DESCRIPTION AIR CONDITIONER Technical Field [0001] The present invention relates to an air conditioner in which an indoor heat exchanger and a radiation panel are disposed in parallel. Background Art [0002] As an air conditioner, one provided with an indoor unit having an indoor heat exchanger and a radiation panel, and an outdoor unit that supplies and circulates a refrigerant to the indoor heat exchanger and the radiation panel is known (e.g., PTL 1) . In a refrigerant circuit of this air conditioner, a channel provided with the indoor heat exchanger and a channel provided with the radiation panel are disposed in parallel, and an expansion valve (decompression structure) for adjusting the pressure in the refrigerant circuit is provided in each of these channels. Citation List Patent Literature [00031 PTL 1: Japanese Unexamined Patent Publication No. 280762/1993 1 (Tokukaihei 5-280762) Summary of Invention Technical Problem [0004] However, for example, controlling a discharge temperature of a compressor in the air conditioner requires controlling of two expansion valves provided in the two channels described above, and this made control complicated. Therefore, it has been difficult to perform proper control in a short time. [00051 In view of this, it is an object of the present invention to provide an air conditioner that is easily controlled. Solution to Problem [00061 An air conditioner related to a first aspect of the present invention includes: an indoor unit, an outdoor unit, and a refrigerant circuit connecting the indoor unit with the outdoor unit, wherein the indoor unit having an indoor heat exchanger provided in the indoor unit so as to oppose a fan, anda radiation panel provided on a surface of the indoor unit, wherein the refrigerant circuit includes a principal channel in which a decompression structure, an outdoor heat exchanger, and a compressor are provided in this order; a first channel provided with the indoor heat exchanger, which connects a branching 2 section and a merging section that are on the downstream side and on the upstream side of the compressor in the principal channel, respectively, during a heating operation; and a second channel provided with a radiation panel, which connects the branching section and the merging section in parallel to the first channel, during the heating operation, and wherein the first channel or the second channel is provided with a valve structure. [0007] With the air conditioner, simply by controlling the decompression structure provided in the principal channel, the pressure in the refrigerant circuit is depressurized. Therefore, control (e.g., control based on the discharge temperature of the compressor, or the like) is made easier as compared with a case in which the decompression structure is provided in the first channel and the second channel. Further, since the valve structure is provided in the first channel or the second channel, the flow rate of the refrigerant flowing in the indoor heat exchanger or the radiation panel can be adjusted. [00081 An air conditioner related to a second aspect of the present invention is an air conditioner of the first aspect adapted so that the valve structure is provided in the second channel. 3 [00091 In the air conditioner, the valve structure is provided in the second channel. Therefore, the flow rate of the refrigerant flowing in the radiation panel can be adjusted. Further, by closing the valve structure, it is possible to cause the refrigerant to flow not in the radiation panel, but only in the indoor heat exchanger. [0010] An air conditioner related to a third aspect of the present invention is an air conditioner of the second aspect adapted so that the valve structure is on the downstream side of the radiation panel in the second channel during the heating operation. [0011] In the air conditioner, the valve structure is provided on the downstream side of the radiation panel, relative to the flowing direction of the refrigerant during the heating operation. It is therefore possible to lower the temperature of the refrigerant passing the valve structure as compared with a case of providing the valve structure on the upstream side of the radiation panel. It is therefore possible to improve the durability of the valve structure. Further, when the valve structure is closed to perform the cooling operation, it is possible to prevent a low-temperature refrigerant from flowing into the radiation panel. Therefore, dew condensation on the 4 radiation panel is prevented. [0012] An air conditioner related to a fourth aspect of the present invention is an air conditioner of any one of the first to third aspects adapted so that the outdoor unit has the compressor, the outdoor heat exchanger, and the decompression structure, and the indoor unit has the valve structure. [0013] With the air conditioner, the decompression structure is provided in the outdoor unit. Therefore, sound accompanied by switching operation of the decompression structure is not sensible inside the room. In short, it is possible to prevent the noise when performing the switching operation of the decompression structure. Advantageous Effects of Invention [0014] As hereinabove described, the present invention brings about the following effects. [0015] With the first aspect of the present invention, simply by controlling the decompression structure provided in the principal channel, the pressure in the refrigerant circuit is depressurized. Therefore, control (e.g., control based on the discharge temperature of the compressor, or the like) is made 5 easier as compared with a case in which the decompression structure is provided in the first channel and the second channel. Further, since the valve structure is provided in the first channel or the second channel, the flow rate of the refrigerant flowing in the indoor heat exchanger or the radiation panel can be adjusted. [0016] In the second aspect of the present invention, the valve structure is provided in the second channel. Therefore, the flow rate of the refrigerant flowing in the radiation panel can be adjusted. Further, by closing the valve structure, it is possible to cause the refrigerant to flow not in the radiation panel, but only in the indoor heat exchanger. [0017] In the third aspect of the present invention, the valve structure is provided on the downstream side of the radiation panel, relative to the flowing direction of the refrigerant during the heating operation. It is therefore possible to lower the temperature of the refrigerant passing the valve structure as compared with a case of providing the valve structure on the upstream side of the radiation panel. It is therefore possible to improve the durability of the valve structure. Further, when the valve structure is closed to perform the cooling operation, it is possible to prevent a low-temperature refrigerant from flowing into the radiation 6 panel. Therefore, dew condensation on the radiation panel is prevented. [0018] With the fourth aspect of the present invention, the decompression structure is provided in the outdoor unit. Therefore, sound accompanied by switching operation of the decompression structure is not sensible inside the room. In short, it is possible to prevent the noise when performing the switching operation of the decompression structure. Brief Description of Drawings [0019] Fig. 1 is a circuit diagram illustrating a schematic configuration of an air conditioner according to an embodiment of the present invention, and is a view illustrating a flow of a refrigerant when an indoor motor-operated valve is opened to perform a cooling operation or a heating operation. Fig. 2 is a circuit diagram illustrating the schematic configuration of the air conditioner according to the embodiment of the present invention, and is a view illustrating the flow of the refrigerant when the indoor motor-operated valve is opened to perform the heating operation. Fig. 3 is a block diagram illustrating a schematic configuration of a controller that controls the air conditioner. 7 Fig. 4 is a circuit diagram illustrating a schematic configuration of an air conditioner according to another embodiment of the present invention. Description of Embodiments [0020] Hereinafter, an air conditioner 1 according to an embodiment of the present invention will be described. [0021] As illustrated in Figs. 1 and 2, the air conditioner 1 of the embodiment includes an indoor unit 2 that is installed inside a room, an outdoor unit 3 that is installed outside the room, and a remote controller 4 (see Fig. 3). The indoor unit 2 includes an indoor heat exchanger 20, an indoor fan 21 that is disposed near the indoor heat exchanger 20, a radiation panel 22, an indoor motor-operated valve (a valve structure) 23, and an indoor temperature sensor 24 that detects an indoor temperature. The outdoor unit 3 includes a compressor 30, a four-way valve 31, an outdoor heat exchanger 32, an outdoor fan 33 that is disposed near the outdoor heat exchanger 32, and an outdoor motor-operated valve (a decompression structure) 34. The indoor unit 2 and the outdoor unit 3 are connected to each other by a circular refrigerant circuit 10. The refrigerant circuit 10 includes a principal channel 11, a first channel 12, and a second channel 13. [0022] 8 The outdoor motor-operated valve 34, the outdoor heat exchanger 32, and the compressor 30 are provided in this order in the principal channel 11. The four-way valve 31 is provided in the principal channel 11, and one of a discharge side and an intake side of the compressor 30 is connected to the outdoor heat exchanger 32 by switching the four-way valve 31. In the principal channel 11, an accumulator 35 is provided between the intake side of the compressor 30 and the four-way valve 31, and a discharge temperature sensor 36 is provided between the discharge side of the compressor 30 and the four-way valve 31. An outdoor heat exchanger temperature sensor 28 is attached to the outdoor heat exchanger 32. An opening degree of the outdoor motor-operated valve 34 can be changed, and the outdoor motor-operated valve 34 serves as the decompression structure. In the principal channel 11, when the intake side of the compressor 30 is connected to the outdoor heat exchanger 32 (during the heating operation in Fig. 2), a branching section 11a is on a downstream side of the compressor 30, and a merging section lb is on an upstream side of the outdoor motor-operated valve 34. [0023] The first channel 12 and the second channel 13 are provided between the branching section 11a and the merging section 11b, and connected in parallel. The indoor heat exchanger 20 is provided in the first channel 12, and the radiation panel 22 9 and the indoor motor-operated valve 23 are provided in the second channel 13 in the order from the side of the branching section 11a. In the embodiment, a channel between the branching section 11a and the merging section lb, excluding the first channel 12 and the second channel 13, constitutes the principal channel in the refrigerant circuit 10. [0024] The indoor heat exchanger 20 is provided so as to be opposed to the indoor fan 21 in the indoor unit 2, and the indoor heat exchanger 20 is disposed on a windward side of the indoor fan 21. Accordingly, in the indoor unit 2, air heated or cooled by heat exchange with the indoor heat exchanger 20 is blown into the room as warm air or cool air by the indoor fan 21, therebyperforming warm-air heating or cooling. Anindoorheat exchanger temperature sensor 27 is provided in the indoor heat exchanger 20. [0025] The radiation panel 22 is disposed on a surface of the indoor unit 2, and a pipe fitting in which the refrigerant flows is provided on a rear side of the radiation panel 22. Accordingly, in the indoor unit 2, heat of the refrigerant flowing in the pipe fitting of the radiation panel 22 is radiated into the room, thereby performing radiation heating. A panel incoming temperature sensor 25 and a panel outgoing temperature 10 sensor 26 are provided on both sides of the radiation panel 22 in the second channel 13, respectively. [0026] The indoor motor-operated valve 23 is provided in order to adjust a flow rate of the refrigerant supplied to the radiation panel 22. The indoor motor-operated valve 23 is on the downstream side of the radiation panel 22 in a refrigerant flowing direction during a radiation heating operation and a radiation breeze heating operation. [0027] The air conditioner 1 of the embodiment can perform a cooling operation, a warm-air heating operation, the radiation heating operation, and the radiation breeze heating operation. The cooling operation is an operation in which the cooling is performed by causing the refrigerant to flow not in the radiation panel 22 but in the indoor heat exchanger 20. The warm-air heating operation is an operation in which the warm-air heating is performed by causing the refrigerant to flow not in the radiation panel 22 but in the indoor heat exchanger 20. The radiation heating operation is an operation in which the radiation heating is performed by causing the refrigerant to flow in the radiation panel 22 while the warm-air heating is performed by causing the refrigerant to flow in the indoor heat exchanger 20. The radiation breeze heating operation is an operation in which the radiation heating is 11 performed by causing the refrigerant to flow in the radiation panel 22 while the warm-air heating is performed by a constant air quantity lower than an air quantity during the warm-air heating operation and radiation heating operation. [0028] A flow of the refrigerant in the refrigerant circuit 10 during each operation will be described with reference to Figs. 1 and 2. During the cooling operation, the indoor motor-operated valve 23 is closed, and the four-way valve 31 is switched to a state indicated by a broken line in Fig. 1. Therefore, as indicated by a broken-line arrow in Fig. 1, the high-temperature, high-pressure refrigerant discharged from the compressor 30 flows in the outdoor heat exchanger 32 through the four-way valve 31. The refrigerant condensed by the outdoor heat exchanger 32 flows in the indoor heat exchanger 20 after being decompressed by the outdoor motor-operated valve 34. The refrigerant vaporized in the indoor heat exchanger 20 flows in the compressor 30 through the four-way valve 31 and accumulator 35. [0029] During the warm-air heating operation, the indoor motor-operated valve 23 is closed, and the four-way valve 31 is switched to a state indicated by a solid line in Fig. 1. Therefore, as indicated by a solid-line arrow in Fig. 1, the 12 high-temperature, high-pressure refrigerant discharged from the compressor 30 flows in the indoor heat exchanger 20 through the four-way valve 31. The refrigerant condensed by the indoor heat exchanger 20 flows in the outdoor heat exchanger 32 after being decompressedby the outdoor motor-operated valve 34. The refrigerant vaporized in the outdoor heat exchanger 32 flows in the compressor 30 through the four-way valve 31 and accumulator 35. [00301 During the radiation heating operation and the radiation breeze heating operation, the indoor motor-operated valve 23 is opened, and the four-way valve 31 is switched to a state indicated by a solid line in Fig. 2. Therefore, as indicated by a solid-line arrow in Fig. 2, the high-temperature, high-pressure refrigerant discharged from the compressor 30 flows in the indoor heat exchanger 20 and radiation panel 22 through the four-way valve 31. The refrigerant condensed by the indoor heat exchanger 20 and radiation panel 22 flows in the outdoor heat exchanger 32 after being decompressed by the outdoor motor-operated valve 34. The refrigerant vaporized in the outdoor heat exchanger 32 flows in the compressor 30 through the four-way valve 31 and accumulator 35. [0031] Using the remote controller 4, a user performs a manipulation of operation start/stop, a setting of an operation 13 mode, a setting of a target temperature (an indoor setting temperature) of an indoor temperature, a setting of a blowing air quantity, and the like. During the warm-air heating operation and the cooling operation, "air quantity automatic" or "strong" to "weak" can be selected as the air quantity setting. In the embodiment, during the radiation heating operation and the radiation breeze heating operation, the air quantity is automatically controlled. [0032] A controller 5 that controls the air conditioner 1 will be described below with reference to Fig. 3. As illustrated in Fig. 3, the controller 5 includes a storage (storage means) 50, an indoor motor-operated valve controller 52, an indoor fan controller 53, a compressor controller (control means) 54, and an outdoor motor-operated valve controller 55. [00331 Various operation settings for the air conditioner 1, a control program, a data table necessary to execute the control program, and the like are stored in the storage 50. The operation settings include those, such as the target temperature (the indoor setting temperature) of the indoor temperature, which are set such that the user manipulates the remote controller 4 and those previously set to the air conditioner 1. In the air conditioner 1 of the embodiment, 14 a target temperature range of the radiation panel 22 is previously set to a given temperature range (for example, 50 to 550C) . The target temperature range of the radiation panel 22 may be set by the manipulation of the remote controller 4. [0034] The indoor motor-operated valve controller 52 controls the opening degree of the indoor motor-operated valve 23. During the cooling operation or the warm-air heating operation, the indoor motor-operated valve controller 52 closes the indoor motor-operated valve 23. During the radiation heating operation or the radiation breeze heating operation, the indoor motor-operated valve controller 52 controls the opening degree of the indoor motor-operated valve 23 based on the temperature at the radiation panel 22. Specifically, based on an average value of the temperatures detected by the panel incoming temperature sensor 25 and panel outgoing temperature sensor 26, the indoor motor-operated valve controller 52 calculates a surface temperature (a predicted value) at the radiation panel 22, and the indoor motor-operated valve controller 52 controls the opening degree of the indoor motor-operated valve 23 such that the predicted value (hereinafter simply referred to as a radiation panel temperature) of the surface temperature at the radiation panel 22 falls within the panel target temperature range (for example, 50 to 550C). In the embodiment, the temperatures detected by both the panel incoming 15 temperature sensor 25 and panel outgoing temperature sensor 26 are used to calculate the radiation panel temperature. Alternatively, the temperatures detected only by the panel incoming temperature sensor 25 may be used or the temperatures detected only by the panel outgoing temperature sensor 26 may be used. [00351 The indoor fan controller 53 controls a rotation speed of the indoor fan 21. During an air-quantity automatic operation in the warm-air heating operation and cooling operation or during the radiation heating operation, the indoor fan controller 53 controls the rotation speed of the indoor fan 21 based on the indoor temperature detected by the indoor temperature sensor 24 or the indoor setting temperature. The indoor fan 21 is controlled at the rotation speed corresponding to a previously-set fan tap, in the case that "strong" to "weak" are set to the air quantity setting during the warm-air heating operation or cooling operation, or in the case of the radiation breeze heating operation. [00361 The compressor controller 54 controls an operation frequency based on the indoor temperature, the indoor setting temperature, a heat exchanger temperature detected by the indoor heat exchanger temperature sensor 27, and the like. 16 [0037] The outdoor motor-operated valve controller 55 controls the opening degree of the outdoor motor-operated valve 34. Particularly, the outdoor motor-operated valve controller 55 controls the opening degree of the outdoor motor-operated valve 34 such that the temperature detected by the discharge temperature sensor 36 becomes an optimum temperature in the operation state. The optimum temperature is determined based on the temperature detected by the indoor heat exchanger temperature sensor 27, the temperature detected by the outdoor heat exchanger temperature sensor 28, and the like. [00381 According to the air conditioner 1 of the embodiment described above, pressure in the refrigerant circuit 10 can be depressed only by controlling the decompression structure (outdoor motor-operated valve) 34 provided in the principal channel 11. Therefore, the control can easily be performed compared with the case that the decompression structure is provided in each of the first channel 12 and the second channel 13. [00391 In the embodiment, the indoor motor-operated valve 23 is provided in the second channel 13. Therefore, the flow rate of the refrigerant flowing in the radiation panel 22 can be adjusted. By closing the indoor motor-operated valve 23, the 17 refrigerant flows not in the radiation panel 22 but only in the indoor heat exchanger 20. [0040 ] In the embodiment, the indoor motor-operated valve 23 is on the downstream side of the radiation panel 22 with respect to the refrigerant flowing direction during the radiation heating operation and radiation breeze heating operation. Accordingly, the temperature of the refrigerant passing through the indoor motor-operated valve 23 can be lowered compared with the case that the indoor motor-operated valve 23 is provided on the upstream side of the radiation panel 22. Therefore, durability of the indoor motor-operated valve 23 can be improved. The flow of the low-temperature refrigerant in the radiation panel 22 can completely be blocked when the indoor motor-operated valve 23 is closed to perform the cooling operation, so that dew condensation of the radiation panel 22 can be prevented. [0041] In a traditional air conditioner having the decompression structure in the indoor unit, in which the decompression structure is disposed in each of the channel having the radiation panel and the channel having the indoor heat exchanger, there is a problem that a noise caused by the switching of the decompression structure is generated in the room. It is possible to provide, in the outdoor unit, the 18 merging section of the channel in which the radiation panel is provided and the channel in which the indoor heat exchanger is provided, and provide the decompression structure in the outdoor unit. However, in this case, the number of pipe fittings connecting the indoor unit and the outdoor unit is increased. On the other hand, in the embodiment, because the outdoor motor-operated valve 34 is provided in the principal channel 11, the outdoor motor-operated valve 34 can be provided in the outdoor unit 3 without increasing the number of pipe fittings connecting the indoor unit 2 and the outdoor unit 3. For this reason, the user hardly hears the noise caused by the switching of the outdoor motor-operated valve 34. That is, the noise can be prevented in the room during the switching of the outdoor motor-operated valve 34. [0 0 42] The embodiment of the present invention is described above with reference to the drawings. However, it is noted that the specific configuration is not limited to the embodiment. The scope of the present invention is determined by not the description of the embodiment but claims of the present invention, and all modifications on meanings and ranges equivalent to the claims of the present invention are also included in the present invention. [0043] 19 In the above embodiment, the indoor motor-operated valve 23 is on the downstream side of the radiation panel 22 in the refrigerant flowing direction during the radiation heating operation and radiation breeze heating operation. Alternatively, the indoor motor-operated valve 23 may be on the upstream side of the radiation panel 22. [0044] In the above embodiment, the air conditioner 1 can perform the warm-air heating operation in which the warm-air heating is performed by causing the refrigerant to flow not in the radiation panel 22 but in the indoor heat exchanger 20. In the case that the warm-air heating operation is not performed, like an air conditioner 101 in Fig. 4, a check valve 129 may be provided between the radiation panel 22 and merging section lb in a second channel 113 instead of providing the indoor motor-operated valve 23 in the second channel 13, while an indoor motor-operated valve 123 is provided in a first channel 112. In the second channel 113, the check valve 129 causes the refrigerant to flow not toward the radiation panel 22 from the merging section lb but only toward the merging section lb from the radiation panel 22. The solid-line arrow in Fig. 4 indicates the flow of the refrigerant during the radiation heating operation or radiation breeze heating operation, and the broken-line arrow in Fig. 4 indicates the flow of the refrigerant during the cooling operation. The flow rate of 20 the refrigerant supplied to the indoor heat exchanger 20 can be adjusted by the indoor motor-operated valve 123. Additionally, when the indoor motor-operated valve 123 is closed, only the radiation heating can be performed by causing the refrigerant to flow not in the indoor heat exchanger 20 but in the radiation panel 22. During the cooling operation, the check valve 129 can prevent the low-temperature refrigerant from flowing in the radiation panel 22. In Fig. 4, the indoor motor-operated valve 123 is provided on the side of the merging section lb of the indoor heat exchanger 20. Alternatively, the indoor motor-operated valve 123 may be provided on the side of the branching section 11a. INDUSTRIAL APPLICABILITY [0045] The use of the present invention can easily control the air conditioner. DESCRIPTION OF REFERENCE SIGNS [0046] 1 air conditioner 2 indoor unit 3 outdoor unit 20 indoor heat exchanger 22 radiation panel 21 23 indoor motor-operated valve (valve structure) 30 compressor 32 outdoor heat exchanger 34 outdoor motor-operated valve (decompression structure) 22