upstream side of the heat exchanger related to heat medium  15   b  in the flow of the heat-source side refrigerant in the cooling only operation mode. It is desirable that the expansion units  16  be formed with something that may variably control its opening degree; for example, an electronic expansion valve or the like. 
     The on-off units  17  open and close channels to which the on-off units  17  are provided. The on-off unit  17   a  is provided on an upstream side of the expansion units  16  in the flow of the heat-source side refrigerant in the cooling only operation mode. The on-off unit  17   b  is provided on a downstream side of the expansion unit  16   a  in the flow of the heat-source side refrigerant in the heating only operation mode. It is desirable that the on-off units  17  be formed with, for example, a two-way valve or the like. 
     The second refrigerant channel switch units  18  select the flow of the refrigerant in the heating only operation mode, the flow of the refrigerant in the cooling only operation mode, or the flow of the refrigerant in the cooling and heating mixed operation mode. The second refrigerant channel switch unit  18   b  connects the high-pressure side refrigerant pipe  4   a ( 2 ) and the heat exchanger related to heat medium  15   b  in the heating only operation mode. The second refrigerant channel switch unit  18   a  connects the heat exchanger related to heat medium  15   a  and the low-pressure side refrigerant pipe  4   b ( 2 ) in the cooling only operation mode and in the cooling and heating mixed operation mode. It is desirable that the second refrigerant channel switch units  18  be formed with, for example, a four-way valve or the like. 
     The pumps  21  make the heat medium that flows in the heat medium pipes  5  circulate. The pump  21   a  is connected to a pipe that connects the heat exchanger related to heat medium  15   a  and the second heat medium channel switch units  23 , among the heat medium pipes  5 . The pump  21   b  is connected to a pipe that connects the heat exchanger related to heat medium  15   b  and the second heat medium channel switch units  23 , among the heat medium pipes  5 . It is desirable that the two pumps  21  be formed with, for example, a pump that may control its capacity, or the like. Note that the pump  21   a  may be connected to a pipe that connects the heat exchanger related to heat medium  15   a  and the first heat medium channel switch units  22 , among the heat medium pipes  5 . Moreover, the pump  21   b  may be connected to a pipe that connects the heat exchanger related to heat medium  15   b  and the first heat medium channel switch units  22 , among the heat medium pipes  5 . 
     The first heat medium channel switch units  22  perform switching between channels of the heat medium. The first heat medium channel switch units  22 , the number of which corresponds to the number (here, four) of the indoor units  2  provided, are provided. One of three-way terminals of each of the first heat medium channel switch units  22  is connected to the heat exchanger related to heat medium  15   a , another one of three-way terminals thereof is connected to the heat exchanger related to heat medium  15   b , and the other one of three-way terminals thereof is connected to a corresponding one of the heat medium flow control devices  25 . The first heat medium channel switch units  22  are each provided on an outlet side of a corresponding heat medium passage of the use-side heat exchangers  26 . Note that the first heat medium channel switch units are the first heat medium channel switch unit  22   a , the first heat medium channel switch unit  22   b , the first heat medium channel switch unit  22   c , and the first heat medium channel switch unit  22   d  from the bottom of the sheet in accordance with the indoor units  2 . It is desirable that the first heat medium channel switch units  22  be formed with, for example, a three-way valve or the like. 
     The second heat medium channel switch units  23  perform switching between channels of the heat medium. The second heat medium channel switch units  23 , the number of which corresponds to the number (here, four) of the indoor units  2  provided, are provided. One of three-way terminals of each of the second heat medium channel switch units  23  is connected to the heat exchanger related to heat medium  15   a , another one of three-way terminals thereof is connected to the heat exchanger related to heat medium  15   b , and the other one of three-way terminals thereof is connected to a corresponding one of the use-side heat exchangers  26 . The second heat medium channel switch units  23  are each provided on an inlet side of the corresponding heat medium passage of the use-side heat exchangers  26 . Note that the second heat medium channel switch units are the second heat medium channel switch unit  23   a , the second heat medium channel switch unit  23   b , the second heat medium channel switch unit  23   c , and the second heat medium channel switch unit  23   d  from the bottom of the sheet in accordance with the indoor units  2 . It is desirable that the second heat medium channel switch units  23  be formed with, for example, a three-way valve or the like. 
     The heat medium flow control devices  25  control the amount of the heat medium that flows in the heat medium pipes  5 . The heat medium flow control devices  25 , the number of which corresponds to the number (here, four) of the indoor units  2  provided, are provided. One terminal of each of the heat medium flow control devices  25  is connected to a corresponding one of the use-side heat exchangers  26 , and the other terminal thereof is connected to a corresponding one of the first heat medium channel switch units  22 . The heat medium flow control devices  25  are each provided on the outlet side of the corresponding heat medium passage of the use-side heat exchangers  26 . Note that the heat medium flow control devices are illustrated as the heat medium flow control device  25   a , the heat medium flow control device  25   b , the heat medium flow control device  25   c , and the heat medium flow control device  25   d  from the bottom of the sheet in accordance with the indoor units  2 . Moreover, the heat medium flow control devices  25  may be each provided on the inlet side of the corresponding heat medium passage of the use-side heat exchangers  26 . It is desirable that the heat medium flow control devices  25  be formed with, for example, a two-way valve that may control, for example, its opening area or the like. 
     Moreover, the heat medium relay unit  3  includes various detection units (two first temperature sensors  31   a  and  31   b , four second temperature sensors  34   a  to  34   d , four third temperature sensors  35   a  to  35   d , and a pressure sensor  36  in  FIG. 5 ). Information (temperature information and pressure information) detected by these detection units is sent to a control device (not illustrated) that performs central control on the operation of the air-conditioning apparatus  100 , and is used to control a driving frequency of the compressor  10 , the rotation speed of the air-sending device, which is not illustrated in the drawings but provided near the heat-source side heat exchanger  12  and the use-side heat exchangers  26 , switching performed by the first refrigerant channel switch unit  11 , a driving frequency of the pumps  21 , switching performed by the second refrigerant channel switch units  18 , switching between channels of heat medium circuits, and the like. 
     The two first temperature sensors  31   a  and  31   b  (also simply referred to as first temperature sensors  31 ) detect the temperature of the heat medium that has flown out from the heat exchangers related to heat medium  15 , that is, the temperature of the heat medium at the outlets of the heat exchangers related to heat medium  15 . The first temperature sensor  31   a  is provided on the heat medium pipe  5  that is on the inlet side of the pump  21   a . The first temperature sensor  31   b  is provided on the heat medium pipe  5  that is on the inlet side of the pump  21   b . It is desirable that the first temperature sensors  31  be formed with, for example, a thermistor or the like. 
     The four second temperature sensors  34   a  to  34   d  (also simply referred to as second temperature sensors  34 ) are provided between the first heat medium channel switch units  22  and the heat medium flow control devices  25 , and detect temperatures of the heat medium that flows out from the use-side heat exchangers  26 . The second temperature sensors  34  are provided in accordance with the number (here, four) of the provided indoor units  2 . Note that the second temperature sensors are illustrated as the second temperature sensor  34   a , the second temperature sensor  34   b , the second temperature sensor  34   c , and the second temperature sensor  34   d  from the bottom of the sheet in accordance with the indoor units  2 . It is desirable that the second temperature sensors  34  be formed with, for example, a thermistor or the like. 
     The four third temperature sensors  35   a  to  35   d  (also simply referred to as third temperature sensors  35 ) are provided on inlet sides or outlet sides of the heat exchangers related to heat medium  15  through which the heat-source side refrigerant flows, and detect the temperatures of the heat-source side refrigerant that flows into the heat exchangers related to heat medium  15  or the temperatures of the heat-source side refrigerant that flows out from the heat exchangers related to heat medium  15 . The third temperature sensor  35   a  is provided between the heat exchanger related to heat medium  15   a  and the second refrigerant channel switch unit  18   a . The third temperature sensor  35   b  is provided between the heat exchanger related to heat medium  15   a  and the expansion unit  16   a . The third temperature sensor  35   c  is provided between the heat exchanger related to heat medium  15   b  and the second refrigerant channel switch unit  18   b . The third temperature sensor  35   d  is provided between the heat exchanger related to heat medium  15   b  and the expansion unit  16   b . It is desirable that the third temperature sensors  35  be formed with, for example, a thermistor or the like. 
     The pressure sensor  36  is provided between the heat exchanger related to heat medium  15   b  and the expansion unit  16   b , similarly to the setting position of the third temperature sensor  35   d , and detects the pressure applied on the heat-source side refrigerant that flows between the heat exchanger related to heat medium  15   b  and the expansion unit  16   b.    
     Moreover, a main controller, not illustrated, is configured with a microcomputer and the like to be provided, and controls the driving frequency of the compressor  10 , the rotation speed (including ON/OFF) of the air-sending device (not illustrated), switching performed by the first refrigerant channel switch unit, driving of the pumps  21 , the opening degree of the expansion units  16 , opening and closing of the on-off units  17 , switching performed by the second refrigerant channel switch units  18 , switching performed by the first heat medium channel switch units  22 , switching performed by the second heat medium channel switch units  23 , and the opening degree of the heat medium flow control devices  25  and the like, on the basis of the information detected by the various detection units and instructions sent from a remote controller, thereby executing each operation mode described below. Note that a controller may be provided in each unit, or may be provided in the outdoor unit  1  or in the heat medium relay unit  3 . 
     The heat medium pipes  5  in which the heat medium flows include a pipe that is connected to the heat exchanger related to heat medium  15   a  and a pipe that is connected to the heat exchanger related to heat medium  15   b . The heat medium pipes  5  is divided into channels (here, each into four channels), the number of which corresponds to the number of indoor units  2  connected to the heat medium relay unit  3 . Then, the heat medium pipes  5  are connected to the first heat medium channel switch units  22  and the second heat medium channel switch units  23 . Whether the heat medium from the heat exchanger related to heat medium  15   a  flows into the use-side heat exchangers  26  or the heat medium from the heat exchanger related to heat medium  15   b  flows into the use-side heat exchangers  26  is set by controlling the first heat medium channel switch units  22  and the second heat medium channel switch units  23 . 
     Then, the air-conditioning apparatus  100  includes the refrigerant circuit A, which has a structure in which the compressor  10 , the first refrigerant channel switch unit  11 , the heat-source side heat exchanger  12 , the on-off units  17 , the expansion units  16 , passages of the heat exchangers related to heat medium  15  for the heat-source side refrigerant, the second refrigerant channel switch units  18 , and the accumulator  19  are connected with the refrigerant pipes  4 . Moreover, the heat medium circulation circuit B has a structure in which passages of the heat exchangers related to heat medium  15  for the heat medium, the pumps  21 , the first heat medium channel switch units  22 , the heat medium flow control devices  25 , the use-side heat exchangers  26 , and the second heat medium channel switch units  23  are connected with the heat medium pipes  5 . That is, each of the heat exchangers related to heat medium  15  is connected to a plurality of use-side heat exchangers  26  in parallel, and thus the heat medium circulation circuit B has a plurality of systems. 
     Thus, in the air-conditioning apparatus  100 , the outdoor unit  1  and the heat medium relay unit  3  are connected through the heat exchanger related to heat medium  15   a  and the heat exchanger related to heat medium  15   b  provided in the heat medium relay unit  3 , and the heat medium relay unit  3  and the indoor units  2  are connected through the heat exchanger related to heat medium  15   a  and the heat exchanger related to heat medium  15   b . That is, in the air-conditioning apparatus  100 , the heat exchanger related to heat medium  15   a  and the heat exchanger related to heat medium  15   b  exchange heat between the heat-source side refrigerant circulating in the refrigerant circuit A and the heat medium that circulates in the heat medium circulation circuit B. 
     On the basis of instructions from each of indoor units  2 , this air-conditioning apparatus  100  enable to perform a cooling operation or a heating operation possible by using the indoor unit  2 . That is, the air-conditioning apparatus  100  can perform the same operation in the indoor units  2  or can perform individual operation in each of the indoor units  2  differently. 
     Operation modes performed by the air-conditioning apparatus  100  contain the cooling only operation mode in which all driving indoor units  2  execute the cooling operation, the heating only operation mode in which all driving indoor units  2  execute the heating operation, the cooling main operation mode as the cooling and heating mixed operation mode with a greater cooling load, and the heating main operation mode as the cooling and heating mixed operation mode with a greater heating load. In the following, each of the operation modes will be described in terms of the flow of the heat-source side refrigerant and that of the heat medium. 
     [Cooling Only Operation Mode] 
       FIG. 3  is a diagram of a refrigerant circuit illustrating a flow of the refrigerant in the air-conditioning apparatus  100  in the cooling only operation mode. In  FIG. 3 , the cooling only operation mode is described in an example of the case where a cooling load is generated only in the use-side heat exchanger  26   a  and the use-side heat exchanger  26   b . Note that, in  FIG. 3 , pipes drawn with a thick line indicate pipes in which the refrigerant (the heat-source side refrigerant and the heat medium) flows. Moreover, in  FIG. 3 , the direction in which the heat-source side refrigerant flows is indicated by arrows drawn with a solid line, and the direction in which the heat medium flows is indicated by arrows drawn with a broken line. 
     In the case of the cooling only operation mode illustrated in  FIG. 3 , the outdoor unit  1  makes the first refrigerant channel switch unit  11  perform switching in such a manner that the heat-source side refrigerant discharged from the compressor  10  flows into the heat-source side heat exchanger  12 . The heat medium relay unit  3  drives the pump  21   a  and the pump  21   b , opens the heat medium flow control device  25   a  and the heat medium flow control device  25   b , fully opens the heat medium flow control device  25   c  and the heat medium flow control device  25   d , and circulates the heat medium between the heat exchanger related to heat medium  15   a  and both the use-side heat exchanger  26   a  and use-side heat exchanger  26   b  and between the heat exchanger related to heat medium  15   b  and both the use-side heat exchanger  26   a  and use-side heat exchanger  26   b.    
     At first, the flow of the heat-source side refrigerant in the refrigerant circuit A is described. 
     A low-temperature low-pressure refrigerant is compressed by the compressor  10 , and becomes a high-temperature high-pressure refrigerant gas, which is then discharged. The high-temperature high-pressure refrigerant gas that has been discharged from the compressor  10  flows into the heat-source side heat exchanger  12  through the first refrigerant channel switch unit  11 . The heat is transferred to the outside air in the heat-source side heat exchanger  12 , and consequently, the high-temperature high-pressure refrigerant gas becomes a high-pressure liquid refrigerant. 
     The liquid refrigerant that has flown out from the heat-source side heat exchanger  12  flows out from the outdoor unit  1  through the pipe that is provided with the check valve  13   a  and the high-pressure side refrigerant pipe  4   a ( 1 ), and flows into the heat medium relay unit  3  through the high-pressure side refrigerant pipe  4   a ( 2 ). 
     The high-pressure liquid refrigerant that has flown into the heat medium relay unit  3  flows through the on-off unit  17   a , then is divided and flows into the expansion unit  16   a  and the expansion unit  16   b  to be expanded. As a result, a low-temperature low-pressure two-phase refrigerant is obtained. Note that the on-off unit  17   b  is closed. 
     The low-temperature low-pressure two-phase refrigerant flows into each of the heat exchanger related to heat medium  15   a  and heat exchanger related to heat medium  15   b  that function as an evaporator. Then, the low-temperature low-pressure two-phase refrigerant becomes a low-temperature low-pressure refrigerant gas while absorbing heat from the heat medium that circulates in the heat medium circulation circuit B, and consequently, cooling the heat medium. 
     The refrigerant gas that has flown out from the heat exchanger related to heat medium  15   a  and the heat exchanger related to heat medium  15   b  flows out from the heat medium relay unit  3  through the second refrigerant channel switch unit  18   a  and the second refrigerant channel switch unit  18   b , and then flows again into the outdoor unit  1  through the low-pressure side refrigerant pipe  4   b ( 2 ). 
     The refrigerant gas that has flown into the outdoor unit  1  is sucked again by the compressor  10  through the low-pressure side refrigerant pipe  4   b ( 1 ), the pipe to which the check valve  13   d  is connected, the first refrigerant channel switch unit  11 , and the accumulator  19 . 
     The opening degree of the expansion unit  16   a  is controlled in such a manner that superheating (a degree of superheat) obtained as the difference between the temperature detected by the third temperature sensor  35   a  and the temperature detected by the third temperature sensor  35   b  is constant. Similarly, the opening degree of the expansion unit  16   b  is controlled in such a manner that superheating obtained as the difference between the temperature detected by the third temperature sensor  35   c  and the temperature detected by the third temperature sensor  35   d  is constant. 
     Next, the flow of the heat medium in the heat medium circulation circuit B will be described. 
     In the cooling only operation mode, the cooling energy of the heat-source side refrigerant is transferred to the heat medium in both the heat exchanger related to heat medium  15   a  and the heat exchanger related to heat medium  15   b , and the cooled heat medium is made to flow in the heat medium pipes  5  by the pump  21   a  and pump  21   b . The heat medium that has flown out by being compressed by the pump  21   a  and the pump  21   b  flows into the use-side heat exchanger  26   a  and the use-side heat exchanger  26   b  through the second heat medium channel switch unit  23   a  and the second heat medium channel switch unit  23   b . Then, the heat medium removes heat from the room air in the use-side heat exchanger  26   a  and the use-side heat exchanger  26   b , and consequently, cooling for the room space  7  is performed. 
     Then, the heat medium flows out from the use-side heat exchanger  26   a  and the use-side heat exchanger  26   b  and flows into the heat medium flow control device  25   a  and the heat medium flow control device  25   b . In this case, the amount of the flowing heat medium is controlled by the operation of the heat medium flow control device  25   a  and heat medium flow control device  25   b  so as to be the amount that is necessary to meet an air conditioning load that is needed in the room, and flows into the use-side heat exchanger  26   a  and the use-side heat exchanger  26   b.    
     The heat medium that has flown out from the heat medium flow control device  25   a  flows into the heat exchanger related to heat medium  15   a  and the heat exchanger related to heat medium  15   b  through the first heat medium channel switch unit  22   a , and is sucked again by the pump  21   a  and the pump  21   b.    
     Note that, in the heat medium pipes  5  of the use-side heat exchangers  26 , the heat medium flows in the direction from the second heat medium channel switch units  23  to the first heat medium channel switch units  22  via the heat medium flow control devices  25 . Moreover, the air conditioning load can meet an amount needed in the room space  7  by performing control in such a manner that the differences between the temperature detected by the first temperature sensor  31   a  and the temperatures detected by the second temperature sensors  34  or the differences between the temperature detected by the first temperature sensor  31   b  and the temperatures detected by the second temperature sensors  34  are to have a target value. As an output temperature of the heat exchangers related to heat medium  15 , either the temperature detected by the first temperature sensor  31   a  or that detected by the first temperature sensor  31   b  may be used, or the average of these temperatures may be used. In this case, the first heat medium channel switch units  22  and the second heat medium channel switch units  23  have an intermediate opening degree in such a manner that the heat medium flows in both channels for the heat exchanger related to heat medium  15   a  and channels for the heat exchanger related to heat medium  15   b.    
     In the case where the cooling only operation mode is performed, since no heat medium need to flow into the use-side heat exchangers  26  that have no heating load (thermostat off is included), the heat medium flow control devices  25  close the channels so that the heat medium does not flow into the use-side heat exchangers  26 . In  FIG. 3 , since the use-side heat exchanger  26   a  and the use-side heat exchanger  26   b  have a heating load, the heat medium flows into the use-side heat exchanger  26   a  and the use-side heat exchanger  26   b , while the use-side heat exchanger  26   c  and the use-side heat exchanger  26   d  have no heating load, and the corresponding heat medium flow control device  25   c  and the corresponding heat medium flow control device  25   d  are fully closed. In the case where a heating load is generated in the use-side heat exchanger  26   c , the heat medium flow control device  25   c  is to be opened so as to circulate the heat medium. In the case where a heating load is generated in the use-side heat exchanger  26   d , the heat medium flow control device  25   d  is to be opened so as to circulate the heat medium. 
     Note that this operation may be similarly applied in other operation modes. 
     [Heating Only Operation Mode] 
       FIG. 4  is a diagram of a refrigerant circuit illustrating a flow of the refrigerant in the air-conditioning apparatus  100  in the heating only operation mode. In  FIG. 4 , the heating only operation mode is described in an example of the case where a heating load is generated only in the use-side heat exchanger  26   a  and the use-side heat exchanger  26   b . Note that, in  FIG. 4 , pipes drawn with a thick line indicate pipes in which the refrigerant (the heat-source side refrigerant and the heat medium) flows. Moreover, in  FIG. 4 , the direction in which the heat-source side refrigerant flows is indicated by arrows drawn with a solid line, and the direction in which the heat medium flows is indicated by arrows drawn with a broken line. 
     In the case of the heating only operation mode illustrated in  FIG. 4 , the outdoor unit  1  makes the first refrigerant channel switch unit  11  perform switching in such a manner that the heat-source side refrigerant discharged from the compressor  10  flows into the heat medium relay unit  3  without flowing through the heat-source side heat exchanger  12 . The heat medium relay unit  3  drives the pump  21   a  and the pump  21   b , opens the heat medium flow control device  25   a  and the heat medium flow control device  25   b , fully closes the heat medium flow control device  25   c  and the heat medium flow control device  25   d , and circulates the heat medium between the heat exchanger related to heat medium  15   a  and both the use-side heat exchanger  26   a  and use-side heat exchanger  26   b  and between the heat exchanger related to heat medium  15   b  and both the use-side heat exchanger  26   a  and use-side heat exchanger  26   b.    
     At first, the flow of the heat-source side refrigerant in the refrigerant circuit A is described. 
     A low-temperature low-pressure refrigerant is compressed by the compressor  10 , and becomes a high-temperature high-pressure refrigerant gas, which is then discharged. The high-temperature high-pressure refrigerant gas that has been discharged from the compressor  10  flows out from the outdoor unit  1  through the first refrigerant channel switch unit  11 , the first connection pipe  42   a , and the high-pressure side refrigerant pipe  4   a ( 1 ). The high-temperature high-pressure refrigerant gas that has flown out from the outdoor unit  1  flows into the heat medium relay unit  3  through the high-pressure side refrigerant pipe  4   a ( 2 ). The high-temperature high-pressure refrigerant gas that has flown into the heat medium relay unit  3  is divided and flows into the heat exchanger related to heat medium  15   a  and the heat exchanger related to heat medium  15   b  through the second refrigerant channel switch unit  18   a  and the second refrigerant channel switch unit  18   b.    
     The high-temperature high-pressure refrigerant that has flown into the heat exchanger related to heat medium  15   a  and the heat exchanger related to heat medium  15   b  becomes a high-pressure refrigerant whose temperature has decreased while transferring heat to the heat medium that circulates in the heat medium circulation circuit B. The liquid refrigerant that has flown out from the heat exchanger related to heat medium  15   a  and the heat exchanger related to heat medium  15   b  is expanded by the expansion unit  16   a  and the expansion unit  16   b , and becomes a low-temperature low-pressure two-phase refrigerant. The low-temperature low-pressure two-phase refrigerant flows out from the heat medium relay unit  3  through the on-off unit  17   b  and flows again into the outdoor unit  1  through the low-pressure side refrigerant pipe  4   b ( 2 ). Note that the on-off unit  17   a  is closed and the on-off unit  17   b  is opened. 
     The refrigerant that has flown into the outdoor unit  1  flows into the heat-source side heat exchanger  12 , which operates as an evaporator, through the low-pressure side refrigerant pipe  4   b ( 1 ) and the second connection pipe  42   b . Then, the refrigerant that has flown into the heat-source side heat exchanger  12  removes heat from the outside air in the heat-source side heat exchanger  12 , and becomes a low-temperature low-pressure refrigerant gas. The low-temperature low-pressure refrigerant gas that has flown out from the heat-source side heat exchanger  12  is sucked again by the compressor  10  through the first refrigerant channel switch unit  11  and the accumulator  19 . 
     Note that the refrigerant that has flown into the low-pressure side refrigerant pipe  4   b ( 1 ) flows into the second connection pipe  42   b  (the check valve  13   c ) but does not flow into the check valve  13   d . This is because the refrigerant that flows at a point P 1  illustrated in  FIG. 4  is a higher pressure refrigerant than the refrigerant that flows at a point P 3 , and thus the check valve  13   d  closes. 
     The refrigerant that has flown into the second connection pipe  42   b  flows into the heat-source side heat exchanger  12  but does not flow into the check valve  13   a . This is because the refrigerant that flows at a point P 4  illustrated in  FIG. 4  is a higher-pressure refrigerant than the refrigerant that flows at a point P 2 , and thus the check valve  13   a  closes. 
     The opening degree of the expansion unit  16   a  is controlled in such a manner that subcooling (a degree of subcooling) obtained as the difference between a saturation temperature, into which the pressure detected by the pressure sensor  36  is converted, and the temperature detected by the third temperature sensor  35   b  is constant. Similarly, the opening degree of the expansion unit  16   b  is controlled in such a manner that subcooling obtained as the difference between the saturation temperature, into which the pressure detected by the pressure sensor  36  is converted, and the temperature detected by the third temperature sensor  35   d  is constant. Note that, in the case where the temperature at a middle position of the heat exchangers related to heat medium  15  is measured, the temperature at the middle position may be used instead of using the pressure sensor  36 , thereby reducing the cost of the system. 
     Next, the flow of the heat medium in the heat medium circulation circuit B will be described. 
     In the heating only operation mode, the heating energy of the heat-source side refrigerant is transferred to the heat medium in both the heat exchanger related to heat medium  15   a  and the heat exchanger related to heat medium  15   b , and the heated heat medium is made to flow in the heat medium pipes  5  by the pump  21   a  and pump  21   b . The heat medium that has flown out by being compressed by the pump  21   a  and the pump  21   b  flows into the use-side heat exchanger  26   a  and the use-side heat exchanger  26   b  through the second heat medium channel switch unit  23   a  and the second heat medium channel switch unit  23   b . Then, the heat medium transfers heat to the room air in the use-side heat exchanger  26   a  and the use-side heat exchanger  26   b , and consequently, heating for the room space  7  is performed. 
     Then, the heat medium flows out from the use-side heat exchanger  26   a  and the use-side heat exchanger  26   b  and flows into the heat medium flow control device  25   a  and the heat medium flow control device  25   b . In this case, the amount of the flowing heat medium is controlled by the operation of the heat medium flow control device  25   a  and heat medium flow control device  25   b  so as to be the amount that is necessary to meet an air conditioning load that is needed in the room, and flows into the use-side heat exchanger  26   a  and the use-side heat exchanger  26   b . The heat medium that has flown out from the heat medium flow control device  25   a  and the heat medium flow control device  25   b  flows into the heat exchanger related to heat medium  15   a  and the heat exchanger related to heat medium  15   b  through the first heat medium channel switch unit  22   a  and the first heat medium channel switch unit  22   b , and is sucked again by the pump  21   a  and the pump  21   b.    
     Note that, in the heat medium pipes  5  of the use-side heat exchangers  26 , the heat medium flows in the direction from the second heat medium channel switch units  23  to the first heat medium channel switch units  22  via the heat medium flow control devices  25 . Moreover, the air conditioning load can meet an amount needed in the room space  7  by performing control in such a manner that the differences between the temperature detected by the first temperature sensor  31   a  and the temperatures detected by the second temperature sensors  34  or the differences between the temperature detected by the first temperature sensor  31   b  and the temperatures detected by the second temperature sensors  34  are to have a target value. As a temperature at the outlets of the heat exchangers related to heat medium  15 , either the temperature detected by the first temperature sensor  31   a  or that detected by the first temperature sensor  31   b  may be used, or the average of these temperatures may be used. 
     In this case, the first heat medium channel switch units  22  and the second heat medium channel switch units  23  have an intermediate opening degree in such a manner that the heat medium flows in both the channels for the heat exchanger related to heat medium  15   a  and the heat exchanger related to heat medium  15   b . Moreover, it is primarily desirable that the use-side heat exchanger  26   a  is controlled by the difference between the temperature at the inlet and the temperature at the outlet thereof; however, the temperatures of the heat medium on the inlet sides of the use-side heat exchangers  26  are almost the same as the temperature detected by the first temperature sensor  31   b . The number of temperature sensors may be reduced by using the first temperature sensor  31   b , thereby reducing the cost of the system. 
     [Cooling Main Operation Mode] 
       FIG. 5  is a diagram of a refrigerant circuit illustrating a flow of the refrigerant in the air-conditioning apparatus  100  in the cooling main operation mode. In  FIG. 5 , the cooling main operation mode is described in an example of the case where a cooling load is generated in the use-side heat exchanger  26   a  and a heating load is generated in the use-side heat exchanger  26   b . Note that, in  FIG. 5 , pipes drawn with a thick line indicate pipes in which the refrigerant (the heat-source side refrigerant and the heat medium) flows. Moreover, in  FIG. 5 , the direction in which the heat-source side refrigerant flows is indicated by arrows drawn with a solid line, and the direction in which the heat medium flows is indicated by arrows drawn with a broken line. 
     In the case of the cooling main operation mode illustrated in  FIG. 5 , the outdoor unit  1  makes the first refrigerant channel switch unit  11  perform switching in such a manner that the heat-source side refrigerant discharged from the compressor  10  flows into the heat-source side heat exchanger  12 . The heat medium relay unit  3  drives the pump  21   a  and the pump  21   b , opens the heat medium flow control device  25   a  and the heat medium flow control device  25   b , fully closes the heat medium flow control device  25   c  and the heat medium flow control device  25   d , and circulates the heat medium between the heat exchanger related to heat medium  15   a  and the use-side heat exchanger  26   a  and between the heat exchanger related to heat medium  15   b  and the use-side heat exchanger  26   b.    
     At first, the flow of the heat-source side refrigerant in the refrigerant circuit A is described. 
     A low-temperature low-pressure refrigerant is compressed by the compressor  10 , and becomes a high-temperature high-pressure refrigerant gas, which is then discharged. The high-temperature high-pressure refrigerant gas that has been discharged from the compressor  10  flows into the heat-source side heat exchanger  12  through the first refrigerant channel switch unit  11 . The high-temperature high-pressure refrigerant gas becomes a liquid refrigerant while transferring heat to the outside air in the heat-source side heat exchanger  12 . 
     The high-pressure liquid refrigerant that has flown out from the heat-source side heat exchanger  12  flows out from the outdoor unit  1  through the pipe that is provided with the check valve  13   a  and the high-pressure side refrigerant pipe  4   a ( 1 ), and flows into the heat medium relay unit  3  through the high-pressure side refrigerant pipe  4   a ( 2 ). The high-pressure liquid refrigerant that has flown into the heat medium relay unit  3  flows through the second refrigerant channel switch unit  18   b  and flows into the heat exchanger related to heat medium  15   b , which functions as a condenser. Note that the on-off units  17  are closed. 
     The refrigerant that has flown into the heat exchanger related to heat medium  15   b  becomes a further low-temperature refrigerant while transferring heat to the heat medium that circulates in the heat medium circulation circuit B. The refrigerant that has flown out from the heat exchanger related to heat medium  15   b  is expanded by the expansion unit  16   b  and becomes a low-pressure two-phase refrigerant. The low-pressure two-phase refrigerant flows into the heat exchanger related to heat medium  15   a , which functions as an evaporator, through the expansion unit  16   a . The low-pressure two-phase refrigerant that has flown into the heat exchanger related to heat medium  15   a  becomes a low-pressure refrigerant gas while cooling the heat medium that circulates in the heat medium circulation circuit B by absorbing heat from the heat medium. The refrigerant gas that has flown out from the heat exchanger related to heat medium  15   a  flows out from the heat medium relay unit  3  through the second refrigerant channel switch unit  18   a , and flows again into the outdoor unit  1  through the low-pressure side refrigerant pipe  4   b ( 2 ). The refrigerant that has flown into the outdoor unit  1  is sucked again by the compressor  10  through the low-pressure side refrigerant pipe  4   b ( 1 ); the pipe that is provided with the check valve  13   d ; the first refrigerant channel switch unit  11 ; and the accumulator  19 . 
     The opening degree of the expansion unit  16   b  is controlled in such a manner that superheating obtained as the difference between the temperature detected by the third temperature sensor  35   a  and the temperature detected by the third temperature sensor  35   b  is constant. Here, the expansion unit  16   a  is fully opened, the on-off unit  17   a  and the on-off unit  17   b  are closed. Note that the opening degree of the expansion unit  16   b  may be controlled in such a manner that subcooling obtained as the difference between a saturation temperature, into which the pressure detected by the pressure sensor  36  is converted, and the temperature detected by the third temperature sensor  35   d  is constant. Moreover, the expansion unit  16   b  may be fully opened, and superheating or subcooling may be controlled by the expansion unit  16   a.    
     Next, the flow of the heat medium in the heat medium circulation circuit B will be described. 
     In the cooling main operation mode, the heating energy of the heat-source side refrigerant is transferred to the heat medium in the heat exchanger related to heat medium  15   b , and the heated heat medium is made to flow in the heat medium pipes  5  by the pump  21   b . Moreover, in the cooling main operation mode, the cooling energy of the heat-source side refrigerant is transferred to the heat medium in the heat exchanger related to heat medium  15   a , and the cooled heat medium is made to flow in the heat medium pipes  5  by the pump  21   a . The heat medium that has flown out by being compressed by the pump  21   a  and the pump  21   b  flows into the use-side heat exchanger  26   a  and the use-side heat exchanger  26   b  through the second heat medium channel switch unit  23   a  and the second heat medium channel switch unit  23   b.    
     The heat medium transfers heat to the room air in the use-side heat exchanger  26   b , and consequently, heating for the room space  7  is performed. Moreover, the heat medium removes heat from the room air in the use-side heat exchanger  26   a , and consequently, cooling for the room space  7  is performed. In this case, the amount of the flowing heat medium is controlled by the operation of the heat medium flow control device  25   a  and heat medium flow control device  25   b  so as to be the amount that is necessary to meet an air conditioning load that is needed in the room, and flows into the use-side heat exchanger  26   a  and the use-side heat exchanger  26   b . The heat medium that has flown through the use-side heat exchanger  26   b  and whose temperature has slightly decreased flows into the heat exchanger related to heat medium  15   b  through the heat medium flow control device  25   b  and the first heat medium channel switch unit  22   b , and is sucked again by the pump  21   b . The heat medium that has flown through the use-side heat exchanger  26   a  and the whose temperature has slightly increased flows into the heat exchanger related to heat medium  15   a  through the heat medium flow control device  25   a  and the first heat medium channel switch unit  22   a , and is sucked again by the pump  21   a.    
     During this time, the warm heat medium is not mixed with the cold heat medium by the operation of the first heat medium channel switch units  22  and second heat medium channel switch units  23 . The warm heat medium is guided to the use-side heat exchanger  26  that has a heating load, and the cold heat medium is guided to the use-side heat exchanger  26  that has a cooling load. Note that, in the heat medium pipes  5  of the use-side heat exchangers  26 , the heat medium flows in the direction from the second heat medium channel switch units  23  to the first heat medium channel switch units  22  via the heat medium flow control devices  25  on both heating and cooling sides. Moreover, the air conditioning load can meet an amount needed in the room space  7  by performing control in such a manner that the differences between the temperature detected by the first temperature sensor  31   b  and the temperatures detected by the second temperature sensors  34  on the heating side and the differences between the temperatures detected by the second temperature sensors  34  and the temperature detected by the first temperature sensor  31   a  on the cooling side are kept as a target value. 
     [Heating Main Operation Mode] 
       FIG. 6  is a diagram of a refrigerant circuit illustrating a flow of the refrigerant in the air-conditioning apparatus  100  in the heating main operation mode. In  FIG. 6 , the heating main operation mode is described in an example of the case where a heating load is generated in the use-side heat exchanger  26   a  and a cooling load is generated in the use-side heat exchanger  26   b . Note that, in  FIG. 6 , pipes drawn with a thick line indicate pipes in which the refrigerant (the heat-source side refrigerant and the heat medium) flows. Moreover, in  FIG. 6 , the direction in which the heat-source side refrigerant flows is indicated by arrows drawn with a solid line, and the direction in which the heat medium flows is indicated by arrows drawn with a broken line. 
     In the case of the heating main operation mode illustrated in  FIG. 6 , the outdoor unit  1  makes the first refrigerant channel switch unit  11  perform switching in such a manner that the heat-source side refrigerant discharged from the compressor  10  flows into the heat medium relay unit  3  without flowing through the heat-source side heat exchanger  12 . The heat medium relay unit  3  drives the pump  21   a  and the pump  21   b , opens the heat medium flow control device  25   a  and the heat medium flow control device  25   b , fully closes the heat medium flow control device  25   c  and the heat medium flow control device  25   d , and circulates the heat medium between the heat exchanger related to heat medium  15   a  and the use-side heat exchanger  26   b  and between the heat exchanger related to heat medium  15   b  and the use-side heat exchanger  26   a.    
     At first, the flow of the heat-source side refrigerant in the refrigerant circuit A is described. 
     A low-temperature low-pressure refrigerant is compressed by the compressor  10 , and becomes a high-temperature high-pressure refrigerant gas, which is then discharged. The high-temperature high-pressure refrigerant gas that has been discharged from the compressor  10  flows out from the outdoor unit  1  through the first refrigerant channel switch unit  11 , the first connection pipe  42   a , and the high-pressure side refrigerant pipe  4   a ( 1 ). The high-temperature high-pressure refrigerant gas that has flown out from the outdoor unit  1  flows into the heat medium relay unit  3  through the high-pressure side refrigerant pipe  4   a ( 2 ). The high-temperature high-pressure refrigerant gas that has flown into the heat medium relay unit  3  flows into the heat exchanger related to heat medium  15   b , which functions as a condenser, through the second refrigerant channel switch unit  18   b . Note that the on-off units  17  are closed. 
     The refrigerant gas that has flown into the heat exchanger related to heat medium  15   b  becomes a low-temperature refrigerant in a supercritical state while the refrigerant gas transfers heat to the heat medium that circulates in the heat medium circulation circuit B. The refrigerant that has flown out from the heat exchanger related to heat medium  15   b  is expanded by the expansion unit  16   b  and becomes a low-pressure two-phase refrigerant. The low-pressure two-phase refrigerant flows into the heat exchanger related to heat medium  15   a , which functions as an evaporator, through the expansion unit  16   a . The low-pressure two-phase refrigerant that has flown into the heat exchanger related to heat medium  15   a  evaporates by absorbing heat from the heat medium that circulates in the heat medium circulation circuit B, and consequently, cools the heat medium. The low-pressure two-phase refrigerant flows out from the heat exchanger related to heat medium  15   a , flows out from the heat medium relay unit  3  through the second refrigerant channel switch unit  18   a , and flows again into the outdoor unit  1  through the low-pressure side refrigerant pipe  4   b ( 2 ). 
     The refrigerant that has flown into the outdoor unit  1  flows into the heat-source side heat exchanger  12 , which functions as an evaporator, through the low-pressure side refrigerant pipe  4   b ( 1 ) and the second connection pipe  42   b . The refrigerant that has flown into the heat-source side heat exchanger  12  removes heat from the outside air in the heat-source side heat exchanger  12 , and consequently, becomes a low-temperature low-pressure refrigerant gas. The low-temperature low-pressure refrigerant gas that has flown out from the heat-source side heat exchanger  12  is sucked again by the compressor  10  through the first refrigerant channel switch unit  11  and the accumulator  19 . 
     The opening degree of the expansion unit  16   b  is controlled in such a manner that subcooling obtained as the difference between a saturation temperature, into which the pressure detected by the pressure sensor  36  is converted, and the temperature detected by the third temperature sensor  35   b  is constant. Here, the expansion unit  16   a  is fully opened, and the on-off unit  17   b  is closed. Note that the expansion unit  16   b  may be fully opened and subcooling may be controlled by the expansion unit  16   a.    
     Next, the flow of the heat medium in the heat medium circulation circuit B will be described. 
     In the heating main operation mode, the heating energy of the heat-source side refrigerant is transferred to the heat medium in the heat exchanger related to heat medium  15   b , and the heated heat medium is made to flow in the heat medium pipes  5  by the pump  21   b . Moreover, in the heating main operation mode, the cooling energy of the heat-source side refrigerant is transferred to the heat medium in the heat exchanger related to heat medium  15   a , and the cooled heat medium is made to flow in the heat medium pipes  5  by the pump  21   a . The heat medium that has flown out by being compressed by the pump  21   a  and the pump  21   b  flows into the use-side heat exchanger  26   a  and the use-side heat exchanger  26   b  through the second heat medium channel switch unit  23   a  and the second heat medium channel switch unit  23   b.    
     The heat medium removes heat from the room air in the use-side heat exchanger  26   b , and consequently, cooling for the room space  7  is performed. Moreover, the heat medium transfers heat to the room air in the use-side heat exchanger  26   a , and consequently, heating for the room space  7  is performed. In this case, the amount of the heat medium that flows is controlled by the operation of the heat medium flow control device  25   a  and heat medium flow control device  25   b  so as to be the amount that is necessary to meet an air conditioning load that is needed in the room, and flows into the use-side heat exchanger  26   a  and the use-side heat exchanger  26   b . The heat medium that has flown through the use-side heat exchanger  26   b  and the whose temperature has slightly increased flows into the heat exchanger related to heat medium  15   a  through the heat medium flow control device  25   b  and the first heat medium channel switch unit  22   b , and is sucked again by the pump  21   a . The heat medium that has flown through the use-side heat exchanger  26   a  and the whose temperature has slightly decreased flows into the heat exchanger related to heat medium  15   b  through the heat medium flow control device  25   a  and the first heat medium channel switch unit  22   a , and is sucked again by the pump  21   b.    
     During this time, the warm heat medium is not mixed with the cold heat medium by the operation of the first heat medium channel switch units  22  and second heat medium channel switch units  23 . The warm heat medium is guided to the use-side heat exchanger  26  that has a heating load, and the cold heat medium is guided to the use-side heat exchanger  26  that has a cooling load. Note that, in the heat medium pipes  5  of the use-side heat exchangers  26 , the heat medium flows in the direction from the second heat medium channel switch units  23  to the first heat medium channel switch units  22  via the heat medium flow control devices  25  on both heating and cooling sides. Moreover, the air conditioning load can meet an amount needed in the room space  7  by performing control in such a manner that the differences between the temperature detected by the first temperature sensor  31   b  and the temperatures detected by the second temperature sensors  34  on the heating side and the differences between the temperatures detected by the second temperature sensors  34  and the temperature detected by the first temperature sensor  31   a  on the cooling side are kept as a target value. 
     [Refrigerant Pipes  4 ] 
     As described above, the air-conditioning apparatus  100  according to Embodiment is provided with some operation modes. In these operation modes, the heat-source side refrigerant flows in the refrigerant pipes  4  that connect the outdoor unit  1  and the heat medium relay unit  3 . 
     [Heat Medium Pipes  5 ] 
     In some of the operation modes performed by the air-conditioning apparatus  100  according to Embodiment, a heat medium such as water and an antifreezing solution flows in the heat medium pipes  5  that connect the heat medium relay unit  3  and the indoor units  2 . 
     [Heat-Source Side Refrigerant] 
     In the air-conditioning apparatus  100  according to Embodiment, a refrigerant that has a small global warming potential is used as the heat-source side refrigerant. Examples of such a refrigerant include HFO1234yf, refrigerant mixture using HFO1234yf, HFO1234ze, other tetrafluoropropene-based refrigerants, R32, HC, carbon dioxide, a refrigerant mixture containing at least one of the above-described refrigerants as a component, and the like. 
     [Heat Medium] 
     As the heat medium, for example, brine (an antifreezing solution), water, a mixed solution of brine and water, a mixed solution in which water and an additive that has a high anti-corrosive effect are mixed, and the like may be used. Thus, in the air-conditioning apparatus  100 , even in the case where the heat medium leaks into the room space  7  through the indoor unit  2 , since the heat medium with a high degree of safety is used, the degree of safety is improved. 
     The air-conditioning apparatus  100  which employs the above-described structure makes it possible to detect leaking of the refrigerant from the refrigerant circuit (the refrigerant circuit A), and consequently, the degree of safety is greatly improved. Moreover, by using a refrigerant that can transition into a supercritical state, the air-conditioning apparatus  100  can reduce the environmental load. 
     The air-conditioning apparatus  100  has been described as an air-conditioning apparatus that can perform the cooling and heating mixed operation; however, the air-conditioning apparatus  100  is not limited thereto. For example, the present invention may be applied to air-conditioning apparatuses that have a structure in which one heat exchanger related to heat medium  15  and one expansion unit  16  are included and a plurality of use-side heat exchangers  26  and a plurality of heat medium flow control devices  25  are connected to the heat exchanger related to heat medium  15  and the expansion unit  16  in parallel and that may only perform either the cooling operation or the heating operation. 
     Moreover, the present invention may be applied to the case in which one use-side heat exchanger  26  and one heat medium flow control device  25  are connected. Furthermore, it is clear that, a plurality of units that function the same as the heat exchangers related to heat medium  15  and the expansion units  16  may be provided. Furthermore, the example in which the heat medium flow control devices  25  are built in the heat medium relay unit  3  has been described; however, the structure is not limited thereto and the heat medium flow control devices  25  may be built in the indoor units  2 . The heat medium relay unit  3  and the indoor units  2  may also be separately provided. 
     Moreover, in general, the heat-source side heat exchanger  12  and the use-side heat exchangers  26  are equipped with an air-sending device to accelerate condensation and evaporation by blowing air; however, the structure is not limited thereto. For example, something like a panel heater that utilizes eradiation may also be used as the use-side heat exchangers  26  or water-cooled type heat exchangers in which heat is transferred by using water or an antifreezing solution may also be used as the heat-source side heat exchanger  12 . That is, a structure that can transfer or remove heat, regardless of the kind of units, may be used as the heat-source side heat exchanger  12  and the use-side heat exchangers  26 . 
     In the air-conditioning apparatus  100  according to Embodiment, the examples in which four use-side heat exchangers  26  are used has been described; however the number of the use-side heat exchangers  26  is not limited thereto. Moreover, the examples in which two heat exchangers related to heat medium  15  are used has been described; however, as a matter of course, the number of the heat exchangers related to heat medium  15  is not limited thereto. If the air-conditioning apparatus  100  is configured to be able to cool or/and heat the heat medium, the number of the heat exchangers related to heat medium  15  does not matter. Furthermore, the number of the pump  21   a  and that of the pump  21   b  do not have to be one, and a plurality of pumps, each of which has a small capacity, may be arranged and connected in parallel. 
       FIG. 8  is a diagram illustrating an example of the structure of another refrigerant circuit of the air-conditioning apparatus  100  according to Embodiment of the present invention. A difference between  FIG. 8  and  FIGS. 2 to 6  is that a direct expansion type air-conditioning apparatus that does not have the heat medium circulation circuit B is used. Furthermore, four expansion units  14   a  to  14   d  are provided. Even in the air-conditioning apparatus  100  having this structure, the effects similar in the cases of  FIGS. 2 to 6  are obtained by the operation of the concentration detection unit  39 , the concentration calculation unit  41 , the first cutoff unit  37 , the second cutoff unit  38  and the cutoff control unit  40 . 
     Note that the examples in which the concentration detection unit  39  is arranged in or near the outdoor unit  1  (a heat source unit) have been described; however, the concentration detection unit  39  may be arranged in or near the indoor units  2 . As a result, leaking of the refrigerant from the indoor units  2  may also be handled.