Patent Publication Number: US-2023141194-A1

Title: Gas furnace and air conditioner having the same

Description:
BACKGROUND OF THE DISCLOSURE 
     Field of the Disclosure 
     The present disclosure relates to a gas furnace and an air conditioner having the same. 
     Related Art 
     In general, an air conditioner refers to an apparatus for cools and heating an indoor space through compression, condensation, expansion, and evaporation of refrigerant. The air conditioner can improve indoor air quality by exchanging indoor unit with outdoor air through a ventilator. In addition, the ventilator may increase the temperature of air supplied to the indoor space by using high-temperature combustion gas of a gas furnace. 
     International patent application WO 2005-095870 A1 (published on Oct. 13, 2005) discloses a gas furnace in which a plurality of burners are classified into two groups and thermal power of each group is independently controlled. 
     Specifically, the manifold of the gas furnace is divided into two sections by a separator plate, and each of the two sections communicates with each of the two groups. Also, the gas furnace has a first gas valve for supplying fuel to one of the two sections, and a second gas valve for supplying fuel to the other of the two sections. That is, the above gas furnace independently controls the thermal power of each group using the first gas valve and the second gas valve, and adjusts the thermal power of the gas furnace in stages. 
     However, this control may require the gas furnace to have at least two gas valves. In this case, compared to a case where the gas furnace is provided with a single gas valve, not only the cost of an added gas valve itself, but also the cost accompanied therewith (that is, the cost of a fuel supply pipe connecting the added gas valve and the manifold, the cost related to a structure for installing the added gas valve in the gas furnace, etc.) are increased. 
     In addition, an igniter and a flame detector are required for each group of burners in the gas furnace, thereby increasing the cost and leading to inconvenience to control each igniter and each flame detector individually. 
     In addition, the gas furnace has a difficulty in independently controlling at least two gas valves. In particular, when an air ratio of burners is to be controlled with one inducer, it may be difficult for the inducer to synchronize the air ratio of one group of burners with the air ratio of the other group. In other words, in order for the burners to have a constant air ratio, it is necessary to match an opening degree of a second gas valve corresponding to one group of the burners to an opening degree of a first gas valve corresponding to the other group. In this case, in a case where the first gas valve is a dual stage valve capable of controlling an opening degree in two stages (i.e., open fully (100%) and open in half (50%)), even if the second gas valve is a modulating valve capable of controlling an opening degree to a lower level than the first gas valve, it may be difficult to reduce the opening degree of the second gas valve to 50% or less. That is, it may be difficult for the gas furnace to provide thermal power below a certain level because an opening degree of any one of the independently controlled gas valves is restricted by another valve when all of the burners are operated. 
     SUMMARY OF THE DISCLOSURE 
     An aspect of the present disclosure is to solve the above and other problems. 
     Another aspect of the present disclosure provides a gas furnace capable of providing a user with thermal comfort and reducing heating cost and energy by implementing a high Top Down Ratio (TDR). Here, the TDR refers to a ratio of maximum thermal power to minimum thermal power. 
     Yet another aspect of the present disclosure provides a gas furnace capable of controlling thermal power in stages in a wide range. 
     Yet another aspect of the present disclosure provides a mechanism capable of supplying fuel to only some burners using a minimum number of valves in order to implement a thermal power below a reference thermal power. 
     Yet another aspect of the present disclosure provides various methods for controlling the above mechanism according to a required thermal power. 
     Yet another aspect of the present disclosure provides a structure capable of minimizing the number of igniters and flame detectors provided in a plurality of burners. 
     According to one aspect of the present disclosure, there is provided a gas furnace including: a fuel valve; a manifold providing a passage of fuel passing through the fuel valve; a plurality of burners provided to burn fuel provided from the manifold and spaced apart from each other in one direction; a plurality of heat exchangers providing a passage of combustion gas generated by the plurality of burners; and a blower for causing a flow of air passing around the heat exchanger. 
     According to another aspect of the present disclosure, the manifold may include: a first tube having one end connected to the fuel valve and forming a first passage; a second tube extending in the one direction, forming a second passage, and facing at least one of the plurality of burners; a third tube extending in the one direction, forming a third passage, and facing remaining of the plurality of burners; and a three-way valve connected to the first tube, the second tube, and the third tube. 
     According to another aspect of the present disclosure, the three-way valve may guides fuel passing through the first passage to flow to the second passage and the third passage or to the third passage. 
     According to another aspect of the present disclosure, the three-way valve may be positioned between the second tube and the third tube. 
     According to another aspect of the present disclosure, the plurality of burners may include: a first burner group with burners facing the second tube; and a second burners group with burners facing the third tube. 
     According to another aspect of the present disclosure, a number of the burners in the first burner group may be equal to a number of the burners in the second burner group. 
     According to another aspect of the present disclosure, the gas furnace may further include: a plurality of first nozzles connected to the second tube, facing the burners in the first burner group, and spaced apart from the burners in the first burner group; and a plurality of second nozzles connected to the third tube, facing the burners in the second burner group, and spaced apart from the burners in the second burner group. 
     According to another aspect of the present disclosure, each of the burners of the first group of burners may further include: a venturi portion forming an entry of a corresponding one of the burners in the first burner group; a head portion forming an exit of the corresponding one of the burners of the first burner group; a retainer inserted into the head part; and a holder connected to the venturi portion and having a corresponding one of the plurality of first nozzles inserted thereinto and fixed thereto. 
     According to another aspect of the present disclosure, the gas furnace may include: a flange connected to the plurality of burners between the plurality of burners and forming a flame propagation port; and a burner box accommodating the plurality of burners and the flange. 
     According to another aspect of the present disclosure, the gas furnace may further include: an igniter mounted on the burner box and adjacent to an exit of a burner located furthest from the first group of burners among the burners of the second group of burners. 
     According to another aspect of the present disclosure, the gas furnace may further include: a flame detector mounted to the burner box and adjacent an exit of a burner positioned farthest from the second burner group among the burners in the first burner group. 
     According to another aspect of the present disclosure, the gas furnace may further include: an auxiliary detector mounted to the burner box and adjacent an exit of a burner positioned closest to the first burner group among the burners in the second burner group. 
     According to another aspect of the present disclosure, the igniter, the flame detector, and the auxiliary detector may be detachably mounted to the burner box. 
     According to another aspect of the present disclosure, the plurality of heat exchangers may include: a first heat exchanger group with heat exchangers in communication with the burners in the first burner group; and a second heat exchanger group with heat exchangers in communication with the burners in the second burner group. 
     According to another aspect of the present disclosure, the gas furnace may further include an inducer for causing a flow of combustion gas through the heat exchangers in the first heat exchanger group and the heat exchangers in the second heat exchanger group. 
     According to another aspect of the present disclosure, the heat exchangers in the first heat exchanger group and the heat exchangers in the second heat exchanger group may extend in the other direction crossing the one direction. 
     According to another aspect of the present disclosure, the blower may be positioned to be biased toward the second heat exchanger group with respect to a reference line extending in the other direction between the first heat exchanger group and the second heat exchanger group. 
     According to another aspect of the present disclosure, the three-way valve may further include a connector having a first part connected to the first tube, a second part connected to the second tube, and a third part connected to the third tube. 
     According to another aspect of the present disclosure, the three-way valve may further include a ball rotatably coupled to an inside of the connector, and having a first opening, a second opening, and a third opening facing in different directions. 
     According to another aspect of the present disclosure, a positional relationship between the second part and the third part with respect to the first part may be identical to a positional relationship between the second opening and the third opening with respect to the first opening. 
     According to another aspect of the present disclosure, the ball may be rotatable between a first position and a second position. 
     According to another aspect of the present disclosure, in response to the ball being located at the first position, the first opening may face the first part, the second opening may face the second part, and the third opening may face the third part. 
     According to another aspect of the present disclosure, in response to the ball being located at the second position, the second opening may face the first part, the first opening may face the third part, and the third opening may face an inner wall of the connector. 
     According to another aspect of the present disclosure, the three-way valve may further include: a rotary motor providing a rotational force; a shaft provided to be rotatable by power from the rotary motor, fixed to the ball through the connector, and providing a central axis of rotation of the ball; and a controller configured to control operation of the rotary motor. 
     According to another aspect of the present disclosure, a diameter of the ball may be greater than an inner diameter of the first part, an inner diameter of the second part, and an inner diameter of the third part. 
     According to another aspect of the present disclosure, the connector further may further include a groove positioned between the second part and the third part and corresponding to a surface of the ball. 
     According to another aspect of the present disclosure, the ball may further include a curved portion facing the first opening with respect to the second opening and the third opening, and a part of the curved portion may be inserted into the second part in response to the ball being located at the second position. 
     According to another aspect of the present disclosure, there is provided an air conditioner having an outdoor unit and a ventilator that are connected to each other through a refrigerant pipe. The ventilator may include: an air supply fan for causing a flow of air along an air supply passage; an exhaust fan for causing a flow of air along an exhaust passage separated from the air supply passage; a plurality of coils located in the air supply passage and having refrigerant flowing therethrough; and a gas furnace positioned downstream of the plurality of coils in the air supply passage. 
     A gas furnace and an air conditioner having the same according to the present disclosure may have effects as below. 
     According to at least one of the embodiments of the present disclosure, it is possible to implement a high Top Down Ratio (TDR) by supplying fuel to some burners using a three-way valve. That is, it is possible to provide a gas furnace capable of providing a user with thermal comfort and reducing heating cost and energy. 
     According to at least one of the embodiments of the present disclosure, it is possible to control the intensity of thermal power by adjusting an opening degree of a fuel valve or by selecting burners to which fuel is supplied through a three-way valve. That is, it is possible to provide a gas furnace in which thermal power can be controlled in stages in a wide range. 
     According to at least one of the embodiments of the present disclosure, it is possible to control a passage of a three-way valve based on rotation of a ball of the three-way valve. That is, it is possible to provide a mechanism capable of supplying fuel to some burners using a minimum number of valves. 
     According to at least one of the embodiments of the present disclosure, it is possible to use a rotary motor of the three-way valve to control a rotating direction of a ball connected thereto. That is, various methods for controlling the aforementioned mechanism may be provided according to required thermal power. 
     According to at least one of the embodiments of the present disclosure, a passage blocked by a three-way valve may be closed due to a close contact structure of a ball of the three-way valve. 
     According to at least one of the embodiments of the present disclosure, it is possible to provide a structure capable of minimizing the number of igniters and flame detectors in a plurality of burners due to characteristics of flame propagation between the plurality of burners. 
     According to at least one of the embodiments of the present disclosure, a flame detector may be detachably mounted to a burner box. That is, when implementing a high TDR, the user may be able to select burners to which fuel can be supplied. 
     According to at least one of the embodiments of the present disclosure, a blower may be disposed to be biased toward one side of the plurality of heat exchangers. That is, when the high TDR is implemented, a rate of heat transfer with air flowing by the blower of some heat exchangers through which combustion gas flows may be improved. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS.  1  and  2    are views showing an internal configuration of an air conditioner according to an embodiment of the present disclosure. 
         FIGS.  3  and  4    are views showing a gas furnace according to an embodiment of the present disclosure. 
         FIG.  5    is a view showing an internal configuration of a gas furnace according to an embodiment of the present disclosure. 
         FIGS.  6  and  7    are views for explaining a three-way valve according to an embodiment of the present disclosure. 
         FIG.  8    is a diagram illustrating a control configuration of a gas furnace according to an embodiment of the present disclosure. 
         FIG.  9    is a flowchart illustrating a method for controlling a gas furnace according to an embodiment of the present disclosure. 
         FIGS.  10 A and  10 B  are tables for explaining a Top Down Ratio (TDR) of a gas furnace according to an embodiment of the present disclosure. 
         FIGS.  11  to  13    are views for explaining a passage of a three-way valve according to an embodiment of the present disclosure: specifically, 
         FIG.  11    is a view for explaining a case where fuel having passed through a first tube is guided to a second tube and a third tube; 
         FIG.  12    is a view for explaining a case where fuel having passed through the first tube is guided to the third tube, and 
         FIG.  13    is a view for explaining a case where fuel having passed through the first tube is guided to the second tube. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Hereinafter, the embodiments disclosed in the present specification will be described in detail with reference to the accompanying drawings, but the same or similar components are assigned the same reference numerals regardless of reference numerals, and redundant description thereof will be omitted. 
     The suffixes “module” and “part” for components used in the following description are given or mixed in consideration of only the ease of writing the specification, and do not have distinct meanings or roles by themselves. 
     In addition, in describing the embodiments disclosed in the present specification, if it is determined that detailed descriptions of related known technologies may obscure the gist of the embodiments disclosed in this specification, the detailed description thereof will be omitted. In addition, the accompanying drawings are only for easy understanding of the embodiments disclosed in the present specification, and the technical spirit disclosed in the present specification is not limited by the accompanying drawings, and all changes included in the spirit and scope of the present disclosure, should be understood to include equivalents or substitutes. 
     Terms including ordinal numbers such as first, second, etc. may be used to describe various elements, but the elements are not limited by the terms. The above terms are used only for the purpose of distinguishing one component from another. 
     When a component is referred to as being “connected” or “connected” to another component, it may be directly connected or connected to the other component, but it is understood that other components may exist in between. On the other hand, when it is said that a certain component is “directly connected” or “directly connected” to another component, it should be understood that the other component does not exist in the middle. 
     A singular expression includes a plural expression unless the context clearly dictates otherwise. 
     In the following description, even if an embodiment is described with reference to specific figure, a reference numeral not indicated in the specific figure may be referred to if necessary, and the reference numeral not indicated in the specific figure may be used when indicated in the other figure. 
     The directions of upward (U, y), downward (D), leftward (Le, x), rightward (Ri), forward (F, z), and rear direction (R) indicated in  FIG.  2    are used for convenience of explanation, and the technical spirit of the present disclosure is not limited thereby. 
     Referring to  FIGS.  1  and  2   , an air conditioner  1  may include an outdoor unit  20  and a ventilator  10 . The outdoor unit  20  may include a compressor (not shown) for compressing refrigerant and an outdoor heat exchanger (not shown) for performing heat exchange between refrigerant and outdoor air. The outdoor unit  20  may be connected to the ventilator  10  through a refrigerant pipe  11   a . The refrigerant may circulate the outdoor unit  20  and the ventilator  10  through the refrigerant pipe. A housing  10 H of the ventilator  10  may form the exterior of the ventilator  10 . 
     The housing  10 H may include a first long side LS 1  and a second long side LS 2  opposite to the first long side LS 1 . The first long side LS 1  and the second long side LS 2  may be collectively referred to as a long side LS 1  and LS 2 . The housing  10 H may include a first short side SS 1  adjacent to the long side LS 1  and LS 2  and a second short side SS 2  opposite the first short side SS 1 . The first short side SS 1  and the second short side SS 2  may be collectively referred to as a short side SS 1  and SS 2 . 
     A direction vertically to the long side LS 1  and LS 2  and the short side SS 1  and SS 2  may be referred to as a first direction DR 1  or a left-right direction. A direction parallel to the short side SS 1  and SS 2  may be referred to as a second direction DR 2  or an up-down direction. A direction parallel to the long side LS 1  and LS 2  may be referred to as a third direction DR 3  or a front-rear direction. 
     A side of the first long side LS 1  may be referred to as an upper side (U, y), and a side of the second long side LS 2  may be referred to as a lower side (D). A side of the first short side SS 1  may be referred to as a front side (F, z), and a side of the second short side SS 2  may be referred to as a rear side (R). In the first direction DR 1 , a direction toward one end of the short side SS 1  and SS 2  may be referred to as a left side (Le, x), and a direction toward the other end of the short side SS 1  and SS 2  may be referred to as a right side (Ri). 
     The ventilator  10  may include a refrigerant distributor  11 , a plurality of heat exchangers  12 ,  13 ,  14 ,  15 , and  19 , a blower  16 , a partition  17 , and an exhaust fan  18 . The refrigerant distributor  11 , the plurality of heat exchangers  12 ,  13 ,  14 ,  15 , and  19 , the blower  16 , the partition  17 , and the exhaust fan  18  may be installed inside the housing  10 H. 
     An air supply passage OA-SA may be formed between a first inlet  10   i  and a first outlet (not shown). The first inlet  10   i  may be formed to penetrate the second short side SS 2  and may be adjacent to the first long side LS 1 . The first outlet may be formed to penetrate the second long side LS 2  and may be adjacent to the first short side SS 1 . Outdoor air OA may be introduced into the first inlet  10   i , and the first inlet  10   i  may be referred to as an outdoor air inlet. Supply air SA may be supplied into a room through the first outlet, and the first outlet may be referred to as a supply air outlet. 
     The blower  16  may be adjacent to the first outlet and located in the air supply passage OA-SA. The blower  16  may cause a flow of air along the air supply passage OA-SA. The blower  16  may be referred to as an air supply fan or a plug fan. Meanwhile, an air supply duct (not shown) may be connected to the second long side LS 2  and may communicate with the first outlet and the indoor space. For example, the air volume per minute of the blower  16  may be 3,000 to 5,000 cubic feet per minute (CFM). 
     The exhaust passage RA-EA may be formed between the second inlet  10   p  and the second outlet  10   g . The second inlet  10   p  may be formed to penetrate the second long side LS 2  and may be spaced apart from the first outlet. The second outlet  10   g  may be formed through the second short side SS 2  and may be adjacent to the second long side LS 2 . The bet (RA, room air, or return air) may be introduced into the second inlet  10   p , and the second inlet  10   p  may be referred to as a bet inlet. Exhaust air (EA) may be discharged to the outside through the second outlet  10   g , and the second outlet  10   g  may be referred to as an exhaust outlet. 
     The exhaust fan  18  may be located in the exhaust passage RA-EA adjacent to the second discharge port  10   g . The exhaust fan  18  may cause a flow of air along the exhaust passage RA-EA. The exhaust fan  18  may be referred to as a blower or a plug fan. On the other hand, the inner duct (not shown) may be connected to the second long side (LS 2 ), it may be in communication with the second inlet ( 10   p ) and the indoor space. 
     The partition wall  17  may divide the inner space of the housing  10 H into a space in which the air supply passage OA-SA is formed and a space in which the exhaust passage RA-SA is formed. The partition wall  17  may be installed near the second inlet  10   p  of the housing  10 H, and may include an inclined portion (unsigned) and a horizontal portion (unsigned). Accordingly, the air supply passage OA-SA may be located above the partition wall  17 , and the exhaust passage RA-SA may be located below the partition wall  17 . 
     The refrigerant distributor  11  may be adjacent to the first long side LS 1  and the first short side SS 1 . One side of the refrigerant distributor  11  may be connected to the refrigerant pipe ( 11   a ). The other side of the refrigerant distributor  11  may be connected to a plurality of pipes  11   b ,  11   c ,  11   d , and  11   e . For example, the refrigerant distributor  11  may open and close the passage of each pipe through a solenoid valve. Here, each pipe  11   b ,  11   c ,  11   d , or  11   e  may include a refrigerant pipe providing a passage of refrigerant supplied to each heat exchanger  12 ,  14 ,  15 , or  19 , and a refrigerant pipe providing a passage of refrigerant passing through each heat exchanger  12 ,  14 ,  15 , or  19 . In addition, each expansion valve (not shown) may expand the refrigerant flowing through each of the pipes  11   b ,  11   c ,  11   d , and  11   e . For example, the expansion valve may be an Electronic Expansion Valve (EEV) capable of adjusting the opening degree. In this case, when the expansion valve is fully opened, the expansion valve may not expand the refrigerant. 
     The preheater  12  may be located in the air supply passage OA-SA adjacent to the first inlet  10   i . A preheater  12  may be disposed vertically within the housing  10 H. A first pipe  11   b  may provide a refrigerant passage connecting the refrigerant distributor  11  and the preheater  12 . Accordingly, the preheater  12  may heat air introduced into the first inlet  10   i . The preheater  12  may be referred to as a preheat coil. 
     The heat exchanger  14  may be located downstream of the preheater  12  in the air supply passage OA-SA. The heat exchanger  14  may be vertically disposed within the housing  10 H. A size of the heat exchanger  14  may be larger than a size of the preheater  12 . The second pipe  11   c  may provide a refrigerant passage connecting the refrigerant distributor  11  and the heat exchanger  14 . The heat exchanger  14  may be referred to as a main heat exchanger or a cooling/heating coil. 
     A reheater  15  may be located downstream of the heat exchanger  14  in the air supply passage OA-SA. The reheater  15  may be vertically disposed within the housing  10 H. A size of the reheater  15  may be smaller than a size of the heat exchanger  14 . The third pipe  11   d  may provide a refrigerant passage connecting the refrigerant distributor  11  and the reheater  15 . The reheater  15  may be referred to as a reheat coil. Meanwhile, the reheater  15  may be operated based on a set indoor temperature and a set humidity. The reheater  15  may face the blower  16  with respect to a base  10 W on which the reheater  15  is installed. 
     A recovery coil  19  may be located in an exhaust passage RA-EA adjacent to the exhaust fan  18 . The recovery coil  19  may be vertically disposed within the housing  10 H. A fourth pipe  11   e  may provide a refrigerant passage connecting the refrigerant distributor  11  and the recovery coil  19 . Meanwhile, a heat transfer direction of the recovery coil  19  to air may be opposite to a heat transfer direction of the heat exchanger  14  to air. 
     A part of the recovery wheel  13  may be located in the air supply passage OA-SA between the preheater  12  and the heat exchanger  14 , and the other part of the recovery wheel  13  may be located in the exhaust passage RA-EA between the recovery coil  19  and the inclined portion of the partition wall  17 . The recovery wheel  13  may be referred to as an energy recovery wheel (ERW). 
     In this case, the recovery wheel  13  may have a flat cylinder shape as a whole. A honeycomb structure may be formed inside the recovery wheel  13 , and air may pass through the honeycomb structure. The recovery wheel  13  may be rotated at a low speed. Accordingly, the recovery wheel  13  may recover sensible heat and latent heat by using temperature difference and humidity difference between the outdoor air OA and the indoor air RA. 
     Referring to  FIGS.  2  and  3   , the blower  16  may include a motor  16   a , a hub  16   b , a shroud  16   c , and a plurality of blades  16   d . The hub  16   b , the shroud  16   c , and the plurality of blades  16   d  may be collectively referred to as an impeller  16   a ,  16   b , and  16   c.    
     The motor  16   a  may provide a rotational force. The motor  16   a  may be a centrifugal fan motor. The motor  16   a  may form a front end of the blower  16 , and a rotational shaft of the motor  16   a  may extend rearward from the motor  16   a . A longitudinal direction of the rotational shaft of the motor  16   a  may be referred to as an axial direction of the blower  16 . 
     The hub  16   b  may be located at the rear of the motor  16   a  and may be fixed to the rotational shaft of the motor  16   a . The hub  16   b  may have a disk shape. 
     The shroud  16   c  may be located at the rear of the hub  16   b  and may have a ring plate shape. The shroud  16   c  may be rotatably coupled to the base  10 W. For example, an inlet (unsigned) may be fixed to a front surface of the base  10 W between the shroud  16   c  and the base  10 W, and may have a hyperbolic cylinder or funnel shape. In this case, the shroud  16   c  may be rotatably coupled to the inlet. A hole formed inside the shroud  16   c , an inner space of the inlet, and a hole (not shown) formed in the base  10 W may communicate with one another and be located in the air supply passage OA-SA (see  FIG.  1   ). 
     The plurality of blades  16   d  may be located between an inner periphery and an outer periphery of the ring-shaped shroud  16   c . The plurality of blades  16   d  may be coupled to the hub  16   b  and the shroud  16   c  between the hub  16   b  and the shroud  16   c . The plurality of blades  16   d  may be formed integrally with the shroud  16   c  and the hub  16   b.    
     In addition, the plurality of blades  16   d  may be spaced apart from each other in a rotating direction of the rotational shaft of the motor  16   a . Each of the plurality of blades  16   d  may be convexly curved in the rotating direction of the rotational shaft (see  FIGS.  4  and  5   ). Among the plurality of blades  16   d , a blade positioned close to a mount plate  110  to be described later may be convex toward the mount plate  110 . 
     Accordingly, when the impeller  16   a ,  16   b , and  16   c  is rotated in a clockwise direction in response to driving of the motor  16   a , air may be introduced in an axial direction of the blower  16  through a hole of the base  10 W and may be pressed by the plurality of blades  16   d  to be discharged in a radial direction of the blower  16 . In this case, a flow of air discharged by the blower  16  may be concentrated on the left side of the blower  16  rather than the right side of the blower  16 . 
     A horizontal plate  10   a  may be vertically disposed on a front surface of the base  10 W, and may be coupled to the front surface of the base  10 W. The horizontal plate  10   a  may be located above the blower  16 . The horizontal plate  10   a  may be referred to as a first horizontal wall or a first panel. Meanwhile, a frame  16   e  may form a skeleton of the blower  16 , and a motor mount  1600  on which the motor  16   a  is mounted may be coupled to the frame  16   a . The frame  16   e  may be coupled to the bottom of the horizontal plate  10   a.    
     A top plate  10   b  may be disposed vertically to the front surface of the base  10 W, and may be coupled to the front surface of the base  10 W. The top plate  10   b  may be located below of the blower  16 . The top plate  10   b  may be referred to as a second horizontal wall or a second panel. A top hole  100   a  may be formed to penetrate the top plate  10   b  in the up-down direction. The top hole  100   a  may be formed to be long in the left-right direction. In the up-down direction, at least a portion of the top hole  100   a  may overlap the blower  16 . 
     A bottom plate  10   c  may be disposed vertically to the front surface of the base W, and may be coupled to the front surface of the base  10 W. The bottom plate  10   c  may face the horizontal plate  10   a  with respect to the top plate  10   b . The bottom plate  10   c  may form a part of the second long side LS 2  of the housing  10 H. The bottom hole  100   b  may be formed to penetrate the bottom plate  10   c  in the up-down direction. The bottom hole  100   b  may be formed to be long in the left-right direction. In the up-down direction, the bottom hole  100   b  may face the top hole  100   a.    
     The side plate  10   d  may be disposed vertically to the front surface of the base W, and may be coupled to the front surface of the base W. The side plate  10   d  may be coupled to a right side of the horizontal plate  10   a , a right side of the top plate  10   b , and a right side of the bottom plate  10   c . A side hole  100   c  may be formed to penetrate the side plate  10   d  in the left-right direction. The side hole  100   c  may be formed to be long in the front-rear direction. The side hole  100   c  may be located between a right side of the top plate  10   b  and a right side of the bottom plate  10   c.    
     The mount plate  110  may include a first plate  111  and a second plate  112 . The first plate  111  may be vertically disposed on the front surface of the base W and an upper surface of the bottom plate  10   c , and may be coupled to the front surface of the base W and the upper surface of the bottom plate  10   c . The first plate  111  may be coupled to a left side of the top plate  10   b . The second plate  112  may extend obliquely in a direction away from the blower  16  from an upper end of the first plate  111 . In this case, a left side of the base  10 W, a left side of the horizontal plate  10   a , a left side of the second plate  112 , and a left side of the bottom plate  10   c  may be connected to a left side of the housing  10 H. 
     A first space  101 S may be formed between the horizontal plate  10   a  and the top plate  10   b . A vertical plate (not shown) may be connected to a front end of the horizontal plate  10   a  and a front end of the top plate  10   b , and may close a front side of the first space  101 S. The first space  101 S may communicate with the top hole  100   a.    
     A second space  102 S may be formed between the top plate  10   b  and the bottom plate  10   c . The vertical plate may be connected to a front end of the top plate  10   b  and a front end of the bottom plate  10   c , and may close the front side of the second space  102 S. The second space  102 S may communicate with the bottom hole  100   b  and the side hole  100   c.    
     For example, the bottom hole  100   b  may be opened, and the side hole  100   c  may be closed. The side hole  100   c  may be closed by a detachable cover (not shown) or may not be initially formed in the side plate  10   d.    
     As another example, the bottom hole  100   b  may be closed, and the side hole  100   c  may be opened. The bottom hole  100   b  may be closed by a detachable cover (not shown) or may not be initially formed in the bottom plate  10   c.    
     Referring to  FIGS.  3  and  4   , the gas furnace  100  may include a fuel valve  120 , a manifold  130 , a burner  140 , a heat exchanger  150 , a collect box  160 , and an inducer  170 . 
     The fuel valve  120  may supply fuel from a fuel source (not shown) connected to the fuel valve  120  to the manifold  130 , or may block the supply of the fuel to the manifold  130 . For example, the fuel may be Liquefied Natural Gas (LNG) or Liquefied Petroleum Gas (LPG). Meanwhile, by adjusting an opening degree of the fuel valve  120 , it is possible to adjust an amount of the fuel supplied to the manifold  130 . In other words, thermal power of the gas furnace  100  may be adjusted in stages using the fuel valve  120 . The fuel valve  120  may be referred to as a modulating valve. 
     The burner  140  may be supplied with the fuel from the manifold  130 . The burner  140  may burn the fuel. When the fuel is burned, a flame and high-temperature combustion gas may be generated. For example, the burner  140  may be provided in plural. A plurality of burners  140  may be installed inside a burner box  140   a . The burner box  140   a  may be installed on the left of the first plate  111  of the mount plate  110 . 
     The heat exchanger  150  may be located in the second space  102 S between the top plate  10   b  and the bottom plate  10   c . The heat exchanger  150  may provide a passage for the combustion gas. One end of the heat exchanger  150  may be coupled to the right of the first plate  111  of the mount plate  110 . The other end of the heat exchanger  150  may be spaced apart from the one end of the heat exchanger  150 , and may be coupled to the right of the first plate  111 . 
     In addition, the heat exchanger  150  may be provided in plural. The number of heat exchangers  150  may be equal to the number of burners  140 . A plurality of heat exchangers  151 ,  152 ,  153 ,  154 ,  155 , and  156  may be connected to the plurality of burners  140 , respectively. The plurality of heat exchangers  151 ,  152 ,  153 ,  154 ,  155 , and  156  may be spaced apart from each other in the front-rear direction. 
     In addition, the heat exchanger  150  may be a tubular type heat exchanger. The heat exchanger  150  may include a first tube  150   a , a bend  150   b , and a second tube  150   c . The passage of the combustion gas may be formed inside the first tube  150   a , inside the bend  150   b , and inside the second tube  150   c . For example, a diameter of the first tube  150   a  may be substantially equal to a diameter of the bend  150   b  and a diameter of the second tube  150   c.    
     The first tube  150   a  may extend long in the left-right direction. The left end of the first tube  150   a  may form the one end of the heat exchanger  150 , and may be referred to as an inlet of the heat exchanger  150 . The inlet of the heat exchanger  150  may communicate with the burner  140  through the first hole  111   a . Here, the first hole  111   a  may be formed to pass through the first plate  111  in the left-right direction, and may be located between the inlet of the heat exchanger  150  and the burner  140 . For example, the inlet of the heat exchanger  150  may be spaced apart from the burner  140 . That is, air may be introduced into the burner  140  between the entry of the heat exchanger  150  and the burner  140 , and the corresponding air may be referred to as secondary air. 
     The second tube  150   c  may extend long in the left-right direction. The second tube  150   c  may be spaced upward from the first tube  150   a . A left end of the second tube  150   c  may form the other end of the heat exchanger  150 , and may be referred to as an exit of the heat exchanger  150 . The exit of the heat exchanger may communicate with the inside of the collect box  160  to be described later through a second hole  111   b . In this case, the second hole  111   b  may be formed to penetrate the first plate  111  in the left-right direction, and may be located between the exit of the heat exchanger  150  and the collect box  160 . 
     The bend  150   b  may be connected to a right end of the first tube  150   a  and a right end of the second tube  150   c . The bend  150   b  may be convex to the right. The bend  150   b  may transfer the combustion gas passing through the first tube  150   a  to the second tube  150   c . Accordingly, the combustion gas may flow to the right in the first tube  150   a , and may flow to the left in the second tube  150   b . The bend  150   b  may be referred to as a U-shaped bend. 
     Meanwhile, according to an embodiment, a bend connected to the left end of the second tube  150   c  and convex to the left, and a tube connected to the bend and disposed in parallel with the second tube  150   c  may be added. 
     The collect box  160  may be located above the burner box  140   a  and may be installed on the left of the first plate  111  of the mount plate  110 . The combustion gas passing through the heat exchanger  150  may be introduced into the inside of the collect box  160 . 
     The inducer  170  may be installed on the left of the collect box  160 . The inlet of the inducer  170  may communicate with the inside of the collect box  160 . An exit  171  of the inducer  170  may be connected to an exhaust pipe  180  (see  FIG.  2   ). The inducer  170  may cause the combustion gas to flow through the heat exchanger  150 , the collector box  160 , the inducer  170 , and the exhaust pipe  180 . In addition, the inducer  170  may cause a fluid to flow through the burner  140 . The inducer  170  may be referred to as a fan, a blower, or an induced draft motor (IDM). 
     The exhaust pipe  180  (see  FIG.  2   ) may extend upward from the exit  171  of the inducer  170 . The exhaust pipe  180  may pass through the second plate  112 , the horizontal plate  10   a , and the first long side LS 1  of the mount plate  110 , and may discharge the combustion gas to the outside. The combustion gas flowing through the exhaust pipe  180  may be referred to as exhaust gas. 
     Accordingly, the air discharged from the blower  16  may pass around the heat exchanger  150  through the top hole  100   a , and may be supplied into an indoor space through the bottom hole  100   b  or the side hole  100   c . In this case, the air passing around the heat exchanger  150  may receive thermal energy from the combustion gas flowing along the heat exchanger  150 . That is, the temperature of the air may be increased while the air passes around the heat exchanger  150 . 
     Meanwhile, the gas furnace  100  may include a roll-out switch, a limit switch, a pressure switch, and the like. 
     Referring to  FIGS.  4  and  5   , a plurality of burners  141 ,  142 ,  143 ,  144 ,  145 , and  146  may have the same shape. The plurality of burners  141 ,  142 ,  143 ,  144 ,  145 , and  146  may be spaced apart from each other in the front-rear direction. 
     Each of the plurality of burners  141 ,  142 ,  143 ,  144 ,  145 , and  146  may include a venturi portion  140   v  and a head portion  140   h . The holder  140   s  may be connected to an entry  140   i  of the venturi portion  140   v , and may face and be spaced apart from the entry  140   i  of the venturi portion  140   v . The holder  140   s  may be referred to as a spud. For example, ribs  140   r  may be formed by being recessed inward of the head portion  140   h  from a side surface of the head portion  140   h . For example, a retainer (not shown) may be inserted into the head portion  140   h  and seated on the ribs  140   r , and a flame to be described above and below may be seated on the retainer. 
     Meanwhile, a flange  140   f  may be connected to the plurality of burners  141 ,  142 ,  143 ,  144 ,  145 ,  146  between the plurality of burners  141 ,  142 ,  143 ,  144 ,  145 ,  146 . In this case, a flame propagation port may be formed in the flange  140   f  between the plurality of burners  141 ,  142 ,  143 ,  144 ,  145  and  146 , and may provide a channel for flame propagation to be described later. 
     The plurality of burners  141 ,  142 ,  143 ,  144 ,  145 , and  146  may be classified into two groups. A first burner  141 , a second burner  142 , a third burner  143 , a fourth burner  144 , a fifth burner  145 , and a sixth burner  146  may be sequentially arranged in the front-rear direction. In this case, the first burner  141 , the second burner  142 , and the third burner  143  may be classified as a first burner group  140 G 1 , and the fourth burner  144 , the fifth burner  145 , and the sixth burner  146  may be classified as a second burner group  140 G 2 . 
     A connector  133  may be positioned between the first burner group  140 G 1  and the second burner group  140 G 2 . The connector  133  may include a first part  133   a , a second part  133   b , and a third part  133   c . The connector  133  may be referred to as a housing or a tube fitting. 
     The first part  133   a  may form a first end of the connector  133 , and the first end may be a left end of the connector  133 . The second part  133   b  may form a second end of the connector  133 , and the second end may be a front end of the connector  133 . The third part  133   c  may form a third end of the connector  133 , and the third end may be a right end of the connector  133 . The first part  133   a , the second part  133   b , and the third part  133   c  may communicate with each other. 
     The manifold  130  may include a first tube  130   a , a second tube  130   b , and a third tube  130   c . The first tube  130   a , the second tube  130   b , and the third tube  130   c  may be connected to each other through the connector  133 . The connector  133  may be positioned between the second tube  130   b  and the third tube  130   c . For example, the first tube  130   a  may be bent at least once, and the second tube  130   b  and the third tube  130   c  may extend in the front-rear direction. 
     The first tube  130   a  may have one end of the first tube  130   a  connected to the fuel valve  120  and the other end connected to the first part  133   a , and may form a first passage through which fuel may flow. The second tube  130   b  may have one end of the second tube  130   b  connected to the second part  133   b  and the other end of the second tube  130   b  blocked, and may form a second passage through which fuel may flow. The second tube  130   b  may extend in the front-rear direction, which is a direction in which the burners  141 ,  142 , and  143  in the first burner group  140 G 1  are arranged, and may face the burners  141 ,  142 , and  143  in the first burner group  140 G 1 . The third tube  130   c  may have one end connected to the third part  133   c  and the other end blocked, and may form a third passage through which fuel may flow. The third tube  130   c  may extend in the front-rear direction, which is a direction in which the burners  144 ,  145 , and  146  in the second burner group  140 G 2  are arranged, and may face the burners  144 ,  145 , and  146  in the second burner group  140 G 2 . That is, the third tube  130   c  may face the second tube  130   b  with respect to the connector  133 . 
     A plurality of first nozzles  131   a ,  131   b , and  131   c  may be coupled to the second tube  130   b , and may be inserted into and fixed to respective holders  130   s  of the burners  141 ,  142 , and  143  in the first burner group  140 G 1 . A plurality of second nozzles  132   a ,  132   b , and  132   c  may be coupled to the third tube  130   c , and may be inserted into and fixed to respective holders  140 S of the burners  144 ,  145 , and  146  in the second burner group  140 G 2 . 
     Accordingly, fuel provided from the fuel valve  120  to the first tube  130   a  may be provided to the second tube  130   b  and the third tube  130   c  through the connector  133 . 
     In addition, the plurality of first nozzles  131   a ,  131   b  and  131   c  may inject the fuel of the second tube  130   b  to the burners  141 ,  142 , and  143  in the first burner group  140 G 1 . In this case, between the plurality of first nozzles  131   a ,  131   b , and  131   c  and the burners  141 ,  142 , and  143  in the first burner group  140 G 1 , primary air may be entrained into the burners  141 ,  142 , and  143  due to the inertial force and viscous force of the injected fuel. 
     In addition, the plurality of second nozzles  132   a ,  132   b , and  132   c  may inject the fuel of the third tube  130   c  to the burners  144 ,  145 , and  146  in the second burner group  140 G 2 . In this case, between the plurality of second nozzles  132   a ,  132   b , and  132   c  and the burners  144 ,  145 , and  146  in the second burner group  140 G 2 , primary air is entrained into the burners  144 ,  145 , and  146  due to the inertial force and viscous force of the injected fuel. 
     That is, the primary air and the fuel may pass through the plurality of burners  141 ,  142 ,  143 ,  144 ,  145  and  146  to form a mixture, and the mixture may be burned together with the secondary air. Aflame formed through such combustion may be referred to as a partially premixed flame. 
     Meanwhile, an igniter  140   b  may be detachably mounted to the burner box  140   a  and may be adjacent to an exit of a burner positioned at one end of the plurality of burners  140 . The igniter  140   b  may be adjacent to an exit of a burner positioned farthest from the first burner group  140 G 1  among the burners  144 ,  145 , and  146  in the second burner group  140 G 2 . That is, the igniter  140   b  may be adjacent to an exit of the sixth burner  146  and may burn fuel that has passed through the sixth burner  146 . The flame formed at the exit of the sixth burner  146  may be propagated to the exits of the remaining burners  145 ,  144 ,  143 ,  142  and  141  through the flame propagation port described above with reference to  FIG.  5   . The propagated flame may burn fuel that has passed through the remaining burners  145 ,  144 ,  143 ,  142  and  141 . 
     Meanwhile, a flame detector  140   c  may be detachably mounted to the burner box  140   a , and may be adjacent to an exit of a burner positioned at the other end of the plurality of burners  140 . The flame detector  140   c  may be adjacent to an exit of a burner positioned farthest from the second burner group  140 G 2  among the burners  141 ,  142 , and  143  in the first burner group  140 G 1 . That is, the flame detector  140   c  may be adjacent to an exit  141   e  of the first burner  141  and may detect whether a flame is formed at the exit  141   e  of the first burner  141 . When the flame detector  140   c  detects a flame of the first burner  141 , it is considered that a flame is formed in the remaining burners  142 ,  143 ,  144 ,  145 , and  146  as a result of combustion due to the characteristics of flame propagation described above. 
     Referring to  FIGS.  6  and  7   , a ball  134  may be positioned between the first part  133   a , the second part  133   b , and the third part  133   c , and may be rotatably coupled to the inside of the connector  133 . The ball  134  may have a hollow sphere shape. The ball  134  may be opened in three directions. The inside of the ball  134  may communicate with the inside of the parts  133   a ,  133   b  and  133   c  of the connector  133  through openings  134   a ,  134   b  and  134   c  of the ball  134 . A direction in which the ball  134  is opened may correspond to a direction in which each of the parts  133   a ,  133   b  and  133   c  extends from the connector  133 . That is, the above-described positional relationship between the second part  133   b  and the third part  133   c  with respect to the first part  133   a  may be identical to a positional relationship between the second opening  134   b  and the third opening  134   c  with respect to the first opening  134   a.    
     A shaft  134   s  may extend in the up-down direction. The shaft  134   s  may pass through the connector  133  and may be fixed to the ball  134 . The shaft  134   s  may provide a central axis of rotation Ax of the ball  134 . For example, the shaft  134   s  may be fixed to a rotational shaft of a rotary motor  135 . For another example, the shaft  134   s  may be a rotational shaft of the rotary motor  135 . The rotary motor  135  may be an electric motor capable of adjusting a direction of rotation and an angle of rotation. 
     A motor mount  133   m  may be coupled to an upper side of the connector  133 , and the shaft  134   s  may pass therethrough. The rotary motor  135  may be mounted on the motor mount  133   m.    
     Accordingly, when the rotary motor  135  is driven, the ball  134  may be rotated with respect to the central axis of rotation Ax. 
     The aforementioned connector  133 , ball  134 , the shaft  134   s , and the rotary motor  135  may be collectively referred to as a three-way valve  133 ,  134 ,  134   s , and  135 . In addition, as described above and below, the three-way valve  133 ,  134 ,  134   s , and  135  has an advantage of efficiently connecting or blocking a passage with a simple structure. However, the three-way valve applicable to the present disclosure is not limited thereto, and a generally used three-way valve may also be applied to the present disclosure. 
     Referring to  FIG.  8   , a controller C of the gas furnace may receive information from a thermostat TS provided in an indoor space through a communication part T. For example, information received from the thermostat TS may include information such as a heating signal, heating intensity, a desired indoor temperature, or a current indoor temperature. 
     The controller C may receive information on operation of the gas furnace from a sensor SS. For example, the sensor SS may detect temperature of air introduced into or discharged from the blower  16 , or may sense temperature of air that has passed through the heat exchanger  150 . 
     The igniter  140   b  and the flame detector  140   c  may be electrically connected to the controller C. That is, the controller C may control operation of the igniter  140   b  and may receive information on whether or not a flame is detected from the flame detector  140   c.    
     The blower  16 , the inducer  170 , and the fuel valve  120  may be electrically connected to the controller C. That is, the controller C may control a revolution per minute (RPM) of the blower  16 , an RPM of the inducer  170 , and an opening degree of the fuel valve  120 . 
     The rotary motor  135  may be electrically connected to the controller C. That is, the controller C may control a direction of rotation and an angle of rotation of the rotary motor  135 . 
     The memory M may be electrically connected to the controller C. The memory M may store information related to operation of the gas furnace, information related to a control operation of the controller C, and the like, and may provide the stored information to the controller C. 
     Referring to  FIGS.  8  and  9   , the controller C may perform an initial thermal power operation in S 1  That is, in consideration of an air ratio and stable ignition, an initial operation may be performed with an initial thermal power that corresponds to ¾ of a maximum thermal power. 
     After S 1 , the controller C may detect a required thermal power in S 10 . For example, a required thermal power Ld may be a thermal power that is arbitrarily input by a user through the thermostat TS. In another example, a required thermal power Ld may increase as a difference (hereinafter, referred to as temperature difference) between a desired indoor temperature input to the thermostat TS and a current indoor temperature sensed by a thermocouple of the thermostat TS is larger. In yet another example, a required thermal power Ld may be determined based on a temperature difference and temperature information of air flowing into the blower  16  sensed by the sensor SS. 
     After S 10 , the controller C may determine whether the required thermal power Ld is greater than or equal to a reference thermal power L 1  and less than or equal to a maximum thermal power Lm (S 20 ). For example, the reference thermal power L 1  may be ¼ of the maximum thermal power Lm. 
     Referring to  FIGS.  5  and  10   , the maximum thermal power Lm of the gas furnace may be 100,000 Btu/h. When the number of burners  141 ,  142 ,  143 ,  144 ,  145 , and  146  of the gas furnace is 6, a maximum thermal power per burner may be 16,667 Btu/h. In addition, considering flame stability of the burner, the minimum force per burner may be 4,167 Btu/h. That is, a maximum TDR per burner may be limited to 4 (=16,667/4,167). Here, the TDR refers to a ratio of maximum thermal power to minimum thermal power. 
     Referring to (a) of  FIG.  10   , a required thermal power Ld of the gas furnace may be 25,000 Btu/h. In this case, a TDR of the gas furnace may be 4 (=100,000/25,000). In order to implement the above required thermal power Ld, the first burner group  140 G 1  and the second burner group  140 G 2  may each have a thermal power of 12,500 Btu/h. In other words, each of the burners  141 ,  142 , and  143  in the first burner group  140 G 1  may have a thermal power of 4,167 Btu/h, and each of the burners  144 ,  145 , and  146  in the second burner group  140 G 2  may have a thermal power of 4,167 Btu/h. 
     Referring to (b) of  FIG.  10   , the required thermal power Ld of the gas furnace may be 12,500 Btu/h. In this case, the TDR of the gas furnace may be 8 (=100,000/12,500). In order to implement the above required thermal power Ld, the first burner group  140 G 1  may have no thermal power, but the second burner group  140 G 2  may have a thermal power of 12,500 Btu/h. In other words, each of the burners  144 ,  145 , and  146  in the second burner group  140 G 2  may have a thermal power of 4,167 Btu/h. 
     Accordingly, although the maximum TDR per burner is limited to 4, it is possible to implement the maximum TDR of the gas furnace up to 8. In this case, the reference thermal power L 1  may be determined to be ¼ of the maximum thermal power Lm, and the minimum thermal power L 0  of the gas furnace may be ⅛ of the maximum thermal power Lm. 
     When the required thermal power Ld is greater than or equal to the reference thermal power L 1  and less than or equal to the maximum thermal power Lm (Yes in S 20 ), the controller C may perform a first operation mode S 21 , S 22 , and S 23  which will be described later with reference to  FIGS.  9  and  11    (see  FIG.  9   ). 
     When the required thermal power Ld is less than the reference thermal power L 1  (No in S 20 ), the required thermal power Ld may fall within a range from the minimum thermal power L 0  to the reference thermal power L 1  (S 24 ) and the controller C may perform a second operation mode S 25 , S 26 , and S 27 , which will be described later with reference to  FIGS.  9  and  12    (see  FIG.  9   ). 
     Referring to  FIGS.  9  and  11   , in the first operation mode S 21 , S 22 , and S 23 , the controller C (see  FIG.  8   ) may control a direction of rotation and an angle of rotation of the rotary motor  135  so that the first part  133   a  communicates with the second part  133   b  and the third part  133   c . In this case, some F 2  of fuel F 1  passing through the first tube  130   a  may be provided to the second tube  130   b  and the first burner group  140 G 1  through the ball  134 , and the remaining F 3  may be provided to the third tube  130   c  and the second burner group  140 G 2  through the ball  134  (S 21 ). 
     Specifically, the first opening  134   a  of the ball  134  may face a passage of the first part  133   a . The second opening  134   b  of the ball  134  may face a passage of the second part  133   b . The third opening  134   c  of the ball  134  may face a passage of the third part  133   c . In this case, the ball  134  may be located at a first position. 
     In response to the required thermal power Ld, the controller C may control an opening degree of the fuel valve  120  (see  FIG.  4   ) and the RPM of the inducer  170  (see  FIG.  4   ) (S 22  and S 23 ). That is, as the required thermal power Ld is smaller, the opening degree of the fuel valve  120  and the RPM of the inducer  170  may be reduced. 
     Accordingly, the gas furnace may implement the required thermal power Ld in a range between the reference thermal power L 1  and the maximum thermal power Lm. 
     Referring to  FIGS.  9  and  12   , in the second operation mode S 25 , S 26 , and S 27 , the controller C (see  FIG.  8   ) may control a direction of rotation and an angle of rotation of the rotary motor  135  so that the first part  133   a  communicates with the third part  133   c  while not communicating with the second part  133   b . In this case, the fuel F 1  passing through the first tube  130   a  may be provided to the third tube  130   c  and the second burner group  140 G 2  through the ball  134  (S 25 ). 
     Specifically, the first opening  134   a  of the ball  134  may face the passage of the third part  133   c . The second opening  134   b  of the ball  134  may face the passage of the first part  133   a . The third opening  134   c  of the ball  134  may face an inner wall of the connector  133 . The ball  134  may close the passage of the second part  133   b . In this case, the ball  134  may be located at a second position. 
     In response to the required thermal power Ld, the controller C may control an opening degree of the fuel valve  120  (see  FIG.  4   ) and the RPM of the inducer  170  (see  FIG.  4   ) (S 26  and S 27 ). That is, as the required thermal power Ld is smaller, the opening degree of the fuel valve  120  and the RPM of the inducer  170  may be reduced. 
     Accordingly, the gas furnace may implement the required thermal power Ld in a range between the minimum force L 0  and the reference thermal power L 1 . 
     Referring back to  FIGS.  11  and  12   , a diameter of the ball  134  may be greater than an inner diameter of the first part  133   a , an inner diameter of the second part  133   b , and an inner diameter of the third part  133   c . In addition, a curved portion  134   d  of the ball  134  may face the first opening  134   a  with respect to the second opening  134   b  and the third opening  134   c . Accordingly, when the ball  134  is located at the second position, a portion of the curved portion  134   d  may be inserted into the second part  133   b  and the passage of the second part  133   b  may be sealed. 
     A groove  133   d  may be formed in a portion of the connector  133  positioned between the second part  133   b  and the third part  133   c  and may have a shape corresponding to a surface of the ball  134 . Accordingly, the groove  133   d  may contact the ball  134  rotating between the first position and the second position, and may support the rotation of the ball  134 . 
     Referring back to  FIGS.  5  and  12    again, the aforementioned igniter  140   b  may be adjacent to an exit of a burner located at one end of the second burner group  140 G 2 . In this case, an auxiliary detector  140   d  may be detachably mounted to the burner box  140   a , and may be adjacent to an exit of a burner located at the other end of the second burner group  140 G 2 . The auxiliary detector  140   d  may be adjacent to an exit of a burner located closest to the first burner group  140 G 1  among the burners  144 ,  145 , and  146  in the second burner group  140 G 2 . That is, the igniter  140   b  may be adjacent to the exit of the sixth burner  146 , and the auxiliary detector  140   d  may be adjacent to the exit of the fourth burner  144 . 
     Accordingly, in the second operation mode S 25 , S 26 , and S 27  described above with reference to  FIGS.  9  and  12   , a flame formed at the exit of the sixth burner  146  by the igniter  140   b  may be propagated to the exits of the remaining burners  145  and  144  in the second burner group  140 G 2 . The propagated flame may burn the fuel that has passed through the remaining burners  145  and  144 . In addition, the auxiliary detector  140   d  may detect whether a flame is formed at the exit of the fourth burner  144 . When the auxiliary detector  140   d  detects the flame of the fourth burner  144 , it may be considered that a flame is formed in the remaining burner  145  as a result of the combustion due to the characteristics of flame propagation described above. 
     The controller C (see  FIG.  8   ) may be electrically connected to the auxiliary detector  140   d , and may receive information as to whether or not a flame is detected from the auxiliary detector  140   d.    
     Referring back to  FIGS.  3  and  4   , a first heat exchanger  151 , a second heat exchanger  152 , and a third heat exchanger  153  may be classified as a first heat exchanger group  151 ,  152 , and  153 , and a fourth heat exchanger  154 , a fifth heat exchanger  155 , and a sixth heat exchanger  156  may be classified as a second heat exchanger groups  154 ,  155 , and  156 . 
     The first heat exchanger group  151 ,  152 , and  153  may communicate with the first burner group  140 G 1  (see  FIG.  5   ), and the second heat exchanger group  154 ,  155 , and  156  may communicate with the second burner group  140 G 2 . 
     The reference line CL may extend between the first heat exchanger group  151 ,  152 , and  153  and the second heat exchanger group  154 ,  155 , and  156  in a longitudinal direction of the heat exchanger  150 , that is, in the left-right direction. The reference line CL may be referred to as a central line of the heat exchanger. 
     In this case, the blower  16  may be positioned to be biased toward the second heat exchanger group  154 ,  155 , and  156  with respect to the reference line CL (see  FIG.  4 E ). That is, air flowing by the blower  16  may be concentrated toward the second heat exchanger group  154 ,  155 , and  156  rather than the first heat exchanger group  151 ,  152 , and  153 . 
     Accordingly, in the second operation mode S 25 , S 26 , and S 27  described above with reference to  FIGS.  9  and  12   , the air passing around the heat exchanger  150  by the blower  16  may easily receive thermal energy from high-temperature combustion gas passing through the second burner group  140 G 2  (see  FIG.  5   ). 
     Referring to  FIGS.  9  and  13   , unlike the above description, the controller C (see  FIG.  8   ) may control a direction of rotation and an angle of rotation of the rotary motor  135  so that the first part  133   a  and the second part  133   b  are allowed to communicate with each other while not communicating with the third part  133   c , in S 25 . In this case, the fuel F 1  passing through the first tube  130   a  may be provided to the second tube  133   b  and the first burner group  140 G 1  through the ball  134 . 
     Specifically, the first opening  134   a  of the ball  134  may face the passage of the second part  133   b . The second opening  134   b  of the ball  134  may face the groove  133   d . The third opening  134   c  of the ball  134  may face the passage of the first part  133   a . A part of the curved portion  134   d  of the ball  134  may be inserted into the third part  133   c , and the ball  134  may close the passage of the third part  133   c.    
     In this case, unlike the above description provided with reference to  FIG.  5   , the igniter  140   b  may be adjacent to the exit of the first burner  141 , the flame detector  140   c  may be adjacent to the exit of the sixth burner  146 , and the auxiliary detector  140   d  may be adjacent to the exit of the third burner  143 . 
     That is, in S 25 , which group of the first burner group  140 G 1  and the second burner group  140 G 2  is to be supplied with fuel may be determined depending on the positions of the igniter  140   b , the flame detector  140   c , and the auxiliary detector  140   d , an installation environment of the gas furnace, or a position of the blower  16  relative to the heat exchanger  150 . Alternatively, if necessary, the first burner group  140 G 1  and the second burner group  140 G 2  may be alternately supplied with fuel in S 25 , and the positions of the igniter  140   b , the flame detector  140   c , and the auxiliary detector  140   d  may be changed together whenever the group to which is supplied is changed. 
     Certain embodiments or other embodiments of the disclosure described above are not mutually exclusive or distinct from each other. Any or all components of the embodiments of the disclosure described above may be combined with another or combined with each other in configuration or function. 
     For example, a configuration “A” described in one embodiment of the disclosure and the drawings and a configuration “B” described in another embodiment of the disclosure and the drawings may be combined with each other. Namely, although the combination between the configurations is not directly described, the combination is possible except in the case where it is described that the combination is impossible. 
     Although embodiments have been described with reference to a number of illustrative embodiments thereof, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the scope of the principles of this disclosure.