Patent Publication Number: US-2023132467-A1

Title: Heat exchanger and water heater

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2021-180258 filed on Nov. 4, 2021. The entire contents of the priority application are incorporated herein by reference. 
     TECHNICAL FIELD 
     The technology described herein relates to a heat exchanger and a water heater. 
     BACKGROUND 
     Conventionally, a primary heat exchanger is known as a heat exchanger used in a water heater. Such a primary heat exchanger can be made of stainless steel and includes an inner casing, fins disposed in the inner casing, and heat transfer tubes. The inner casing is formed in a square cylindrical shape through which combustion exhaust gas from a burner passes downward. The fins are disposed in a lower position of the inner casing and arranged at intervals along a thickness direction of the fins, which corresponds to a right-left direction of the primary heat exchanger. Each of the fins includes two edge portions with respect to a front-rear direction of the primary heat exchanger and the two end portions are fixed to inner wall surfaces of the inner casing. The heat transfer tubes extend through the fins, which are disposed in the inner casing, in the right-left direction. An example of such a system is disclosed in Japanese Unexamined Patent Application Publication No. 2020-143841. 
     In the primary heat exchanger described above, the combustion exhaust gas from the burner flows downstream by a pressure generated by a fan and passes through between the fins. The heat of the combustion exhaust gas is transferred to the heat transfer tubes via the fins and the heat is further transferred from the heat transfer tubes to water passing through the heat transfer tubes. The temperature of the combustion exhaust gas flowing through the fins on the upstream side is higher than the temperature of the combustion exhaust gas flowing through the fins on the downstream side. Therefore, an upstream side (upper side) portion of the fin has higher temperature than a downstream side (lower side) portion of the fin. Consequently, upper parts of the heat transfer tubes penetrating the fins thermally expand larger than lower parts of the heat transfer tubes penetrating the fins. The heat transfer tubes tend to deform with upward warping. 
     The heat transfer tubes disposed in a middle of the inner casing are apart from the edge potions of the fins that are fixed to the inner wall surfaces. Therefore, the heat transfer tubes disposed in the middle of the inner casing are likely to deform freely. Consequently, the heat transfer tubes disposed in the middle of the inner casing are likely to be allowed to deform with the upward warping, as described above. On the other hand, the heat transfer tubes disposed in front and rear end portions of the inner casing are near the edge portions of the fins that are fixed to the inner wall surfaces. This restricts deformations of the heat transfer tubes disposed in the front and rear end portions of the inner casing. Therefore, the heat transfer tubes disposed in the front and rear end portions of the inner casing are less likely to be allowed to deform with the upward warping. For this reason, thermal expansion of the heat transfer tubes may cause stress concentration on the heat transfer tubes disposed in the front and rear end portions of the inner casing and this may damage the heat transfer tubes. Such a problem is particularly noticeable in the heat exchanger made of stainless steel that has lower heat conductivity and toughness compared to copper etc. 
     SUMMARY 
     The technology described herein was made in view of the above circumstances. An object is to provide a heat exchanger in which heat transfer tubes are less likely to be damaged and to provide a water heater including the heat exchanger. 
     A heat exchanger according to a first aspect in the present disclosure includes a case, fins, and heat transfer tubes. The case is rectangular cylindrical and made of stainless steel, in which a combustion exhaust gas passes downward. The case includes a first wall, a second wall, a third wall, and a fourth wall. The first wall and the second wall face each other in a first direction. The third wall and the fourth wall face each other in a second direction. The third wall connects one side edges of the first wall and the second wall. The fourth wall connects other side edges of the first wall and the second wall. The fins are made of stainless steel and disposed in a lower portion of the case and arranged in the first direction. Each of the fins has an elongated shape extending in the second direction and includes a first end and a second end that is an opposite end from the first end. Each of the fins includes through holes penetrating each of the fins in the first direction. The heat transfer tubes are made of stainless steel and arranged in the second direction and inserted in the through holes. In this heat exchanger, each of the fins includes heat receiving portions, a first recessed portion, a second recessed portion, connecting portions, a first edge portion, and a second edge portion. The heat receiving portions have a frame shape and extend around the through holes. The heat receiving portions are arranged in the second direction and include a first heat receiving portion disposed at the first end of each of the fins and a second heat receiving portion disposed at the second end of each of the fins. The first recessed portion is included in the first heat receiving portion and recessed from an outer edge of an upper part of the first heat receiving portion. The second recessed portion is included in the second heat receiving portion and recessed from an outer edge of an upper part of the second heat receiving portion. The connecting portions connect the heat receiving portions adjacent to each other in the second direction. The first edge portion protrudes in the first direction from the first end of each of the fins and is fixed to the third wall. The second edge portion protrudes in the first direction from the second end of each of the fins and is fixed to the fourth wall. 
     In a second aspect of the disclosure based on the first aspect, the first recessed portion is included in a portion of the first heat receiving portion that is an opposite side from the first edge portion in the second direction. The second recessed portion is included in a portion of the second heat receiving portion that is an opposite side from the second edge portion in the second direction. 
     In a third aspect of the disclosure based on the first or the second aspect, the first recessed portion is near one of the connecting portions connecting the first heat receiving portion and one of the heat receiving portions. 
     In a fourth aspect of the disclosure based on any one of the first to the third aspects, the first recessed portion includes recessed portions and the first heat receiving portion further includes a protrusion portion disposed between two of the recessed portions adjacent to each other. 
     A fifth aspect of the disclosure based on the fourth aspect, the through holes include a first through hole around which the first heat receiving portion extends. The protrusion portion has a length extending along a hole edge of the first through hole. Each of the recessed portions has a recessed bottom having a length extending along the hole edge of the first through hole. The length of the protrusion portion is smaller than the length of the recessed bottom of each of the recessed portions. 
     A sixth aspect of the disclosure is directed to a water heater that includes a burner and the heat exchanger according to any one of the first to the fifth aspects. 
     According to the technology described herein, a heat exchanger in which damage of heat transfer tubes are suppressed and a water heater with the heat exchanger can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a front view of a water heater according to one embodiment with a front cover being not illustrated. 
         FIG.  2    is a front view of the water heater with a resin sheet, a controller, and a display operation panel being not illustrated. 
         FIG.  3    is a perspective view of a primary heat exchanger. 
         FIG.  4    is a right-side view of the primary heat exchanger with lid parts of headers being not illustrated. 
         FIG.  5    is a left-side view of the primary heat exchanger with lid parts of headers being not illustrated. 
         FIG.  6    is a cross-sectional view taken along A-A line in  FIG.  2   . 
         FIG.  7    is an expanded view of  FIG.  6    illustrating a surrounding portion near a thorough-hole formed in a front end portion of a fin. 
         FIG.  8    is a perspective view of the fin with a repeated structure being not illustrated. 
     
    
    
     DETAILED DESCRIPTION 
     &lt;Embodiment&gt; 
     One embodiment will be described below with reference to  FIGS.  1  to  8   . Regarding components having the same configuration, some of the components may be indicated by reference signs and others may not be indicated by the reference signs. 
     [Whole Structure of Water Heater] 
       FIG.  1    is a front view of a water heater  1  and shows the water heater  1  with a front cover being removed.  FIG.  2    shows the water heater  1  and does not illustrate a resin sheet  26 , a controller  12 , and a display operation panel  13 , which are shown in  FIG.  1   . 
     The water heater  1  includes an outer casing  2  and an inner body  3 . The outer casing  2  has a rectangular cylindrical shape and has an opening at a front face. The inner body  3  is housed in the outer casing  2 . The inner body  3  includes a burner  4 , a primary heat exchanger  5  (one example of a heat exchanger), and a secondary heat exchanger  6  in this order from an upper side. The water heater  1  is a downstream combustion type water heater where combustion exhaust gas from the burner  4  flows from above to below. 
     An exhaust portion  7 , a fan unit  8 , and a gas supply unit  10  are arranged in the outer casing  2 . The exhaust portion  7  extends from a lower part of the inner body  3  toward a rear side and then toward an upper side. The fan unit  8  is connected to the burner  4  in a right portion of the inner body  3 . The gas supply unit  10  is connected to the fan unit  8  under the fan unit  8 . The gas supply unit  10  supplies fuel gas from a gas introduce pipe  11  to the fan unit  8  via a gas governor  9 . The controller  12  including an electrical board is disposed in a lower right portion of the inner body  3 . The display operation panel  13  is disposed in a lower middle portion of the inner body  3 . The display operation panel  13  is not covered with the front cover. 
     [Burner] 
     The burner  4  is a totally primary air type burner in which a mixture of combustion gas and all air required for burning burns. The burner  4  includes an upper casing  14  that has a laterally-long rectangular plan-view shape. The upper casing  14  has a predetermined depth in the upper-bottom direction and openings in an upper surface and a lower surface of the upper casing  14 . The opening in the upper surface of the upper casing  14  is closed by a chamber  15  that is upwardly protruded and connected to the fan unit  8 . A frame hole plate (not shown) which has flame holes is disposed in the opening in the lower surface of the upper casing  14 . The mixture burns at the surface of a frame hole plate (the lower surface of upper casing  14 ). 
     The fan unit  8  includes a fan case  16  having a circular plan-view shape and a fan disposed in the fan case  16 . A fan motor  17  that rotationally drives the fan is disposed on an upper side and at a middle with respect to the fan case  16 . 
     [Primary Heat Exchanger] 
     As shown in  FIG.  3   , the primary heat exchanger  5  includes an inner casing  20  (an example of a case), fins  21 , heat transfer tubes  22  (eight in this embodiment), and water flow pipes  23  (six in this embodiment). The inner casing  20  is rectangular cylindrical. The inner casing  20  is fixed to the burner  4 . In the inner casing  20 , combustion exhaust gas generated at the burner  4  flows from above to below (see  FIG.  2   ). A heat of the combustion exhaust gas is transferred to water passing through the heat transfer tubes  22 . 
     The inner casing  20 , the fins  21 , the heat transfer tubes  22 , and the water flow pipes  23  are made of stainless steel. 
     As shown in  FIG.  1   , a belt-like resin sheet  26  is wound around an upper part of an outer peripheral surface of the inner casing  20 . The resin sheet  26  includes conductive patterns of a meandering shape stretched over an approximately whole surface of the resin sheet  26 . The resin sheet  26  allows detection of leakage of combustion exhaust gas from the inner casing  20 . 
     As shown in  FIG.  3   , the inner casing  20  includes a first wall  31 , a second wall  32 , a third wall  33 , and a fourth wall  34 . The first wall  31  and the second wall  32  face each other in a right-left direction (an example of a first direction). One (front) end part of the first wall  31  and one (front) end part of the second wall  32  are jointed via the third wall  33 . The other (rear) end part of the first wall  31  and the other (rear) end part of the second wall  32  are jointed via the fourth wall  34 . The third wall  33  and the fourth wall  34  face each other in a front-rear direction (an example of a second direction). In this embodiment, the first wall  31  is a right side wall of the inner casing  20 , and the second wall  32  is a left side wall of the inner casing  20 . The third wall  33  is a front side wall of the inner casing  20 , and the fourth wall  34  is a rear side wall of the inner casing  20 . 
     As shown in  FIGS.  3  and  6   , the fins  21  (only some of the fins  21  are shown) are disposed in a lower portion of the inner casing  20 . The fins  21  are arranged in parallel to each other at predetermined intervals along the right-left direction. The heat transfer tubes  22  have linear shapes extending along the right-left direction. The heat transfer tubes  22  are disposed in the lower portion of the inner casing  20  and arranged in parallel in the front-rear direction. The heat transfer tubes  22  penetrate the fins  21  in the right-left direction. As shown in  FIGS.  4  to  6   , a cross-sectional shape of each of the heat transfer tubes  22  is an elliptic shape having a major axis in a vertical direction. In each of the heat transfer tubes  22 , a flow velocity control member  22 A is disposed for controlling a flow velocity of water and agitating water. 
     As shown in  FIGS.  3  and  6   , the water flow pipes  23  are located at upper positions with respect to the heat transfer tubes  22 . Three water flow pipes  23  are arranged in parallel along the vertical direction at predetermined intervals on each of outer wall surfaces of the third wall  33  and the fourth wall  34  of the inner casing  20 . The water flow pipes  23  have a linear shape extending in the right-left direction. A cross-sectional shape of each of the water flow pipes  23  is circular. 
     As shown in  FIGS.  3  to  5   , lower side headers  40  are attached to lower parts of the first wall  31  and the second wall  32 . In  FIGS.  4  and  5   , lid parts of headers  40 ,  42 ,  43 ,  45  are not illustrated in order to clearly show connections between the heat transfer tubes  22  or water flow pipes  23  and headers  40 ,  42 ,  43 ,  45 . 
     As shown in  FIGS.  4  and  5   , each of the lower side headers  40  connects right or left ends of two of the heat transfer tubes  22  that are adjacent to each other in the front-rear direction. As a result, the heat transfer tubes  22  are connected to each other and forms a meandering pathway for water. However, the lower side header  40  disposed at the rearmost position on the first wall  31  is connected only to a right end of the heat transfer tube  22  disposed at the rearmost position among the heat transfer tubes  22 . As shown in  FIG.  3   , the lower side header  40  disposed at the rearmost position on the first wall  31  is connected to a joint tube  41 . As shown in  FIG.  1   , the joint tube  41  is connected to the secondary heat exchanger  6  via a connection pipe  24 . 
     As shown in  FIG.  4   , a right end of the heat transfer tube  22  disposed at the frontmost position among the heat transfer tubes  22  is connected to right ends of three front side water flow pipes  23  via a front side header  42  extending in the vertical direction. 
     As shown in  FIGS.  3  and  4   , a first header  43  is attached to an upper part of the first wall  31 . The first header  43  is connected to right ends of rear side water flow pipes  23 . As shown in  FIG.  3   , a joint tube  44  is mounted in a right upper part of the first header  43 . As shown in  FIG.  2   , the joint tube  44  is connected to a hot water outlet pipe  25 . 
     As shown in  FIG.  5   , a second header  45  is attached to an upper part of the second wall  32 . The second header  45  is connected to left ends of front side water flow pipes  23  and left ends of rear side water flow pipes  23 . Namely, the second header  45  connects left ends of three front side water flow pipes  32  and three rear side water flow pipes  32 . 
     Therefore, water flows through a water flow path in the primary heat exchanger  5  as follows. 
     Water flowing into the rearmost heat transfer tube  22  from the connection pipe  24  passes through the heat transfer tubes  22  via the lower side headers  40  to the front side in a meandering way. Then, water flowing out from the frontmost heat transfer tube  22  flows into the three front side water flow pipes  23  via the front side header  42  and moves to the left side through the three front side water flow pipes  23 . Then, the water flows into the three rear side water flow pipes  23  via the second header  45  and moves to the right side through the three rear side water flow pipes  23 . Water flowing out from the three rear side water flow pipes  23  flows out from the hot water outlet pipe  25  via the first header  43 . 
     [Fins, Through Holes] 
     A structure of each of the fins  21  will be described below with reference to  FIGS.  6  to  8   . 
     The fin  21  is a laterally long plate member extending in the front-rear direction. As shown in  FIG.  6   , eight oval shaped through holes  50  are formed in the fin  21  and disposed at intervals in the front-rear direction. The heat transfer tubes  22  are inserted into the through holes  50  of the fins  21 . 
     [Heat Receiving Portions, Connecting Portions] 
     As shown in  FIG.  8   , burring parts  51  protrude leftward from inner edges of the through holes  50  of the fins  21 . Each of the fins  21  includes flange shaped heat receiving portions  52 ,  52 F,  52 R. The fin  21  includes the heat receiving portion  52 F and the heat receiving portion  52 R at front and rear ends, respectively, and receiving portions  52  between the heat receiving portion  52 F and the heat receiving portion  52 R. The heat receiving portions  52 ,  52 F,  52 R extend outward in a radial direction of the through holes  50  from hole edges of the through holes  50 , respectively. The fins  21  include the heat receiving portions  52 ,  52 F,  52 R and connecting portions  53  that connect the heat receiving portions  52 , the heat receiving portion  52  and the heat receiving portion  52 F, the heat receiving portion  52  and the heat receiving portion  52 F. Each of the heat receiving portions  52 ,  52 F,  52 R has an ovel frame shape and includes the through hole  50 . The burring parts  51  protrude from hole edges of the through holes  50 . The heat receiving portions  52 ,  52 F,  52 R receive heat from combustion exhaust gas flowing in the inner casing  20  from the above to the below, transfer the heat to the heat transfer tubes  22  inserted in the through holes  50 , and warm up water in the heat transfer tubes  22 . The heat receiving portions  52 ,  52 F,  52 R are connected by the connecting portions  53  at lower portions of the heat receiving portions  52 ,  52 F,  52 R. 
     [Edge Portions] 
     Each of the fins  21  includes a first edge portion  54 A on the front end and a second edge portion  54 B on the rear end. The first edge portion  54 A protrudes leftward from a front end of the fin  21  and the second edge portion  54 B protrudes leftward from a rear end of the fin  21 . Namely, the first edge portion  54 A protrudes from a front edge of the heat receiving portion  52 F and the second edge portion  54 B protrudes from a rear edge of the heat receiving portion  52 R. As shown in  FIG.  6   , the first edge portion MA is disposed to abut on a lower part of an inner wall surface of the third wall  33  and fixed. The second edge portion MB is disposed to abut on a lower part of an inner wall surface of the fourth wall  34  and fixed. Each of the fins  21  further includes a third edge portion MC and a fourth edge portion MD that protrude leftward from the front end and the rear end of the fin  21 , respectively. The third edge portion MC is on an upper side of the first edge portion MA and spaced away from the first edge portion MA. The fourth edge portion MD is on an upper side of the second edge portion MB and spaced away from the first edge portion MB. A recess MR is between the first edge portion MA and the third edge portion MC and between the second edge portion MB and the fourth edge portion MD. 
     [Cavity Portions] 
     As shown in  FIG.  8   , the heat receiving portions  52 F,  52 R of each of the fins  21  include cavity portions  55 . The cavity portions  55  are included in the upper parts of the heat receiving portions  52 F,  52 R and recessed inward in the radial direction of the through holes  50 . The cavity portions  55  are recessed portions that are recessed inward in the radial direction of the through holes  50  from outer edges of the heat receiving portions  52 F,  52 R. The recessed portions included in the heat receiving portion  52 F are included in a first recessed portion. The recessed portions included in the heat receiving portion  52 R are included in a second recessed portion. By providing the cavity portions  55 , the volume of the upper parts of the heat receiving portions  52 F,  52 R decreases and therefore, quantity of the heat that the upper parts of the heat receiving portions  52 F,  52 R receive from combustion exhaust gas passing through the inner casing  20  is reduced (see  FIGS.  6  and  7   ). Therefore, quantity of the heat that upper parts of the heat transfer tubes  22  receive from the upper parts of the heat receiving portions  52 F,  52 R decreases. Heat of the combustion exhaust gas that is not recovered by the upper parts of the heat receiving portions  52 F,  52 R is recovered by lower parts of the heat receiving portions  52 F,  52 R, and the heat thus recovered is transferred to lower parts of the heat transfer tubes  22 . Thus, by providing the cavity portions  55  on the upper parts of the heat receiving portions  52 F,  52 R, the heat that the lower parts of the heat transfer tubes  22  receive increases. 
     [Temperature Difference Suppression Portions] 
     When combustion exhaust gas passes through the inner casing  20  from the above to the below, upper parts of the heat transfer tubes  22  become hotter than lower parts of those. Consequently, the heat transfer tubes  22  tend to deform with upward warping. In this embodiment, however, the cavity portions  55  function as temperature difference suppression portions that reduces temperature difference between the upper parts and the lower parts of the heat transfer tubes  22  and therefore, the heat transfer tubes  22  disposed at front and rear ends of the fins  21  are less likely to be deformed. 
     The heat transfer tubes  22  disposed at the front and rear ends of the fins  21  are close to the first edge portion  54 A and the second edge portion  54 B with which the fins  21  are fixed to the inner casing  20 . Therefore, the deformation of the heat transfer tubes  22  at the front and rear ends caused by thermal expansion tends to be limited by a fixing structure of the first edge portion  54 A, the second edge portions  54 B, and the inner casing  20 . However, by providing the cavity portions  55 , the deformation of the heat transfer tubes  22  at the front and rear ends is suppressed, and the heat transfer tubes  22  at the front and rear ends are less likely to be damaged by the thermal expansion. 
     The heat receiving portions  52  that are disposed between the heat receiving portions  52 F,  52 R do not include the cavity portions  55 . Namely, the heat receiving portions  52  disposed in a middle inside the inner casing  20  do not include the cavity portions  55  (see FIG.  6 ). The deformation of the heat receiving portions  52  disposed in the middle of the inner casing  20  is not limited by their surroundings. Therefore, the heat transfer tubes  22  disposed in the middle of the inner casing  20  are allowed to deform and hardly damaged. 
     Thus, since the heat receiving portions  52  disposed in the middle of the inner casing  20  do not include the cavity portions  55 , reduction in the quantity of the heat that the fins  21  receive from combustion exhaust gas is suppressed and lowering of heat efficiency of the primary heat exchanger  5  is suppressed. 
     As shown in  FIGS.  6  and  8   , the heat receiving portions  52 F disposed at the front ends include the cavity portions  55  on rear upper parts of the heat receiving portions  52 F. The heat receiving portions  52 R disposed at the rear ends include the cavity portions  55  on front upper parts of the heat receiving portions  52 R. Namely, in the heat receiving portions  52 F,  52 R, the cavity portions  55  are disposed apart from the first edge portion  54 A and the second edge portion  54 B, respectively, in the front-rear direction. 
     The cavity portions  55  included in the heat receiving portion  52 F is near the connecting portion  53  connecting the heat receiving portion  52 F and the heat receiving portion  52 . The cavity portions  55  included in the heat receiving portion  52 R is near the connecting portion  53  connecting the heat receiving portion  52 R and the heat receiving portion  52 . 
     Normally, as shown in  FIG.  7   , combustion exhaust gas is less likely to pass through a space SP between the heat transfer tube  22  and each of the first edge portion  54 A, the third edge portion  54 C. A portion of an upper part of the heat receiving portion  52 F near the first edge portion  54 A and the third edge portion  54 C (front side part in  FIG.  7   ) receives smaller amount of heat than the amount of heat that a portion of the upper part of the heat receiving portion  52 F on an opposite side from the first edge portion  54 A and the third edge portion  54 C in the front-rear direction (rear side part in  FIG.  7   ) receives. Therefore, with the heat receiving portion  52 F not including the cavity portion  55  in a portion close to the first edge portion MA but including the cavity portion  55  in a portion apart from the first edge portion  54 A and the third edge portion  54 C in the front-rear direction, the quantity of heat that the front upper part of the heat receiving portion  52 F receives from combustion exhaust gas and the quantity of heat that the rear upper part of the heat receiving portion  52 F receives from combustion exhaust gas can be balanced. Furthermore, the damage of the heat transfer tube  22  caused by thermal expansion is further suppressed. The heat receiving portion  52 R has same configuration and effects as those of the heat receiving portion  52 F. 
     [Protrusion Portions] 
     The heat receiving portion  52 F,  52 R includes the cavity portions  55  that are adjacent to each other. In this embodiment, as shown in  FIG.  7   , the front side heat receiving portion  52 F includes two cavity portions  55  and a protrusion portion  57  between the two cavity portions  55 . The protrusion portion  57  protrudes outward in the radial direction of the through hole  50  from the hole edge of the through hole  50 . The rear side heat receiving portion  52 R also includes two cavity portions  55  and the protrusion portion  57  (see  FIG.  6   ). The protrusion portion  57  has a length extending along the hole edge of the through hole  50 . The cavity portion  55  has a recessed bottom having a length extending along the hole edge of the through hole  50 . The length of the protrusion portion  57  is much smaller than the length of the recessed bottom of the cavity portion  55 . 
     With the cavity portions  55 , the heat that the heat receiving portion  52 F,  52 R receives may be excessively decreased. However, with the protrusion portion  57 , such excessive decrease in the heat that the heat receiving portion  52 F,  52 R receives is suppressed. The protrusion portion  57  can be used as a holding part with which the fin  21  can be held when the through holes  50  and the burring parts  51  are formed in a process of producing the fin  21  by processing a metal plate. 
     [Secondary Heat Exchanger] 
     As shown in  FIG.  2   , the secondary heat exchanger  6  includes a lower casing  80  that is rectangular cylindrical and communicates with the inner casing  20 . In the lower casing  80 , heat transfer plates with concave/convex structures are stacked with a predetermined space in between. Internal flow paths are formed between the heat transfer plates. The internal flow paths, which are continuous to each other, are provided with inlets connected to a water supply pipe  81  and outlets connected to the connection pipe  24 . 
     The exhaust portion  7  includes a drain receiver  82  and an exhaust duct  83 . The drain receiver  82  is attached to a lower surface of the lower casing  80  of the secondary heat exchanger  6 . The exhaust duct  83  is erected from a rear part of the drain receiver  82 . A bottom part of the drain receiver  82  is connected to a neutralizer  85  via a drain discharge pipe  84 . 
     The exhaust duct  83  is made of synthetic resin. The exhaust duct  83  is laterally long and rectangular cylindrical. An upper cover  86  is joined to an opening edge of an opening formed at the upper end of the exhaust duct  83 . The upper cover  86  has an exhaust cylinder  87  with a cylindrical shape that protrudes from an upper surface of the outer casing  2 . 
     [Explanation of Operation of Water Heater] 
     In the water heater  1 , when water is introduced into the instrument, the controller  12  drives the fan motor  17  to rotate the fan with a rotation rate according to a combustion amount requested by a remote controller etc. Then, an air is sucked into the fan unit  8  such that an air amount is proportional to the rotation rate of the fan. Simultaneously, a fuel gas is supplied from the gas introduce pipe  11 , and the gas governor  9  controls a pressure of the fuel gas. After that, the fuel gas is mixed with an air that is introduced via a venturi provided at a suction side of the fan unit  8 , and a mixture of the fuel gas and the air is produced. The mixture produced is discharged from a discharge port of the fan case  16  into the chamber  15  of the burner  4  and suppled in the upper casing  14 . The mixture is ejected from each of the flame holes of the flame hole plate and burns by being ignited by ignition electrodes. 
     Combustion exhaust gas from the burner  4  passes through a space between the fins  21  in the inner casing  20  of the primary heat exchanger  5  from the above to the below and exchanges heat with water flowing in the heat transfer tube  22 , and sensible heat is recovered. 
     At this occasion, with the cavity portions  55  (the temperature difference suppression portions), temperature difference between the upper parts and the lower parts of the heat transfer tubes  22  disposed at front and rear ends can be decreased and therefore, the heat transfer tubes  22  disposed at front and rear ends are less likely to be damaged by thermal expansion. 
     Combustion exhaust gas discharged from the primary heat exchanger  5  passes through spaces between the heat transfer plates in the lower casing  80  of the secondary heat exchanger  6  and exchanges heat with water flowing in the internal flowing paths of the heat transfer plates, and latent heat is recovered. 
     Combustion exhaust gas having passed through the lower casing  80  enters the drain receiver  82  of the exhaust portion  7 , moves to a rear part of the drain receiver  82 , ascends in the exhaust duct  83 , and is discharged from the exhaust cylinder  87  to the outside. A drain generated in the secondary heat exchanger  6  drops into the drain receiver  82  and is discharged to the outside of the instrument through the drain discharge pipe  84  and the neutralizer  85 . 
     [Effects of Embodiment] 
     As above, in the primary heat exchanger  5  of this embodiment, the fins  21  include the heat receiving portions  52 ,  52 F,  52 R, the first edge portion  54 A, the second edge portion  54 B, and the temperature difference suppression portions (the cavity portions  55 ). The heat receiving portions  52 ,  52 F,  52 R are flange shaped and extend outward in the radial direction of the through holes  50  from the hole rim parts of the through holes  50 , respectively. The first edge portion  54 A and the second edge portion  54 B are disposed at front and rear ends of each of the fins  21 . The first edge portion  54 A is fixed to the third wall  33  and the second edge portion  54 B is fixed to the fourth wall  34 . The temperature difference suppression portions reduce a temperature difference between the upper parts and the lower parts of the two of the heat transfer tubes  22  disposed at front and rear ends. 
     According to this configuration, the temperature difference suppression portions can reduce the temperature difference between the upper parts and the lower parts of the two of the heat transfer tubes  22  disposed at front and rear ends. Therefore, the transfer tubes  22  disposed at front and rear ends are less likely to be damaged by thermal expansion. 
     In this embodiment, as an example of the temperature difference suppression portions, the cavity portions  55  are formed in upper parts of the heat receiving portions  52 F,  52 R disposed at front and rear ends of the fins  21 . The cavity portions  55  are recessed inward in the radial direction of the through holes  50 . Therefore, with the cavity portions  55  having a simple structure, the heat that the upper parts of the heat receiving portions  52 F,  52 R receive from combustion exhaust gas can be reduced and thus the temperature difference between the upper parts and the lower parts of the two of the heat transfer tubes  22  disposed at the front and rear ends can be reduced. 
     In this embodiment, the heat receiving portion  52 F,  52 R includes the cavity portions  55  in a portion of the upper portion thereof that is away from the corresponding edge portion MA, MB. Although the combustion exhaust gas is less likely to pass near the portion of the heat receiving portion  52 F,  52 R close to the edge portion MA, MB, with the cavity portions  55 , heat that the portion of the heat receiving portion  52 F,  52 R close to the edge portion MA, MB receives from combustion exhaust gas and heat that the portion of the heat receiving portion  52 F,  52 R away from the edge portion MA, MB receives from combustion exhaust gas can be uniformed. This further suppresses a damage of the heat transfer tube  22  by thermal expansion. 
     In this embodiment, the heat receiving portion  52 F,  52 R includes the cavity portions  55  that are provided adjacent to each other and the protrusion portion  57  between the cavity portions  55 . Therefore, the protrusion portion  57  recovers heat from combustion exhaust gas and suppresses extreme decrease of heat efficiency that is caused by formation of the temperature difference suppression portions. The protrusion portion  57  is used as the holding part with which the fin  21  can be held when the through holes  50  and the burring parts  51  are formed in a process of producing the fin  21  by processing a metal plate. 
     The water heater  1  described in this embodiment includes the burner  4  and the primary heat exchanger  5  above described. Therefore, it is possible to provide the water heater  1  that includes the primary heat exchanger  5  including the heat transfer tubes  22  that are less likely to be damaged. 
     &lt;Other embodiments&gt; 
     The technology described herein is not limited to the embodiments described above with reference to the drawings. For example, features of the embodiments described above or below can be combined as far as they are compatible. Any feature of the embodiments described above or below that is not explicitly stated as essential may be omitted. The above embodiments may be altered as following. 
     A water heater may include only a primary heat exchanger. 
     A heat exchanger of present disclosure may be different types of heat exchangers including heat exchangers of a circulation type for bath and central heating. 
     The technical scope of present disclosure is not limited to the embodiment described above and may include all modifications in the scope of claim or its equivalent scope.