Patent Publication Number: US-9890952-B2

Title: Exhaust structure for combustion apparatus and construction method thereof

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an exhaust structure for combustion apparatus and a construction method thereof. 
     Description of the Background Art 
     In markets, there are locations where an exhaust pipe (a B vent) already placed in a building cannot be removed from a point of view of maintaining appearance of buildings. At such a location, a combustion apparatus can be replaced by introducing a new exhaust tube (a flexible exhaust tube) into the exhaust pipe and an exhaust terminal (a rain cap) which have already been placed. 
     The technique related to an exhaust pipe of the combustion apparatus is disclosed, for example, in Japanese Patent Laying-Open No. 2000-18566, Japanese Patent Laying-Open No. 2008-82613, and the like. Also, a method for inserting an exhaust tube into an already-placed exhaust pipe is disclosed, for example, in Japanese Utility Model Publication No. 02-47343, Japanese Patent Laying-Open No. 2003-343790 and the like. 
     When a new exhaust tube is inserted into the already-placed exhaust pipe and exhaust terminal from inside the building, the end of the exhaust tube may come into contact with a ceiling wall of the already-placed exhaust terminal. In this case, the exhaust port of combustion gas in the exhaust tube is blocked by the ceiling wall of the exhaust terminal, which leads to a problem that combustion gas cannot be sufficiently emitted from the exhaust tube. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in light of the above-described problems. An object of the present invention is to provide an exhaust structure for combustion apparatus capable of stably emitting combustion gas to the outside of the building without changing the external appearance of the building, and a construction method thereof. 
     An exhaust structure for combustion apparatus according to the present invention includes a combustion apparatus, an exhaust tube, an exhaust pipe, and an exhaust adapter. The exhaust tube includes one end portion and the other end portion and is connected at the one end portion to the combustion apparatus, of which inside is defined as an exhaust passage for combustion gas. The exhaust pipe has an outer diameter greater than that of the exhaust tube, and a part of the exhaust tube on a side of the other end portion is introduced into the exhaust pipe. The exhaust adapter is attached between an outer circumferential surface of the exhaust tube and an inner circumferential surface of the exhaust pipe. On a side close to the other end portion in the exhaust tube relative to the exhaust adapter, an exhaust passage hole is provided separately from an opening in the other end portion of the exhaust tube, has an opening area greater than a cross-sectional area of the exhaust passage, and is in communication with the exhaust passage. 
     According to the exhaust structure for combustion apparatus of the present invention, the exhaust passage hole is provided on the side close to the other end portion in the exhaust tube relative to the exhaust adapter and separately from the opening in the other end portion of the exhaust tube. Accordingly, even if the other end portion of the exhaust tube is blocked by a ceiling wall of the exhaust terminal, combustion gas can still be emitted to the outside of the building through the exhaust passage hole. 
     Furthermore, the exhaust passage hole has an opening area greater than the cross-sectional area of the exhaust passage inside the exhaust tube. Accordingly, exhaust resistance is not caused by the exhaust passage hole, so that combustion gas can be stably emitted to the outside of the building through the exhaust passage hole. 
     In the above-described exhaust structure for combustion apparatus, the exhaust passage hole is a through hole portion provided in the exhaust tube. 
     According to such a simple configuration in which a through hole is provided in the exhaust tube in this way, combustion gas can be stably emitted to the outside of the building. 
     In the above-described exhaust structure for combustion apparatus, the through hole portion includes a plurality of through holes provided in the exhaust tube so as to be spaced apart from each other. 
     By providing the through hole portion consisting of a plurality of separate through holes in this way, the strength of the exhaust tube required for introduction into the exhaust pipe can be readily ensured. Furthermore, by providing the through hole portion consisting of a plurality of separate through holes, emission of combustion gas from the exhaust tube can be readily controlled. 
     In the above-described exhaust structure for combustion apparatus, a space formation member attached to the other end portion of the exhaust tube is further provided. The exhaust passage hole is provided in the space formation member. 
     In this way, the space formation member is provided separately from the exhaust tube and the exhaust passage hole is provided in this space formation member, thereby improving the flexibility of each of the shape and the arrangement position of the exhaust passage hole. Therefore, it becomes possible to achieve a structure that is more suitable for emission of combustion gas. 
     In the above-described exhaust structure for combustion apparatus, the space formation member includes an exhaust tube support portion supported by the exhaust tube, and a frame portion formed in a shape of an arch-shaped plate and supported by the exhaust tube support portion. The exhaust passage hole is surrounded by the frame portion and the exhaust tube support portion. 
     By using such a frame portion formed in the shape of an arch-shaped plate, the top portion of the arch can be brought into contact with the ceiling wall of the exhaust terminal while combustion gas can be emitted through the exhaust passage hole surrounded by the frame portion and the exhaust tube support portion. 
     In the above-described exhaust structure for combustion apparatus, the space formation member has an exhaust tube support portion supported by the exhaust tube and a protruding portion protruding from the exhaust tube support portion to a side opposite to the exhaust tube. The exhaust passage hole is provided in the protruding portion. 
     By using such a protruding portion, the top portion of this protruding portion can be brought into contact with the ceiling wall of the exhaust terminal while combustion gas can be emitted through the exhaust passage hole. 
     In the above-described exhaust structure for combustion apparatus, the protruding portion includes a reduced diameter portion, a flange portion extending radially outward from the reduced diameter portion, and a curved portion curved in a convex shape toward a side opposite to the reduced diameter portion relative to the flange portion. The exhaust passage hole is provided in one or more selected from a group consisting of the reduced diameter portion, the flange portion and the curved portion. 
     Since the curved portion of the space formation member is curved in a convex shape in this way, the contact resistance caused between the curved portion and the inner circumferential surface of the exhaust pipe can be reduced when the other end portion of the exhaust tube having this space formation member attached thereto is inserted into the exhaust pipe. Consequently, the exhaust tube having the space formation member attached thereto can be smoothly inserted into the exhaust pipe. 
     In the above-described exhaust structure for combustion apparatus, the exhaust passage hole is provided at a position where the exhaust passage hole cannot be seen when the protruding portion is viewed from a side of the protruding portion toward a side of the exhaust tube support portion. 
     Accordingly, when the other end portion of the exhaust tube having this space formation member attached thereto is inserted into the exhaust pipe, foreign substances are less likely to come into the exhaust tube through the exhaust passage hole of the space formation member. 
     In the above-described exhaust structure for combustion apparatus, the protruding portion includes a cylindrical portion extending from the exhaust tube support portion and a tip end portion formed at a tip end of the cylindrical portion. The exhaust passage hole is provided in at least one of the cylindrical portion and the tip end portion. 
     The space formation member has the cylindrical portion and the tip end portion in this way. Accordingly, the other end portion of the exhaust tube having this space formation member attached thereto is inserted into the exhaust pipe, so that the exhaust tube can be inserted into the exhaust pipe in a highly straight manner. 
     In the above-described exhaust structure for combustion apparatus, the exhaust passage hole is a hole provided by opening a cut portion in the exhaust tube by compressing the exhaust tube, the cut portion being formed by cutting the exhaust tube from the outer circumferential surface to an inner circumferential surface. 
     This cut portion is a hole that is closed before the other end portion of the exhaust tube comes into contact with the ceiling wall of the exhaust terminal, and opened when the other end portion comes into contact with the ceiling wall to thereby exert compression force on the exhaust tube. Accordingly, when the exhaust tube is inserted into the exhaust pipe, the cut portion is closed, so that foreign substances can be prevented from coming into the exhaust tube through this cut portion. Furthermore, after insertion of the exhaust tube into the exhaust pipe is completed, the cut portion is opened by compression force, so that combustion gas can be emitted from the exhaust tube. 
     A construction method of an exhaust structure for combustion apparatus according to the present invention provides a method of constructing an exhaust structure for combustion apparatus, by which an exhaust tube is inserted into an exhaust pipe extending from inside to outside of a building. An exhaust terminal is provided at a tip end portion of the exhaust pipe on the outside. The construction method includes the following steps. 
     An exhaust passage hole is provided in the exhaust tube having one end portion and the other end portion so as to be located on a side of the other end portion separately from an opening in the other end portion of the exhaust tube. The exhaust passage hole has an opening area greater than a cross-sectional area of an exhaust passage and is in communication with an inner passage of the exhaust tube. The exhaust tube is inserted into the exhaust pipe until a part of the exhaust tube on the side of the other end portion provided with the exhaust passage hole comes into contact with a ceiling wall of the exhaust terminal. The one end portion of the exhaust tube is connected to the combustion apparatus. 
     According to the construction method of an exhaust structure for combustion apparatus of the present invention, the exhaust tube only has to be inserted into the exhaust pipe from the inside of the building, and connected to the combustion apparatus inside the building. Accordingly, a contractor does not have to climb on to the roof of the building to remove the exhaust terminal from the exhaust pipe, so that the working performance for construction is significantly improved. 
     Furthermore, the exhaust passage hole is provided, separately from the opening in the other end portion of the exhaust tube, on the side close to the other end portion in the exhaust tube relative to the exhaust adapter. Accordingly, even if the other end portion of the exhaust tube is blocked by the ceiling wall of the exhaust terminal, combustion gas can be emitted to the outside of the building through the exhaust passage hole. 
     Furthermore, the exhaust passage hole has an opening area greater than the cross-sectional area of the exhaust passage inside the exhaust tube. Accordingly, exhaust resistance is not caused by the exhaust passage hole, so that combustion gas can be stably emitted to the outside of the building through the exhaust passage hole. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram schematically showing the state where an exhaust structure for combustion apparatus in the first embodiment of the present invention is placed in a building. 
         FIG. 2  is an exploded perspective view schematically showing the configuration of the exhaust structure for combustion apparatus in the first embodiment of the present invention. 
         FIG. 3  is a cross-sectional view showing, in an enlarged manner, a region III in  FIG. 1  schematically showing the configuration of the exhaust structure for combustion apparatus in the first embodiment of the present invention. 
         FIG. 4  is a front view schematically showing a configuration of a water heater included as an example of a combustion apparatus in the exhaust structure for combustion apparatus in the first embodiment of the present invention. 
         FIG. 5  is a partial cross-sectional side view schematically showing the configuration of the water heater shown in  FIG. 4 . 
         FIG. 6  is a cross-sectional view schematically showing the first step of the construction method of the exhaust structure for combustion apparatus in the first embodiment of the present invention. 
         FIG. 7  is a cross-sectional view schematically showing the second step of the construction method of the exhaust structure for combustion apparatus in the first embodiment of the present invention. 
         FIG. 8  is an exploded perspective view schematically showing the configuration of an exhaust structure for combustion apparatus in the second embodiment of the present invention. 
         FIG. 9  is a perspective view schematically showing the configuration of a space formation member included in the exhaust structure for combustion apparatus in the second embodiment of the present invention. 
         FIG. 10  is a plan view schematically showing the configuration of the space formation member included in the exhaust structure for combustion apparatus in the second embodiment of the present invention. 
         FIG. 11  is a cross-sectional view schematically showing the configuration of the exhaust structure for combustion apparatus in the second embodiment of the present invention. 
         FIG. 12  is an exploded perspective view schematically showing the configuration of an exhaust structure for combustion apparatus in the third embodiment of the present invention. 
         FIG. 13  is a perspective view schematically showing the configuration of the first example of a space formation member included in the exhaust structure for combustion apparatus in the third embodiment of the present invention. 
         FIG. 14  is a cross-sectional view schematically showing the state where the first example of the space formation member included in the exhaust structure for combustion apparatus in the third embodiment of the present invention is attached to an exhaust tube. 
         FIG. 15  is a bottom view schematically showing the configuration of the first example of the space formation member included in the exhaust structure for combustion apparatus in the third embodiment of the present invention. 
         FIG. 16  is a cross-sectional view schematically showing the state where the second example of the space formation member included in the exhaust structure for combustion apparatus in the third embodiment of the present invention is attached to the exhaust tube. 
         FIG. 17  is a cross-sectional view schematically showing the state where the third example of the space formation member included in the exhaust structure for combustion apparatus in the third embodiment of the present invention is attached to the exhaust tube. 
         FIG. 18  is a perspective view schematically showing the configuration of the fourth example of the space formation member included in the exhaust structure for combustion apparatus in the third embodiment of the present invention. 
         FIG. 19  is a cross-sectional view schematically showing the state where the fourth example of the space formation member included in the exhaust structure for combustion apparatus in the third embodiment of the present invention is attached to the exhaust tube. 
         FIG. 20  is a plan view schematically showing the configuration of the fourth example of the space formation member included in the exhaust structure for combustion apparatus in the third embodiment of the present invention. 
         FIG. 21  is an exploded perspective view schematically showing the configuration of an exhaust structure for combustion apparatus in the fourth embodiment of the present invention. 
         FIG. 22  is a perspective view schematically showing the configuration of the first example of a space formation member included in the exhaust structure for combustion apparatus in the fourth embodiment of the present invention. 
         FIG. 23  is a cross-sectional view schematically showing the state where the first example of the space formation member included in the exhaust structure for combustion apparatus in the fourth embodiment of the present invention is attached to the exhaust tube. 
         FIG. 24  is a perspective view schematically showing the configuration of the second example of the space formation member included in the exhaust structure for combustion apparatus in the fourth embodiment of the present invention. 
         FIG. 25  is a cross-sectional view schematically showing the state where the second example of the space formation member included in the exhaust structure for combustion apparatus in the fourth embodiment of the present invention is attached to the exhaust tube. 
         FIG. 26  is an exploded perspective view schematically showing the configuration of an exhaust structure for combustion apparatus in the fifth embodiment of the present invention. 
         FIG. 27  is a cross-sectional view schematically showing the configuration of the exhaust structure for combustion apparatus in the fifth embodiment of the present invention. 
         FIG. 28  is a cross-sectional view schematically showing the state where a cut portion used for a tube-diameter reducing process is formed in the other end portion of the exhaust tube. 
         FIG. 29  is a cross-sectional view schematically showing the state where the other end portion of the exhaust tube is subjected to the tube-diameter reducing process. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The embodiments of the present invention will be hereinafter described with reference to the accompanying drawings. 
     First Embodiment 
     An exhaust structure for combustion apparatus in the present embodiment will be first described with reference to  FIGS. 1 to 3 . 
     Referring to  FIGS. 1 to 3 , an exhaust structure for combustion apparatus  100  in the present embodiment mainly has an exhaust adapter  3 , an exhaust tube  20 , an exhaust pipe  30 , an exhaust terminal  40 , and a combustion apparatus  50 . This exhaust structure for combustion apparatus  100  serves to emit combustion gas produced in combustion apparatus  50  to the outside of a building  200 . 
     Combustion apparatus  50  is placed inside building  200 . This combustion apparatus  50  serves as a water heater that heats water, for example, with combustion gas, and may be a heating apparatus or the like that warms up the inside of the building with combustion gas. Furthermore, in the case where a water heater is used as combustion apparatus  50 , this water heater may be, for example, a water heater of a latent heat recovery type adapted to an exhaust suction and combustion system. 
     Exhaust tube  20  includes one end portion  20   a  and the other end portion  20   b . One end portion  20   a  of exhaust tube  20  is connected to combustion apparatus  50  and the other end portion  20   b  of exhaust tube  20  extends to the outside of the building. The inside of exhaust tube  20  is defined as an exhaust path for the combustion gas exhausted from combustion apparatus  50 . Thus, the combustion gas produced in combustion apparatus  50  can be guided to the outside through exhaust tube  20 . Although exhaust tube  20  is implemented as a flexible pipe such as an accordion pipe, it may be a spiral pipe. 
     Exhaust pipe  30  is attached to building  200  so as to extend from the inside to the outside, for example, through a roof  110  of building  200 . Exhaust pipe  30  may extend from the inside to the outside through a wall. Exhaust pipe  30  has an outer diameter greater than that of exhaust tube  20 . Into this exhaust pipe  30 , a part of exhaust tube  20  on the other end portion  20   b  side is introduced. 
     Exhaust terminal  40  is attached to a tip end of exhaust pipe  30  on the outside of the building. This exhaust terminal  40  has a ceiling wall  40   b , a bottom wall  40   c , a circumferential wall  40   d , and a connection pipe portion  40   e  ( FIG. 3 ). Ceiling wall  40   b  is attached to an upper end of circumferential wall  40   d , and bottom wall  40   c  is attached to a lower end of circumferential wall  40   d . Connection pipe portion  40   e  is formed in a cylindrical shape and attached to bottom wall  40   c  so as to penetrate bottom wall  40   c . In circumferential wall  40   d , an exhaust port  40   a  for exhausting the combustion gas to the outside (to the outside of the building) is formed. This exhaust port  40   a  allows the combustion gas guided through exhaust tube  20  to be exhausted from exhaust terminal  40  to the outside of building  200  through exhaust pipe  30 . 
     Connection pipe portion  40   e  of exhaust terminal  40  is connected to exhaust pipe  30 . This connection pipe portion  40   e  may be an outer cover attached on an outer circumferential side of exhaust pipe  30  or an inner cover attached on an inner circumferential side of exhaust pipe  30 . Exhaust terminal  40  is made, for example, of such a material as aluminum or stainless steel. 
     Exhaust adapter  3  serves to fix exhaust tube  20  to exhaust pipe  30 . This exhaust adapter  3  is fitted to an outer circumferential surface of exhaust tube  20  and fitted to an inner circumferential surface of exhaust pipe  30 . Exhaust adapter  3  has an inner circumferential surface pressing the outer circumferential surface of exhaust tube  20  while it is fitted to the outer circumferential surface of exhaust tube  20 , and has an outer circumferential surface pressing the inner circumferential surface of exhaust pipe  30  while it is fitted to the inner circumferential surface of exhaust pipe  30 . 
     Thus, the inner circumferential surface of exhaust adapter  3  is in intimate contact with the outer circumferential surface of exhaust tube  20  while the outer circumferential surface of exhaust adapter  3  is in intimate contact with the inner circumferential surface of exhaust pipe  30 . Therefore, exhaust tube  20  can firmly be fixed to exhaust pipe  30  with exhaust adapter  3 , and exhaust adapter  3  can prevent leakage of the combustion gas or drainage water from between exhaust tube  20  and exhaust pipe  30  and hence backflow thereof into the inside of building. 
     Exhaust tube  20  has an exhaust passage hole  1   a  on the side close to the other end portion  20   b  relative to the position at which exhaust adapter  3  is attached to exhaust tube  20 . This exhaust passage hole  1   a  is provided separately from an opening  20   b   1  provided in the other end portion  20   b  of exhaust tube  20 , has an opening area greater than the cross-sectional area of the exhaust passage inside exhaust tube  20 , and is in communication with the exhaust passage. In this case, the cross-sectional area of the exhaust passage of exhaust tube  20  means an area of the internal space of exhaust tube  20  in the state where exhaust tube  20  is vertically cut in the direction in which exhaust tube  20  extends. 
     Exhaust passage hole  1   a  is, for example, a through hole portion  1   a  formed in the outer circumferential surface of exhaust tube  20 . This through hole portion  1   a  includes a plurality of through holes  1   a   1  that are provided at a distance from each other in the outer circumferential surface of exhaust tube  20 . A plurality of (for example, eight) through holes  1   a   1  are arranged along the circumferential direction of exhaust tube  20  while a plurality of (for example, three) through holes  1   a   1  are arranged along the direction in which exhaust tube  20  extends. The plurality of through holes  1   a   1  may be arranged at regular intervals (every 45°) along the circumferential direction of exhaust tube  20  or may be arranged at regular intervals along the direction in which exhaust tube  20  extends. The total sum of the opening areas of the plurality of through holes  1   a   1  is greater than the cross-sectional area of the exhaust passage inside exhaust tube  20 . 
     It is to be noted that the number of through holes  1   a   1  arranged in the circumferential direction and in the direction in which exhaust tube  20  extend is not limited to the above. Although each of the plurality of through holes  1   a   1  is implemented as a through hole in a shape, for example, of a circle (a perfect circle, an oval shape, or an elliptical shape), the shape is not limited as such and the through hole may be implemented as a through hole in a polygonal shape such as a triangular shape and a rectangular shape, or a through hole in any shape. 
     Exhaust tube  20  is provided with a drainage water recovering hole  2  at the height level between exhaust adapter  3  and an upper end  30   a  of exhaust pipe  30 . The drainage water accumulated in a space above exhaust adapter  3  and between the outer circumferential surface of exhaust tube  20  and the inner circumferential surface of exhaust pipe  30  can be recovered in exhaust tube  20  through this drainage water recovering hole  2 . The number of drainage water recovering holes  2  provided in exhaust tube  20  may be more than one. For example, three drainage water recovering holes  2  may be arranged at regular intervals (every 120°) in the circumferential direction of exhaust tube  20 . 
     Combustion apparatus  50  used in exhaust structure for combustion apparatus  100  set forth above may be a water heater of a latent heat recovery type, for example, adapted to an exhaust suction and combustion system, as described above. The configuration of water heater  50  of the latent heat recovery type adapted to the exhaust suction and combustion system will be described below with reference to  FIGS. 4 and 5 . 
     Referring to  FIGS. 4 and 5 , water heater  50  mainly has a burner  51 , a primary heat exchanger  52 , a secondary heat exchanger  53 , an exhaust box  54 , a fan  55 , a connection pipe  56 , a drainage water tank  57 , a housing  58 , and pipes  61  to  66 . 
     Burner  51  serves to produce a combustion gas by burning a fuel gas. A gas supply pipe  62  is connected to burner  51 . This gas supply pipe  62  serves to supply a fuel gas to burner  51 . A gas valve (not shown) implemented, for example, by an electromagnetic valve is attached to this gas supply pipe  62 . 
     A spark plug  51   a  is arranged above burner  51 . This spark plug  51   a  serves to ignite an air fuel mixture injected from burner  51  to thereby produce a flame, by generating sparks between the plug and a target (not shown) provided in burner  51  by activating an ignition device (an igniter). Burner  51  generates a quantity of heat by burning a fuel gas supplied from gas supply pipe  62  (which is called a combustion operation). 
     Primary heat exchanger  52  is a heat exchanger of a sensible heat recovery type. This primary heat exchanger  52  mainly has a plurality of plate-shaped fins  52   b , a heat conduction pipe  52   a  penetrating the plurality of plate-shaped fins  52   b , and a case  52   c  accommodating fins  52   b  and heat conduction pipe  52   a . Primary heat exchanger  52  exchanges heat with the combustion gas generated by burner  51 , and specifically, it serves to heat water which flows through heat conduction pipe  52   a  of primary heat exchanger  52  with the quantity of heat generated as a result of the combustion operation of burner  51 . 
     Secondary heat exchanger  53  is a heat exchanger of a latent heat recovery type. This secondary heat exchanger  53  is located downstream of primary heat exchanger  52  in a flow of the combustion gas and connected in series with primary heat exchanger  52 . Since water heater  50  according to the present embodiment thus has secondary heat exchanger  53  of a latent heat recovery type, it is water heater  50  of the latent heat recovery type. 
     Secondary heat exchanger  53  mainly has a drainage water discharge port  53   a , a heat conduction pipe  53   b , a sidewall  53   c , a bottom wall  53   d , and an upper wall  53   g . Heat conduction pipe  53   b  is layered as it is spirally wound. Sidewall  53   c , bottom wall  53   d , and upper wall  53   g  are arranged to surround heat conduction pipe  53   b.    
     In secondary heat exchanger  53 , water which flows through heat conduction pipe  53   b  is pre-heated (heated) through heat exchange with the combustion gas of which heat has been exchanged in primary heat exchanger  52 . As a temperature of the combustion gas is lowered to approximately 60° C. through this process, moisture contained in the combustion gas is condensed so that latent heat can be obtained. In addition, latent heat is recovered in secondary heat exchanger  53  and moisture contained in the combustion gas is condensed, whereby drainage water is produced. 
     Bottom wall  53   d  serves as a partition between primary heat exchanger  52  and secondary heat exchanger  53 , and it also serves as an upper wall of primary heat exchanger  52 . This bottom wall  53   d  is provided with an opening portion  53   e , and this opening portion  53   e  allows communication between a space where heat conduction pipe  52   a  of primary heat exchanger  52  is arranged and a space where heat conduction pipe  53   b  of secondary heat exchanger  53  is arranged. As shown with hollow arrows in  FIG. 5 , the combustion gas can flow from primary heat exchanger  52  to secondary heat exchanger  53  through opening portion  53   e . In this embodiment, although one common component is employed for bottom wall  53   d  of secondary heat exchanger  53  and the upper wall of primary heat exchanger  52  for the sake of simplification, an exhaust collection and guide member may be connected between primary heat exchanger  52  and secondary heat exchanger  53 . 
     Upper wall  53   g  is provided with an opening portion  53   h , and this opening portion  53   h  allows communication between the space where heat conduction pipe  53   b  of secondary heat exchanger  53  is arranged and an internal space in exhaust box  54 . As shown with hollow arrows in  FIG. 5 , the combustion gas can flow from secondary heat exchanger  53  into the internal space in exhaust box  54  through opening portion  53   h.    
     Drainage water discharge port  53   a  is provided in sidewall  53   c  or bottom wall  53   d . This drainage water discharge port  53   a  opens at the lowest position in the space surrounded by side wall  53   c , bottom wall  53   d  and upper wall  53   g  (at the lowermost position in the vertical direction in the state of placement of the water heater), which is lower than the lowermost portion of heat conduction pipe  53   b . Thus, drainage water produced in secondary heat exchanger  53  can be guided to drainage water discharge port  53   a  along bottom wall  53   d  and sidewall  53   c  as shown with a black arrow in  FIG. 5 . 
     Exhaust box  54  forms a path for a flow of the combustion gas between secondary heat exchanger  53  and fan  55 . This exhaust box  54  can guide the combustion gas of which heat has been exchanged in secondary heat exchanger  53  to fan  55 . Exhaust box  54  is attached to secondary heat exchanger  53  and located downstream of secondary heat exchanger  53  in the flow of the combustion gas. 
     Exhaust box  54  mainly has a box main body  54   a  and a fan connection portion  54   b . An internal space in box main body  54   a  communicates through opening portion  53   h  in secondary heat exchanger  53  with the internal space where heat conduction pipe  53   b  of secondary heat exchanger  53  is arranged. Fan connection portion  54   b  is provided so as to protrude from an upper portion of box main body  54   a . This fan connection portion  54   b  has, for example, a cylindrical shape, and an internal space  54   ba  thereof communicates with the internal space in box main body  54   a.    
     Fan  55  serves to emit the combustion gas (of which heat has been exchanged in secondary heat exchanger  53 ) which has passed through secondary heat exchanger  53  to the outside of water heater  50  by suctioning the combustion gas. This fan  55  is located downstream of exhaust box  54  and secondary heat exchanger  53  in the flow of the combustion gas. Namely, in water heater  50 , burner  51 , primary heat exchanger  52 , secondary heat exchanger  53 , exhaust box  54 , and fan  55  are arranged in this order from upstream to downstream in the flow of the combustion gas produced in burner  51 . Since the combustion gas is suctioned and exhausted by means of fan  55  as above in this arrangement, water heater  50  in the present embodiment is a water heater adapted to an exhaust suction and combustion system. 
     Fan  55  mainly has an impeller  55   a , a fan case  55   b , a drive source  55   c , and a rotation shaft  55   d . Fan case  55   b  is attached to fan connection portion  54   b  of exhaust box  54  such that an internal space in fan case  55   b  and the internal space in fan connection portion  54   b  communicate with each other. Thus, as shown with hollow arrows in  FIG. 5 , the combustion gas can be suctioned from box main body  54   a  of exhaust box  54  through fan connection portion  54   b  into fan case  55   b.    
     Impeller  55   a  is arranged in fan case  55   b . This impeller  55   a  is connected to drive source  55   c  with rotation shaft  55   d  interposed therebetween. Thus, impeller  55   a  is provided with drive force from drive source  55   c  and can rotate around rotation shaft  55   d . With rotation of impeller  55   a , the combustion gas in exhaust box  54  can be suctioned from the inner circumferential side of impeller  55   a  and can be emitted toward the outer circumferential side of impeller  55   a.    
     Connection pipe  56  is connected to a region within fan case  55   b , on the outer circumferential side of a region where impeller  55   a  is arranged. Therefore, the combustion gas emitted to the outer circumferential side of impeller  55   a  by impeller  55   a  of fan  55  can be emitted into exhaust tube  20  through connection pipe  56 . 
     The combustion gas produced by burner  51  as above is suctioned by fan  55  with rotation of impeller  55   a  above, so that the combustion gas can reach fan  55  after passage through primary heat exchanger  52 , secondary heat exchanger  53 , and exhaust box  54  in this order as shown with the hollow arrows in the figure and can be exhausted to the outside of water heater  50 . 
     Drainage water tank  57  serves to store drainage water produced in secondary heat exchanger  53 . This drainage water tank  57  is connected to secondary heat exchanger  53  through pipe  61 . Pipe  61  is connected to drainage water discharge port  53   a  of secondary heat exchanger  53 . Thus, the drainage water produced in secondary heat exchanger  53  can be discharged to drainage water tank  57 . Pipe  66  extending to the outside of water heater  50  is connected to drainage water tank  57 . The drainage water stored in drainage water tank  57  can be discharged to the outside of water heater  50  through this pipe  66 . 
     This drainage water tank  57  has a water seal structure. Namely, drainage water tank  57  has such a structure that, as a prescribed amount of drainage water is stored in drainage water tank  57 , the stored drainage water cannot allow air to pass through drainage water tank  57 . With such a water seal structure of drainage water tank  57 , entry of air outside water heater  50  (outside air) into water heater  50  (secondary heat exchanger  53 ) through drainage water tank  57  via pipe  66  can be prevented. 
     Water supply pipe  63  is connected to one end of heat conduction pipe  53   b  of secondary heat exchanger  53  and hot water delivery pipe  64  is connected to one end of heat conduction pipe  52   a  of primary heat exchanger  52 . The other end of heat conduction pipe  52   a  of primary heat exchanger  52  and the other end of heat conduction pipe  53   b  of secondary heat exchanger  53  are connected to each other through connection pipe  65 . Each of gas supply pipe  62 , water supply pipe  63 , and hot water delivery pipe  64  mentioned above leads to the outside, for example, in a top portion of water heater  50 . Burner  51 , primary heat exchanger  52 , secondary heat exchanger  53 , exhaust box  54 , fan  55 , drainage water tank  57 , and the like are arranged in housing  58 . 
     Then, the method of constructing the exhaust structure for combustion apparatus according to the present embodiment will be hereinafter described with reference to  FIGS. 6, 7 and 3 . 
     Referring to  FIG. 6 , exhaust pipe  30  is placed in a roof  110  of building  200  so as to extend from the inside of building  200  to the outside thereof. Exhaust terminal  40  is attached to a tip end portion (near an upper end  30   a ) of this exhaust pipe  30  on the outside of the building. In this state, a preparation for inserting exhaust tube  20  into exhaust pipe  30  is made. In preparation for this, through hole portion  1   a , for example, consisting of a plurality of through holes  1   a   1  is provided as an exhaust passage hole on the other end portion  20   b  side of exhaust tube  20 . As described above, through hole portion  1   a  is provided separately from opening  20   b   1  in the other end portion  20   b  of exhaust tube  20 , and formed so as to have an opening area greater than the cross-sectional area of the exhaust passage inside exhaust tube  20  and so as to be in communication with the internal passage of exhaust tube  20 . Furthermore, drainage water recovering hole  2  is provided on the side close to one end portion  20   a  of exhaust tube  20  relative to the plurality of through holes  1   a   1 . 
     Furthermore, on the side close to one end portion  20   a  in exhaust tube  20  relative to drainage water recovering hole  2 , exhaust adapter  3  is fitted on the outer circumferential surface of exhaust tube  20 . Exhaust adapter  3  is fitted on the outer circumferential surface of exhaust tube  20  such that the inner circumferential surface of exhaust adapter  3  presses the outer circumferential surface of exhaust tube  20 . 
     Referring to  FIG. 7 , exhaust tube  20 , which is provided with through hole portion  1   a  and drainage water recovering hole  2  described above and equipped with exhaust adapter  3 , is inserted into exhaust pipe  30 . In this insertion, exhaust adapter  3  is inserted into exhaust pipe  30  such that the outer circumferential surface of exhaust adapter  3  presses the inner circumferential surface of exhaust pipe  30 . 
     Referring to  FIG. 3 , exhaust tube  20  is inserted into exhaust pipe  30  until the other end portion  20   b  of exhaust tube  20  comes into contact with ceiling wall  40   b  of exhaust terminal  40 . Then, when one end portion  20   a  of exhaust tube  20  is connected to the combustion apparatus (connection pipe  56  of water heater  50 :  FIG. 4 ), construction of exhaust structure for combustion apparatus  100  in the present embodiment is completed. 
     Then, the functions and effects of the present embodiment will be described with reference to  FIG. 3 . 
     According to the present embodiment, as shown in  FIG. 3 , through hole portion  1   a  is provided as an exhaust passage hole on the side close to the other end portion  20   b  in exhaust tube  20  relative to exhaust adapter  3 , and separately from opening  20   b   1  in the other end portion  20   b  of exhaust tube  20 . Accordingly, even if the other end portion  20   b  of exhaust tube  20  is blocked by ceiling wall  40   b  of exhaust terminal  40 , combustion gas can be emitted to the outside of the building via through hole portion  1   a  as an exhaust passage hole. 
     Furthermore, through hole portion  1   a  as an exhaust passage hole has an opening area greater than the cross-sectional area of the exhaust passage inside exhaust tube  20 . Accordingly, exhaust resistance against the combustion gas is not caused by through hole portion  1   a , so that combustion gas can be stably emitted to the outside of the building via through hole portion  1   a.    
     Furthermore, since exhaust passage hole  1   a  is formed by through hole portion  1   a  provided in the outer circumferential surface of exhaust tube  20 , combustion gas can be stably emitted to the outside of the building in such a simple configuration of providing through hole portion  1   a . Furthermore, since through hole portion  1   a  includes a plurality of through holes  1   a   1  that are provided at a distance from each other in the outer circumferential surface of exhaust tube  20 , the strength of exhaust tube  20  required for insertion into exhaust pipe  30  can be readily ensured. Furthermore, by providing through hole portion  1   a  consisting of a plurality of separate through holes  1   a   1 , emission of combustion gas from exhaust tube  20  can be readily controlled. 
     Second Embodiment 
     Then, the configuration of the exhaust structure for combustion apparatus in the second embodiment will be hereinafter described with reference to  FIGS. 8 to 11 . 
     Referring to  FIGS. 8 to 10 , the configuration of exhaust structure for combustion apparatus  100  in the present embodiment is different from the configuration of the first embodiment in that exhaust tube  20  is not provided with a through hole portion, but instead, a space formation member  1  is attached to the other end portion  20   b  of exhaust tube  20 . Space formation member  1  has an exhaust tube support portion  1   b  and a plate-shaped frame portion  1   c . Exhaust tube support portion  1   b  is formed in an annular shape, and fitted on the outer circumferential surface of exhaust tube  20  and thereby supported by exhaust tube  20 . Frame portion  1   c  extends above the region on the inner circumferential side in annular-shaped exhaust tube support portion  1   b  and is connected to one end portion and the other end portion of exhaust tube support portion  1   b  that are opposed to each other, thereby forming an arch shape. 
     In the present embodiment, two frame portions  1   c  are connected to exhaust tube support portion  1   b . Two frame portions  1   c  are arranged so as to be orthogonal to each other in a plan view as shown in  FIG. 10 . The number of frame portions  1   c  is not limited to two, but may be one, or may be three or more. 
     Exhaust passage hole  1   a  is provided in space formation member  1  and provided with a plurality of openings  1   a   1 . Each of the plurality of openings  1   a   1  is provided so as to be surrounded by frame portion  1   c  and exhaust tube support portion  1   b . As shown in  FIG. 9 , the opening area of each opening  1   a   1  is equal to an area of a plane that is surrounded by frame portion  1   c  and exhaust tube support portion  1   b  and curved along a dome shape formed by frame portion  1   c  (a plane with hatching lines in  FIG. 9 ). The opening area of exhaust passage hole  1   a  is equal to the total sum of the opening areas of the plurality of openings  1   a   1 . 
     Referring to  FIG. 11 , exhaust tube  20  to which space formation member  1  is attached is inserted into exhaust pipe  30  until the arch-shaped top portion of frame portion  1   c  comes into contact with ceiling wall  40   b  of exhaust terminal  40 . 
     Since the configuration of exhaust structure for combustion apparatus  100  in the present embodiment other than those described above is almost the same as the configuration of the first embodiment, the same components are designated by the same reference characters, and the description thereof will not be repeated. 
     In the present embodiment, space formation member  1  is provided separately from exhaust tube  20  and exhaust passage hole  1   a  is provided in this space formation member  1 , thereby improving the flexibility of each of the shape and the arrangement position of exhaust passage hole  1   a . Therefore, it becomes possible to achieve a structure that is more suitable for emission of combustion gas. 
     Furthermore, since the space formation member includes a frame portion formed in the shape of an arch-shaped plate, the top portion of this arch can be brought into contact with ceiling wall  40   b  of exhaust terminal  40  as shown in  FIG. 11  while combustion gas can be emitted through exhaust passage hole  1   a  between frame portion  1   c  and exhaust tube support portion  1   b.    
     Also, according to the present embodiment, the total sum of the opening areas of openings  1   a   1  included in exhaust passage hole  1   a  is greater than the cross-sectional area of the exhaust passage inside exhaust tube  20  as in the first embodiment. Accordingly, exhaust resistance against combustion gas is not caused by exhaust passage hole  1   a  consisting of the plurality of openings  1   a   1 , so that combustion gas can be stably emitted to the outside of the building through the plurality of openings  1   a   1 . 
     Third Embodiment 
     Then, the configuration of the exhaust structure for combustion apparatus in the third embodiment will be hereinafter described with reference to  FIGS. 12 to 20 . 
     Referring to  FIGS. 12 to 15 , the configuration of exhaust structure for combustion apparatus  100  in the present embodiment is different from the configuration of the first embodiment in that exhaust tube  20  is not provided with a through hole portion, but instead, space formation member  1  is attached to the other end portion  20   b  of exhaust tube  20 . Space formation member  1  has an exhaust tube support portion  1   d  attached to exhaust tube  20  and a protruding portion  1   e ,  1   f  protruding from exhaust tube support portion  1   d  to the side opposite to exhaust tube  20 . Exhaust passage hole  1   a  is provided in protruding portion  1   e ,  1   f.    
     Exhaust tube support portion  1   d  is provided at its end with an engaging portion  1   d   1  protruding to the outer circumferential side. By hooking this engaging portion  1   d   1  on the inner circumferential surface of exhaust tube  20 , exhaust tube support portion  1   d  is supported by the inner circumferential surface of exhaust tube  20 . 
     Protruding portion  1   e ,  1   f  has a flange portion  1   e  and a curved portion  1   f . Flange portion  1   e  is connected to exhaust tube support portion  1   d  and extends radially outward. Curved portion  1   f  is connected to the outer circumferential edge of flange portion  1   e , and has a convex shape curved toward the side opposite to exhaust tube support portion  1   d  relative to flange portion  1   e.    
     As shown in  FIGS. 14 and 15 , exhaust passage hole  1   a  is provided in flange portion  1   e  and has a plurality of openings  1   a   1 . A plurality of (for example, four) openings  1   a   1  are arranged along the circumference of flange portion  1   e  as shown in  FIG. 15 . The opening area of exhaust passage hole  1   a  is equal to the total sum of the opening areas of the plurality of openings  1   a   1 . Since exhaust passage hole  1   a  consisting of the plurality of openings  1   a   1  is provided in flange portion  1   e , exhaust passage hole  1   a  is located at a position where exhaust passage hole  1   a  cannot be seen when curved portion  1   f  is viewed from the curved portion  1   f  side toward the exhaust tube support portion  1   d  side as indicated by an arrow B in  FIG. 14 . 
     Furthermore, as shown in  FIG. 16 or 17 , a reduced diameter portion  1   g  may be additionally provided between exhaust tube support portion  1   d  and flange portion  1   e , and a plurality of openings  1   a   1  constituting exhaust passage hole  1   a  may be provided in reduced diameter portion  1   g . Reduced diameter portion  1   g  shown in  FIG. 16  is formed in an annular shape and has the same diameter as that of exhaust tube support portion  1   d . Reduced diameter portion  1   g  shown in  FIG. 17  is formed in an annular shape and has a diameter larger than that of exhaust tube support portion  1   d . Accordingly, in space formation member  1  shown in  FIG. 17 , a first flange portion  1   ea  extending in the radial direction is provided between exhaust tube support portion  1   d  and reduced diameter portion  1   g  while a second flange portion  1   eb  extending in the radial direction is provided between reduced diameter portion  1   g  and curved portion  1   f . In this configuration, the end face of the other end portion  20   b  of exhaust tube  20  is brought into contact with first flange portion  1   ea , so that exhaust passage hole  1   a  consisting of the plurality of openings  1   a   1  can be prevented from being blocked by exhaust tube  20 . 
     Also as shown in  FIGS. 18 to 20 , a plurality of openings  1   a   1  constituting exhaust passage hole  1   a  may be provided in curved portion  1   f . Each of the plurality of openings  1   a   1  is provided by cutting a part of the surface portion of curved portion  1   f  and bending this part. Accordingly, a bent portion  1   f   1  is located below opening  1   a   1  (on the exhaust tube support portion  1   d  side) and bent inside the dome shape of curved portion  1   f  (on the inner circumferential side). 
     By forming bent portion  1   f   1  in this way, only bent portion  1   f   1  can be seen but the exhaust passage of exhaust tube  20  cannot be seen through opening  1   a   1  when curved portion  1   f  is viewed from the curved portion  1   f  side toward the exhaust tube support portion  1   d  side as indicated by an arrow B shown in  FIG. 19 . 
     Since the configuration of exhaust structure for combustion apparatus  100  in the present embodiment other than those described above is almost the same as the configuration of the first embodiment, the same components are designated by the same reference characters, and the description thereof will not be repeated. 
     In the present embodiment, space formation member  1  has protruding portion  1   e ,  1   f  (alternatively, protruding portion  1   e ,  1   f ,  1   g ) provided with exhaust passage hole  1   a . Accordingly, the top portion of protruding portion  1   e ,  1   f  (alternatively, protruding portion  1   e ,  1   f ,  1   g ) (the top portion of curved portion  1   f ) can be brought into contact with ceiling wall  40   b  of exhaust terminal  40  while combustion gas can be emitted through exhaust passage hole  1   a.    
     Furthermore, curved portion  1   f  of space formation member  1  is curved in a convex shape. Accordingly, when the other end portion  20   b  of exhaust tube  20  having this space formation member  1  attached thereto is inserted into exhaust pipe  30 , the contact resistance between curved portion  1   f  and the inner circumferential surface of exhaust pipe  30  can be reduced. Furthermore, even if exhaust pipe  30  is curved, exhaust tube  20  can be smoothly moved through exhaust pipe  30 . Consequently, exhaust tube  20  having space formation member  1  attached thereto can be smoothly inserted into exhaust pipe  30  and moved therethrough. 
     Furthermore, in the configuration shown in  FIGS. 12 to 15 , the configuration shown in  FIG. 16 , and the configuration shown in  FIG. 17 , a plurality of openings  1   a   1  constituting exhaust passage hole  1   a  each are provided at a position where the plurality of openings  1   a   1  cannot be seen when curved portion  1   f  is viewed from the curved portion  1   f  side toward the exhaust tube support portion  1   d  side. Accordingly, when the other end portion  20   b  of exhaust tube  20  having this space formation member  1  attached thereto is inserted into exhaust pipe  30 , foreign substances are less likely to come into exhaust tube  20  through each opening  1   a   1  of space formation member  1 . 
     Furthermore, in the configuration shown in  FIGS. 18 to 20 , only bent portion  1   f   1  can be seen but the exhaust passage of exhaust tube  20  cannot be seen through the plurality of openings  1   a   1  constituting exhaust passage hole  1   a  when curved portion  1   f  is viewed from the curved portion  1   f  side toward the exhaust tube support portion  1   d  side. Accordingly, when the other end portion  20   b  of exhaust tube  20  having this space formation member  1  attached thereto is inserted into exhaust pipe  30 , foreign substances are less likely to come into exhaust tube  20  through each opening  1   a   1  of space formation member  1  because bent portion  1   f   1  becomes an obstacle. 
     Fourth Embodiment 
     Then, the configuration of the exhaust structure for combustion apparatus in the fourth embodiment will be hereinafter described with reference to  FIGS. 21 to 25 . 
     Referring to  FIGS. 21 to 23 , the configuration of exhaust structure for combustion apparatus  100  in the present embodiment is different from the configuration of the first embodiment in that a through hole portion is not provided in exhaust tube  20 , but instead, space formation member  1  is attached to the other end portion  20   b  of exhaust tube  20 . Space formation member  1  includes an exhaust tube support portion  1   d  attached to exhaust tube  20  and protruding portion  1   e ,  1   h ,  1   i  protruding from exhaust tube support portion  1   d  to the side opposite to exhaust tube  20 . 
     Protruding portion  1   e ,  1   h ,  1   i  has a flange portion  1   e , a cylindrical portion  1   h , and a tip end portion  1   i . Flange portion  1   e  extends radially outward from exhaust tube support portion  1   d . Cylindrical portion  1   h  is connected to the outer circumferential edge of flange portion  1   e . Tip end portion  1   i  is connected to the end of cylindrical portion  1   h  and formed in a conical shape, for example. Exhaust passage hole  1   a  is provided in cylindrical portion  1   h  and includes a plurality of openings  1   a   1 . As shown in  FIG. 15 , the plurality of openings  1   a   1  are arranged along the circumference of cylindrical portion  1   h  and also arranged along the direction in which cylindrical portion  1   h  extends (in the axial direction). The opening area of exhaust passage hole  1   a  is equal to the total sum of the opening areas of the plurality of openings  1   a   1 . 
     Each of the plurality of openings  1   a   1  is formed by cutting a part of the surface portion of cylindrical portion  1   h  and bending this part. Bent portion  1   h   1  obtained by this bending is located, for example, below opening  1   a   1  (on the exhaust tube support portion  1   d  side) and bent inward (on the inner circumferential side) relative to the outer circumferential surface having a cylindrical shape formed by cylindrical portion  1   h.    
     By forming bent portion  1   h   1  in this way, each of the plurality of openings  1   a   1  cannot be seen when cylindrical portion  1   h  is viewed from the cylindrical portion  1   h  side toward the exhaust tube support portion  1   d  side as shown by an arrow B shown in  FIG. 23 . 
     Furthermore, as shown in  FIGS. 24 and 25 , bent portion  1   h   1  may be located above opening  1   a   1  (on the tip end portion  1   i  side) and may be bent outward (on the outer circumferential side) relative to the outer circumferential surface having a cylindrical shape formed by cylindrical portion  1   h . Also in this configuration, due to bent portion  1   h   1  formed in this way, the plurality of openings  1   a   1  cannot be seen when cylindrical portion  1   h  is viewed from the cylindrical portion  1   h  side toward the exhaust tube support portion  1   d  side as indicated by arrow B shown in  FIG. 25 . 
     Although an explanation has been given in the above with regard to the configuration in which exhaust passage hole  1   a  is provided in cylindrical portion  1   h , exhaust passage hole  1   a  only has to be provided at least one of cylindrical portions  1   h  and tip end portion  1   i.    
     Since the configuration of exhaust structure for combustion apparatus  100  in the present embodiment other than those described above is almost the same as the configuration of the first embodiment, the same components are designated by the same reference characters, and the description thereof will not be repeated. 
     Although an explanation has been given in the above-described embodiment with regard to the case where tip end portion  1   i  has a conical shape, the shape of tip end portion  1   i  is not limited to a conical shape but may be a hemispherical shape or an elliptical hemispherical shape. 
     In the present embodiment, exhaust passage hole  1   a  is provided in at least one of cylindrical portions  1   h  and tip end portion  1   i  of space formation member  1 . Accordingly, the top portion of tip end portion  1   i  can be brought into contact with ceiling wall  40   b  of exhaust terminal  40  while combustion gas can be emitted through exhaust passage hole  1   a.    
     Furthermore, each of openings  1   a   1  is provided at a position where each opening  1   a   1  cannot be seen when cylindrical portion  1   h  is viewed from the cylindrical portion  1   h  side toward the exhaust tube support portion  1   d  side as indicated by arrow B shown in each of  FIGS. 23 and 25 . Accordingly, when the other end portion  20   b  of exhaust tube  20  having this space formation member  1  attached thereto is inserted into exhaust pipe  30 , foreign substances are less likely to come into exhaust tube  20  through each of the plurality of openings  1   a   1  of space formation member  1 . 
     Furthermore, space formation member  1  has cylindrical portion  1   h  and tip end portion  1   i . Accordingly, when the other end portion  20   b  of exhaust tube  20  having this space formation member  1  attached thereto is inserted into exhaust pipe  30 , exhaust tube  20  can be inserted into exhaust pipe  30  in a highly straight manner. 
     Fifth Embodiment 
     Then, the configuration of an exhaust structure for combustion apparatus in the fifth embodiment will be hereinafter described with reference to  FIGS. 26 and 27 . 
     Referring to  FIGS. 26 and 27 , the configuration of an exhaust structure for combustion apparatus  100  in the present embodiment is different from the configuration of the first embodiment in that exhaust tube  20  is provided with a plurality of cut portions  1   a   1  as an exhaust passage hole  1   a  in place of a through hole portion. Each of the plurality of cut portions  1   a   1  is obtained by cutting exhaust tube  20  from its outer circumferential surface to its inner circumferential surface. Each cut portion  1   a  is provided as a hole that is closed before the other end portion  20   b  of exhaust tube  20  comes into contact with ceiling wall  40   b  of exhaust terminal  40  but opened when the other end portion  20   b  of exhaust tube  20  comes into contact with ceiling wall  40   b  and compression force is exerted on exhaust tube  20 , as shown in  FIG. 27 . 
     Exhaust passage hole  1   a  in the present embodiment consists of a plurality of cut portions  1   a   1  that are opened when exhaust tube  20  receives compression force. In the state where each cut portion  1   a   1  is opened in this way, the total sum of the opening areas of the plurality of cut portions  1   a   1  that are opened is greater than the cross-sectional area of the exhaust passage of exhaust tube  20 . 
     Since the configuration of exhaust structure for combustion apparatus  100  in the present embodiment other than those described above is almost the same as the configuration of the first embodiment, the same components are designated by the same reference characters, and the description thereof will not be repeated. 
     In the present embodiment, each cut portion  1   a   1  is closed before the other end portion  20   b  of exhaust tube  20  comes into contact with ceiling wall  40   b  of exhaust terminal  40 , as shown in  FIG. 26 . In this way, since each cut portion  1   a   1  is closed when inserting exhaust tube  20  into exhaust pipe  30 , foreign substances can be prevented from coming into exhaust tube  20  through this each cut portion  1   a   1 . Furthermore, when exhaust tube  20  comes into contact with ceiling wall  40   b  and compression force is exerted on exhaust tube  20 , each cut portion  1   a   1  is opened, as shown in  FIG. 27 . After insertion of exhaust tube  20  into exhaust pipe  30  is completed in this way, each cut portion  1   a   1  is opened by compression force. Accordingly, even if the other end portion  20   b  of exhaust tube  20  is blocked by ceiling wall  40   b , combustion gas can be emitted from inside exhaust tube  20 . 
     Others 
     In the above-described first to fifth embodiments, the other end portion  20   b  of exhaust tube  20  may be subjected to a tube-diameter reducing process so as to reduce the diameter of the other end portion  20   b  of exhaust tube  20  toward its end. This tube-diameter reducing process is carried out as in the following manner: for example, as shown in  FIG. 28 , a plurality of V-shaped cut portions  20   b   2  are formed in the other end portion  20   b  of exhaust tube  20 , and then, the ends of a plurality of tip end portions  20   b   3  separated by these V-shaped cut portions  20   b   2  are inclined toward the inner circumferential side as shown in  FIG. 29 , so that tip end portions  20   b   3  adjacent to each other are connected to each other. By this tube-diameter reducing process, the other end portion  20   b  of exhaust tube  20  is reduced in diameter toward its end as shown in  FIG. 29 , so that the other end portion  20   b  can be readily inserted into exhaust pipe  30 . 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the scope of the present invention being interpreted by the terms of the appended claims.