EP0778450B1 - Apparatus for producing domestic hot water - Google Patents

Description

The invention relates to generating devices domestic hot water using combustion of a gas.

• un circuit d'eau fermé dit de "réchauffage sanitaire" comportant lui-même une pompe de circulation, une tubulure associée à un brûleur à gaz dans une chambre de combustion et formant avec ce brûleur un échangeur principal, et le primaire d'un échangeur thermique auxiliaire,
• un circuit d'eau de puisage comprenant une arrivée d'eau froide, le secondaire de l'échangeur thermique auxiliaire et une sortie de puisage,
• et un circuit de chauffage central monté en parallèle sur le tronçon, du circuit fermé, comprenant la pompe et la tubulure, la connexion d'une extrémité dudit circuit de chauffage central avec ledit tronçon étant effectuée par une vanne à trois voies.

It is more particularly aimed, among these appliances, at those known as "mixed", which also make it possible to supply hot water to a central heating circuit and which include:

• a closed water circuit called "sanitary heating" itself comprising a circulation pump, a pipe associated with a gas burner in a combustion chamber and forming with this burner a main exchanger, and the primary of an exchanger auxiliary thermal,
• a drawing water circuit comprising a cold water inlet, the secondary of the auxiliary heat exchanger and a drawing outlet,
• and a central heating circuit mounted in parallel on the section of the closed circuit, comprising the pump and the pipe, the connection of one end of said central heating circuit with said section being effected by a three-way valve.

With such a mixed appliance, the water which circulates in the central heating circuit is also allowed at circulate in the domestic heating circuit and it the draw-off circuit should therefore be separate from this double circuit, which avoids any water pollution drawn.

In the usual embodiments of appliances of the kind in question, any hot water drawing, triggered by the opening of a tap connected at the end downstream of the withdrawal outlet, results in following operations: excitation of the pump, ignition of the burner, heating of the water circulating in the circuit for space heating and transfer of calories from this water to the water of the drawing circuit, in the exchanger auxiliary.

This process is not instantaneous so the water drawn is not hot from the start: the user must first run a significant water flow before that it doesn't get really hot.

This drawback is important for drawdowns requiring only a small flow of hot water: it is even current that, for such withdrawals, the user is content ultimately cold or lukewarm water drawn off during first seconds of its drawing, which reduces interest of the installation.

To remedy this drawback, it has already been proposed to make clear to the heat exchanger auxiliary a balloon of limited capacity filled with water sanitary to draw, the temperature of this water being maintained at a value corresponding to the desired drawing, for example of the order of 60 degrees, said balloon being crossed by a section of the domestic heating circuit.

Such a solution is advantageous in that the water drawn from the balloon is quickly available to the relatively high temperature desired for it.

But this solution has the disadvantage next: the temperature of the water drawn drops sharply after distribution of the entire volume contained in the ball at the start of the drawing, so that the capacity of this balloon must be relatively large, and typically greater than 10 liters, to satisfy the majority of said drawings.

To remedy this drawback further, and therefore ability to reduce tank capacity to lower volume at 10 liters, it was proposed to make clear at the auxiliary heat exchanger, in addition to the tank and in series with it, a direct exchanger comprising two sections belonging respectively to the domestic heating circuit and to the drawing circuit, sections brought into mutual thermal contact against the current.

This improved solution is not without interest

• d'une part, dans le circuit de réchauffage sanitaire, en amont du ballon,
• et, d'autre part, dans le circuit de puisage, en aval de ce ballon.

However, in the embodiments which have been proposed for this, the direct exchanger was placed,

• on the one hand, in the sanitary heating circuit, upstream of the tank,
• and, on the other hand, in the drawing circuit, downstream of this balloon.

• le tronçon, de l'échangeur direct, qui appartient au circuit de réchauffage sanitaire, reçoit directement l'eau très chaude sortant de l'échangeur principal, ce qui risque de l'entartrer, surtout s'il s'agit d'un dispositif de construction compacte,
• lors d'un puisage, l'eau sanitaire qui circule dans cet échangeur direct après avoir été préchauffée en traversant le ballon risque d'être surchauffée et donc de brûler l'usager,
• Peau qui parvient au ballon en provenance de l'échangeur principal commence toujours par céder des calories dans l'échangeur direct, même lors des périodes où cet échangeur n'est pas utile, savoir en dehors des périodes de puisage et au début de chaque puisage,
• l'eau froide en provenance de l'extérieur est toujours directement introduite dans le ballon sans pré-chauffage, ce qui peut engendrer dans ce ballon des « trains froids » indésirables.

Such a construction has a number of drawbacks, and in particular the following:

• the section of the direct exchanger, which belongs to the domestic heating circuit, directly receives very hot water leaving the main exchanger, which risks scaling it up, especially if it is a device compact construction,
• during a drawing, the sanitary water which circulates in this direct exchanger after having been preheated by crossing the tank risks being overheated and therefore burning the user,
• Skin that reaches the balloon coming from the main exchanger always starts by giving up calories in the direct exchanger, even during periods when this exchanger is not useful, namely outside of the drawdown periods and at the start of each drawdown. ,
• cold water from outside is always directly introduced into the tank without pre-heating, which can cause undesirable "cold trains" in this tank.

• un circuit dit de « réchauffage sanitaire », qui est un circuit d'eau fermé comportant une pompe de circulation, une tubulure associée à un brûleur à gaz dans une chambre de combustion et formant avec ledit brûleur un échangeur principal, et le primaire d'un échangeur thermique direct à contre-courant ;
• un circuit d'eau de puisage comportant une arrivée d'eau froide, le secondaire de l'échangeur thermique auxiliaire et une sortie de puisage ; et
• un circuit de chauffage central monté en parallèle sur un tronçon dudit circuit de réchauffage sanitaire, ledit tronçon comprenant desdites pompe et tubulure.

FR-A-2 569 469 also discloses a “mixed” type hot water generator for the production of domestic hot water and heating, using the combustion of a gas and comprising:

• a so-called “domestic heating” circuit, which is a closed water circuit comprising a circulation pump, a pipe associated with a gas burner in a combustion chamber and forming with said burner a main exchanger, and the primary a direct counter-current heat exchanger;
• a drawing water circuit comprising a cold water inlet, the secondary of the auxiliary heat exchanger and a drawing outlet; and
• a central heating circuit mounted in parallel on a section of said domestic heating circuit, said section comprising said pump and tubing.

But the generating apparatus according to the aforementioned document ensures the production simultaneous domestic hot water and heating hot water, and requires a high power condensing boiler and a storage tank of relatively large capacity.

The invention aims, among other things, to overcome the drawbacks of the devices cited above.

To this end, the hot water generating device according to the invention, of the type known from FR-A-2 569 469 and as presented above, is characterized in that that said direct counter-current exchanger is mounted downstream with respect to a tank of reduced capacity on said sanitary heating circuit and upstream with respect to said tank on said drawing water circuit, and said section of said domestic heating circuit is connected to one end of said central heating circuit by a three-way valve, the control of which allows to selectively control the production of domestic hot water in said auxiliary heat exchanger or sending hot water into said central heating circuit.

• le volume du ballon est de l'ordre de 6 litres,
• l'échangeur direct à contre-courant est d'un type compact à plaques,
• l'échangeur direct à contre-courant est noyé à l'intérieur du ballon,
• l'appareil générateur comprend une sonde de température disposée à l'intérieur du ballon et des moyens asservis aux franchissements vers le bas d'un seuil de consigne de température par la température d'eau détectée par cette sonde de façon à mettre en fonctionnement le brûleur à une allure maximale ou à une allure réduite selon qu'un seuil de consigne de débit est alors dépassé ou non par le débit d'eau détecté par un débitmètre sur le circuit d'eau de puisage,
• l'appareil générateur selon l'alinéa précédent comprend une seconde sonde de température disposée sur le circuit d'eau de puisage à la sortie de l'échangeur direct à contre-courant et associée aux moyens asservis de façon à exploiter en outre, pour la mise en fonctionnement du brûleur, les franchissements vers le bas d'un second seuil de consigne de température par les températures d'eau détectées par la seconde sonde de température.

In preferred embodiments, use is also made of one and / or the other of the following arrangements:

• the volume of the flask is around 6 liters,
• the direct counter-current exchanger is of a compact plate type,
• the direct counter-current exchanger is embedded inside the tank,
• the generating device comprises a temperature probe placed inside the tank and means slaved to the crossing down of a temperature set point by the water temperature detected by this probe so as to put the burner at a maximum rate or at a reduced rate depending on whether a flow setpoint threshold is then exceeded or not by the water flow detected by a flow meter on the drawing-off water circuit,
• the generating device according to the preceding paragraph comprises a second temperature probe arranged on the water draw-off circuit at the outlet of the direct counter-current exchanger and associated with the servo means so as to further use, for the putting the burner into operation, the crossing down of a second temperature set point by the water temperatures detected by the second temperature probe.

The invention comprises, apart from these main provisions, certain other arrangements which are preferably used at the same time and which will be more explicitly discussed below.

In the following, we will describe a mode of preferred embodiment of the invention with reference to drawing attached hereto in a manner which is of course not limiting.

Figure 1, of this drawing, shows schematically a mixed water heater conforming to the invention usable both for drawing water domestic hot water and for supplying a closed circuit central heating, as well as the associated installation to this device.

Figure 2 is a perspective view of a portion of such a device.

• un générateur de calories comportant lui-même, dans une chambre de combustion 2, un brûleur à gaz 3 et une tubulure 4 exposée aux flammes et/ou aux gaz chauds engendrés par le brûleur, tubulure destinée à être parcourue par l'eau à chauffer et constituant avec le brûleur un échangeur principal de calories,
• une pompe de circulation 5 raccordée à la tubulure 4,
• un tronçon de conduite 6 formant avec la tubulure 4 et la pompe 5 un circuit fermé, dit de réchauffage sanitaire, ledit tronçon 6 constituant le primaire d'un échangeur de chaleur auxiliaire E,
• et un circuit de puisage comprenant, entre son arrivée d'eau froide 7 et une sortie d'eau chaude 8, un tronçon 9 constituant le secondaire de l'échangeur de chaleur auxiliaire E.

In a manner known per se, the heating appliance considered comprises, integrated inside the same envelope 1:

• a calorie generator itself comprising, in a combustion chamber 2, a gas burner 3 and a pipe 4 exposed to the flames and / or to the hot gases generated by the burner, pipe intended to be traversed by the water to be heated and constituting with the burner a main heat exchanger,
• a circulation pump 5 connected to the pipe 4,
• a pipe section 6 forming with the tubing 4 and the pump 5 a closed circuit, called sanitary heating, said section 6 constituting the primary of an auxiliary heat exchanger E,
• and a drawing circuit comprising, between its cold water inlet 7 and a hot water outlet 8, a section 9 constituting the secondary of the auxiliary heat exchanger E.

The invention relates essentially to said exchanger auxiliary heat E.

• d'une part, un petit ballon 10 propre à contenir une réserve d'eau chaude sanitaire de volume réduit immédiatement disponible pour le puisage, ledit ballon étant traversé par une boucle 11 faisant partie du tronçon 6 et son volume intérieur, qui fait partie du tronçon 9, étant raccordé en un point bas 12 à l'arrivée d'eau froide 7, et en un point haut 13 au départ de l'eau chaude 8,
• et d'autre part, un échangeur thermique 14, dit échangeur direct ci-après, présentant une forte capacité volumique d'échange thermique, ledit échangeur étant de préférence du type à plaques et constitué par deux portions de canalisation 15 et 16 faisant partie respectivement des deux tronçons 6 et 9 et mises en contact thermique mutuel à contre-courant.

This exchanger E here includes:

• on the one hand, a small tank 10 capable of containing a reserve of domestic hot water of reduced volume immediately available for drawing, said tank being crossed by a loop 11 forming part of the section 6 and its internal volume, which is part of the section 9, being connected at a low point 12 at the cold water inlet 7, and at a high point 13 at the hot water outlet 8,
• and on the other hand, a heat exchanger 14, called direct exchanger below, having a high volume heat exchange capacity, said exchanger preferably being of the plate type and constituted by two portions of pipe 15 and 16 forming part respectively of the two sections 6 and 9 and brought into mutual thermal contact against the current.

The tank 10 and the direct exchanger 14 are mounted in series as follows.

First, the two portions 11 and 15 of the section 6 of the domestic heating circuit 4, 5, 6 are crossed in this order by the water driven by the pump 5.

Secondly, with regard to section 9 of the drawing circuit, its two portions constituted respectively by section 16 and by the interior volume at balloon 10 are crossed in this order by the water of drawing.

In other words, when the burner 3 is on, very hot water leaving the main exchanger 4 and circulating in the domestic heating circuit first crosses the heating loop 11 of its balloon 10 before crossing the direct exchanger 14.

As for the cold water introduced into the device, it always begins by crossing the direct interchange 14 before entering the ball 10.

These traffic orders present certain advantages explained below.

We still see in Figure 1 several organs or assemblies outside the enclosure 1 and connected to the circuits described above.

• d'une source d'eau froide sous pression 17 raccordée à l'arrivée d'eau froide 7,
• d'un robinet de puisage 18 raccordé à la sortie d'eau chaude 8,
• et d'un tronçon de circuit de chauffage central 19 comprenant des radiateurs 20 et monté en parallèle sur l'ensemble constitué par la tubulure 4 et par la pompe 5, l'une des connexions dudit tronçon 19 avec cet ensemble étant réalisée au moyen d'une vanne à trois voies et deux positions 21 permettant de relier sélectivement ledit ensemble soit au tronçon 6 de réchauffage sanitaire, soit audit tronçon 19 de chauffage central.

It's about :

• a source of cold water under pressure 17 connected to the cold water inlet 7,
• a draw-off tap 18 connected to the hot water outlet 8,
• and of a section of central heating circuit 19 comprising radiators 20 and mounted in parallel on the assembly constituted by the pipe 4 and by the pump 5, one of the connections of said section 19 with this assembly being produced by means of 'a three-way valve and two positions 21 for selectively connecting said assembly either to section 6 of domestic heating, or to said section 19 of central heating.

We also see in Figure 1 a valve safety 22 connected to the hot water outlet 8, a flow meter 23 mounted on the cold water inlet 7 and a temperature 24 mounted in the tank 10.

The flow meter 23 and the probe 24 are associated respectively to means (not shown) allowing to enslave the calorific power generated by the 2-3-4 boiler with parameters (flow rates and temperatures) that they detect.

An advantageous mode of action of these means is the following: the crossing down of a threshold of setpoint given T by the water temperature detected by the probe 24 results in starting the burner 3 to its maximum gait or at a reduced gait (corresponding for example at a calorific power of the order of third of maximum power) depending on whether the water flow detected by the flow meter 23 may or may not exceed a threshold of instruction given D.

The operation of the installation is next.

It is first assumed that the valve to three way 21 is arranged as shown on Figure 1, for which there is no request for central heating.

Initially, the draw-off valve 18 is closed, the burner 3 is off and the pump 5 is stopped.

This initial state assumes that the water reserve R contained in the flask 10 is brought to a temperature sufficient, for example of the order of 60 °, to respond the needs for drawing domestic hot water.

If, under these conditions, said temperature, detected at 24, goes below the set point predetermined T, this passage automatically triggers excitation of pump 5 and ignition of burner 3 to a reduced gait - corresponding for example to a flow gas of the order of a third of the maximum flow rate, which makes circulate hot water in the heating circuit 4, 5, 6, until the temperature of the water reserve R has again reached the value desired, which automatically translates into a new pump 5 stopped and by further switching off the burner 3.

If a user wishes to draw hot water sanitary, he just has to open the tap 18.

This opening has no consequences as long as the water temperature detected by probe 24 remains above setpoint T: hot water from of the reserve R is then drawn and is replaced as and measure with cold water so that the temperature of this reserve gradually decreases.

It is only when said temperature passes below the set point T that the excitation of the pump 5 is triggered, as well as a burner ignition 3 at a speed which depends on the flow rate, the power calorific generated by the burner being maximum or at otherwise reduced depending on whether threshold D is then exceeded or not by said flow of the drawing.

Said ignition, as well as the circulation of water hot in the domestic heating circuit, are then maintained for the duration of the relevant drawdown.

This is where the direct exchanger 14 comes in.

Indeed, the drawing water begins by crossing this exchanger before reaching balloon 10 and, given the relatively high temperature of the heating water which crosses the portion 15 of said direct exchanger, the water of drawing which crosses the portion 16 of said exchanger is itself heated before reaching balloon 10: its introduction into this balloon therefore does not cool substantially the water from the reserve R of said balloon and the temperature of the water drawn can then remain at a sufficient value during all heating, and even beyond the duration corresponding to a total distribution of the volume of reserve R.

In other words, this installation presents both one of the essential advantages of the facilities equipped with large hot water tanks, know availability immediate hot water from the start of each drawing, and this without having the drawback of said installations due to the size and price of such a large ball.

• l'eau de réchauffage qui circule dans le primaire 15 dudit échangeur 14 ne pénètre dans ce primaire qu'après avoir traversé la boucle 11 de réchauffage du ballon 10 et n'est donc jamais portée à une température très élevée, ce qui est à éviter en raison des risques d'entartrage dudit primaire 15,
• les calories destinées au réchauffage du circuit sanitaire et provenant de l'échangeur principal 4, sont, dans tous les cas, distribuées en premier lieu au ballon 10, puisque la boucle 11 est traversée par cette eau avant le primaire 15, ce qui permet d'éviter toute dispersion inutile de calories dans ledit primaire, en particulier au cours des périodes pour lesquelles l'échangeur auxiliaire 14 n'est pas exploité, comme c'est le cas en dehors des puisages ou tout à fait au début de chaque puisage,
• du fait que l'eau froide sanitaire provenant du réseau extérieur 17 pénètre dans le ballon 10 seulement après avoir traversé l'échangeur direct 14, le débit de cette eau qui pénètre effectivement dans le ballon est en général déjà préchauffé, ce qui évite au maximum l'introduction de "trains froids" d'eau dans le ballon, préjudiciables bien entendu à la tenue en température de la réserve d'eau R y contenue,
• ladite localisation de l'échangeur direct en amont du ballon sur le circuit sanitaire écarte en outre les risques de surchauffe de l'eau puisée qui pouvaient être observés lorsque cette localisation était combinée avec un montage de l'échangeur direct en amont du ballon sur le circuit de réchauffage sanitaire.

In addition, due to the judicious order adopted for the series connections of the tank 10 and the direct exchanger 14, the following advantages are observed:

• the heating water which circulates in the primary 15 of said exchanger 14 only enters this primary after having passed through the balloon heating loop 11 and is therefore never brought to a very high temperature, which is to be avoided because of the risks of scaling of said primary 15,
• the calories intended for the heating of the sanitary circuit and coming from the main exchanger 4, are, in all cases, distributed firstly to the balloon 10, since the loop 11 is crossed by this water before the primary 15, which makes it possible to avoid any unnecessary dispersion of calories in said primary, in particular during the periods for which the auxiliary exchanger 14 is not used, as is the case outside the drawdowns or entirely at the start of each drawdown,
• the fact that the domestic cold water from the external network 17 enters the tank 10 only after passing through the direct exchanger 14, the flow of this water which actually enters the tank is generally already preheated, which avoids as much as possible the introduction of "cold trains" of water into the balloon, which are of course detrimental to the temperature resistance of the water reserve R contained therein,
• said location of the direct exchanger upstream of the tank on the sanitary circuit further eliminates the risks of overheating of the drawn water which could be observed when this location was combined with mounting of the direct exchanger upstream of the tank on the domestic heating circuit.

If the three-way valve 21 is put in its position other than that illustrated, for which the assembly 4-5 is connected to the central heating circuit 19, it is this circuit which is reheated from the burner 3 any known desirable manner, tentative priorities can be automatically assigned to the service of the drawing circuit as required.

The whole of the auxiliary exchanger E, comprising in particular the tank 10 and the plate exchanger 14, can be collected in a very small volume, by example at the base of casing 1 of the device generating calories.

We have shown in Figure 2 an example of such development having given all satisfaction, for which the balloon 10 has the general shape of a cylinder of horizontal axis revolution and is associated in a very small volume, not only at the direct exchanger 14, but also at pump 5, at valve 21 and at the various accessories and service pipes for the appliance.

One of said accessories, visible at 25, is a flow safety device mounted just upstream of pump 5, on the water intake section of this pump which receives both water back from the circuit heating 19 and that of the primary circuit 6 of the exchanger auxiliary E.

A avec le réseau extérieur d'eau froide 17,

B avec les robinets de puisage 18,

C avec la portion de départ du circuit de chauffage 19,

D avec la portion de retour dudit circuit 19.

We also see in Figure 2 the different ends of pipes A, B, C and D which are to be connected respectively:

A with the outside cold water network 17,

B with the taps 18,

C with the starting portion of the heating circuit 19,

D with the return portion of said circuit 19.

These four ends A, B, C and D are aligned along the same horizontal line, above a location intended to receive a connection strip appropriate (not shown) itself carried by the housing 1.

As a result of what, and whatever the mode of adopted implementation, we finally have a device domestic hot water generator whose constitution, the operation and the benefits result sufficiently of the above.

• celles où l'échangeur direct relativement compact 14 serait immergé dans le ballon 10,
• et celles où la sonde de température 24 placée à l'intérieur du ballon 12 serait secondée par une autre sonde de température 26 placée sur le circuit de puisage à la sortie de l'échangeur 14, en un emplacement, dudit circuit de puisage, susceptible à la fois d'être réchauffé en permanence par les calories provenant du ballon et d'être refroidi très peu de temps après le début de chaque puisage : cette sonde 26 est associée aux moyens d'asservissement décrits ci-dessus de façon à déclencher un réchauffage dès qu'un second seuil de consigne T' est franchi vers le bas par la température de l'eau détectée par elle, ce qui permet de réduire le temps de réponse de l'installation dû à l'inertie thermique de la réserve d'eau R contenue dans le ballon ; dans un tel cas il peut être avantageux d'asservir entre eux les deux seuils de consigne T et T' de façon telle que le réglage de l'un des deux entraíne automatiquement le réglage corrélatif de l'autre.

As it goes without saying, and as it already follows from the above, the invention is in no way limited to those of its modes of application and embodiments which have been more especially envisaged; on the contrary, it embraces all variants, in particular:

• those where the relatively compact direct exchanger 14 would be immersed in the tank 10,
• and those where the temperature probe 24 placed inside the tank 12 would be seconded by another temperature probe 26 placed on the drawing circuit at the outlet of the exchanger 14, at a location, of said drawing circuit, susceptible both to be constantly reheated by the calories coming from the balloon and to be cooled very shortly after the start of each drawing: this probe 26 is associated with the control means described above so as to trigger a reheating as soon as a second setpoint threshold T 'is crossed downwards by the water temperature detected by it, which makes it possible to reduce the response time of the installation due to the thermal inertia of the reserve d water R contained in the flask; in such a case it may be advantageous to slave between them the two set points T and T 'so that the adjustment of one of the two automatically causes the correlative adjustment of the other.

Claims (6)

1. A "combination" type hot water generating device for the production of hot water for sanitary and heating purposes using the combustion of a gas and comprising:

   a so-called "sanitary re-heating" circuit (6) which is a closed water circuit comprising a feed pump (5), a tube (4) associated with a gas burner (3) in a combustion chamber (2) and forming a principal exchanger with said burner (3), and the primary stage of a direct counter-current heat exchanger (14);

   a feeder water circuit (9) comprising a cold water supply (7), the secondary stage of the auxiliary heat exchanger (E) and a feeder draw-off (8); and

   a central heating circuit (19) fitted in parallel on a section of the said sanitary re-heating circuit (6), the said section comprising the said pump (5) and the said tube (4),

   characterised in that the said direct counter-current exchanger (14) is fitted downstream of a reduced capacity tank (10) on the said sanitary re-heating circuit (6) and upstream of the said tank (10) on the said feeder water circuit (9), and that the said section of the said sanitary re-heating circuit (6) is connected to one end of the said central heating circuit (19) by means of a three-way valve (21) with a switch which makes it possible to control either the production of sanitary hot water in the said auxiliary heat exchanger (E) or the supply of hot water to the said central heating circuit (19).
2. A device according to claim 1, characterised in that the volume of the tank (10) is of the order of 6 litres.
3. A device according to any one of the preceding claims characterised in that the direct counter-current exchanger (14) is of a compact plate type.
4. A device according to any one of the preceding claims characterised in that the direct counter-current exchanger (14) is immersed inside the tank (10).
5. A device according to any one of the preceding claims characterised in that it comprises a temperature sensor (24) disposed inside the said tank (10) and means controlled by the points at which the water temperature detected by the said sensor (24) falls below a set-point temperature (T) in such a manner as to operate the said burner (3) at a maximum speed or a reduced speed depending on whether or not a set-point flow rate (D) has been exceeded by the water flow rate detected by a flow rate meter (23) on the feeder water circuit (19).
6. A device according to claim 5 characterised in that it comprises a second temperature sensor (26) disposed on the said feeder water circuit (19) at the draw-off from the said direct counter-current exchanger (14) and associated with the said controlled means in such a manner as to exploit for the operation of the said burner (3) inter alia the points at which the water temperatures detected by the said second temperature sensor (26) fall below a second set-point temperature (T').

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