FR2708339A1 - Closed fireplace for a chimney - Google Patents

Abstract

The closed fireplace (F) for a chimney comprises a combustion chamber (1) bounded by two side walls (2a, 2b), a rear wall (3), a hearth (4), an upper wall (5) including at least one opening provided with an orientable flap (22) for removing smoke into a chimney stack, and a front door (6), in particular glazed, which can be opened, in particular for loading the fireplace with fuel. At least one of the two side walls (2a, 2b) of the combustion chamber (1) is thermally insulated.

Description

CLOSED FIREPLACE FOR FIREPLACE
The invention relates to a closed hearth, for a chimney, of the type which includes a combustion chamber delimited by two side walls, a rear wall, a hearth, an upper wall comprising at least one opening provided with an adjustable flap. , for the evacuation of smoke in a chimney flue, and a front door, in particular a glass door, capable of being opened in particular to load the hearth with fuel.

 Closed hearths of this type make it possible to obtain advantageous heating yields and have known wide developments.

 An important problem which arises, with such a hearth, is to ensure, when the user so desires, slow combustion, without risk of extinction, while the hearth operates at reduced power, shutter closed. It is desired, for example, that such a closed hearth makes it possible to ensure, with a determined load of wood, of the order of 20 to 25 kg, a high number of hours of combustion, for example 10 hours of combustion.

 Another problem which arises with closed hearths provided with a glass door, is that of the blackening of the glass by deposit of carbon or other combustion residues.

 The object of the invention is, above all, to provide a closed hearth making it possible to ensure proper operation in slow combustion, closed draft flap. It is further desired that, in reduced speed, the blackening of the door window be reduced, preferably eliminated.

 In addition to the requirements expressed above, there is the requirement of a simple and robust manufacturing fireplace, and an acceptable cost price.

 According to the invention, a closed hearth for a chimney, comprising a combustion chamber delimited by two side walls, a rear wall, a hearth, an upper wall comprising at least one opening provided with an orientable flap for discharging the fumes in a chimney flue, and a front door, in particular a glass door, is characterized in that at least one of the two side walls of the combustion chamber is thermally insulated.

 Preferably, the two side walls are thermally insulated.

 This blocks part of the thermal radiation from the combustion chamber and maintains the temperature there at a level sufficient to prevent extinction at reduced speed.

 Each side wall preferably comprises two metal elementary walls, parallel and spaced from one another, and a thermally insulating filling placed between these elementary walls.

This filling is advantageously constituted by ceramic fiber.

 Advantageously, the hearth comprises, in the upper part of the combustion chamber, a deflector covering the major part of the combustion chamber, extending transversely to the side walls and, in a direction parallel to these side walls, up to in the vicinity of the rear wall, leaving a space for the passage of smoke, the assembly being such that when the shutter is in the closed position, this passage located at the rear of the deflector is the only one offered to combustion gases.

 The deflector comprises a plate substantially in the form of a bowl open upwards, this plate extending to the side walls, but stopping a short distance from the rear wall, while another upper plate, substantially horizontal, covers said bowl extending to the rear wall, but without reaching the side walls so that passages are offered to the combustion gases on the sides of said plate.

 The upper plate has an opening in a semicircle, to allow the entry of part of the draft flap when it passes into the open position, this opening in a semicircle being bordered, according to the diameter constituting its edge facing the door, by a transverse rib extending substantially up to the upper wall of the hearth.

 The deflector, at its front part, is separated from the upper wall of the hearth, which defines a possible passage for the fumes, and an articulated pediment, extending transversely, is capable of closing this passage when the draft flap is closed , the movements of this pediment being subject to those of the shutter, this pediment being able to open when the shutter is open, to allow direct passage of the combustion gases towards the chimney flue.

 A linkage is provided to control the movements of the pediment to those of the shutter and that this linkage comprises a control button forming a return in front of the door so as to prevent the opening of said door when the shutter is not open.

 When the hearth comprises a window pane mounted in a door, advantageously, the pane is tightly mounted in the frame of the door and a building clearance is provided between the upper and lower horizontal edge of the door and the neighboring edge of the wall of the hearth in order to determine a continuous passage for the air which comes to lick the glass and opposes the blackening of this glass.

 The clearance between the upper edge of the door and the edge adjacent to the wall of the hearth can be greater at the ends thanks to recesses provided in the wall of the hearth.

 The hearth can include a fairing to determine air passages to be heated and distributed in a room advantageously, this fairing defines separate exchangers, namely a rear exchanger formed between the rear wall of the hearth and a fairing wall parallel to this wall, and closed at the two transverse ends in vertical directions, and two lateral / upper exchangers, the air flows of which open into a tube concentric with a tube of larger diameter collecting the air from the first exchanger.

 Each lateral / upper exchanger is delimited by a dihedron of the fairing and a side wall of the hearth, and, in the upper part, by a cap connected to the dihedron and covering a rear corner zone of the upper wall of the hearth, zone below from which are directed the combustion gases of the hearth to ensure a significant heat transmission in this upper exchanger.

 Each rear corner zone of the upper wall of the hearth may include studs (or equivalent projections) promoting thermal exchanges with the air which circulates.

 The invention consists, apart from the arrangements set out above, in a certain number of other arrangements which will be more explicitly discussed below in connection with an exemplary embodiment described with reference to the attached drawings, but which n 'is in no way limiting.

 Figure 1 of these drawings is a perspective view with parts broken away, of a closed hearth according to the invention.

 Figure 2 is a partial section of the hearth of Figure 1 along a vertical median plane parallel to the side walls.

 Figure 3 is a top view of the upper wall of the fireplace, the upper fairing being removed.

 Figure 4 is a top view of the deflector, the upper wall of the hearth being removed.

 Figure 5 is a schematic elevational view of the fireplace door.

 Figure 6 is a schematic section along line VI-VI Figure 5.

 Figure 7 is a schematic section along line VII-VII, Figure 5.

 Figure 8 is a section along the line VIII-Vffi, Figure 5.

 Figure 9, finally, is a section along the line IX-IX, Figure 5.

 Referring to the drawings, in particular to Figures 1 to 4, one can see a closed hearth F substantially in the shape of a rectangular parallelepiped, constituting an insert intended to be placed in a chimney, not shown.

 The hearth F is surrounded by a fairing C which will be described later.

 The hearth F comprises a combustion chamber 1 delimited by two side walls 2a 2b a rear wall 3, a hearth 4, an upper wall 5 and a front door 6 provided with a window 7.

 The walls of the combustion chamber are made up of cast iron elements. The vertical rear wall 3 is detachably connected to the side walls and to the upper wall. This rear wall 3 has, on its outer surface, vertical ribs 8 (see Figure 3) forming fins to improve heat exchange.

 The upper wall 5 may have a substantially horizontal mean plane, as in the example in FIG. 1, or be constituted by a sort of trunk of pyramid forming a drain. The hearth 4 is generally provided with a grid, not shown in the drawing, for the evacuation of the ashes collected, below the hearth, by a drawer (not shown).

 The front door 6 can be opened by a rotational movement around a vertical axis located on the left side according to the illustrated embodiment. Alternatively, the door 6 can be opened by a vertical sliding movement. The opening of door 6 allows, in particular, to load the combustion chamber with firewood.

 The two side walls 2a 2b are vertical, slightly converging towards the rear (see Figure 3).

 These two side walls 2a 2b are thermally insulated.

Each side wall comprises two elementary metal walls 9, 10, parallel and spaced from one another by a distance d, for example of the order of 25 mm. A thermally insulating filling 11 is placed in the space between these elementary walls 9, 10. The filling 11 is advantageously constituted by ceramic fiber. Preferably, the wall 9 is made of cast iron and the wall 10 is made of steel.

 The insulating side walls 2a 2b block the thermal radiation and make it possible to maintain a sufficient temperature in the combustion chamber 1 even when the hearth is operating at idle.

 To complete the action of the insulating side walls 2a -, a deflector 12 covering the major part of the combustion chamber is provided in the upper part of the combustion chamber 1.

This deflector 12, as visible in FIG. 4, extends transversely to the side walls 2a 2b over most of the depth of the hearth F, that is to say along most of the distance between the door 6 and the rear wall 3. In a direction parallel to the side walls 2a 2b, the deflector 12 extends to the vicinity of the rear wall 3, leaving a space e between this wall 3 and the rear edge 13 of the deflector 12, for the passage of smoke. The straight rear edge 13 has, at its two ends, set back relative to the ends of the plate 3, a recess 14 at a right angle which is connected to the lateral edge 15 of the deflector 12 located in the immediate vicinity of a side wall 2a 2b . The recesses 14 at right angles provide a passage of larger section, for the fumes (or combustion gases), at each interior angle of the combustion chamber.

 The deflector 12 advantageously comprises a plate 16, for example of sheet steel, in the form of a bowl open upwards, with a longitudinal section substantially in the shape of a V which is largely open and the tip of which has been cut. The front transverse edge 17 of the deflector 12 is set back, from a distance g (see FIG. 4), relative to the bearing plane of the door 6 in the closed position. This distance g is of the order of a few centimeters.

 The deflector 12 is surmounted by an upper deflector constituted by a substantially horizontal plate 18 which covers the bowl constituting the deflector 12. As visible in FIG. 4, the plate 18 extends to the rear wall 3, its rear edge 19 being adjacent to this wall. On the other hand, over most of the depth of the hearth from the rear wall 3, towards the door 6, the plate 18 does not extend to the side walls 2a 2b. Each lateral edge of this plate 18 has a part 20 set back relative to the lateral walls -, 2b so as to define a passage 21 of substantially trapezoidal outline for the fumes. These parts 20, slightly inclined relative to the median plane of the hearth, extend forwardly, beyond the axis A of articulation of a draft flap 22 provided in the upper wall 5. The axis A is parallel to the plane of door 6 when it is closed. The front part 23 of each lateral edge of the plate 18 is located in the immediate vicinity of the walls 2a 2b. As can be seen in FIG. 2, the front transverse edge of the plate 18 is in the same vertical plane as the front edge 17 of the plate 12 and is therefore also set back a distance g from the support plane of the door.

 The rear edge 13 of the deflector 12 is located at a distance j (FIG. 2) from the underside of the plate 18, so that a passage for the fumes or combustion gases exists between this edge 13 and the plate 18.

The flap 22 is constituted by a disc wedged in rotation on the axis
A, from which a forward segment has been deleted. When the flap 22 is in the closed position, as illustrated in FIG. 3, a passage 24 in the form of a circular segment exists between the flap and the edge of the opening 25 provided in the upper plate 5. The passage 24 is symmetrical by with respect to a vertical plane orthogonal to axis A and passing through the center of the shutter.

 The distance s (see FIG. 2) between the plate 18 and the upper wall 5 is less than the radius of the flap 22. To allow the flap 22 to open, with its rear edge lowered and its cut front part raised, an opening 26 in a semicircle is provided in the plate 18. This opening 26 has a radius slightly greater than that of the flap 22 while its center is located substantially vertical to that of the flap. The front edge 27 of this opening 26 is rectilinear and formed by a diameter, parallel to the rear wall 3.

 A vertical rib 28, forming part of the plate 18, extends along the edge 27. The height of this rib 28 is such that its upper edge is practically in contact with the flap 22 in the closed position (see FIG. 2) so as to constitute a transverse barrier forcing gases or fumes to bypass it before escaping through the opening 24.

 The plate 18 further comprises, on its upper surface, two vertical fins 29, symmetrical with respect to the plane orthogonal to the axis A, passing through the center of the flap 22. Each fin 29 starts from the lateral edge of the plate 18, slightly set back from its front end and goes substantially towards the center of the flap 22 to stop slightly behind the outline of the opening 25. These fins 29 contribute to guiding the fumes.

 Because the deflector 12 and the upper deflector 18 are set back a distance g relative to the closed door 6, and the fact that the upper deflector 18 is located at a distance s from the upper wall 5, a possible passage exists for combustion gases towards the opening 24. A pediment 30 is provided for closing this passage, in front of the deflector 12, when the flap 22 is in the closed position.

The pediment 30 is constituted by a plate, in particular made of cast iron, the upper edge of which is locked in rotation on a horizontal transverse articulation axis 31 located in the vicinity of the front wall of the hearth. The lower edge of the pediment 30, as illustrated in FIG. 2, bears against the front edge of the deflector 12, the pediment 30 then being inclined from front to back. This pediment 30 covers the entire transverse extent of the hearth F.

 The rotational movements of the pediment 30 are controlled by those of the flap 22. When the flap 22 is closed, as illustrated in FIG. 2, the pediment 30 is itself in the closed position. On the other hand, when the flap 22 is open, the pediment 30 is itself in the open position, that is to say that it is situated substantially in the vertical plane passing through the axis 31. This control is achieved by simply using a mechanical linkage visible in Figure 2 located outside the combustion chamber, on the right side as shown in Figures 3 and 4. A flange 32, made of sheet metal, is integral in rotation of one end of the axis 31, and therefore of the pediment 30. A rod 33 is articulated at one end 34 on the flange 32, at a distance from the axis 31.

 The other end of the rod 33 is linked by a hinge 35 to the end of a crank 36 locked in rotation on the axis A of the flap 22. The crank 36 is thus linked in rotation to this flap 22. The crank 36 is substantially parallel to the straight line passing through the axis 31 and the articulation 36 while the rod 33 is substantially parallel to a straight line resting on the axes A and 31.

 The control of the flange 32 by the user is ensured by means of a rod 37 extending downwards, the upper end of which is linked by an articulation 38 with the lower part of the flange 32.

The articulation 38 is located below the articulation 34. The rod 37 at its lower end is provided with a rotating roller 39, suitable for providing rolling support against a vertical support face 40 provided at the outside of the combustion chamber, on one side of the hearth, the right side in the example considered. The lower end of the rod 37 is extended by a horizontal part 41 which projects beyond the door 6 in the closed position. This horizontal extension 41, which is integral with the rod 37, comprises at its front end a sort of button 42 forming a return in front of the door 6 when it is closed, as illustrated in FIG. 4. The entire linkage is arranged so that this button 42 opposes the opening of the door 6 when the flap 22 is closed. The opening of the door 6 is only possible for the open position of the flap 22, position for which the pediment 30 is also in the open position; the rod 41 and the return 42 forming a control button are then at a level lower than the lower level of the door 6 which can be opened.

 The window 7 of the door 6 is mounted in a sealed manner (see FIGS. 8 and 9) in the frame of the door using a peripheral seal 43. The door 6 is located in front of a front wall 44, forming a frame, of the hearth, which front wall has an opening 45 of dimensions slightly smaller than those of the door.

This front wall 44 has a rim 46 orthogonal to the mean plane of the wall and surrounding the door 6. The vertical hinge axis of the door 6 is located on the left side according to the example shown, and is carried by the wall frontal 44.

 When the door 6 is closed, its two vertical sides, as illustrated in FIG. 9, come to bear in sealed manner against the front wall 44, thanks to a sealing bead 47 which can be fixed on the door 6 or on the wall 44.

 The upper horizontal edge, as illustrated in FIG. 8, as well as the lower horizontal edge of the door 6 remain spaced from the external face of the wall 44, in the closed position of the door, by a distance h ′, h corresponding to a well-defined building game.

The inner face of the upper and lower edges of the door 6 may be provided with a plate 48 covering the horizontal edges of the window 7 and making it possible to determine the clearance h or h 'with good precision. This game defines a vertical gap which communicates in the upper part for the upper edge of the door and in the lower part for the lower edge, with a space 49 which flares outwards and which communicates with the atmosphere. The space 49 is limited by a face of the rim 46 and an edge of the door 6.

 The clearance h, h 'is continuous over the entire transverse extent of the door as illustrated in FIGS. 6 and 7.

 For the upper edge of the door 6 (FIG. 6), the clearance h ′ becomes greater and equal to h at the two ends of the door to facilitate the entry of air into the corner area and reduce the edge effects.

The increase in the clearance, at the ends, is ensured by recesses 50, 51 in the wall 44. The clearance h ′ is advantageously of the order of 2 mm while the clearance h at the level of the recesses 50, 51 and of the lower part is around 4 mm.

 The air inlets thus defined, precisely, over the entire extent of the upper and lower horizontal edge of the door 6 ensure an internal sweeping of the window 7 and considerably reduce deposits and therefore the blackening of this window.

 The shroud C surrounds the hearth so as to define heat exchangers between the air circulating outside the hearth, and the combustion gases circulating inside the hearth.

 The fairing C is made, for example, of sheet steel. The lower edge 52 of the fairing is located a few centimeters above the bottom of the hearth F so that an air inlet 53 exists along the entire lower periphery of the fairing C. The fairing C comprises a substantially vertical wall 54 (see Figure 3) which surrounds most of the rear wall 3 of the hearth so as to determine a rear exchanger in which the rising air heats up in contact with the wall 3 and the fins 8. The wall 54 at the top is connects to two cylindrical caps 55 covering two rear corner zones of the upper wall 5, located on either side of the flap 2. These caps 55 are provided with a frustoconical outlet 56, for example with a diameter of 150 mm intended to be connected to a duct (not shown) for distributing hot air in the building.

 The fairing C further comprises two parts 57 substantially in the form of a right dihedron, surrounding the side walls 2a 2b and the two rear angles of the hearth F. In the upper part, each dihedron 57 is connected to a cap 58 inside the cap 55. This cap 58 covers a corner angle 59 of the upper wall 5, provided with frustoconical pins 60 distributed substantially concentrically around a central zone 61 projecting upwards. The upper wall 5 further comprises transverse ribs n, located in front of the zones 59, suitable for channeling the hot air into the upper heat exchangers delimited by the caps 58 and the wall 5. The two zones 59 are located on the right side and on the other side of the flap 22, towards the rear corners of the upper plate 5. The cap 58 is connected to a frustoconical outlet 62, concentric with the outlet 56 but of smaller diameter. The flow of hot air passing through the outlet 62 mixes with the concentric annular flow of the outlet 56.

 It should be noted that the rear exchanger formed by the wall 54 and the plate 3 is separated from the lateral / upper exchangers delimited by the dihedrons 57, the caps 58 and the zones 59.

 That said, the operation and use of the closed hearth F are as follows.

 We consider the hearth F loaded with firewood in the process of burning, while the door 6 is closed and the flap 22 is horizontal, that is to say in the closed position corresponding to the reduced speed.

 The side walls 2a 2b thermally insulated and the deflector assembly 12, 18 limit the heat exchange by radiation so that the temperature in the combustion chamber, even at reduced speed, is maintained at a high value, preventing extinction.

 The fumes or combustion gases rise against the rear wall 3 heated both by convection and by radiation. The fumes bypass the rear edge 13 of the deflector 12 as indicated by the arrow 63 in FIG. 1, passing through the rear space e (FIG. 4) and through the recesses 14. The combustion gases are diverted towards the side walls 2a , 2b by the plate 18 and flow, through the passages 21, to arrive above the plate 18 as indicated by the arrows 64 and 65 in FIG. 4.

 The combustion gases therefore circulate below the upper wall 5 mainly in the areas surmounted by the pins 60 and serving to heat the air in the upper exchangers.

 The combustion gases also pass over the plate 18 through the semicircular opening 26 as indicated by the arrows 66 in FIG. 4. The rib 28 diverts these combustion gases towards the zones of the upper wall 5 surmounted by pins 60. The combustion gases make a substantial half-turn to return according to the arrows 67 towards the opening 24, located in front of the flap 22, allowing the combustion gases to escape through the chimney flue connected to the sleeve bordering the opening 25.

 The ribs 29 also contribute to maintaining the circulation of the combustion gases in the areas surmounted by the pins 60 which is favorable to the heat exchange with the air circulating above the wall 5.

 The hot air produced in the space between the rear wall 3 and the wall 54 is collected inside the cap 55 and s' flows in the annular space limited by the outlet 56 and the outer surface of the outlet 62 .

 Air also rises between the dihedral-shaped walls 57 and the side walls 2a 2b. In the upper part, the air circulation curves towards the horizontal, substantially 90, to bathe the spikes 60 and the central zone 61, before escaping through the outlet 62 and mixing with the flow of air from the rear exchanger.

 The heat transmission in the zones 59 particularly heated by the combustion gases takes place under good conditions with good efficiency.

 Because the temperature inside the combustion chamber 1 is kept at a relatively high temperature, the condensations of vapor on the internal face of the glass 7, condensation generating dirt, are reduced.

 In addition, the vertical scanning of the internal face of the window 7 by the incoming air according to the arrows E (FIGS. 1 and 5) by the clearance h controlled, also contributes to reducing if not eliminating the blackening of the window. The sweeping is accentuated in the vicinity of the two vertical edges of the pane thanks to the larger section of the passages 50, 51 (FIG. 6) allowing, for example, to double the air flow in areas of the pane 7 which are particularly subject to soiling.

 When the flap 22 is opened by lowering the button 42, the pediment 30 (see FIG. 2) assumes a vertical position and frees a direct passage, in front of the deflectors 12 and 18, for the gases towards the opening 25 and the chimney.

 The opening of the door 6 requires lowering the button 42 to release the bottom of the door. The lowering of the button 42, by the linkage, opens the flap 22 and puts the pediment 30 in the vertical position, which prevents, or at least considerably reduces, the smoke outlet into the room through the open door.

 The shutter 22 is closed by lifting the button 42.

Claims (14)

 1. Closed hearth, for a chimney, comprising a combustion chamber delimited by two side walls, a rear wall, a hearth, an upper wall comprising at least one opening provided with an adjustable flap, for the evacuation of smoke in a duct chimney, and a front door, in particular glazed, which can be opened, in particular for charging the hearth with fuel,  characterized in that at least one of the two side walls (2a, 2b) of the combustion chamber is thermally insulated.  2. Hearth according to claim 1, characterized in that the two side walls (2a, 2b) are thermally insulated.  3. Hearth according to claim 1 or 2, characterized in that each side wall (2a, 2! Z) comprises two elementary metal walls (9,10), parallel and spaced from one another, and a filling (11) thermally insulating placed between these elementary walls.  4. Hearth according to claim 3, characterized in that the filling (11) consists of ceramic fiber.  5. Hearth according to one of the preceding claims, characterized in that it comprises, in the upper part of the combustion chamber, a deflector (12) covering the major part of the combustion chamber, extending transversely, up to the side walls (2a, 2b) and, in a direction parallel to these side walls, to the vicinity of the rear wall (3) leaving a space (e) for the passage of combustion gases, the assembly being such that when the flap (22) is in the closed position, this passage (ç) located at the rear of the deflector (12) is the only one offered to the combustion gases.  6. Hearth according to claim 5, characterized in that the deflector (12) comprises a plate (16) substantially in the form of a bowl open upwards, this plate (16) extending to the side walls (2a, 2b), but stopping at a short distance (6 from the rear wall (3), while another upper plate (18), substantially horizontal, covers said bowl extending to the rear wall (3) , but without reaching the side walls so that passages (21) are offered to the combustion gases on the sides of said plate (18).  7. Hearth according to claim 6, characterized in that the upper plate (18) has an opening (26) in a semicircle, to allow the entry of part of the draft flap (22) when it passes in the open position, this semicircular opening (26) being bordered, along the diameter constituting its edge facing the door  (6), by a transverse rib (28) extending substantially up to the upper wall (5) of the hearth.  8. Closed hearth according to one of claims 5 to 7, characterized in that the deflector (12), at its front part (17), is spaced from the upper wall of the hearth, which defines a possible passage for the smoke, and that an articulated pediment (30), extending transversely, is capable of closing this passage when the draft flap (22) is closed, the movements of this pediment (30) being controlled by those of the flap (22 ), this pediment (30) being able to open when the flap (22) is open, to allow direct passage of the combustion gases towards the chimney flue.  9. Hearth according to claim 8, characterized in that a linkage (33, 36, 37) is provided to control the movements of the pediment (30) to those of the flap (22) and that this linkage comprises a control button (42) forming a return in front of the door (6) so as to prevent the opening of said door (6) when the flap (22) is not open.  10. Hearth according to one of the preceding claims, comprising a window (7) mounted in a door (6) articulated around a vertical axis at the front of the hearth, characterized in that the window (7) is mounted tightly within the framework of the door and that a clearance CK, Jh of construction is provided between the upper and lower horizontal edge of the door and the adjacent edge of the wall (44) of the hearth in order to determine a continuous passage for the air which comes to lick the window and opposes the blackening of this window.  11. Closed hearth claim 10, characterized in that the clearance between the upper edge of the door (6) and the edge adjacent to the wall of the hearth is greater (h) at the ends thanks to recesses (50, 51) provided in the wall (44) of the hearth.  12. Hearth according to one of the preceding claims, comprising a shroud (C) for determining air passages to be heated and distributed in a room, characterized in that this shroud (C) defines separate exchangers, namely a rear exchanger formed between the rear wall (3) of the hearth and a fairing wall (54) parallel to this wall, and closed at the two transverse ends in vertical directions, and two upper lateral exchangers whose air flows open into a tubing (62) concentric with a tubing (56) of larger diameter collecting the air from the first exchanger.  13. Closed hearth according to claim 12, characterized in that each lateral / upper exchanger is delimited by a dihedral (57) of the fairing (C) and a lateral wall (2a, 2O of the hearth, and, in the upper part, by a cap (58) connected to the dihedral (57) and covering a rear corner zone (59) of the upper wall (5) of the hearth, zone below which the combustion gases of the hearth are directed to ensure heat transmission important in this upper exchanger.  14. Closed hearth according to claim 12, characterized in that each rear corner area (59) has pins (60), or equivalent projections, promoting heat exchange with the circulating air.