[DESCRIPTION] [Invention Title] DUAL VENTURI FOR COMBUSTOR [Technical Field] 5 The present invention relates to a dual venturi for a combustion apparatus, and more particularly, to a dual venturi for a combustion apparatus, capable of effectively controlling a heating value since respective amounts of gas and air supplied to a burner provided in a water heater are 10 controlled in a two-step manner and a motor is coupled to a damper so that the damper simultaneously opens or closes inlets for secondary air and gas along with rotation of the damper by driving of the motor. [Background Art] 15 In general, combustion apparatuses such as boilers and water heaters for the purpose of use of heating and hot water are classified into an oil boiler, a gas boiler, an electric boiler and a water heater depending on the fuel it is supplied with, and are variously developed and used 20 according to installation applications. In such combustion apparatuses, particularly the gas boiler and the water heater typically use a bunsen burner or a premixed burner to burn gas fuel. The premixed burner among others has a combustion method of mixing gas and air 25 in a mixing ratio for optimal combustion and then supplying -1Ia mixture (air + gas) to a flame hole section so that the mixture is burned. In addition, the performance of the combustion apparatuses is evaluated as a TDR (Turn-Down Ratio). The 5 TDR refers to "a ratio of maximum gas consumption to minimum gas consumption" in a gas combustion device in which an amount of gas is variably controlled. For example, when the maximum gas consumption is 24,000 kcal/h and the minimum gas consumption is 8,000 kcal/h, the TDR is 3:1. The TDR is 10 constrained by whether flame is stably maintained to some degree under minimum gas consumption. In the gas boiler and the water heater, convenience for use of heating and hot water is increased as the TDR becomes greater. That is, when the burner is actuated in a region 15 in which the TDR is small (namely, the minimum gas consumption is high) and loads of heating and hot water are small, the combustion apparatuses are frequently turned on/off. For this reason, variation in temperature control is increased and durability of the apparatuses is 20 deteriorated. Thus, in order to improve these problems, various methods for improving the TDR of the burner applied to the combustion apparatuses have been developed. In such a modulating control burner, valves allowing for supply of gas are mainly classified into an electrical 25 modulating gas valve controlled by current values and a - 2 pneumatic modulating gas valve controlled by differential pressures generated during supply of air. The pneumatic modulating gas valve controls an amount of gas supplied to the burner by differential pressures 5 generated when air required for combustion is supplied to the burner by a blower. In this case, air and gas required for combustion are mixed in a gas-air mixer and then supplied to the burner in a mixture (air + gas) form. In the gas-air mixer of the gas burner using the above 10 pneumatic modulating gas valve, the TDR is basically constrained by a factor such as a relation between gas consumption Q and differential pressure AP. The relation between flow rate and differential pressure of a fluid is generally as follows. 15 Q-kVAP 15' That is, as seen in the above relational equation, in order to double the flow rate of the fluid, the differential pressure has to be quadrupled. Accordingly, the ratio of pressure differences has to 20 be defined as 9:1 for defining the TDR as 3:1 and the ratio of pressure differences has to be defined as 100:1 for defining the TDR as 10:1. However, there is a problem in that it is impossible to infinitely increase gas supply pressure. -3- In order to resolve the problem in which the gas supply pressure may not be infinitely increased, there is disclosed a method which increases the TDR of the gas burner by respectively dividing passages, through which air and gas 5 are supplied, into two or more regions and by opening and closing each passage of gas injected into the burner. [Disclosure] [Technical Problem] The present invention has been made in view of the 10 above problems, and it is an object of the present invention to provide a dual venturi for a combustion apparatus, capable of achieving high operation durability, easy manufacturing, and a reduction in manufacturing cost while the dual venturi is compact by simplification of a 15 complicated structure. An additional or alternative object is to provide the public with a useful choice. [Technical Solution] In accordance with an aspect of the present invention, a dual venturi for a combustion apparatus includes a housing 20 having a first gas inlet and a second gas inlet formed at one side of an upper portion thereof and divided therein by a partition wall to have a first passage and a second passage, and an opening and closing unit provided within the housing, wherein the opening and closing unit allows gas 25 introduced through the second gas inlet to be introduced - 4 into the second passage and simultaneously opens the second passage to allow air to flow into the second passage, or prevents gas introduced through the second gas inlet from being introduced into the second passage and simultaneously 5 closes the second passage to prevent air from flowing into the second passage, wherein the opening and closing unit is connected to the second gas inlet, the opening and closing unit comprising: a moving body that is reciprocable vertically with respect to a flow direction of air that 10 passes through the second passage to allow the gas introduced through the second gas inlet to be introduced into the second passage or not according to the reciprocation; and a valve body rotated by a motor, wherein the valve body is connected with the moving body to 15 reciprocate the moving body according to the rotation, the valve body opening or closing the second passage according to the rotation. In an embodiment, the opening and closing unit may include a tubular guide tube having one or more concave 20 guides formed on an inner wall thereof; a spring provided within the moving body to elastically support the moving body; and a cap supporting the spring and coupled to an upper portion of the guide tube, wherein the moving body is tubular shaped, the moving body comprising: convex guides 25 that are formed on an outer wall thereof and correspond to - 5 the concave guides so that the moving body moves within the guide tube along a longitudinal direction of the guide tube; a gas outlet that is formed at a distal end thereof and communicates with the second gas inlet; and a moving body 5 cam that is formed on an outer surface thereof and has a cam shape, and wherein the valve body comprises: a damper that is formed on an outer surface thereof and opens or closes the second passage; a gas outlet that is formed at a side of the damper; and a valve body cam that is formed at an inner 10 side thereof in a shape corresponding to the moving body cam such that the valve body cam engages with the moving body cam. In an embodiment, when the valve body is rotated by the motor, respective convex portions of the moving body cam and 15 the valve body cam come into contact with each other and move the moving body such that such that gas passing through the gas outlet of the moving body is introduced into the second passage through the gas outlet of the valve bodyand simultaneously the second passage is opened as the damper of 20 the valve body is rotated. In an embodiment, the valve body may further include a sealing member for sealing an inner lower end thereof. In an embodiment, the motor may be a synchronous motor. [Advantageous Effects] 25 In accordance with the present invention having the - 6 above features, the following effects may be obtained. First, heating values such as low heating values or high heating values may be selectively generated by a water heater as necessary and a user may control the heating 5 values such as low heating values or high heating values as necessary. Therefore, fuel costs may be reduced. Secondly, an inner portion of a housing is divided by a partition wall to form a first passage and a second passage so that only primary air and gas flow in the first passage 10 and only secondary air and gas flow in the second passage. Therefore, it may be possible to easily adjust a TDR by regulating the flows of air and gas in the second passage. Thirdly, since a second gas outlet is opened and closed and at the same time the second passage is also opened and 15 closed by rotation of a valve body, a structure may be significantly simplified. [Description of Drawings] FIG. 1A is a perspective view illustrating a dual venturi for a combustion apparatus according to an 20 embodiment of the present invention. FIG. 1B is a cross-sectional view taken along line A-A of FIG. 1A. FIG. 2A is a cross-sectional view taken along line B-B of FIG. 1A. 25 FIG. 2B is a view illustrating a state in which a -7 moving body is moved upward in FIG. 2A. FIGS. 3A and 3B are perspective views illustrating the moving body of FIG. 1B. FIG. 4 is a perspective view illustrating a valve body 5 of FIG. 1B. FIGS. 5A and 5B are perspective views illustrating an opening and closing unit. FIG. 6 is a cross-sectional view taken along line C-C of FIG. 5A. 10 FIG. 7 is a view illustrating a position relation between a motor and a limited switch of the motor included in the dual venturi for a combustion apparatus according to the embodiment of the present invention. [Mode for Invention] 15 Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings. The present invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. 20 Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art. Accordingly, the drawings are not necessarily to scale and in some instances, proportions may have been 25 exaggerated in order to clearly illustrate features of the - 8 embodiments. Throughout the disclosure, like reference numerals refer to like parts throughout the various figures and embodiments of the present invention. In addition, detailed descriptions of functions and constructions well 5 known in the art may be omitted to avoid unnecessarily obscuring the gist of the present invention. Hereinafter, a dual venturi for a combustion apparatus according to an exemplary embodiment of the present invention will be described in detail with reference to the 10 accompanying drawings. FIG. 1A is a perspective view illustrating a dual venturi for a combustion apparatus according to an embodiment of the present invention. FIG. 1B is a cross sectional view taken along line A-A of FIG. 1A. FIG. 2A is 15 a cross-sectional view taken along line B-B of FIG. 1A. FIG. 2B is a view illustrating a state in which a moving body is moved upward in FIG. 2A. FIGS. 3A and 3B are perspective views illustrating the moving body of FIG. 1B. FIG. 4 is a perspective view illustrating a valve body of FIG. 1B. FIGS. 20 5A and 5B are perspective views illustrating an opening and closing unit. FIG. 6 is a cross-sectional view taken along line C-C of FIG. 5A. FIG. 7 is a view illustrating a position relation between a motor and a limited switch of the motor included in the dual venturi for a combustion 25 apparatus according to the embodiment of the present - 9 invention. Referring to FIGS. 1 to 6, the dual venturi for a combustion apparatus according to the embodiment of the present invention includes a cylindrical housing 10 which 5 has a first gas inlet 14 and second gas inlet 15 formed at one side of an upper portion thereof and is divided therein by a partition wall 13 to have a first passage 11 and a second passage 12. In addition, a discharge port 16 formed at the housing 10 10 is coupled with a turbofan (not shown) for supplying a mixture (gas + air) introduced from the first and second passages 11 and 12 to a burner. Meanwhile, the housing 10 is coupled, at an inner central portion thereof, with an opening and closing unit 15 100 which is connected to the second gas inlet 15 at an upper portion thereof such that gas is introduced into the opening and closing unit 100 and controls the flows of air and gas by closing or opening the second passage 12 along with rotation of a damper 142 by a motor 180 provided at an 20 outer side of the housing 10. In addition, the motor 180 is preferably configured of a synchronous motor. This enables manufacturing costs to be reduced by generally using the cheap synchronous motor. The opening and closing unit 100 includes a tubular 25 guide tube 130 having one or more concave guides 131 formed - 10 on an inner wall thereof, and a tubular moving body 120 which has convex guides 122 formed on an outer wall thereof to correspond to the concave guides 131 so that the moving body 120 moves within the guide tube 130 in a longitudinal 5 direction of the guide tube 130, a gas outlet 123 formed at a distal end thereof to communicate with the second gas inlet 15, and a moving body cam 121 having a cam shape formed on an outer surface thereof. An inner surface of the guide tube 130 and an outer surface of the moving body 120 10 are formed in a stepped shape, and a sealing ring 146 is coupled to the stepped portion to prevent leakage of gas. In addition, the opening and closing unit 100 includes a valve body 140 which is coupled to a lower portion of the moving body 120, has the damper 142 formed on an outer 15 surface thereof, a gas outlet 143 formed at a side of the damper 142, and a valve body cam 141 formed at an inner side thereof in a shape corresponding to the moving body cam 121 such that the valve body cam 141 engages with the moving body cam 121, and is coupled to the motor 180. 20 Furthermore, the opening and closing unit 100 includes a spring 160 formed inside the moving body 120 to elastically support the moving body 120 and a cap 170 which supports the spring 160 and is coupled to an upper portion of the guide tube 130. 25 Meanwhile, a connection member 190 is coupled to a - 11 lower portion of the valve body 140. The connection member 190 is coupled to a rotary shaft 181 of the motor 180 to transfer driving force generated by the motor 180 to the valve body 140. In addition, a sealing member 145 for 5 sealing an inner lower end of the valve body 140 is inserted into a lower end of the valve body 140. Meanwhile, the outer side of the housing 10 is further provided with the motor 180 for rotating the valve body 140 coupled to the connection member 190 by coupling of the 10 rotary shaft 181 and the connection member 190. The motor 180 provides torque to the valve body 140. The rotary shaft 181 has protrusions 182 formed at intervals of 90 degrees and the protrusions 182 operate a limit switch 201 along with rotation of the motor 180 such that a rotation angle of 15 the valve body 140 is controlled. The limit switch 201 is provided inside a switch box 200 and the switch box 200 is interposed between the housing 10 and the motor 180. Hereinafter, an operation state of the dual venturi for a combustion apparatus of the present invention having the 20 above configuration will be described. First, an operation in which only primary gas and air are supplied from a water heater is performed in such a way that the damper 142 of the valve body 140 is rotated perpendicular to a direction in which air and gas in the 25 second passage 12 flow so that the second passage 12 is - 12 blocked, and at the same time a concave portion of the moving body cam 121 and a convex portion of the valve body cam 141 are in contact with a convex portion of the moving body cam 121 and a concave portion of the valve body cam 141, 5 thereby allowing the gas outlet 123 of the moving body 120 to come into contact with the sealing member 145 so that the second gas passage is maintained in a blocked state and the gas outlet 123 is also maintained in a blocked state, as shown in FIGS. 2A and 5A. 10 Accordingly, since a mixture of air and gas is introduced into the turbofan only through the first gas inlet 14 and the first passage 11, a combustion apparatus may be actuated by means of a low heating value. Meanwhile, in order to actuate the combustion apparatus 15 by means of a high heating value, power is applied to the motor 180 and the motor 180 rotates the valve body 140 by 90 degrees such that the damper 142 is rotated to correspond to a longitudinal direction of the second passage 12, as shown in FIGS. 2B and 5B. 20 At the same time, since the valve body 140 is rotated and the valve body cam 141 formed therein is also rotated, the respective convex portions (tip portions) of the valve body cam 141 and the moving body cam 121 come into contact with each other, thereby allowing the moving body cam 121 to 25 be pushed up along with rotation of the valve body cam 141. - 13 - In this case, since the convex guides 122 are inserted into the concave guides 131, the moving body 120 is easily moved upward within the guide tube 130. Accordingly, since the valve body 140 is rotated in a 5 direction equal to the longitudinal direction of the second passage 12, the second passage 12 is opened and secondary gas is simultaneously introduced through the gas outlet 143 by decoupling of the lower end of the gas outlet 123 of the moving body 120 from the sealing member 145, as shown in 10 FIGS. 2B and 5B. Here, the secondary gas is mixed with the air and gas introduced through the second passage 12 and is then mixed with the air and gas introduced through the first passage 11 and the first gas inlet 14, so that a large amount of mixture is generated and the mixture is introduced 15 into the turbofan. As a result, the combustion apparatus may be actuated by means of a high heating value. Subsequently, when the motor 180 rotates the valve body 140 by 90 degrees in order to actuate the combustion apparatus by means of a low heating value again, the valve 20 body 140 is returned to the state shown in FIGS. 1B, 2A, and 5A. Consequently, the second passage 12 and the gas outlet 143 are blocked and, as such, the combustion apparatus is actuated by means of a low heating value. Here, the spring 160 provided in the moving body 120 serves to increase 25 restoration of the moving body 120 when the second passage - 14 - 12 is closed by rotation of the valve body 140. Hereinafter, a description will be given with respect to the limit switch 201 for controlling rotation of the motor 180 driving the damper such that the combustion 5 apparatus is actuated by means of the low heating value or the high heating value. Referring to FIG. 7, the rotary shaft 181 of the motor 180 has the protrusions 182 which are protrudingly formed on an outer peripheral surface thereof at intervals of 90 10 degrees and the limit switch 201 has movable protrusions 202 formed to be located on the same circumference as the protrusions 182. When the protrusions 182 press the movable protrusions 202 while rotating at intervals of 90 degrees, the limit switch 201 causes a short circuit current and, as 15 such, rotation of the motor 180 is stopped. Therefore, when the combustion apparatus is actuated, the protrusions 182 are rotated by 90 degrees to press the movable protrusions 202. Then, the limit switch 201 is turned off to stop rotation of the motor 180 and the valve 20 body 140 is also stopped. As a result, the second passage 12 is opened or closed. Although the present invention has been described with respect to the illustrative embodiments of the dual venturi for a combustion apparatus, it should be understood that 25 numerous other modifications and applications may be devised - 15 by those skilled in the art that will fall within the intrinsic aspects of the embodiments. More particularly, various variations and modifications are possible in concrete constituent elements of the embodiments. In 5 addition, it is to be understood that differences relevant to all variations, equivalents, and alternatives fall within the spirit and scope of the present disclosure defined in the appended claims. The term 'comprising' as used in this specification and 10 claims means 'consisting at least in part of'. When interpreting statements in this specification and claims which include the term 'comprising', other features besides the features prefaced by this term in each statement can also be present. Related terms such as 'comprise' and 15 'comprised' are to be interpreted in a similar manner. [Description of Reference Numerals] 10: housing, 11: first passage 12: second passage, 13: partition wall 14: first gas inlet, 15: second gas inlet 20 100: opening and closing unit, 120: moving body 121: moving body cam, 122: convex guide 130: guide tube, 131: concave guide 140: valve body, 141: valve body cam 142: damper, 143: gas outlet 25 145: sealing member, 146: sealing ring - 16 - 160: spring, 170: cap 180: motor, 181: rotary shaft 182: protrusion, 190: connection member 200: switch case, 201: limit switch 5 202: movable protrusion - 17 -