Abstract:
The invention relates to a high-pressure cleaning appliance, comprising a heatable heat exchanger for heating up a liquid dispensable by the high-pressure cleaning appliance, a blower with a flow channel connected thereto for supplying combustion air to the heat exchanger, and a setting member for setting the amount of combustion air suppliable to the heat exchanger, the setting member having a channel constriction section which is positionable in the flow channel and movable relative thereto for changing the free cross-sectional area of the flow channel. In order to develop a generic high-pressure cleaning appliance further so that the amount of combustion air suppliable to the heat exchanger can be better adjusted, it is proposed that the channel constriction section be configured so as to be slidingly displaceable relative to the flow channel.

Description:
[0001]    This application is a continuation of international application number PCT/EP2010/061378 filed on Aug. 5, 2010 and claims the benefit of German application number 20 2009 014 093.8 filed on Oct. 14, 2009. 
         [0002]    The present disclosure relates to the subject matter disclosed in international application number PCT/EP2010/061378 of Aug. 5, 2010 and German application number 20 2009 014 093.8 of Oct. 14, 2009, which are incorporated herein by reference in their entirety and for all purposes. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    The invention relates to a high-pressure cleaning appliance, comprising a heatable heat exchanger for heating up a liquid dispensable by the high-pressure cleaning appliance, a blower with a flow channel connected thereto for supplying combustion air to the heat exchanger, and a setting member for setting the amount of combustion air suppliable to the heat exchanger, the setting member having a channel constriction section which is positionable in the flow channel and movable relative thereto for changing the free cross-sectional area of the flow channel. 
         [0004]    Such high-pressure cleaning appliances are known, in which the setting member comprises, in each case, a throttle flap with a channel constriction section rotatable relative to the flow channel. The free cross-sectional area of the flow channel differs in accordance with the angular position of the channel constriction section relative to the flow channel, so that the amount of combustion air supplied to the heat exchanger can thereby be set. “The amount of combustion air supplied to the heat exchanger” is, in this case, to be understood as considered per time unit and, therefore, designates the volume flow rate of combustion air. 
         [0005]    High-pressure cleaning appliances equipped with throttle flaps have been found to work well in practice. Even better adjustability of the amount of combustion air that is to be supplied to the heat exchanger would, however, be desirable. 
         [0006]    The object of the present invention is to so develop a generic high-pressure cleaning appliance that the amount of combustion air suppliable to the heat exchanger can be better adjusted. 
       SUMMARY OF THE INVENTION 
       [0007]    This object is accomplished, in accordance with the invention, in a high-pressure cleaning appliance of the kind mentioned at the outset in that the channel constriction section is configured so as to be slidingly displaceable relative to the flow channel. 
         [0008]    A sliding displacement of the channel constriction section relative to the flow channel in the high-pressure cleaning appliance in accordance with the invention can effect a change in the free cross-sectional area of the flow channel, which is linearly dependent upon the path of displacement of the channel constriction section relative to the flow channel. This is not possible with high-pressure cleaning appliances with a channel constriction section rotatable relative to the flow channel that are known from the prior art. There the change in the free cross-sectional area occurs in dependence upon the change in the angle between the channel constriction section and the flow channel. In particular, the change in the free cross-sectional area does not depend linearly on the change in the angle. Owing to the linear dependence of the change in the free cross-sectional area on the path of displacement of the channel constriction section in the high-pressure cleaning appliance in accordance with the invention, the change in the amount and, therefore, also the total amount of combustion air supplied to the heat exchanger can, in practice, be set more precisely. 
         [0009]    In addition, the displaceable configuration of the channel constriction section relative to the flow channel allows a constructional simplification of the setting member. 
         [0010]    It may be provided that the setting member in its entirety is configured as displaceable relative to the flow channel. 
         [0011]    It is advantageous for the channel constriction section to be displaceable transversely to the direction of flow of the combustion air in the flow channel as the space required for installation of the setting member and, in particular, of the channel constriction section can thereby be reduced. It is possible to minimize the installation space especially in the case of a channel constriction section that is displaceable perpendicularly to the direction of flow of the combustion air. 
         [0012]    To enable the amount of combustion air suppliable to the heat exchanger to be set more precisely, it is advantageous for the channel constriction section, and, in particular, even more advantageous for the setting member, to be displaceable in a stepless manner relative to the flow channel, for, in this way, the free cross-sectional area of the flow channel can be changed in a stepless manner. 
         [0013]    The channel constriction section is preferably positionable in at least one direction oriented transversely and, in particular, perpendicularly to the direction of displacement in a manner free of play in the flow channel. In this way, the direction of displacement of the channel constriction section can be clearly defined. In particular, a guide can be formed for the channel constriction section, with, for example, the latter lying against the inside of a wall of the flow channel during the displacement. 
         [0014]    The high-pressure cleaning appliance advantageously comprises at least one stop element for delimiting the path of displacement of the channel constriction section, so that the latter can assume at least one and preferably two definable end positions relative to the flow channel. In one end position, it may, for example, be provided that the free cross-sectional area of the flow channel is minimized, but the latter is not completely closed by the channel constriction section so as not to entirely block the supply of combustion air to the heat exchanger. It may also be provided that in one end position, the channel constriction section does not fully release the entire cross-sectional area of the flow channel. 
         [0015]    It has already been mentioned that the channel constriction section can be guided relative to the flow channel. In particular, the high-pressure cleaning appliance comprises a guide device for guiding the channel constriction section relative to the flow channel. Consequently, a user can easily displace the channel constriction section relative to the flow channel without having to pay attention to the direction of displacement. 
         [0016]    It is advantageous for the guide device to comprise at least one contact element against which the setting member is positionable, during the displacement, with a guide section coupled to the channel constriction section. The at least one contact element is, for example, a rib or bar extending longitudinally in the direction of displacement, against which the guide section lies slidingly during the displacement. The at least one contact element may be designed in such a way that the guide section can be supported thereon in two directions oriented transversely and, in particular, perpendicularly to the direction of displacement. 
         [0017]    It may be provided that the channel constriction section comprises and/or forms the guide section. 
         [0018]    The guide device preferably comprises an elongate hole extending along the direction of displacement of the channel constriction section and a guide element engaging the elongate hole. In this way, it is possible to form a constructionally simple guide in which the guide element, for example, a guide pin, can engage the elongate hole transversely to the direction of displacement in a manner free of play. 
         [0019]    To achieve a simple construction for the guide device, it is advantageous for the guide section to comprise the elongate hole. 
         [0020]    A simple construction of the guide device is further promoted by the guide device being at least partially connected in one piece to the flow channel. It may thus be provided that the aforementioned at least one stop element is connected in one piece to the flow channel and, for example, is made together with the latter from a one-piece plastic molded part. 
         [0021]    The high-pressure cleaning appliance advantageously comprises a locking device for releasably locking the channel constriction section relative to the flow channel. When the channel constriction section is locked relative to the flow channel, it can, in this way, be ensured that the free cross-sectional area of the latter will not change. The amount of combustion air suppliable to the heat exchanger then no longer changes either. In this way, a constant operation of the high-pressure cleaning appliance can be ensured. 
         [0022]    It is advantageous for the locking device to be configured as a clamping device. It has been shown that a simple construction and a compact design of the locking device can thereby be achieved. 
         [0023]    Alternatively or additionally, the locking device may be configured as a latching device. 
         [0024]    The locking device preferably comprises an elongate hole extending along the direction of displacement of the channel constriction section, and a first fixing element passing through the elongate hole, and a second fixing element which interacts with the first fixing element to fix the elongate hole relative to the first and second fixing elements. This makes it possible to impart a simple construction to the locking device. The elongate hole, which may be the aforementioned elongate hole of the guide device, can be fixed by means of and relative to two interacting fixing elements. The fixing elements are, for example, a screw element and a nut element corresponding thereto. This also makes it possible to form the locking device as a clamping device. 
         [0025]    To achieve a simple construction of the locking device, it is advantageous for the setting member to comprise a fixing section which includes the elongate hole and is coupled to the channel constriction section. The fixing section may be the aforementioned guide section of the guide device. 
         [0026]    It may be provided that the channel constriction section comprises and/or forms the guide section. 
         [0027]    It is advantageous for the locking device to be at least partially connected in one piece to the flow channel, for example, in the form of a one-piece plastic molded part. A simple construction is thereby obtained for the locking device. 
         [0028]    It is advantageous for the locking device to be arranged outside of the flow channel, as this facilitates its handling by a user. The channel constriction section can be locked relative to the flow channel without the flow channel having to be opened for this. 
         [0029]    The high-pressure cleaning appliance preferably comprises a setting device for displacing the channel constriction section. The channel constriction section and, particularly preferred, the setting member, can be displaced in a more user-friendly manner relative to the flow channel by the setting device. 
         [0030]    It is advantageous for the setting device to have a setting section coupled to the channel constriction section with at least one setting element having a contact surface oriented transversely to the direction of displacement of the channel constriction section. A user can act upon the contact surface of the at least one setting element with a force oriented in the direction of displacement in order to displace the channel constriction section. The at least one setting element can have a contact surface for displacement of the channel constriction section in a first direction and a further contact surface for displacement of the channel constriction section in the second direction opposite thereto. The setting section may be the aforementioned guide section and/or the aforementioned fixing section. 
         [0031]    It may be provided that the channel constriction section comprises and/or forms the setting section. 
         [0032]    The setting section advantageously comprises a plurality of setting elements which are spaced from one another along the direction of displacement of the channel constriction section. This makes it possible for a user to act upon a plurality of setting elements with an adjusting force in order to displace the channel constriction section. This facilitates handling of the setting member by a user. The plurality of setting elements are, in particular, of identical configuration and/or equidistantly spaced from one another. 
         [0033]    The plurality of setting elements preferably form together with a setting tool allocatable to the setting member a linear drive. This allows particularly user-friendly displacement of the channel constriction section. For example, by way of arrangement of their contact surfaces in a row, the setting elements form a toothed or wave-shaped profile on which the setting tool can roll with a contact surface formed so as to correspond thereto. In this way, the setting section and together with it the channel constriction section can be displaced relative to the flow channel. 
         [0034]    In a simple constructional design of the setting device, the setting section comprises an elongate hole, on one of the rims of which the plurality of setting elements are arranged. The elongate hole may be the aforementioned elongate hole of the guide device and/or the aforementioned elongate hole of the locking device. The setting elements may, for example, be formed as projections, such as teeth or the like, on a rim of the elongate hole. 
         [0035]    It is advantageous for the high-pressure cleaning appliance to comprise a display device for displaying the position of the channel constriction section relative to the flow channel. This makes it easy for a user to recognize which position the channel constriction section is assuming relative to the flow channel, and so the size of its free cross-sectional area is easy to determine. 
         [0036]    The setting member is preferably at least partially of plate-shaped configuration, so that a simple constructional design is imparted to it. It also has a low space requirement and can be produced cost-effectively. 
         [0037]    For the same reasons, the channel constriction section is advantageously of plate-shaped configuration, for example, in the form of a plate which is displaceable relative to the flow channel and engages therein. The plate can lie slidingly against the inner sides of walls of the flow channel during the displacement, so that the channel constriction section is arranged transversely to the direction of displacement in a manner free of play in the flow channel and is guided relative thereto. 
         [0038]    To achieve a simple construction and cost-effective production of the setting member, it is also advantageous for the setting member to comprise a plate-shaped guide section for guiding the channel constriction section and/or a plate-shaped fixing section for releasably locking the channel constriction section and/or a plate-shaped setting section for displacing the channel constriction section. 
         [0039]    It is particularly advantageous for the setting member in its entirety to be of plate-shaped configuration. 
         [0040]    The setting member is preferably of one piece configuration and/or is advantageously made of metal because both a simple constructional design and a cost-effective production of the setting member are thereby achievable. 
         [0041]    A different type of embodiment of the high-pressure cleaning appliance in accordance with the invention comprises a setting member made of plastic material. 
         [0042]    It is advantageous for the channel constriction section to pass through a through-opening formed in a wall of the flow channel. This makes it possible for the channel constriction section to engage in the flow channel so that when the channel constriction section is displaced relative to the flow channel, its free cross-sectional area can be changed. 
         [0043]    It is advantageous for the through-opening to be of slit-shaped configuration, with the channel constriction section advantageously being of plate-shaped configuration. 
         [0044]    The following description of a preferred embodiment of the invention serves to explain the invention in greater detail in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0045]      FIG. 1  shows a perspective view of a high-pressure cleaning appliance in accordance with the invention with a housing comprising a hood; 
           [0046]      FIG. 2  shows a perspective view of the high-pressure cleaning appliance after removal of the hood; 
           [0047]      FIG. 3  shows a horizontal sectional view of part of the high-pressure cleaning appliance; 
           [0048]      FIG. 4  shows an enlarged representation of detail A in  FIG. 3 ; and 
           [0049]      FIG. 5  shows a sectional view taken along line  5 - 5  in  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0050]    A preferred embodiment of a high-pressure cleaning appliance in accordance with the invention is shown in perspective in  FIG. 1  and designated in its entirety by reference numeral  10  therein. It comprises a front side  12 , a rear side  13 , a left side  14 , a right side  15 , an upper side  16  and an underside  17 . 
         [0051]    The high-pressure cleaning appliance  10  comprises a bottom part  18  with a chassis  20  on which two wheels rotatable about a common axis of rotation  22  are held near the rear side  13 . Only one wheel  24  of these is shown in the drawings. The high-pressure cleaning appliance  10  can stand on a set-down surface  26  with the wheel  24  and the wheel that is not shown and with a plurality of support elements formed on the underside  17  of the chassis  20 , only one support foot  25  of which is shown. 
         [0052]    The high-pressure cleaning appliance  10  comprises a housing  28  above the bottom part  18 . The housing  28  comprises a housing wall  30  in the area of the rear side  13  and of the sections of the left side  14  and the right side  15  that face the rear side  13 , and a hood  32  in the area of the front side  12  and of the sections of the left side  14  and the right side  15  that face the front side  12 . The hood  32  covers an interior  33 , shown uncovered in  FIG. 2 , of the high-pressure cleaning appliance  10 . 
         [0053]    In the interior  33 , a plate-shaped support part  34  is supported on the chassis  20  above the bottom part  18 . The support part  34  forms an installation surface for an upright heatable heat exchanger  36 . A pump unit  38  drivable by a motor and held on the chassis  20  near the left side  14  can pressurize cleaning liquid, preferably water, supplied to the high-pressure cleaning appliance  10 . This cleaning liquid can be supplied to the heat exchanger  36  through a liquid conduit, not shown in the drawings, in order to be heated up and so increase the effectiveness of the cleaning. 
         [0054]    A blower  40  which is drivable by a motor is arranged on the chassis  20  near the right side  15 . A flow of combustion air can be generated for the heat exchanger  36  by the blower  40 . In order to supply combustion air to the heat exchanger  36 , a flow channel  44  formed in one piece is connected to a socket-shaped connection element  42  formed by the blower  40 . 
         [0055]    A first channel section  46  of the flow channel  44  engages over the connection element  42  and, starting from the blower  40 , extends in the height direction past the plate-shaped support part  34 . Below a fuel container  48 , arranged on the right side  15  next to the heat exchanger  36 , the flow channel  44  bends approximately at a right angle and extends with a second channel section  50  above the support part  34  in the direction of the left side  14 . The second channel section  50  is connected approximately tangentially to a housing shell  52  of the heat exchanger  36 . Combustion air can be drawn in by the blower  40  through inlet openings  54  at a side wall  56  of the blower  40 , near the right side  15 . 
         [0056]    As can be seen from  FIGS. 3 to 5 , the flow channel  44  comprises a channel wall  58  which in the region of the first channel section  46  is approximately of rectangular cross section. The channel wall  58  comprises a left wall section  60  facing the left side  14 , a right wall section  62  facing the right side  15 , a front wall section  64  facing the front side  12  and a rear wall section  66  facing the rear side  13 . In the rear wall section  66 , a through-opening in the form of a slit  68  ( FIG. 5 ) is formed across the width of the first channel section  46  between the wall sections  60  and  62 . The channel wall  58  encloses a channel interior  70 . 
         [0057]    Typically, the total amount of combustion air which can be made available by the blower  40  is not required for operation of the heat exchanger  36 . The high-pressure cleaning appliance  10  comprises a setting member  72  for setting the amount of combustion air, in relation to a time unit, actually supplied to the heat exchanger  36 . It is arranged ( FIGS. 3 to 5 ) at the flow channel  44  slightly above an upper end of the connection element  42  at the first channel section  46 . 
         [0058]    The setting member  72  is in the form of a plate  74  made of metal and in one piece, with a first plate section  76  and a second plate section  78 , which are joined to each other by a web  80 . The plate  74  is installed horizontally on the high-pressure cleaning appliance  10 . Relative to the flow channel  44 , it is configured, as will be discussed in further detail below, so as to be slidingly displaceable along a direction of displacement indicated by a double arrow  82 , i.e., in the longitudinal direction of the high-pressure cleaning appliance  10 , both in the direction of the front side  12  and in the direction of the rear side  13 . The direction of displacement  82  is oriented perpendicularly to a direction of flow, symbolized by an arrow  84 , of combustion air in the flow channel  44  in the region of the first channel section  46 . 
         [0059]    The first plate section  76  and the second plate section  78  are both of rectangular configuration and are joined to each other by the web  80  at the side of the plate  74  facing the rear side  13 . Aside from that, they are separated from each other by a slit-shaped space  86 . The first plate section  76  can engage in the channel interior  70  by passing through the slit  68  in the rear wall section  66 . The first plate section  76  is of such dimensions that it is arranged within the first channel section  46  in a manner free of play between the wall sections  60  and  62 . The first plate section  76  is also arranged in a manner free of play in the slit  68 . 
         [0060]    The plate  74  can engage around the right wall section  62  owing to the right wall section  62  engaging in the space  86 , and the second plate section  78  being arranged outside of the flow channel  44 , bordering on the right wall section  62 . In the direction of the front side  12 , the second plate section  78  extends beyond the first plate section  76 , and, approximately at its center, it has an elongate hole  88  extending in the direction of displacement  82 . 
         [0061]    The elongate hole  88  has a rim  90 . A plurality of identical, equidistantly spaced recesses  92  are formed in the direction of displacement  82  on the area of the rim  90  enclosing the elongate hole  88  in the direction of the right wall section  62 . The recesses  92  form altogether a wave profile  94 . Viewed differently, the wave profile  94  may also be regarded as being formed by identical projections  96  arranged on the rim  90 . They each have contact surfaces oriented transversely to the direction of displacement  82 . 
         [0062]    A plurality of markings  98  are affixed to the second plate section  78  on its side facing the right side  15 . 
         [0063]    A supporting device  100  for the second plate section  78  is formed in one piece on the first channel section  46  on the its side facing the right side  15  in the area of transition to the connection element  42 . The supporting device  100  has a first strip-shaped contact element  102  formed in the longitudinal direction of the high-pressure cleaning appliance  10 , on which the second plate section  78  lies. A second strip-shaped contact element  104  of the supporting device  100  lies against the side of the second plate section  78  facing the right side  15 . The second contact element  104  carries on its upper side a marking  106  in the form of an arrow. 
         [0064]    Approximately level with the center of the first channel section  46 , in relation to the longitudinal direction of the high-pressure cleaning appliance  10 , the first contact element  102  forms a fixing element  108  in the form of a screw boss  110  forming a nut element. The screw boss  110  can interact with a second fixing element  112  in the form of a screw  114 , which extends through the elongate hole  88  from the top to the bottom in a manner free of play and interacts with the screw boss  110  to fix the second plate section  78  to the supporting device  100  and, therefore, the plate  74  to the flow channel  44 . The screw  114  also forms a guide element, in particular, a guide pin for the elongate hole  88 . 
         [0065]    The first contact element  102  is hollowed out at its side facing the front side  12 , so that it has a recess  116  ( FIG. 4 ) on its upper side facing the second plate section  78 . 
         [0066]    As mentioned above, the amount of combustion air supplied to the heat exchanger  36  can be set by the plate  74  and, in particular, the first plate section  76 . Because the first plate section  76  partly engages in the channel interior  70  of the flow channel  44 , it blocks part of the entire cross-sectional area enclosed by the channel wall  58  in the flow channel  44 . For this reason, the free cross-sectional area of the flow channel  44  through which combustion air can pass is smaller than the entire cross-sectional area enclosed by the channel wall  58 . The first plate section  76  is, therefore, also designated as channel constriction section  118 . 
         [0067]    In accordance with the amount of combustion air to be supplied to the heat exchanger  36  per time unit during operation of the high-pressure cleaning appliance  10 , the plate  74  and, therefore, the channel constriction section  118  can slidingly be displaced relative to the flow channel  44  in the direction of displacement  82 . To enable the plate  74  to be displaced, it will usually be necessary to first release a lock existing between the plate  74  and the flow channel  44 . The plate  74  is locked at the second plate section  78  and, in particular, at the elongate hole  88  by the screw  114  interacting with the screw boss  110 . The second plate section  78  forms together with the screw  114  and the screw boss  110  a locking device  120  of the high-pressure cleaning appliance  10 , and the second plate section  78  is also designated as fixing section  122 . The screw  114  can be released relative to the screw boss  110  with a suitable tool. Here access to the locking device  120  is made easier for the user by it being arranged outside of the flow channel  44 . 
         [0068]    After releasing the screw  114  from the screw boss  110 , the plate  74  can be displaced relative to the flow channel  44 . On the one hand, this can be done by, for example, a user gripping the plate  74  with his hand and moving it. On the other hand, it is, for example, possible for a user to use a user-friendly setting device  124  for displacing the plate  74 . The setting device  124  comprises the projections  96  which form setting elements  126 . A setting tool allocatable to the plate  74  and having a contact surface of complementary configuration to the wave profile  94  can be applied to the setting elements  126 . With the high-pressure cleaning appliance  10 , a tool which is also suited for releasing the screw  114 , in particular, a torx screwdriver can be used in a user-friendly manner. 
         [0069]    The tool can be passed through the elongate hole  88  from the upper side so that its bottom end engages the recess  116 . By turning the tool, its contact surface can roll on the wave profile  94 . The setting elements  126  thereby form with the tool a linear drive for the plate  74 . The plate  74  can thus be displaced in the direction of the front side  12  or the rear side  13  by continued turning of the tool. Because the setting elements  126  are arranged on the second plate section  78 , the latter is also designated as setting section  128 . 
         [0070]    When the plate  74  is being displaced relative to the flow channel  44 , it is guided by a guide device  130 . This comprises the second plate section  78 , for this reason also designated as guide section  132 , the contact elements  102  and  104 , the screw  114 , the wall sections  60  and  62  and the slit  68 . 
         [0071]    When the plate  74  is displaced in the direction of the front side  12 , the free cross-sectional area of the flow channel  44  is, consequently, reduced, and a smaller amount of combustion air is supplied to the heat exchanger  36 . When the plate  74  is displaced in the direction of the rear side  13 , the free cross-sectional area of the flow channel  44  is enlarged, and the amount of combustion air supplied to the heat exchanger  36  is increased. The configuration of the setting member  72  as displaceable plate  74  is advantageous over the setting members of conventional high-pressure cleaning appliances because owing to the sliding displacement of the plate  74  and in particular, the channel constriction section  118 , relative to the flow channel  44 , the change in the free cross-sectional area is linearly dependent upon the path of displacement of the plate  74  relative to the flow channel  44 . 
         [0072]    This is not the case with conventional high-pressure cleaning appliances in which the setting member has a flap pivotable relative to the flow channel. In an implementation of the high-pressure cleaning appliance  10 , this linear dependence of the change in the free cross-sectional area on the path of displacement of the plate  74  has proven advantageous for precisely setting the change in the amount and, therefore, the total amount of combustion air supplied to the heat exchanger  36 . 
         [0073]    In particular, a stepless change in the free cross-sectional area is made possible with the high-pressure cleaning appliance  10 , because the plate  74  and, therefore, the channel constriction section  118  are steplessly displaceable relative to the flow channel  44 . 
         [0074]    The markings  98  on the second plate section  78  and the marking  106  on the second contact element  104  interact to form a display device  134  by means of which the position of the plate  74  and, therefore, of the channel constriction section  118  relative to the flow channel  44  is displayable. The free cross-sectional area of the flow channel  44  can also be determined from this display. 
         [0075]    When the plate  74  assumes the desired position relative to the flow channel  44 , i.e., the free cross-sectional area of the flow channel  44  is set by the channel constriction section  118  such that the desired amount of combustion air is supplied to the heat exchanger  36 , a user can fix the plate  74  again by means of the locking device  120 . 
         [0076]    The rim  90  of the elongate hole  88  forms at its end facing the front side  12  a first stop element  136 , and at its end facing the rear side  13  a second stop element  138 . The path of displacement of the plate  74  relative to the flow channel  44  can be delimited by the stop elements  136  and  138 , which can come to lie against the screw  114 . 
         [0077]    When the second stop element  138  is lying against the screw  114 , the flow channel  44  is not completely blocked by the channel constriction section  118 . 
         [0078]    In this way, it is ensured that a minimum amount of combustion air will in any case be supplied to the heat exchanger  36 . When the first stop element  136  is lying against the screw  114 , the channel constriction section  118  still remains partly positioned within the channel interior  70 . It is thereby ensured that the channel constriction section  118 , when being displaced relative to the flow channel  44 , will not be inadvertently passed through the slit  68 . In this way, an otherwise necessary re-inserting of the channel constriction section  118  into the slit  68  can be avoided.