WO2017175110A1

WO2017175110A1 - An improved thermostat

Info

Publication number: WO2017175110A1
Application number: PCT/IB2017/051889
Authority: WO
Inventors: Paolo LUCHETTA; Werner NOETH
Original assignee: Defendi Italy S.R.L.
Priority date: 2016-04-04
Filing date: 2017-04-03
Publication date: 2017-10-12
Also published as: ITUA20162288A1; EP3440406A1; EP3440406B8; EP3440406B1; ES2790407T3

Abstract

A safety thermostat for gas cooking ovens comprising a body (2) provided with an inlet (13) fluidically connectable to a gas feeding source, characterized in that it comprises within said body: - a first cavity (4) fluidically communicating with said inlet (13) and housing a normally closed safety valve (6) inside, - a second cavity (20) fluidically communicating with said first cavity (4) and housing therein an adjustment valve (22) of the gas flow coming from said first cavity (4), said adjustment valve comprising a slider (22) axially movable within said second cavity between a first closing position of a passage towards a third cavity (56) and a second opening position of said passage, - elastic means (53) acting on said slider (22) to push it towards said second opening position, - at least one third cavity (56, 80), which houses a normally closed cut-off valve (58, 82) and which fluidically communicates with at least one burner (68, 90) of the oven, - a heat sensitive element (40) acting on said slider (22), as a function of the temperature intended to be detected by a sensor (44) applicable to said oven, to push said slider (22) towards said first closing position in contrast with said elastic means (53), - means (24, 38, 39, 48, 61, 92) for causing the opening of said safety valve (6) and of said cut-off valve (58, 82) and the adjustment of the position of said slider (22) to control the gas flow through said at least one third cavity (56, 80) in this manner.

Description

AN IMPROVED THERMOSTAT.

The present invention relates to a safety thermostat for gas cooking ovens.

Safety gas thermostats to be applied to gas cooking ovens are known. They have the function of adjusting the gas flow towards a burner installed inside the oven, so as to ensure that the temperature inside is kept around a desired value which is preset by actuating an adjustment knob placed in the front panel of the oven. From the point of view of operation, they comprise a thermostatic bulb installed inside the oven and a heat sensitive fluid contained in the bulb and operatively connected to a modulating valve arranged inside the body of the thermostat.

Thermostats with two outlets are also known, to feed one or two different oven burners, i.e. the main burner and a second burner for the grill or pilot light functions. They are provided with a safety valve mounted upstream of the gas circuit and a second safety valve, interposed between the first safety valve and the modulating valve and consisting of a traditional conical shutter.

A drawback of these known thermostats is in that the conical shutter does not always ensure the sealing hermeticity when it is closed because of its conformation, particularly in case of accidental pressure increases.

Another drawback is in that they do not allow the making of a simple thermostat in simple, compact and cost-effective manner.

Another drawback is in that they imply limitations to the gas flow rate.

Another drawback is in that they are generally provided with only one gas outlet and would require major and costly changes to be transformed into thermostats with a second gas outlet, often required to feed a pilot light or a grill or an auxiliary burner.

WO2004/088206 describes a gas delivery control assembly which comprises, in succession along the gas path, a first safety valve, a device for adjusting the gas pressure and a second sealing and adjustment valve. In particular, the latter valve comprises two closing means (shutters), one for sealing and one for adjusting the pressure, which act between two respective housings arranged coaxially mutually in series. Furthermore, such valve comprises two springs, each of which acts independently from the other on one of the two shutters so as to close the gas passage defined by the respective housing. Both the shutters of the sealing and adjustment valves are controlled by the same control rod, which acts contrasting the action of the springs to control firstly the opening of only the shutter provided for the sealing and then, only in a later step, the opening of the other shutter, which is however provided to adjust the gas pressure. Furthermore, in the sealing and adjustment valve of WO2004/088206, the shutter provided for sealing has a mushroom-shaped closing portion and therefore does not guarantee sealing hermeticity when it is closed.

It is the object of the invention to suggest a safety thermostat for gas cooking ovens which is free from the drawbacks of the traditional safety thermostats used in this context.

It is another object of the invention to suggest a safety thermostat which can be made with compact size.

It is another object of the invention to suggest a safety thermostat which ensures an excellent gas sealing hermeticity in closing condition. It is another object of the invention to suggest a safety thermostat which with minimum changes can be made with two gas outlets, instead of one.

It is another object of the invention to suggest a thermostat the adjustment of which can be performed in accurate and reliable manner.

It is another object of the invention to suggest a thermostat which displays an alternative and/or improving characterization, both in construction and functional terms, with respect to the traditional ones.

It is another object of the invention to suggest a safety thermostat of simple construction which can be manufactured with low industrial costs.

All these objects, taken individually or in any combination thereof, and others which will be apparent from the following description, are achieved, according to the invention, by a safety thermostat for gas cooking ovens having the features indicated in claim 1 .

The present invention is further explained below by means of two preferred embodiments thereof given by way of non-limiting example only with reference to the accompanying drawings, in which:

Figure 1 shows a perspective view of a thermostat according to the invention in the embodiment with a gas outlet,

Figure 2 shows a plan view thereof,

Figure 3 shows a view from the bottom of the control element,

Figure 4 shows a vertical section view thereof taken along Il l-Il l in figures

2 and 9,

Figure 5 shows a vertical section view thereof taken along IV-IV in figures 2 and 9, Figure 6 shows a vertical section view thereof taken along V-V in figures

2 and 9,

Figure 7 shows a diagrammatic vertical section view of an oven to which the thermostat of figure 1 is appl

Figure 8 shows a perspective view of a thermostat according to the invention in the embodiment with two gas outlets,

Figure 9 shows a plan view thereof,

Figure 10 shows the vertical section thereof taken along IX-IX in figure 9,

Figure 1 1 shows a view from the bottom of the control element,

Figure 12 shows a diagrammatic vertical section view of an oven, to which the thermostat of figure 8 is applied.

As shown in figures 1 - 7, the thermostat according to the invention substantially comprises a body 2, preferably metallic and in particular made of extruded aluminum, in which a series of gas passages and a series of chambers or cavities for housing particular functional components, which will be described in greater detail below, are obtained by mechanical machining.

More specifically, a first cavity 4, preferably of substantially cylindrical shape, is obtained in the metallic body 2 for housing a safety valve 6, which is conveniently of the traditional type and preferably configured to have only two states (i.e. is of the on-off type) either to allow or to prevent the passage of a gas flow according to its state. It comprises a shutter 8, associated with a spring 10, which in absence of external biases, keeps closed a passage between an inlet chamber 12 of the gas and an outlet chamber 14 of the gas itself. The spring 10 is arranged to push the shutter 8 in the same sense in which the gas acts on it. Preferably, the portion of the shutter 8 which acts at the passage between the chambers 12 and 14 is flat and disc-shaped.

The inlet chamber 12 communicates, through a first pipe 16, with an external attachment 13, obtained in the body 2 and fluidically connected to the external feed, while the outlet chamber 14 communicates through a second pipe 18 with a second preferably substantially cylindrical cavity 20 of a slider valve 22, preferably of proportional type, for adjusting the gas flow.

The shutter 8 of the safety valve 6 is axially movable between an extreme closing position, which is kept by the spring 10, as mentioned, and an opposite extreme opening position, which may be reached following the actuation of a first control rod 24, which may apply a thrust higher than the elastic reaction of the spring 10 on the shutter itself.

In the opening position, the valve body 6, which is made of ferrous material, is subject to the attractive action of an electromagnet 26, which is aligned with the shutter 8.

The end of the control rod 24 distant from the shutter 8 is kept by effect of a spring 28 in contact with the lower surface of a control element 30, which preferably comprises a plate, which is conveniently shaped and/or has convenient cam profiles at its lower surface, as will be more apparent below.

This control element 30 is integral with an external pin 32, which exits from the base of a cap 34 applied to the body 2 and is intended to be constrained to a control knob 36.

Preferably, the element 30 and/or the control knob 36 are made of technopolymer. The external pin 32 is integral in the rotation but is free in the axial movements with respect to a first pin 38, which rests, by means of a second pin 39, coaxial thereto, on an expandable element 40. The latter is welded to the second pin 39 and is concerned by an internal communication cavity, by means of a capillary 42, with a sensor (bulb) 44, intended to be inserted in the cavity 46 of the oven, the internal temperature of which must be controlled.

The expandable element 40, the capillary 42 and the sensor (bulb) 44 contain therein a thermically expandable fluid, e.g. a diathermic oil, as described for example in WO2014/064605, which is entirely quoted and incorporated for reference herein.

More in detail, the first pin 38, which is constrained in rotation to the external pin 32, is concerned by a cylindrical axial cavity, in which the end of the second pin 39 is engaged distant from the expandable element 40. Furthermore, the first pin 38 has an external threaded portion which is engaged in a thread obtained in a bushing 48, fixed with respect to the body 2 of the thermostat. In this manner, the rotations of the knob 36 and the first pin 38, integral thereto in rotation by means of the external pin 32, also cause axial movements of the second pin 39 in addition to causing the rotations of the control element 30.

The expandable element 40 is also connected to the slider 22 with the interposition of an elastic element 50, as described in WO2014/064605, mentioned above. In particular, the arrangement of the various parts is such that the resting position of the expandable element 40, which corresponds to the maximum opening position of the slider 22, is predisposed by means of the rotations of the control knob 36, while during operation the expansions of the expandable element 40 cause the displacement of the slider 22 and thus allow to reduce, until canceling out, the gas flow which by crossing the main outlet 52 then reaches the burner to control the temperature inside the oven 46 heated by it in this manner.

The second cavity 20 has a main outlet 52, which can be closed by the slider 22, which is further kept in position away from the outlet 52 elastically by a spring 53.

The outlet 52 communicates, through a pipe 54, with a third cavity 56 for housing a cut-off valve 58. In particular, it comprises a shutter 60 mounted on the end of a second rod 61 associated with a spring 62, which in absence of external biases acts in the sense of keeping closed a passage between an inlet chamber 64 of the gas coming from the pipe 54, and an outlet chamber 66 of the gas towards a main burner 68. Conveniently, the cut-off valve 58 is shaped and controlled to have only two states (i.e. is of the on-off type), and in particular either to allow or to prevent the passage of gas flow according to its state. Preferably, the portion of the shutter 60 which acts at the passage between the chambers 64 and 66 is flat and disc-shaped.

The end of the second rod 61 opposite to the shutter 60 also acts with the lower surface of the control element 30 at the cam profile of a track 75 provided with a circular relief 76.

The same second cavity 20, in which the slider valve 22 for adjusting the gas flow is housed, which is further provided, on a side surface thereof, with another outlet 70, which cannot be closed by the slider 22 and which communicates, by means of an adjustment screw 72 with calibrated hole and a pipe 74, with the outlet pipe 54 of the second cavity 20, constituting a bypass circuit of the slider valve 22 in this manner.

Conveniently, the first cavity 4 and/or the second cavity 20 and/or the third cavity 56 are arranged side-by-side in said body 2. Preferably, said cavities 4, 20 and 56 are arranged mutually parallel and at different heights of the body 2 so as to optimize their arrangement in the body itself and reduce the size of the latter, as a consequence.

The operation of the thermostat according to the invention in the embodiment described hereto is the following.

At rest and with the oven cold, the control knob 36 is maintained in position 0°. In particular, in such position, the safety valve 6 is kept with its shutter 8 in closing position by spring 10 and the cut-off valve 58 is also kept with its shutter 60 in closing condition by spring 62, so as to prevent the gas passage from the external attachment 1 3 to the outlet chamber 66 of the thermostat towards the main burner 68. Conveniently, such gas passage is prevented by a double sealing obtained by means of the action of no fewer than two on-off valves 6 and 58 arranged mutually in series.

In order to access the main burner 68, the user thus axially presses the control knob 36 and at the same time turns it to take it to an operative angle, which in general is either equal to or greater than 52°. In this manner, the control element 30, axially pushed by the knob 36, presses with its lower surface on the upper end of the first control rod 24 to push it axially, i.e. to displace the shutter 8 of the safety valve 6 towards the opening condition, in contrast with the reaction of the spring 1 0. This allows the gas present in the inlet chamber 12, and which comes from the external attachment 13 through the first pipe 16, to pass into the outlet chamber 14 and from here to enter into the second cavity 20 through the second pipe 18. At the same time, the rotation of the first pin 38 causes, by virtue of the threaded coupling between it and the bushing 48, the distancing of the slider 22 from the main opening 52 of the second cavity 20. This allows the gas, which has reached the second cavity 20, to exit through its main opening 52 and the pipe 54 and thus enter into the third cavity 56.

The gap of the main outlet 52 of the second chamber is bound to the axial position of the slider 22, which in turn is bound, through the threaded coupling between the first pin 38, which by means of the second pin 39 is integral with the expandable element 40, and the threaded bushing 48 to the angular position of the control knob 36. More in particular, with the oven cold, the slider 22 is maintained by the spring 53 in the extreme position, which corresponds to the opening condition of the outlet 52. During operation, instead, the axial position of the slider 22 is also bound to the degree of expansion of the expandable element 40 and substantially to the temperature existing inside the cavity 46 of the oven, according to a heat adjustment process known in itself, described for example in WO2014/064605.

In addition to causing the opening of the safety valve 6 by means of the interaction of the upper end of the first rod 24 with the lower surface of the control element 30, following the axial thrust applied on the latter by means of the control knob 46, the rotation of the latter and of the control element 30 also causes, by means of the interaction of the second rod 61 with the cam profile of the track 75 obtained on the lower surface of the control element 30, the opening of the cut-off valve 58, so that the gas which crosses the main opening 52 of the second cavity 20 can reach the inlet chamber 64 of the third cavity 56 and pass into the outlet chamber 66 to then feed the main burner 68. Here the manual or automatic actuation of a traditional plug, associated to the main burner 68 and not shown on the drawings, causes the ignition of the burner itself, the flames of which also act on a traditional thermocouple to generate a supply voltage of the electromagnet 26 and to maintain the safety valve 6 open as a consequence also after releasing the control knob 36.

The cut-off valve 58 instead is kept open also after having released the control knob 36, by the circular relief 76 of the track 75, which in this angular position of the knob 36 and of the element 30 associated with it, acts on the end of the second rod 61 .

Therefore, the maneuver thus described leads to the ignition and keeping of the flames of the main burner 68 of the oven 46, until the environment therein reaches a temperature corresponding to the angular position of the control knob 36, temperature which is then kept by virtue of the expansion to the expandable element 40 according to traditional criteria, which are not an object of the present invention and are not described further.

It is apparent that a different axial position of the expandable element 40 corresponds to a different angular position of the control knob 36, and substantially that the heat regulation of the oven corresponds to a different temperature value.

Then, should the user return the control knob 36 to the initial 0° position, the circular relief 76 of the track 75 of the element 30 ceases to act on the second control rod 61 of the cut-off valve 58, which thus closes and interrupts the gas feed, the consequence of which is the extinguishing of the flames, the closing of the safety valve 6 and substantially the double closing of the gas feeding path from the external attachment 13 to the main burner 68.

Furthermore, it is worth noting that, according to the present invention, when the control knob 36 is usually turned to its 0° position, without being pressed, the cut-off valve 58 moves to the opening condition, while the safety valve 6 remains in the closing condition and therefore there is no passage of gas from the inlet chamber 12 to the outlet chamber 14 of the cavity 4, and therefore there is also no gas which reaches the third cavity 56 to flow towards the main burner 68. Correspondingly, when the control knob 36 is usually pressed, without any rotation with respect to the position of 0°, the safety valve 6 moves to the opening condition, while the cut-off valve 58 remains in closing condition and therefore the gas which reaches the inlet chamber 64 of the third cavity 56 does not pass into the outlet chamber 66 and, therefore, does not reach the main burner 68.

From the above, it is clearly apparent that the safety thermostat according to the invention described hereto is more advantageous than the traditional thermostats, and in particular:

- it is very compact and can be housed in a rather small space,

- it is much safer by virtue of the cut-off valve 58, which ensures a more effective sealing with respect to the sealing offered by a traditional conical shutter,

- it can be manufactured at low industrial cost,

- it allows to feed the burner 68 at a higher gas flow rate.

The thermostat shown in figures 8 - 12 shows the same main and preferential and/or detailed features of the thermostat described above and shown in figures 1 -7, and in particular comprises a double safety valve 6 and cut-off valve 58, placed respectively upstream and downstream with respect to the gas flow adjustment valve 22, and is intended to be used in an oven, in which however a second series of non-modulable flames, which can be used as pilot lights or as flames for a grill 90 or for a second main burner, may be provided in addition to a main burner 68 provided with a series of modulable flames (cf. fig. 12).

In particular, in this second embodiment according to the invention, from the cylindrical side wall of the second cavity 20, which houses the slider 22, a further pipe 78 also departs, the access opening of which is preferably not affected by the axial position of the slider itself. This further pipe 78 leads into a further third cavity 80 (or third additional cavity), preferably obtained in a body 2' distinct from the body 2 and mechanically constrained thereto. However, it is also understood that the bodies 2' and 2 may be made in one piece.

This further third cavity 80 houses a further cut-off valve 82, substantially comprising the same both main and preferential and/or detailed features of the cut-off valve 58 housed in the third main cavity 56. In particular, such further cut-off valve 82 comprises a shutter 84 thereof, which divides an inlet chamber 86 of the further third cavity 80, communicating with the pipe 78, from a gas outlet chamber 88 to an auxiliary burner (grill 90, pilot light or other) (cf. fig. 12).

The shutter 84 is mounted on the end of a second control rod 92 thereof, associated with a spring 94, which in absence of external biases acts in the sense of keeping closed a passage between the inlet chamber 86 and the outlet chamber 88, but may be displaced from such condition by axial thrust at its end distant from the shutter 84 by means of a second circular relief 96 provided in a convenient control element 93 and active for this angular position of the knob 36. In particular, in this second embodiment, the control element 93 substantially corresponds to the control element 30 already provided in the first described embodiment, in which however, in addition to the cam profile of the circular track 75 with the first circular relief 76, an additional track 95 is obtained with a cam profile, which is provided with a second circular relief 96 and which cooperates with the upper end of the control rod 92.

The operation of the thermostat according to the invention in this different embodiment is the following.

Nothing changes with respect to the embodiment shown in fig. 1 -7 and described above for the feeding of the gas to the main burner 68. Furthermore, if it is required to feed the gas to the auxiliary burner 90, the user returns the control knob 36 to 0° and then continues the rotation in the opposite sense, so that the circular relief 96 provided in the control element 93 can axially push the rod 92 and the shutter 84 associated with it can be arranged to open the passage between the chambers 86 and 88.

Evidently, the two burners, main 68 and auxiliary 90, are fed alternatively and furthermore the flames of the main burner 68 are adjustable, while the flames of the auxiliary burner 90 are not adjustable. Furthermore, it is understood that in the case of auxiliary grill burner 90, two ignition plugs and two thermocouples are required, one for each of the two burners 68 and 90, but it may be envisaged that both thermocouples feed the single electromagnet 26 of the safety valve 6. Conveniently, it is also understood that a single plug may be provided if the auxiliary burner 90 is for the pilot lights.

In addition to the advantages offered by the safety thermostat with one outlet described above, the thermostat with two outlets offers the further advantage of being able to feed an auxiliary burner too.

Furthermore, this thermostat with two gas outlets can be obtained very simply starting from a thermostat with a single gas outlet by means of minimal additional machining, consisting in practice in creating the pipe 78 to place the second cavity 20 of the body 2 into communication with the external wall, at which the body 2' or "elbow", at which the further third cavity 80 is obtained, is advantageously applicable by means of a simple mechanical constraint.

Claims

C L A I M S

1 . A safety thermostat for gas cooking ovens comprising a body (2) provided with an inlet (13) fluidically connectable to a gas feeding source, characterized in that it comprises within said body:

- a first cavity (4) fluidically communicating with said inlet (1 3) and housing a normally closed safety valve (6) inside,

- a second cavity (20) fluidically communicating with said first cavity (4) and housing therein an adjustment valve (22) of the gas flow coming from said first cavity (4), said adjustment valve comprising a slider (22) axially movable within said second cavity between a first closing position of a passage towards a third cavity (56) and a second opening position of said passage,

- elastic means (53) acting on said slider (22) to push it towards said second opening position,

- at least one third cavity (56,80), which houses a normally closed cut-off valve (58,82) and which fluidically communicates with at least one burner (68,90) of the oven,

- a heat sensitive element (40) acting on said slider (22), as a function of the temperature intended to be detected by a sensor (44) applicable to said oven, to push said slider (22) towards said first closing position in contrast with said elastic means (53),

- means (24,38,39,48,61 ,92) for causing the opening of said safety valve (6) and of said cut-off valve (58,82) and the adjustment of the position of said slider (22) to control the gas flow through said at least one third cavity (56,80) in this manner.

2. A thermostat according to claim 1 , characterized in that said second cavity (20) and said at least one third cavity (56,80), and preferably also said first cavity (4), are arranged side-by-side inside said body (2).

3. A thermostat according to one or more of the preceding claims, characterized in that said second cavity (20) and said at least one third cavity (56,80), and preferably also said first cavity (4), are arranged mutually parallel at different heights in said body (2).

4. A thermostat according to one or more of the preceding claims, characterized in that said means (24,61 ,92) for causing the opening of said safety valve (6) and of said cut-off valve (58,82) are actuated by a same control element (30,93).

5. A thermostat according to one or more of the preceding claims, characterized in that said means (24,61 ,92) for causing the opening of said safety valve (6) and of said cut-off valve (58,82) are actuated by the same control element (30,93) which is rotational about an axis thereof and is axially movable along said axis.

6. A thermostat according to one or more of the preceding claims 4 or 5, characterized in that said control element (30,93) is configured so that:

- its rotation about an axis thereof causes the opening of said cut-off valve (58) and not of said safety valve (6), or vice versa,

- its translation along said axis causes the opening of said safety valve (6) and not of said cut-off valve (58,82), or vice versa,

- its contemporary rotation and translation along said axis causes the opening of said cut-off valve (58,82) and not of said safety valve (6).

7. A thermostat according to one or more of the preceding claims, characterized in that said safety valve (6) and/or of said cut-off valve (58,82) are of the on-off type and/or in that said gas flow adjustment valve (22) is of the proportional type.

8. A thermostat according to one or more of the preceding claims, characterized in that said cut-off valve (58,82) is housed in said at least one third cavity (56,80) comprising:

- a shutter (60,84) acting on a passage between an inlet chamber (64,86) fluidically communicating with the outlet (52) of said second cavity (20) controlled by said slider (22) and an outlet chamber (66,88) fluidically communicating with said burner (68),

- a control rod (61 ,92) mechanically associated with said shutter (60,84),

- elastic means (62,94) associated with said rod (61 ,92) to push the rod towards said control element (30,93) and said shutter (60,84) towards said passage on which it acts.

9. A thermostat according to one or more of the preceding claims, characterized in that the shutter (60,84) of said cut-off valve (58,82) has a substantially flat development.

10. A thermostat according to one or more of the preceding claims, characterized in that said control element (30,93) comprises a cam profile (75,76,95,96) which cooperates with said rod (61 ,92) of said cut-off valve (58,82) to cause its opening and in that said control element (30,93) is associated with means (32,34,38,39,48) configured to transform a variation of their angular position into a variation of the axial position of the slider (22) of said gas flow adjustment valve.

1 1 . A thermostat according to one or more of the preceding claims, characterized in that the means (24) for causing the opening of said safety valve (6), the means for causing (61 ,92) the opening of said cut-off valve (58,82) and the means (32,34,38,39,48) for adjusting the axial position of said slider (22) are distinct and arranged mutually side-by-side.

12. A thermostat according to one or more of the preceding claims, characterized in that said control element (30,93) is integral in rotation and axial translation with an actuating and adjustment knob (36) comprising a cam profile (75,95) with a circular relief (76,96) cooperating with the end of said rod (61 ,92) of said cut-off valve (58,82) when said knob (36) is turned in a predetermined sense with respect to its home position in order to keep open said cut-off valve (58,82) also when said knob (36), after having been pushed axially in the sense of causing the opening of said cut-off valve (58,82), is released.

13. A thermostat according to one or more of the preceding claims, characterized in that said at least one third cavity comprises:

- a third main cavity (56), housing a first normally closed cut-off valve (58) and fluidically communicating with at least one main burner (68) of the oven, and

- a third additional cavity (80), which houses a second normally closed cutoff valve (82), and which fluidically communicates with at least one auxiliary burner (90) and, by means of a further passage (78), fluidically communicates with said second cavity (20).

14. A thermostat according to the preceding claim, characterized in that said third additional cavity (80) communicates with said second cavity (20) by means of a further passage (78), the opening of which is independent from the axial position of said slider (22) in said second cavity (20).

15. A thermostat according to claims 13 or 14, characterized in that said control element (93) is configured to cause the opening of the cut-off valve (82) of said third additional cavity (80) when said safety valve (6) and said cutoff valve (58) of said third main cavity (56) are closed.

16. A thermostat according to one or more of the preceding claims from 1 3 to 15, characterized in that said third additional cavity (80) is obtained in a second body (2'), which is mechanically constrained to said first body (2), in which said first (4), said second (20) and said third main cavity (56) are obtained.

17. A thermostat according to one or more of the preceding claims from 1 3 to 15, characterized in that said third additional cavity (80) is obtained in the same body (2) in which said first (4), said second (20) and third main cavity (56) are obtained.

18. A thermostat according to one or more of the preceding claims, characterized in that said control element (93) is integral in rotation and axial translation with an actuating and adjustment knob (36) and comprises:

- a first cam profile (75) with a first circular relief (76) cooperating with the end of the rod (61 ,92) of said first cut-off valve (58,82) of said third main cavity (56) when said knob (36) is turned in a predetermined sense with respect to its resting position, so as to keep said first cut-off valve (58) open also when said knob (36), after having been axially pushed in the sense of causing the opening of said first cut-off valve (58) is released,

- a second cam profile (95) with a second circular relief (96) cooperating with the end of the rod (92) of said second cut-off valve (82) of said third additional cavity (80) when said knob (36) is turned in a sense opposite to the previous one with respect to its resting position, so as to keep said second cut-off valve (82) open also when said knob (36) is released, after having been axially pushed in the sense to cause the opening of said second cut-off valve (82).

19. A thermostat according to one or more of the preceding claims, characterized in that said safety valve (6) is provided with an electromagnet (26) supplied by a thermocouple which is positioned inside the oven and which is intended to be struck by at least one flame generated by said main burner (68), said electromagnet (26) being configured to act on the shutter (8) of said safety valve (6) to keep it open.

20. A thermostat according to claim 19, characterized in that said electromagnet (26) is independently supplied by two thermocouples positioned in the oven and positioned respectively near said main burner (68) and said auxiliary burner (90) to be struck by at least one flame generated by said main burner (68) and/or by said auxiliary burner (90), said electromagnet (26) being configured to act on the shutter (8) of said safety valve (6) to keep it open.