Patent Publication Number: US-2015067970-A1

Title: Washing machine and control method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Korean Patent Application No. 10-2013-0108819, filed on Sep. 11, 2013 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     Embodiments of the present disclosure relate to a washing machine having a boiling washing course function to perform washing by heating water in order to increase a washing effect, and a control method thereof. 
     2. Description of the Related Art 
     In general, a washing machine (e.g., fully automatic washing machine) includes a tub to accommodate water (washing water or rinsing water), a washing tub rotatably installed inside the tub to accommodate the laundry and a pulsator rotatably installed inside the washing tub to generate a water flow, and a motor to generate driving force for rotating the washing tub and the pulsator. The washing machine is an apparatus for removing stains from laundry by surface activity of a detergent and a water flow. 
     This washing machine performs washing by a series of operations such as a washing process to separate stains from the laundry using water (specifically, washing water) in which a detergent is dissolved, a rinsing process to rinse a residual detergent or bubbles out of the laundry using water (specifically, rinsing water) which does not contain a detergent, and a spin-dry process to remove water contained in the laundry by high speed rotation. 
     Among the above-mentioned processes, the washing process has a boiling washing course function to perform washing by heating supplied water in order to increase a washing effect of contaminated laundry. 
     In a conventional boiling washing course, initially supplied water is heated and the water is unconditionally drained when the temperature (water temperature) of the water reaches a set temperature. Then, after the water level is reduced to be lower than the level of the initially supplied water, the water is heated up to a target temperature. This is intended to reduce electrical energy consumed for heating the water because plenty of water is used due to the nature of the fully automatic washing machine. 
     However, in the conventional boiling washing course, since the water is unconditionally drained although the level of the initially supplied water is the amount of washing water suitable for the laundry, the amount of the detergent and the initially supplied water for washing the contaminated laundry decreases, and the washing effect may be reduced. 
     SUMMARY 
     Therefore, it is an aspect of the present disclosure to provide a washing machine capable of increasing a washing effect by heating initially supplied water for a predetermined time, determining whether to drain water according to the temperature of the water, and maintaining the amount of the initially supplied water without unconditionally draining the water, and a control method thereof. 
     Additional aspects of the disclosure will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the disclosure. 
     In accordance with one aspect of the present disclosure, a control method of a washing machine including a tub to accommodate water and a heater installed inside the tub to heat the water includes determining whether a currently washing course is a boiling washing course, supplying water into the tub if the currently washing course is the boiling washing course, performing primary heating washing to heat the supplied water for a predetermined time, detecting a temperature of the primarily heated water, and controlling a level of water in the tub for secondary heating washing according to the detected temperature of the water. The controlling the water level includes varying the level of water in the tub according to the temperature of the primarily heated water. 
     The controlling the water level may include comparing the temperature of the primarily heated water with a set temperature, and maintaining a current water level without draining the primarily heated water if the temperature of the primarily heated water is greater than the set temperature. 
     Further, if the temperature of the primarily heated water is equal to or less than the set temperature, the primarily heated water may be drained to lower the water level. 
     The supplying the water into the tub may include supplying water up to a predetermined water level for the boiling washing course. 
     The supplied water may be cold water or hot water. 
     The performing the primary heating washing may include detecting a temperature of the water supplied into the tub, determining a primary heating time according to the detected temperature of the water, and driving the heater according to the determined primary heating time. 
     The draining the water may include draining the water down to a water level lower by one level than the predetermined water level. 
     Further, the control method may further include performing secondary heating washing to heat the primarily heated water up to a target temperature after controlling the water level. 
     The set temperature may be a temperature of the water supplied up to the predetermined water level, which may reach the target temperature within a secondary heating time, and the set temperature may be about 45° C. 
     The target temperature may be equal to or greater than 70° C. which may provide a boiling effect. 
     Further, the control method may further include performing washing for a set time after the secondary heating washing. 
     Further, the control method may further include performing a cooling process by supplying cold water after performing the washing. 
     Further, the control method may further include performing a washing and cooling process by supplying cold water after performing the secondary heating washing. 
     In accordance with another aspect of the present disclosure, a control method of a washing machine includes a tub to accommodate water and a heater installed inside the tub to heat the water determining whether a currently washing course is a boiling washing course, supplying water into the tub if the currently washing course is the boiling washing course, detecting a temperature of the supplied water, determining a primary heating time according to the detected temperature of the water, performing primary heating washing by driving the heater according to the determined primary heating time, and controlling a level of water in the tub for secondary heating washing according to the detected temperature of the water. The controlling the water level includes varying the level of water in the tub according to the temperature of the supplied water. 
     The controlling the water level may include comparing the temperature of the supplied water with a set temperature, and maintaining a current water level without draining the primarily heated water if the temperature of the supplied water is greater than the set temperature. 
     Further, the controlling the water level may include comparing the temperature of the supplied water with a set temperature, and lowering the water level by draining the primarily heated water if the temperature of the supplied water is equal to or less than the set temperature. 
     Further, the control method may further include, after controlling the water level, performing secondary heating washing to heat the primarily heated water up to a target temperature. 
     In accordance with one aspect of the present disclosure, a washing machine includes a tub to accommodate water, a heater installed inside the tub to heat the water, an input unit through which a boiling washing course is selected, a control unit to, if the boiling washing course is selected, perform primary heating washing for a predetermined time after supplying water into the tub, detect a temperature of the primarily heated water, and control a level of water in the tub for secondary heating washing. The control unit may vary the level of water in the tub according to the temperature of the primarily heated water. 
     The control unit may compare the temperature of the supplied water with a set temperature, and maintain a current water level without draining the primarily heated water if the temperature of the supplied water is greater than the set temperature. 
     Further, the control unit may compare the temperature of the supplied water with a set temperature, and lower the water level by draining the primarily heated water if the temperature of the supplied water is equal to or less than the set temperature. 
     Further, after controlling the water level, the control unit may perform secondary heating washing to heat the primarily heated water up to a target temperature. 
     In accordance with another aspect of the present disclosure, a washing machine includes a tub to accommodate water, a heater installed inside the tub to heat the water, an input unit through which a boiling washing course is selected, a control unit to, if the boiling washing course is selected, supply water into the tub, detect a temperature of the supplied water, perform primary heating washing to drive the heater for a primary heating time determined according to the detected temperature of the water, and control a level of water in the tub for secondary heating washing according to the detected temperature of the water. The control unit may vary the level of water in the tub according to the temperature of the supplied water. 
     The control unit may compare the temperature of the supplied water with a set temperature, and maintain a current water level without draining the primarily heated water if the temperature of the supplied water is greater than the set temperature. 
     The control unit may compare the temperature of the supplied water with a set temperature, and lower the water level by draining the primarily heated water if the temperature of the supplied water is equal to or less than the set temperature. 
     According to the proposed washing machine and control method thereof, it is possible to increase a washing effect by heating water for a predetermined time according to the temperature of the initially supplied water, draining the water if the temperature of the heated water is less than the set temperature, and maintaining the amount of the initially supplied water without draining the water if the temperature of the water is equal to or greater than the set temperature. Further, by allowing the water to reach the target temperature of a high temperature within a predetermined washing time while maintaining the amount of the initially supplied water, it is possible to maintain a washing effect due to a washing temperature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects of the disclosure will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a cross-sectional view showing a configuration of a washing machine according to an embodiment of the present disclosure; 
         FIG. 2  is a control block diagram of the washing machine according to the embodiment of the present disclosure; 
         FIG. 3  is a diagram showing the water levels of the washing machine according to the embodiment of the present disclosure; 
         FIGS. 4A and 4B  illustrate an operation flowchart showing a first control algorithm for boiling washing in the washing machine according to the embodiment of the present disclosure; 
         FIGS. 5A and 5B  illustrate an operation flowchart showing a second control algorithm for boiling washing in the washing machine according to the embodiment of the present disclosure; 
         FIGS. 6A and 6B  illustrate an operation flowchart showing a third control algorithm for boiling washing in the washing machine according to the embodiment of the present disclosure; and 
         FIGS. 7A and 7B  illustrate an operation flowchart showing a fourth control algorithm for boiling washing in the washing machine according to the embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the embodiments of the present disclosure, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
       FIG. 1  is a cross-sectional view showing a configuration of a washing machine according to an embodiment of the present disclosure. 
     Referring to  FIG. 1 , a washing machine  1  according to the embodiment of the present disclosure includes a substantially box-shaped cabinet  10  forming an external appearance of the washing machine  1 , a tub  11  installed inside the cabinet  10  to accommodate water (washing water or rinsing water), a washing tub  12  rotatably installed inside the tub  11  to accommodate the laundry and a pulsator  13  rotatably installed inside the washing tub  12  to rotate clockwise or counterclockwise (forward or backward) and generate a water flow. The washing machine has a vertical axis. 
     The washing tub  12  is formed in a cylindrical shape to have an open top portion. On the side surface of the washing tub  12 , plural water-extracting holes  12   a  are formed. A balancer  12   b  may be installed in an upper portion of the washing tub  12  such that the washing tub  12  may be rotated stably during high-speed rotation. 
     On the lower outside of the tub  11 , there are installed a washing motor  14  to generate a driving force for rotating the washing tub  12  and the pulsator  13 , and a power switching device  15  to simultaneously or selectively transfer the driving force generated from the washing motor  14  to the washing tub  12  and the pulsator  13 . 
     A hollow water-extracting shaft  16  is coupled to the washing tub  12 . A washing shaft  17  is installed in the hollow portion of the water-extracting shaft  16 . The washing shaft  17  may be coupled to the pulsator  13  through a washing shaft coupling part  18 . 
     The washing motor  14  is a direct drive (DD) motor having a variable speed function. The washing motor  14  may selectively transfer driving force to the washing tub  12  or the pulsator  13  according to vertical movements. 
     Further, as the washing motor  14 , a universal motor including a field coil and an armature, a brushless direct current (BLDC) motor including a stator and a rotor, or the like may be used. In addition, any motor which is applicable to the washing machine  1  may be used. Alternatively, the washing motor  14  may be configured in a belt manner. 
     A water outlet port  20  is formed at the bottom of the tub  11  to discharge water to the outside, and a first drain pipe  21  is connected to the water outlet port  20 . A drain motor  22  is installed in the first drain pipe  21  to regulate drainage. A second drain pipe  23  for discharging water to the outside is connected to the outlet of the drain motor  22 . 
     Further, on the lower inside of the tub  11 , there are installed a heater  30  to heat the water in the tub  11 , a temperature sensor  32  to detect the temperature of the water in the tub  11 , and a water level sensor  34  to detect a frequency varying according to the water level in order to detect the amount of water (water level) in the tub  11 . 
     On the upper side of the cabinet  10 , there are installed a door  40  which is opened/closed to load laundry into the washing tub  12  or unload the laundry from the washing tub  12 , and a top cover  42  which the door  40  is separated from or seated on according to whether the door  40  is opened or closed. 
     At the top cover  42 , there is formed a laundry input port  46  through which the laundry is loaded into the washing tub  12  or the laundry is unloaded from the washing tub  12 . The laundry input port  46  may be opened and closed by the door  40 . 
     Further, a detergent box  48  for supplying a detergent and softening agent and a water supply device  50  for supplying water (washing water or rinsing water) are installed in the upper part of the cabinet  10 . 
     The water supply device  50  includes first and second water supply pipes  51  and  52  which connect an external water supply pipe to the detergent box  48  to supply water (washing water or rinsing water) into the tub  11 , and first and second water supply valves  53  and  54  which are installed in the first and second water supply pipes  51  and  52  to control the supply of hot or cold water. This configuration enables the water supplied into the tub  11  to pass through the detergent box  48  so that the detergent may be supplied together with the water into the tub  11 . 
       FIG. 2  is a control block diagram of the washing machine according to the embodiment of the present disclosure. 
     Referring to  FIG. 2 , the washing machine  1  according to the embodiment of the present disclosure is configured to include an input unit  70 , a control unit  72 , a memory  74 , a driving unit  76  and a display unit  78 . 
     The input unit  70  enables the user to input a command for performing a washing process, a rinsing process, a spin-dry process and the like of the washing machine  1 . The input unit  70  may be configured as keys, buttons, switches, touch pads and the like, and includes all devices to generate predetermined input data by an operation (e.g., pressing, touching, pressure and rotation). 
     Further, the input unit  70  includes a plurality of buttons (power, reservation, washing, rinsing, water-extraction, water level selection, etc.) for entering a user command related to operation of the washing machine  1 . Among the plurality of buttons, there are provided course selection buttons for selecting a washing course (standard course, wool course, boiling course, etc.) depending on the type of laundry loaded in the washing machine  1 . 
     The course selection buttons provided in the input unit  70  include a boiling course button  71  to select a boiling washing course for performing washing by heating the supplied water in order to increase a washing effect on the contaminated laundry. 
     The boiling course button  71  is provided to perform washing by heating the supplied water in the case of requiring a sterilization effect as well as removal of stains and odors from laundry such as baby clothes and underwear. The boiling course does not require the water to be heated to a boiling point, i.e. 100° C. 
     The control unit  72  is a microcomputer for controlling the overall operations of the washing machine  1  such as washing, rinsing and spin-dry operations according to the operating information inputted from the input unit  70 . The control unit  72  sets the amount of water for washing and rinsing, a target RPM, a motor operation rate (washing motor on-off time), washing and rinsing time and the like according to the weight (load) of the laundry in the selected washing course. 
     Further, the control unit  72  controls a driving timing and driving time of a heater  30  to maximize washing effects by heating the supplied water during boiling washing. 
     Accordingly, if a boiling washing course is selected, the control unit  72  performs primary heating washing for a predetermined washing time by driving the washing motor  14  and the heater  30  after supplying water up to a second water level (middle water level) determined for a boiling washing course. The primary heating time varies depending on the temperature (water temperature) of primarily heated water. As the water temperature is higher, the primary heating time is shorter. When the primary heating time has elapsed, the primary heating washing is stopped by stopping the washing motor  14  and the heater  30 . 
     Then, the control unit  72  determines whether to drain water according to the temperature of the primarily heated water. That is, the control unit  72  determines whether the temperature of the primarily heated water is a water temperature (third temperature; about 45° C.) that may reach a target temperature Ts within a secondary heating time, and determines whether to drain water. 
     For example, if the temperature of the primarily heated water is equal to or less than the third temperature, the control unit  72  performs secondary heating washing to heat the water up to the target temperature by driving the washing motor  14  and the heater  30  again after draining water down to a first water level (low water level). 
     On the other hand, if the temperature of the primarily heated water is greater than the third temperature, the control unit  72  performs secondary heating washing to heat the water up to the target temperature Ts by driving the washing motor  14  and the heater  30  again while maintaining the second water level of the primarily heated water without draining the water. 
     Thus, the control unit  72  performs a boiling course process to increase the washing effect by determining whether to drain water according to the water temperature after heating the primarily heated water for boiling washing, and maintaining the amount of the primarily heated water without unconditionally draining the water. 
     The memory  74  may store control data for controlling the operation of the washing machine  1 , reference data used during the control of the operation of the washing machine  1 , operation data generated while the washing machine  1  performs a predetermined operation, setting information such as setting data inputted via the input unit  70  to enable the washing machine  1  to perform a predetermined operation, use information including the number of times the washing machine  1  performs a specific operation and model information of the washing machine  1 , and failure information including the cause or location of the malfunction when the washing machine  1  malfunctions. 
     The driving unit  76  drives the washing motor  14 , the drain motor  22 , the heater  30 , the first water supply valve  53 , the second water supply valve  54  and the like relating to the operation of the washing machine  1  according to a driving control signal of the control unit  72 . 
     The display unit  78  displays the operating state of the washing machine  1  and the operation status of the user according to a display control signal of the control unit  72 . 
     Hereinafter, an operation process and operational effects of the washing machine and control method thereof according to the embodiment of the present disclosure will be described. 
     When the user intends to wash baby clothes and underwear, the user desires to perform boiling washing such that bacteria in the laundry may be sterilized while removing stains and odors from the laundry. Accordingly, in the present disclosure, a boiling washing course capable of further increasing the washing effect compared to conventional cases while satisfying user demands will be described with reference to  FIGS. 3 to 7 . 
       FIG. 3  is a diagram showing the water levels of the washing machine according to the embodiment of the present disclosure. 
     In  FIG. 3 , the levels of water supplied into the tub  11  are represented as three water levels. The first water level is the lowest water level of 12 L, the second water level is the middle water level of 18 L, and the third water level is the highest water level of 27 L. 
       FIGS. 4A and 4B  illustrate an operation flowchart showing a first control algorithm for boiling washing in the washing machine according to the embodiment of the present disclosure. 
     Referring to  FIGS. 4A and 4B , if the user loads the laundry (specifically, baby clothes) into the washing tub  12  and selects the boiling course button  71  provided in the input unit  70  (operation  100 ), the operation information selected by the user is inputted to the control unit  72  through the input unit  70 . 
     Accordingly, the control unit  72  operates the first water supply valve  53  or the second water supply valve  54  through the driving unit  76  in order to supply water (washing water) required for the boiling washing process. In this case, the operation of the first water supply valve  53  or the second water supply valve  54  varies according to the water supply of hot water or cold water selected by the user. 
     For example, the first water supply valve  53  is operated if the user selects hot water, and the second water supply valve  54  is operated if the user selects cold water. If the user selects hot and cold water, the first water supply valve  53  and the second water supply valve  54  are operated. 
     If the first water supply valve  53  or the second water supply valve  54  is operated, the first water supply valve  53  or the second water supply valve  54  is opened, and the water (washing water) supplied through the external water supply pipe passes through the first water supply pipe  51  or the second water supply pipe  52  and the detergent box  48  and is supplied into the tub  11  together with the detergent (operation  102 ). 
     Accordingly, the control unit  72  detects the level of the water supplied into the tub  11  through the water level sensor  34  and determines whether the water level is the second water level (middle water level when the water levels in the washing machine are divided into three water levels; about 18 L) shown in  FIG. 3  (operation  104 ). The control unit  72  continues a water supply operation until the level of the water supplied into the tub  11  reaches the second water level. 
     If the level of the water supplied into the tub  11  reaches the second water level through the water supply operation, the control unit  72  stops the water supply operation by stopping the first water supply valve  53  or the second water supply valve  54 . 
     Upon completion of the water supply up to the second water level, the control unit  72  detects the temperature (water temperature T) of the water in the tub  11  through the temperature sensor  32  (operation  106 ). 
     The control unit  72  determines whether the water temperature T detected through the temperature sensor  32  is greater than 0° C. (T0) and equal to or less than a first temperature T1 (about 10° C.) (operation  108 ). 
     As the determination result of operation  108 , if it is determined that the water temperature T is greater than 0° C. (T0) and equal to or less than the first temperature T1 (about 10° C.), the control unit  72  determines that the water temperature T is a water temperature in winter, and performs the primary heating washing by driving the washing motor  14  and the heater  30  (operation  110 ). 
     In this case, the control unit  72  determines, as a first time period (about 25 minutes), a time period during which the water must be heated to determine whether to drain water after the primary heating washing under the winter water temperature conditions. Then, the control unit  72  counts the time passed since the start of the primary heating washing, and determines whether the predetermined first time period (about 25 minutes) has elapsed (operation  112 ). 
     As the determination result of operation  112 , if the first time period has not elapsed, the control unit  72  returns to operation  110 , and performs the primary heating washing until the first time period has elapsed. 
     On the other hand, as the determination result of operation  112 , if the first time period has elapsed, the control unit  72  stops the primary heating washing by stopping the washing motor  14  and the heater  30  (operation  114 ). 
     On the other hand, as the determination result of operation  108 , if it is not determined that the water temperature T is greater than 0° C. (T0) and equal to or less than the first temperature T1 (about 10° C.), the control unit  72  determines whether the water temperature T detected through the temperature sensor  32  is greater than the first temperature T1 (about 10° C.) and equal to or less than a second temperature T2 (about 20° C.) (operation  140 ). 
     As the determination result of operation  140 , if it is determined that the water temperature T is greater than the first temperature T1 (about 10° C.) and equal to or less than the second temperature T2 (about 20° C.), the control unit  72  determines that the water temperature T is a water temperature in spring/fall, and performs the primary heating washing by driving the washing motor  14  and the heater  30  (operation  142 ). 
     In this case, the control unit  72  determines, as a second time period (about 20 minutes), a time period during which the water must be heated to determine whether to drain water after the primary heating washing under the spring/fall water temperature conditions. Then, the control unit  72  counts the time passed since the start of the primary heating washing, and determines whether the predetermined second time period (about 20 minutes) has elapsed (operation  144 ). 
     As the determination result of operation  144 , if the second time period has not elapsed, the control unit  72  returns to operation  142 , and performs the primary heating washing until the second time period has elapsed. 
     On the other hand, as the determination result of operation  144 , if the second time period has elapsed, the control unit  72  proceeds to operation  114  and stops the primary heating washing by stopping the washing motor  14  and the heater  30 . 
     On the other hand, as the determination result of operation  140 , if it is not determined that the water temperature T is greater than the first temperature T1 (about 10° C.) and equal to or less than the second temperature T2 (about 20° C.), the control unit  72  determines whether the water temperature T detected through the temperature sensor  32  is greater than the second temperature T2 (about 20° C.) (operation  150 ). 
     As the determination result of operation  150 , if it is determined that the water temperature T is greater than the second temperature T2 (about 20° C.), the control unit  72  determines that the water temperature T is a water temperature in summer, and performs the primary heating washing by driving the washing motor  14  and the heater  30  (operation  152 ). 
     In this case, the control unit  72  determines, as a third time period (about 15 minutes), a time period during which the water must be heated to determine whether to drain water after the primary heating washing under the summer water temperature conditions. Then, the control unit  72  counts the time passed since the start of the primary heating washing, and determines whether the predetermined third time period (about 15 minutes) has elapsed (operation  154 ). 
     As the determination result of operation  154 , if the third time period has not elapsed, the control unit  72  returns to operation  152 , and performs the primary heating washing until the third time period has elapsed. 
     On the other hand, as the determination result of operation  154 , if the third time period has elapsed, the control unit  72  proceeds to operation  114  and stops the primary heating washing by stopping the washing motor  14  and the heater  30 . 
     Thus, the primary heating washing is performed to heat the water for the first time period, the second time period, or the third time period according to the temperature of the primarily heated water. Then, after the primary heating washing, whether to drain water is determined according to the water temperature. 
     Accordingly, the control unit  72  detects the water temperature T of the primarily heated water through the temperature sensor  32  in order to determine whether to drain water according to the temperature of the primarily heated water, and determines whether the detected water temperature T is equal to or less than a predetermined third temperature T3 (water temperature which may reach the target temperature within the secondary heating time; about 45° C.) (operation  116 ). 
     As the determination result of operation  116 , if the water temperature T is equal to or less than the third temperature T3 (about 45° C.), the control unit  72  determines that the temperature of the primarily heated water is not a water temperature which may reach the target temperature Ts within the second heating time (about 35 minutes), and operates the drain motor  22  to drain the primarily heated water (washing water) (operation  118 ). 
     Accordingly, the control unit  72  detects the level of the water remaining in the tub  11  according to the drain operation through the water level sensor  34 , determines whether the water level is the first water level (the lowest water level when the water levels in the washing machine are divided into three water levels; about 12 L) shown in  FIG. 3  (operation  120 ), and continues the drain operation until the water level reaches the first water level. 
     When the level of the water remaining in the tub  11  through the drain operation reaches the first water level, the control unit  72  stops the drain operation of the drain motor  22 . 
     Subsequently, the control unit  72  drives the washing motor  14  and the heater  30  again and performs secondary heating washing to heat the water up to the target temperature Ts (final target temperature capable of providing a boiling effect; about 90° C.) (operation  122 ). 
     On the other hand, as the determination result of operation  116 , if the water temperature T is greater than the third temperature T3 (about 45° C.), the control unit  72  determines that the temperature of the primarily heated water is a water temperature which may reach the target temperature Ts within the second heating time (about 35 minutes), and does not drain the primarily heated water. 
     Accordingly, the control unit  72  proceeds to operation  122  while maintaining the second water level of the primarily heated water, and performs the secondary heating washing to heat the water up to the target temperature Ts by driving the washing motor  14  and the heater  30  again. 
     Thus, if the water temperature increases according to the secondary heating washing, the control unit  72  detects the water temperature T of the secondarily heated water through the temperature sensor  32 , and determines whether the detected water temperature T is equal to or greater than the predetermined target temperature Ts (operation  124 ). 
     As the determination result of operation  124 , if the water temperature T is less than the target temperature Ts, the control unit  72  returns to operation  122 , and continues the secondary heating washing until the water temperature T reaches the target temperature Ts. 
     On the other hand, as the determination result of operation  124 , if the water temperature T is equal to or greater than the target temperature Ts, the control unit  72  stops the secondary heating washing by stopping the washing motor  14  and the heater  30  (operation  125 ). 
     Then, the control unit  72  performs washing by using high-temperature water (washing water) after the water temperature T reaches the target temperature Ts (operation  126 ). In this case, washing is performed for a set time using high-temperature water without additional consumption of electricity by driving only the washing motor  14  without driving the heater  30 , thereby providing a more or less boiling washing effect. 
     The control unit  72  counts the washing progress time in the state of the first water level (or second water level) in which cold water is not supplied after the water temperature T reaches the target temperature Ts, and determines whether a fifth time period (time period during which washing is additionally performed after the water temperature T reaches the target temperature Ts to increase the washing effect, about 10 to 15 minutes) has elapsed (operation  128 ). 
     As the determination result of operation  128 , if the fifth time period has not elapsed, the control unit  72  returns to operation  126 , and performs the subsequent operations. 
     On the other hand, as the determination result of operation  128 , if the fifth time period has elapsed, the control unit  72  operates the second water supply valve  54  to supply cold water (operation  130 ). When high-temperature water is drained after the boiling washing is completed, there is a possibility that the user may get burned. Taking this possibility into consideration, a cooling process is performed by supplying cold water in the final stage of the boiling washing. 
     Thus, the control unit  72  detects the water level, at which cold water is supplied into the tub  11 , through the water level sensor  34 , and determines whether the water level is the third water level (the highest water level when the water levels in the washing machine are divided into three water levels; about 27 L) shown in  FIG. 3  (operation  132 ). Then, the control unit  72  continues a water supply operation until the level of the water supplied into the tub  11  reaches the third water level. That is, in order to reduce the temperature of the water to a temperature (about 55 to 60° C.) suitable for drainage at which there is no risk of burns when water is drained after the boiling washing, cold water is supplied up to the third water level. 
     If the level of the water supplied into the tub  11  reaches the third water level through the water supply operation, the control unit  72  stops the cold water supply operation by closing the second water supply valve  54 . 
     Upon completion of the supply of cold water up to the third water level, the control unit  72  performs a separate cooling process by driving the washing motor  14  for a set time (about 2 minutes) (operation  134 ), and completes the washing process. 
     Meanwhile, in the embodiment of the present disclosure, a case of performing washing for a set time (about 10 to 15 minutes) using high-temperature water by driving only the washing motor  14  without driving the heater  30  after the water temperature T reaches the target temperature Ts has been described as an example, but the present disclosure is not limited thereto. That is, washing may be performed for a set time while maintaining the target temperature Ts by driving the heater  30  as well as the washing motor  14 . 
     Next, a method of providing a cooling effect while performing the washing for the remaining time by supplying cold water after the water temperature T reaches the target temperature Ts will be described with reference to  FIGS. 5A and 5B . 
       FIGS. 5A and 5B  illustrate an operation flowchart showing a second control algorithm for boiling washing in the washing machine according to the embodiment of the present disclosure. A repeated description of the same configurations as those of  FIGS. 4A and 4B  will be omitted. 
     Referring to  FIGS. 5A and 5B , if the user loads the laundry (specifically, baby clothes) into the washing tub  12  and selects the boiling course button  71  provided in the input unit  70  (operation  200 ), the operation information selected by the user is inputted to the control unit  72  through the input unit  70 . 
     Accordingly, the control unit  72  operates the first water supply valve  53  or the second water supply valve  54  through the driving unit  76  in order to supply water (washing water) required for the boiling washing process. 
     If the first water supply valve  53  or the second water supply valve  54  is operated, the first water supply valve  53  or the second water supply valve  54  is opened, and the water (washing water) supplied through the external water supply pipe passes through the first water supply pipe  51  or the second water supply pipe  52  and the detergent box  48  and is supplied into the tub  11  together with the detergent (operation  202 ). 
     Accordingly, the control unit  72  detects the level of the water supplied into the tub  11  through the water level sensor  34  and determines whether the water level is the second water level (operation  204 ). The control unit  72  continues a water supply operation until the level of the water supplied into the tub  11  reaches the second water level. 
     Upon completion of the water supply up to the second water level, the control unit  72  detects the temperature (water temperature T) of the water in the tub  11  through the temperature sensor  32  (operation  206 ). 
     The control unit  72  determines whether the water temperature T detected through the temperature sensor  32  is greater than 0° C. (T0) and equal to or less than a first temperature T1 (about 10° C.) (operation  208 ). 
     As the determination result of operation  208 , if it is determined that the water temperature T is greater than 0° C. (T0) and equal to or less than the first temperature T1 (about 10° C.), the control unit  72  determines that the water temperature T is a water temperature in winter, and performs the primary heating washing by driving the washing motor  14  and the heater  30  (operation  210 ). 
     Then, the control unit  72  counts the time passed since the start of the primary heating washing, and determines whether a predetermined first time period (about 25 minutes) has elapsed (operation  212 ). 
     As the determination result of operation  212 , if the first time period has not elapsed, the control unit  72  performs the primary heating washing until the first time period has elapsed. 
     On the other hand, as the determination result of operation  212 , if the first time period has elapsed, the control unit  72  stops the primary heating washing by stopping the washing motor  14  and the heater  30  (operation  214 ). 
     On the other hand, as the determination result of operation  208 , if it is not determined that the water temperature T is greater than 0° C. (T0) and equal to or less than the first temperature T1 (about 10° C.), the control unit  72  determines whether the water temperature T detected through the temperature sensor  32  is greater than the first temperature T1 (about 10° C.) and equal to or less than a second temperature T2 (about 20° C.) (operation  240 ). 
     As the determination result of operation  240 , if it is determined that the water temperature T is greater than the first temperature T1 (about 10° C.) and equal to or less than the second temperature T2 (about 20° C.), the control unit  72  determines that the water temperature T is a water temperature in spring/fall, and performs the primary heating washing by driving the washing motor  14  and the heater  30  (operation  242 ). 
     In this case, the control unit  72  determines, as a second time period (about 20 minutes), a time period during which the water must be heated to determine whether to drain water after the primary heating washing under the spring/fall water temperature conditions. Then, the control unit  72  counts the time passed since the start of the primary heating washing, and determines whether the predetermined second time period (about 20 minutes) has elapsed (operation  244 ). 
     As the determination result of operation  244 , if the second time period has not elapsed, the control unit  72  performs the primary heating washing until the second time period has elapsed. 
     On the other hand, as the determination result of operation  244 , if the second time period has elapsed, the control unit  72  proceeds to operation  214  and stops the primary heating washing by stopping the washing motor  14  and the heater  30 . 
     On the other hand, as the determination result of operation  240 , if it is not determined that the water temperature T is greater than the first temperature T1 (about 10° C.) and equal to or less than the second temperature T2 (about 20° C.), the control unit  72  determines whether the water temperature T detected through the temperature sensor  32  is greater than the second temperature T2 (about 20° C.) (operation  250 ). 
     As the determination result of operation  250 , if it is determined that the water temperature T is greater than the second temperature T2 (about 20° C.), the control unit  72  determines that the water temperature T is a water temperature in summer, and performs the primary heating washing by driving the washing motor  14  and the heater  30  (operation  252 ). 
     In this case, the control unit  72  determines, as a third time period (about 15 minutes), a time period during which the water must be heated to determine whether to drain water after the primary heating washing under the summer water temperature conditions. Then, the control unit  72  counts the time passed since the start of the primary heating washing, and determines whether the predetermined third time period (about 15 minutes) has elapsed (operation  254 ). 
     As the determination result of operation  254 , if the third time period has not elapsed, the control unit  72  returns to operation  152 , and performs the primary heating washing until the third time period has elapsed. 
     On the other hand, as the determination result of operation  254 , if the third time period has elapsed, the control unit  72  proceeds to operation  214  and stops the primary heating washing by stopping the washing motor  14  and the heater  30 . 
     Thus, the primary heating washing is performed to heat the water for the first time period, the second time period, or the third time period according to the temperature of the primarily heated water. Then, after the primary heating washing, whether to drain water is determined according to the water temperature. 
     Accordingly, the control unit  72  detects the water temperature T of the primarily heated water through the temperature sensor  32  in order to determine whether to drain water according to the temperature of the primarily heated water, and determines whether the detected water temperature T is equal to or less than a predetermined third temperature T3 (about 45° C.) (operation  216 ). 
     As the determination result of operation  216 , if the water temperature T is equal to or less than the third temperature T3 (about 45° C.), the control unit  72  determines that the temperature of the primarily heated water is not a water temperature which may reach the target temperature Ts within the second heating time (about 35 minutes), and operates the drain motor  22  to drain the primarily heated water (washing water) (operation  218 ). 
     Accordingly, the control unit  72  detects the level of the water remaining in the tub  11  according to the drain operation through the water level sensor  34 , determines whether the water level is the first water level (operation  220 ), and continues the drain operation until the water level reaches the first water level. 
     When the level of the water remaining in the tub  11  through the drain operation reaches the first water level, the control unit  72  drives the washing motor  14  and the heater  30  again and performs secondary heating washing to heat the water up to the target temperature Ts (about 90° C.) (operation  222 ). 
     On the other hand, as the determination result of operation  216 , if the water temperature T is greater than the third temperature T3 (about 45° C.), the control unit  72  determines that the temperature of the primarily heated water is a water temperature which may reach the target temperature Ts within the second heating time (about 35 minutes), and does not drain the primarily heated water. 
     Accordingly, the control unit  72  proceeds to operation  222  while maintaining the second water level of the primarily heated water, and performs the secondary heating washing to heat the water up to the target temperature Ts by driving the washing motor  14  and the heater  30  again. 
     Thus, if the water temperature increases according to the secondary heating washing, the control unit  72  detects the water temperature T of the secondarily heated water using the temperature sensor  32 , and determines whether the detected water temperature T is equal to or greater than the predetermined target temperature Ts (operation  224 ). 
     As the determination result of operation  224 , if the water temperature T is less than the target temperature Ts, the control unit  72  returns to operation  222 , and continues the secondary heating washing until the water temperature T reaches the target temperature Ts. 
     On the other hand, as the determination result of operation  224 , if the water temperature T is equal to or greater than the target temperature Ts, the control unit  72  stops the secondary heating washing by stopping the washing motor  14  and the heater  30  (operation  225 ). 
     If the water temperature T reaches the target temperature Ts, the control unit  72  operates the second water supply valve  54  to supply cold water (operation  226 ). When high-temperature water is drained after the boiling washing is completed, there is a possibility that the user may get burned. Taking this possibility into consideration, a cooling process is performed by supplying cold water in the final stage of the boiling washing 
     Thus, the control unit  72  detects the water level, at which cold water is supplied into the tub  11 , through the water level sensor  34 , and determines whether the water level is the third water level (operation  228 ). Then, the control unit  72  continues a water supply operation until the level of the water supplied into the tub  11  reaches the third water level. 
     Upon completion of the supply of cold water up to the third water level, the control unit  72  performs the process to provide a washing and cooling effect by driving the washing motor  14  for the remaining time (about 10 to 15 minutes) (operation  230 ), and completes the washing process. 
     Next, a method of performing a separate cooling process by supplying cold water after washing is performed for a set time after the water temperature T reaches the target temperature Ts will be described with reference to  FIGS. 6A and 6B . 
       FIGS. 6A and 6B  illustrate an operation flowchart showing a third control algorithm for boiling washing in the washing machine according to the embodiment of the present disclosure. A repeated description of the same configuration as that of  FIGS. 4A and 4B  will be omitted. 
     Referring to  FIGS. 6A and 6B , if the user loads the laundry (specifically, baby clothes) into the washing tub  12  and selects the boiling course button  71  provided in the input unit  70  (operation  300 ), the operation information selected by the user is inputted to the control unit  72  through the input unit  70 . 
     Accordingly, the control unit  72  operates the first water supply valve  53  or the second water supply valve  54  through the driving unit  76  in order to supply water (washing water) required for the boiling washing process. 
     If the first water supply valve  53  or the second water supply valve  54  is operated, the first water supply valve  53  or the second water supply valve  54  is opened, and the water (washing water) supplied through the external water supply pipe passes through the first water supply pipe  51  or the second water supply pipe  52  and the detergent box  48  and is supplied into the tub  11  together with the detergent (operation  302 ). 
     Accordingly, the control unit  72  detects the level of the water supplied into the tub  11  through the water level sensor  34  and determines whether the water level is the second water level (operation  304 ). The control unit  72  continues a water supply operation until the level of the water supplied into the tub  11  reaches the second water level. 
     Upon completion of the water supply up to the second water level, the control unit  72  detects the temperature (water temperature T) of the water in the tub  11  through the temperature sensor  32  (operation  306 ). 
     The control unit  72  determines whether the water temperature T detected through the temperature sensor  32  is greater than 0° C. (T0) and equal to or less than a first temperature T1 (about 10° C.) (operation  308 ). 
     As the determination result of operation  308 , if it is determined that the water temperature T is greater than 0° C. (T0) and equal to or less than the first temperature T1 (about 10° C.), the control unit  72  determines that the water temperature T is a water temperature in winter, and performs the primary heating washing by driving the washing motor  14  and the heater  30  (operation  310 ). 
     Then, the control unit  72  counts the time passed since the start of the primary heating washing, and determines whether a predetermined first time period (about 25 minutes) has elapsed (operation  312 ). 
     As the determination result of operation  312 , if the first time period has elapsed, the control unit  72  stops the primary heating washing by stopping the washing motor  14  and the heater  30  (operation  314 ). 
     On the other hand, as the determination result of operation  308 , if it is not determined that the water temperature T is greater than 0° C. (T0) and equal to or less than the first temperature T1 (about 10° C.), the control unit  72  determines whether the water temperature T detected through the temperature sensor  32  is greater than the first temperature T1 (about 10° C.) and equal to or less than a second temperature T2 (about 20° C.) (operation  340 ). 
     As the determination result of operation  340 , if it is determined that the water temperature T is greater than the first temperature T1 (about 10° C.) and equal to or less than the second temperature T2 (about 20° C.), the control unit  72  determines that the water temperature T is a water temperature in spring/fall, and performs the primary heating washing by driving the washing motor  14  and the heater  30  (operation  342 ). 
     Then, the control unit  72  counts the time passed since the start of the primary heating washing, and determines whether a predetermined second time period (about 20 minutes) has elapsed (operation  344 ). 
     As the determination result of operation  344 , if the second time period has elapsed, the control unit  72  proceeds to operation  314  and stops the primary heating washing by stopping the washing motor  14  and the heater  30 . 
     On the other hand, as the determination result of operation  340 , if it is not determined that the water temperature T is greater than the first temperature T1 (about 10° C.) and equal to or less than the second temperature T2 (about 20° C.), the control unit  72  determines whether the water temperature T detected through the temperature sensor  32  is greater than the second temperature T2 (about 20° C.) and equal to or less than a third temperature T3 (about 45° C.) (operation  350 ). 
     As the determination result of operation  350 , if it is determined that the water temperature T is greater than the second temperature T2 (about 20° C.) and equal to or less than the third temperature T3 (about 45° C.), the control unit  72  determines that the water temperature T is a water temperature in summer, and performs the primary heating washing by driving the washing motor  14  and the heater  30  (operation  352 ). 
     Then, the control unit  72  counts the time passed since the start of the primary heating washing, and determines whether a predetermined third time period (about 15 minutes) has elapsed (operation  354 ). 
     As the determination result of operation  354 , if the third time period has elapsed, the control unit  72  proceeds to operation  314  and stops the primary heating washing by stopping the washing motor  14  and the heater  30 . 
     Thus, the primary heating washing is performed to heat the water for the first time period, the second time period, or the third time period according to the temperature of the primarily heated water. The control unit  72  determines that the temperature of the primarily heated water is not a water temperature which may reach the target temperature Ts within the second heating time (about 35 minutes), and operates the drain motor  22  to drain the primarily heated water (washing water) (operation  318 ). 
     Accordingly, the control unit  72  detects the level of the water remaining in the tub  11  according to the drain operation through the water level sensor  34 , determines whether the water level is the first water level (operation  320 ), and continues the drain operation until the water level reaches the first water level. 
     Subsequently, the control unit  72  drives the washing motor  14  and the heater  30  again and performs secondary heating washing to heat the water up to the target temperature Ts (about 90° C.) (operation  322 ). 
     On the other hand, as the determination result of operation  150 , if it is not determined that the water temperature T is greater than the second temperature T2 (about 20° C.) and equal to or less than the third temperature T3 (about 45° C.), the control unit  72  determines whether the water temperature T detected through the temperature sensor  32  is greater than the third temperature T3 (about 45° C.) (operation  360 ). 
     As the determination result of operation  360 , if the water temperature T is greater than the third temperature T3 (about 20° C.), the control unit  72  determines that the water temperature T is a water temperature which may reach the target temperature Ts within the second heating time, and performs the primary heating washing by driving the washing motor  14  and the heater  30  (operation  362 ). 
     Then, the control unit  72  counts the time passed since the start of the primary heating washing, and determines whether a predetermined fourth time period (about 15 minutes) has elapsed (operation  364 ). 
     As the determination result of operation  364 , if the fourth time period has elapsed, the control unit  72  proceeds to operation  366  and stops the primary heating washing by stopping the washing motor  14  and the heater  30 . 
     Thus, the primary heating washing is performed to heat the water for the fourth time period according to the temperature of the primarily heated water. The control unit  72  determines that the temperature of the primarily heated water is a water temperature which may reach the target temperature Ts within the second heating time (about 35 minutes), and does not drain the primarily heated water. 
     Accordingly, the control unit  72  proceeds to operation  322  while maintaining the second water level of the primarily heated water, and performs the secondary heating washing to heat the water up to the target temperature Ts by driving the washing motor  14  and the heater  30  again. 
     Thus, if the water temperature increases according to the secondary heating washing, the control unit  72  detects the water temperature T of the secondarily heated water using the temperature sensor  32 , and determines whether the detected water temperature T is equal to or greater than the predetermined target temperature Ts (operation  324 ). 
     As the determination result of operation  324 , if the water temperature T is less than the target temperature Ts, the control unit  72  returns to operation  322 , and continues the secondary heating washing until the water temperature T reaches the target temperature Ts. 
     On the other hand, as the determination result of operation  324 , if the water temperature T is equal to or greater than the target temperature Ts, the control unit  72  stops the secondary heating washing by stopping the washing motor  14  and the heater  30  (operation  325 ). 
     Then, the control unit  72  performs washing by using high-temperature water (washing water) after the water temperature T reaches the target temperature Ts (operation  326 ). In this case, washing is performed for a set time using high-temperature water without additional consumption of electric energy by driving only the washing motor  14  without driving the heater  30 , thereby providing a boiling washing effect to some extent. 
     The control unit  72  counts the washing progress time in the state of the first water level (or second water level) in which cold water is not supplied after the water temperature T reaches the target temperature Ts, and determines whether a fifth time period (time period during which washing is additionally performed after the water temperature T reaches the target temperature Ts to increase the washing effect, about 10 to 15 minutes) has elapsed (operation  328 ). 
     As the determination result of operation  328 , if the fifth time period has not elapsed, the control unit  72  returns to operation  326 , and performs subsequent operations. 
     On the other hand, as the determination result of operation  328 , if the fifth time period has elapsed, the control unit  72  operates the second water supply valve  54  to supply cold water (operation  330 ). When high-temperature water is drained after the boiling washing is completed, there is a possibility that the user may get burned. Taking this possibility into consideration, a cooling process is performed by supplying cold water in the final stage of the boiling washing 
     Thus, the control unit  72  detects the water level, at which cold water is supplied into the tub  11 , through the water level sensor  34 , and determines whether the water level is the third water level (operation  332 ). Then, the control unit  72  continues a water supply operation until the level of the water supplied into the tub  11  reaches the third water level. That is, in order to reduce the temperature of the water to a temperature (about 55 to 60° C.) suitable for drainage at which there is no risk of burns when water is drained after the boiling washing, cold water is supplied up to the third water level. 
     Upon completion of the supply of cold water up to the third water level, the control unit  72  performs a separate cooling process by driving the washing motor  14  for a set time (about 2 minutes) (operation  334 ), and completes the washing process. 
     Next, a method of providing a cooling effect while performing the washing for the remaining time by supplying cold water after the water temperature T reaches the target temperature Ts will be described with reference to  FIGS. 7A and 7B . 
       FIGS. 7A and 7B  illustrate an operation flowchart showing a fourth control algorithm for boiling washing in the washing machine according to the embodiment of the present disclosure. A repeated description of the same configurations as those of  FIGS. 6A and 6B  will be omitted. 
     Referring to  FIGS. 7A and 7B , if the user loads the laundry (specifically, baby clothes) into the washing tub  12  and selects the boiling course button  71  provided in the input unit  70  (operation  400 ), the operation information selected by the user is inputted to the control unit  72  through the input unit  70 . 
     Accordingly, the control unit  72  operates the first water supply valve  53  or the second water supply valve  54  through the driving unit  76  in order to supply water (washing water) required for the boiling washing process. 
     If the first water supply valve  53  or the second water supply valve  54  is operated, the first water supply valve  53  or the second water supply valve  54  is opened, and the water (washing water) supplied through the external water supply pipe passes through the first water supply pipe  51  or the second water supply pipe  52  and the detergent box  48  and is supplied into the tub  11  together with the detergent (operation  402 ). 
     Accordingly, the control unit  72  detects the level of the water supplied into the tub  11  through the water level sensor  34  and determines whether the water level is the second water level (operation  404 ). The control unit  72  continues a water supply operation until the level of the water supplied into the tub  11  reaches the second water level. 
     Upon completion of the water supply up to the second water level, the control unit  72  detects the temperature (water temperature T) of the water in the tub  11  through the temperature sensor  32  (operation  406 ). 
     The control unit  72  determines whether the water temperature T detected through the temperature sensor  32  is greater than 0° C. (T0) and equal to or less than a first temperature T1 (about 10° C.) (operation  408 ). 
     As the determination result of operation  408 , if it is determined that the water temperature T is greater than 0° C. (T0) and equal to or less than the first temperature T1 (about 10° C.), the control unit  72  determines that the water temperature T is a water temperature in winter, and performs the primary heating washing by driving the washing motor  14  and the heater  30  (operation  410 ). 
     Then, the control unit  72  counts the time passed since the start of the primary heating washing, and determines whether a predetermined first time period (about 25 minutes) has elapsed (operation  412 ). 
     As the determination result of operation  412 , if the first time period has elapsed, the control unit  72  stops the primary heating washing by stopping the washing motor  14  and the heater  30  (operation  414 ). 
     On the other hand, as the determination result of operation  408 , if it is not determined that the water temperature T is greater than 0° C. (T0) and equal to or less than the first temperature T1 (about 10° C.), the control unit  72  determines whether the water temperature T detected through the temperature sensor  32  is greater than the first temperature T1 (about 10° C.) and equal to or less than a second temperature T2 (about 20° C.) (operation  440 ). 
     As the determination result of operation  440 , if it is determined that the water temperature T is greater than the first temperature T1 (about 10° C.) and equal to or less than the second temperature T2 (about 20° C.), the control unit  72  determines that the water temperature T is a water temperature in spring/fall, and performs the primary heating washing by driving the washing motor  14  and the heater  30  (operation  442 ). 
     Then, the control unit  72  counts the time passed since the start of the primary heating washing, and determines whether a predetermined second time period (about 20 minutes) has elapsed (operation  444 ). 
     As the determination result of operation  444 , if the second time period has elapsed, the control unit  72  proceeds to operation  414  and stops the primary heating washing by stopping the washing motor  14  and the heater  30 . 
     On the other hand, as the determination result of operation  440 , if it is not determined that the water temperature T is greater than the first temperature T1 (about 10° C.) and equal to or less than the second temperature T2 (about 20° C.), the control unit  72  determines whether the water temperature T detected through the temperature sensor  32  is greater than the second temperature T2 (about 20° C.) and equal to or less than a third temperature T3 (about 45° C.) (operation  450 ). 
     As the determination result of operation  450 , if it is determined that the water temperature T is greater than the second temperature T2 (about 20° C.) and equal to or less than the third temperature T3 (about 45° C.), the control unit  72  determines that the water temperature T is a water temperature in summer, and performs the primary heating washing by driving the washing motor  14  and the heater  30  (operation  452 ). 
     Then, the control unit  72  counts the time passed since the start of the primary heating washing, and determines whether a predetermined third time period (about 15 minutes) has elapsed (operation  454 ). 
     As the determination result of operation  454 , if the third time period has elapsed, the control unit  72  proceeds to operation  414  and stops the primary heating washing by stopping the washing motor  14  and the heater  30 . 
     Thus, the primary heating washing is performed to heat the water for the first time period, the second time period, or the third time period according to the temperature of the primarily heated water. The control unit  72  determines that the temperature of the primarily heated water is not a water temperature which may reach the target temperature Ts within the second heating time (about 35 minutes), and operates the drain motor  22  to drain the primarily heated water (washing water) (operation  418 ). 
     Accordingly, the control unit  72  detects the level of the water remaining in the tub  11  according to the drain operation through the water level sensor  34 , determines whether the water level is the first water level (operation  420 ), and continues the drain operation until the water level reaches the first water level. 
     Subsequently, the control unit  72  drives the washing motor  14  and the heater  30  again and performs secondary heating washing to heat the water up to the target temperature Ts (about 90° C.) (operation  422 ). 
     On the other hand, as the determination result of operation  450 , if it is not determined that the water temperature T is greater than the second temperature T2 (about 20° C.) and equal to or less than the third temperature T3 (about 45° C.), the control unit  72  determines whether the water temperature T detected through the temperature sensor  32  is greater than the third temperature T3 (about 45° C.) (operation  460 ). 
     As the determination result of operation  460 , if the water temperature T is greater than the third temperature T3 (about 20° C.), the control unit  72  determines that the water temperature T is a water temperature which may reach the target temperature Ts within the second heating time, and performs the primary heating washing by driving the washing motor  14  and the heater  30  (operation  462 ). 
     Then, the control unit  72  counts the time passed since the start of the primary heating washing, and determines whether a predetermined fourth time period (about 15 minutes) has elapsed (operation  464 ). 
     As the determination result of operation  464 , if the fourth time period has elapsed, the control unit  72  stops the primary heating washing by stopping the washing motor  14  and the heater  30  (operation  466 ). 
     Thus, the primary heating washing is performed to heat the water for the fourth time period according to the temperature of the primarily heated water. The control unit  72  determines that the temperature of the primarily heated water is a water temperature which may reach the target temperature Ts within the second heating time (about 35 minutes), and does not drain the primarily heated water. 
     Accordingly, the control unit  72  proceeds to operation  422  while maintaining the second water level of the primarily heated water, and performs the secondary heating washing to heat the water up to the target temperature Ts by driving the washing motor  14  and the heater  30  again. 
     Thus, if the water temperature increases according to the secondary heating washing, the control unit  72  detects the water temperature T of the secondarily heated water through the temperature sensor  32 , and determines whether the detected water temperature T is equal to or greater than the predetermined target temperature Ts (operation  424 ). 
     As the determination result of operation  424 , if the water temperature T is less than the target temperature Ts, the control unit  72  returns to operation  422 , and continues the secondary heating washing until the water temperature T reaches the target temperature Ts. 
     On the other hand, as the determination result of operation  424 , if the water temperature T is equal to or greater than the target temperature Ts, the control unit  72  stops the secondary heating washing by stopping the washing motor  14  and the heater  30  (operation  425 ). 
     If the water temperature T reaches the target temperature Ts, the control unit  72  operates the second water supply valve  54  to supply cold water (operation  426 ). When high-temperature water is drained after the boiling washing is completed, there is a possibility that the user may get burned. Taking this possibility into consideration, a cooling process is performed by supplying cold water in the final stage of the boiling washing. 
     Thus, the control unit  72  detects the water level, at which cold water is supplied into the tub  11 , through the water level sensor  34 , and determines whether the water level is the third water level (operation  428 ). Then, the control unit  72  continues a water supply operation until the level of the water supplied into the tub  11  reaches the third water level. 
     Upon completion of the supply of cold water up to the third water level, the control unit  72  performs the process to provide a washing and cooling effect by driving the washing motor  14  for the remaining time (about 10 to 15 minutes) (operation  430 ), and completes the washing process. 
     Although, in the embodiment of the present disclosure, a case of setting the target temperature Ts to 90° C. has been described as an example, the present disclosure is not limited thereto. That is, even if the target temperature Ts is set to equal to or greater than 70° C. at which the sterilizing effect begins to appear, the same object and effect as those of the present disclosure may be achieved. 
     Although a few embodiments of the present disclosure have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.