Storage boiler of the combined type

Abstract

 A storage boiler (1) of the so-called combined type, comprises a secondary heat exchanger (4) provided with respective first thermal exchange means for heating sanitary water supplied to said exchanger (4), a unit (5) for producing and storing hot sanitary water provided with respective second thermal exchange means (19) for heating the sanitary water stored within said unit (5), and a secondary hydraulic circuit (20) for the circulation of sanitary water, provided with means (27) for selectively drawing off the sanitary water heated by said secondary heat exchanger (4) and/or by said unit (5) for producing and storing hot sanitary water. The storing unit (5) is arranged in the secondary hydraulic circuit (20) for the circulation of sanitary water in parallel to the secondary heat exchanger (4). In the boiler of the invention the second thermal exchange means (19) provided in the unit (5) for producing and storing hot sanitary water is essentially in parallel to the first thermal exchange means, so that heating of the unit (5) for producing and storing hot sanitary water may be independently carried out with respect to the secondary heat exchanger (4).

Description

Field of the invention

[0001] The present invention relates to a storage boiler of the so-called combined type, comprising:

• a secondary heat exchanger provided with respective first thermal exchange means for heating sanitary water supplied to said exchanger;
• a unit for producing and storing hot sanitary water provided with respective second thermal exchange means for heating the sanitary water stored within said unit;
• a secondary hydraulic circuit for the circulation of sanitary water, provided with means for selectively drawing off the sanitary water heated by said secondary heat exchanger and/or by said unit for producing and storing hot sanitary water;
  wherein said unit for producing and storing hot sanitary water is arranged in said secondary hydraulic circuit in parallel to the secondary heat exchanger.

[0002] In the following description and in the following claims, the expression 'boiler of the combined type' is used to indicate a boiler adapted to generate both hot water for space heating (primary water), and hot water for sanitary use (sanitary water).

Prior Art

[0003] In the field of boilers for domestic use, and in particular, of boilers of the combined type, the need is increasingly felt of ensuring both a prompt delivery of hot sanitary water at the desired temperature, and the production of hot primary water for space heating. In order to fulfil this need, the so-called storage boilers, wherein a certain amount of readily-usable hot water is stored within a storage unit and herein maintained at a predetermined temperature so as to provide hot water for sanitary use in relatively short times, have gained ground in the field.

[0004] A kind of storage boilers of the combined type currently available on the market essentially comprises a primary heat exchanger- (usually, of the gas-water type) for producing hot primary water for space heating, and a secondary heat exchanger (usually, of the plate-and-frame type) for instantaneously producing hot sanitary water, this kind of boilers providing for the use, at each water drawing-off by the users, of a predetermined amount of readily-usable hot sanitary water previously heated in a separate storage unit.

[0005] Thus, for example, German patent application DE 3714968 discloses a storage boiler of the combined type wherein a single primary hydraulic circuit is provided for the circulation of hot primary water, which puts in fluid communication the primary heat exchanger with the secondary heat exchanger, and thus, with the storage tank.

[0006] More particularly, the hot primary water leaving the primary heat exchanger is sent to the external heating plant or, alternatively, to the secondary heat exchanger and, from here, to the storage tank which is thus arranged in series with the preceding one. On the contrary, for the circulation of sanitary water, a hydraulic circuit of sanitary water is provided, comprising a first section for supplying sanitary water to the secondary heat exchanger, and a second section, extending in parallel to said first section, for supplying sanitary water to the storage tank. The sanitary water is heated in the secondary heat exchanger and in the storage tank by means of a thermal exchange with the primary water flowing in the primary hydraulic circuit.

[0007] A significant drawback associated to the storage boilers of this kind is related to the fact that the storage tank and the secondary heat exchanger are in series with one another in the primary hydraulic circuit; thus, heating sanitary water in the storage tank inevitably implies the circulation of hot primary water also in the secondary heat exchanger thereby heating the sanitary water contained therein, even when no hot sanitary water is drawn off (and consequently, even when the secondary heat exchanger is not in operation) but there is the need of restoring the desired temperature of the hot water stored within the storage tank.

[0008] This causes the undesired formation of limestone and scaling in the fluid flowpath for the circulation of sanitary water provided in the secondary heat exchanger. Said scaling and limestone hinder the thermal exchange between sanitary water and primary water, thus causing a performance and efficiency reduction of the secondary exchanger, which constantly worsen in a substantially irreversible way in the course of time.

[0009] In fact, the performance and efficiency reduction related to the formation of limestone and scaling in the fluid flowpath for the circulation of sanitary water causes a gradual but irreversible reduction of the flow rate which may be delivered by the secondary exchanger at the maximum design temperature of the latter or, in the alternative, it allows to deliver the maximum design flow rate but at a temperature which is increasingly lower than the maximum temperature reachable when the secondary heat exchanger is new.

[0010] Moreover, the presence of limestone and scaling in the fluid flowpath for the circulation of sanitary water in the secondary exchanger of the boiler inevitably involves an increase in the operating and maintenance costs of the boiler itself, up to the need of replacing the secondary exchanger with the consequent costs.

Summary of the invention

[0011] The technical problem underlying the present invention is that of providing a storage boiler of the combined type which allows to ensure a prompt delivery of hot sanitary water, overcoming at the same time the above-mentioned drawbacks of the prior art.

[0012] This problem is solved according to the invention by a storage boiler of the combined type of the kind described above, which is characterised in that the second thermal exchange means provided in the unit for producing and storing hot sanitary water is essentially in parallel with the first thermal exchange means provided in the secondary heat exchanger.

[0013] According to the invention, it is possible in this way to heat the sanitary water stored within the production and storage unit without having to necessarily heat also the sanitary water present in the secondary heat exchanger, thus advantageously avoiding the aforementioned drawbacks related to the formation of limestone and scaling in the secondary heat exchanger.

Brief description of the drawings

[0014] Additional features and advantages of the present invention will be more readily apparent from the following detailed description of a preferred embodiment, given by way of illustration and not of limitation with reference to the attached drawing, which shows a simplified hydraulic circuit of the boiler according to the invention.

Detailed description of the preferred embodiment

[0015] In figure 1, a storage boiler of the combined type according to a preferred embodiment of the present invention is generally and schematically indicated at 1.

[0016] The boiler 1 comprises a gas-water heat exchanger 2, preferably including a condensation section, adapted to produce hot primary water for space heating (herein referred to as primary exchanger).

[0017] The primary exchanger 2 is in fluid communication, by means of a first hydraulic circuit of hot primary water globally indicated at 3, with an external plant for space heating (not shown) and with a heat exchanger 4, generally of the plate-and-frame type, adapted to produce hot sanitary water (herein referred to as secondary exchanger).

[0018] In a way known per se, the secondary exchanger 4 is provided with respective thermal exchange means, such as for example a fluid flowpath of hot primary water defined in a plurality of hollow spaces formed in the body of the exchanger, which are intended to heat the cold sanitary water coming from the water mains. To this end, the first thermal exchange means provided in the secondary exchanger 4 is associated to the first hydraulic circuit 3 for the circulation of hot primary water to and from the primary heat exchanger 2.

[0019] Two flowpaths of primary water in parallel with one another and respectively indicated at 3a and 3b are defined in the first hydraulic circuit 3.

[0020] A first flowpath 3a is intended to connect the primary exchanger 2 to the external heating plant and comprises a delivery duct 7 to the primary exchanger 2; an outlet duct 8 from the primary exchanger 2, a first three-way diverter valve 9 and a delivery duct 10 to the external heating plant. The aforementioned delivery ducts 7 and 10 to the primary exchanger 2 and to the external heating plant are connected to respective ducts (not shown) of the heating plant by means of a pair of conventional fittings, not shown in the figure.

[0021] A second flowpath 3b connects the primary exchanger 2 to the secondary exchanger 4 and comprises the delivery duct 7 to the primary exchanger 2, the outlet duct 8 from the primary exchanger 2, the first diverter valve 9, a connecting duct 11, a second three-way diverter valve 12, inlet and, respectively, feeding and delivery ducts 13 and 14 to and from the secondary exchanger 4, and a first return line 15 to the primary exchanger 2.

[0022] The boiler 1 of the invention also comprises a unit 5 for producing and storing hot sanitary water capable to produce and store readily-usable hot sanitary water having a predetermined temperature.

[0023] The unit 5 is essentially constituted by a reservoir 33 having an elongated shape with a substantially vertical extension and is provided with respective thermal exchange means adapted to heat the cold sanitary water coming from the water mains.

[0024] In a preferred embodiment, these thermal exchange means are constituted by a heat exchanger 19, comprising a coil 19a, housed in the lower portion of the reservoir 33 and adapted to carry out a thermal exchange between the hot primary water flowing in the coil and the sanitary water stored within the unit 5.

[0025] According to a preferred embodiment of the present invention, shown in figure 1, the heat exchanger 19 provided in unit 5 for producing and storing hot sanitary water is in fluid communication with the primary exchanger 2, with respect to which it is structurally independent, by means of a second hydraulic circuit 6 for the circulation of hot primary water from and to the primary heat exchanger 2 and extending in parallel to the first hydraulic circuit 3 of hot primary water described above.

[0026] Said second hydraulic circuit 6 connects the primary exchanger 2 to the unit 5 for producing and storing hot water by means of the delivery duct 7 to the primary exchanger 2, of the outlet duct 8 from the primary exchanger 2, of the first diverter valve 9, of the duct 11, of the second diverter valve 12, of a feeding duct 16 to the unit 5 and of a second return line 17 to the primary exchanger 2.

[0027] In a conventional way, the heat exchanger 19 is connected to the ducts 16 and 17 of the second hydraulic circuit 6 of hot primary water by means of respective pipe fittings, not shown.

[0028] The return lines 15 and 17 to the primary exchanger 2 from the secondary exchanger 4 and from the storage unit 5 are connected to the delivery duct 7 to the primary exchanger 2 in a known way, so as to define respective loop-closed flowpaths.

[0029] The circulation of primary water in the parallel hydraulic circuits 3 and 6 is promoted by a circulation pump 18 arranged in the delivery duct 7 upstream of the primary exchanger 2.

[0030] The primary water flow is directed towards the fluid flowpaths 3a and/or 3b of the first hydraulic circuit 3 by means of the first valve 9, and towards the flowpath 3b and/or the second hydraulic circuit 6 by means of the second valve 12. In particular, the first valve 9 intercepts the hot primary water flowing in the outlet duct 8 from the primary exchanger 2 and directs the same towards the delivery duct 10 to the external heating plant and/or to the connecting duct 11. The second valve 12 is arranged downstream of the first valve 9 and intercepts the hot primary water flowing in the connecting duct 11, directing the same towards the feeding duct 13 to the secondary exchanger 4 and/or to the feeding duct 16 to the unit 5 for producing and storing hot water.

[0031] According to a preferred embodiment, the first diverter valve 9 is a three-way mixing valve, i.e. a valve capable of directing the hot primary water coming from the primary exchanger partly towards the external heating plant and partly towards the storage unit 5.

[0032] Thus, in the embodiment shown in figure 1, the sanitary water is heated in the secondary exchanger 4 and in the storage unit 5 by means of thermal exchange with the hot primary water respectively circulating in the first circuit 3 and in the second circuit 6 of hot primary water.

[0033] According to an alternative embodiment, not shown, the heat exchanger 19 provided in the unit 5 for producing and storing hot sanitary water may be in fluid communication with the condensation section of the primary exchanger 2, with respect to which the heat exchanger 19 is structurally independent, the hydraulic circuit 6 being in this case connected to said section in parallel to the first hydraulic circuit 3 of hot primary water described above by means of suitable ducts, fittings and valve means.

[0034] In this embodiment, the sanitary water stored within the reservoir 33 may be heated essentially by the condensation section of the primary exchanger 2, whereas its main section is primarily used for providing heat to the heating plant and/or to the secondary exchanger 4.

[0035] According to a further alternative embodiment, also not shown, the thermal-exchange means provided in the unit 5 for producing and storing hot sanitary water may be constituted by a resistor for heating the sanitary water stored therein, or by any other heating means independent of the hydraulic circuit 3 of primary water and, as such, parallel to the same.

[0036] The sanitary water circulates in a hydraulic circuit for sanitary water, globally indicated at 20, comprising a first and a second section 20a, 20b, extending in parallel between a feeding duct 21 for supplying main sanitary water to the boiler 1 and a duct 22 for delivering hot sanitary water.

[0037] The first section 20a of the hydraulic circuit 20 for the circulation of sanitary water comprises a feeding duct 23 to the secondary exchanger 4, extending between the duct 21 for the fluid connection to the water mains and the secondary exchanger 4, a fluid flowpath on the sanitary water side (not shown) defined in the secondary exchanger 4 and an outlet duct 24 from the secondary exchanger 4, extending between the latter and suitable means for selectively drawing off sanitary water heated by the secondary exchanger 4 and/or by the unit 5 for producing and storing hot sanitary water.

[0038] In a preferred embodiment of the present invention, said means is essentially constituted by a three-way diverter valve 27 mounted upstream of the delivery duct 22 of the heated hot sanitary water.

[0039] The second section 20b of the hydraulic circuit 20 for the circulation of sanitary water comprises a feeding duct 25 to the unit 5, extending between the duct 21 connected to the water mains and the unit 5, in which the duct 25 penetrates down to the lower portion of the reservoir 33 which houses the heat exchanger 19, and an outlet duct 26 from the unit 5 extending between said unit and the three-way diverter valve 27.

[0040] The feeding ducts 23 and 25 to the secondary exchanger 4 and to the storage unit 5 are connected in a conventional way to the duct 21 which feeds sanitary water to the boiler 1.

[0041] In a preferred embodiment of the present invention, the three-way valve 27 is a mixing valve, which is capable of both selectively drawing off the sanitary water heated by the secondary exchanger 4 or by the unit 5 for producing and storing hot sanitary water and of simultaneously drawing off the sanitary water heated by the exchanger 4 and by the unit 5 also suitably mixing the water drawn as a function of the temperature desired by the user.

[0042] In order to ensure a regular operation and the desired temperature regulation of the delivered sanitary water, the boiler 1 also comprises temperature sensors 28, 29 and 30 respectively arranged on the outlet duct 8 from the primary exchanger 2, on the outlet duct 29 from the storage unit 5 and on the duct 22 for delivering hot sanitary water. A thermostat 31 is arranged in the storage unit 5 so as to allow to maintain the hot sanitary water stored therein at the desired temperature.

[0043] In addition, a flow sensor 32 is arranged in the duct 21 which feeds sanitary water to the boiler 1.

[0044] The operation of the boiler of the invention, illustrated with reference to the preferred embodiment of figure 1, is the following.

[0045] In the absence of any drawing-off of hot sanitary water and of any reset of the storage unit 5 and thus, in space-heating mode only, the pump 18 promotes the circulation of primary water in the flowpath 3a of the first hydraulic circuit 3. The primary water circulates from the duct 7 to the duct 8 through the primary exchanger 2, wherein it is heated by the hot combustion gases circulating into the same exchanger 2. In this operating mode, the three-way mixing valve 9 closes the duct 11, thus sending the hot primary water towards the delivery duct 10 to the external heating plant. The temperature of the hot primary water produced by the primary exchanger 2 is detected by a sensor 28, arranged in the outlet duct 8 from the exchanger 2, which regulates the operation of the primary exchanger 2 so as to have the hot primary water at the desired temperature.

[0046] In the reset mode of the storage unit 5, the pump 18 promotes the circulation of primary water in the second hydraulic circuit 6 of hot primary water. In this case, the primary water circulates from the primary exchanger 2 towards the duct 11 by means of the valve 9. The diverter valve 12 closes the feeding duct 13 to the secondary exchanger 4 and directs the hot primary water only towards the feeding duct 16 of the unit 5 for storing hot sanitary water, and thus, into the exchanger 19, wherein it releases part of its thermal energy to the sanitary water stored within the unit 5. Then, the primary water continues towards the return line 17 to the primary exchanger 2 wherein the same is heated again.

[0047] In the absence of any drawing-off of sanitary water, the temperature of the sanitary water stored within the unit 8 is maintained at a constant value thanks to the intervention of thermostat 31.

[0048] The presence of the mixing valve 9 instead of a simple diverter valve makes it possible to carry out the reset of the unit 5 without interrupting the thermal service to the heating plant, but only reducing the same for the time strictly necessary to reset the unit 5. This is particularly advantageous if the heating plant operates with a reduced load factor, that is, only uses a minor fraction of the nominal power of the boiler, whereby an interruption in the heating service may cause an undesired fall of the temperature of the heating bodies (and in the long run, also that of the heated space), with a consequent protraction of the time required for restoring the heating plant to its steady operating temperature.

[0049] Advantageously, in the boiler of the invention the heating of sanitary water does not involve the secondary exchanger 4, thus preventing that the latter could be affected by the problems related to the formation of limestone in still-water conditions.

[0050] In the mode of hot sanitary water drawing-off, the three-way mixing valve 9 closes the delivery duct 10 to the external heating plant and directs the hot primary water towards the duct 11, whereas the valve 12 closes the feeding duct 16 to the unit 5 for storing hot sanitary water and sends the hot primary water only towards the secondary exchanger 4 by means of the duct 13.

[0051] In this case, the hot primary water releases part of its thermal energy to the sanitary water drawn from the water mains through the duct 23, and then continues towards the return line 15 to the primary exchanger 2, wherein it is heated again.

[0052] The selection of a determined flowpath for the sanitary water and for the primary water is controlled in a known way by an electronic control unit of the boiler (not shown) and it occurs, for example, when the flow sensor 32 detects a drawing-off of sanitary water in the duct 21 which feeds sanitary water to the boiler 1.

[0053] The boiler of the present invention advantageously allows to deliver hot sanitary water at the temperature and flow rate desired by the user thanks to the possibility of drawing off from the secondary exchanger 4 and/or from the storage unit 5.

[0054] In fact, when the sensor 30 arranged on the duct 22 for delivering hot sanitary water detects a temperature of the sanitary water lower than the desired one, the mixing valve 27 opens the duct 26 so as to draw off hot water also from the unit 5, capable of promptly providing hot water at high temperature.

[0055] Thus, the boiler of the present invention allows to produce in operation hot sanitary water according to three different operating modes, which may be preselected by the user as a function of the desired flow rate of hot sanitary water:

a) by means of the secondary exchanger 4 only;

b) by means of the secondary exchanger 4 and of the unit 5, giving priority to the secondary exchanger 4;

c) by means of the secondary exchanger 4 and of the unit 5, giving priority to the unit 5.

[0056] Mode a) is especially suitable in periods of scarce use of hot water, such as for example, in the summertime. In this case, the sanitary water stored within the unit 5 is not heated, thus limiting both the formation of limestone inside said unit and the heat losses towards the external environment, with an advantageous increase of the global efficiency of the boiler. In addition, it is possible to regulate the temperature of the delivered hot sanitary water by drawing off a small quantity of cold water from the reservoir 33 by means of the valve 27 driven by the electronic control unit of the boiler 1. Among the other things, this allows a continuous replacement of the water stored within the unit 5, thus reducing the proliferation of potentially dangerous micro-organisms in the stored still water.

[0057] Mode b) is especially suitable in the periods of maximum delivery of hot sanitary water. In this case, the thermal energy and the flow rate of the hot water drawn from the storage unit 5 supplement those of the water leaving the secondary exchanger 4.

[0058] Mode c) is suitable for intermediate situations with respect to the aforementioned cases. In this mode, the hot sanitary water is drawn off only from the storage unit 5, without involving the secondary exchanger 4, al least until the energy content in the storage unit 5 is fully exploited (said energy content is detected by the temperature sensor 29 arranged on the outlet duct 26 from the storage unit 5). Thus, in these conditions it is possible to deliver hot sanitary water without interrupting the thermal service to the heating plant.

[0059] Finally, it is to be added that in the modes of sanitary water delivery in which the storage unit 5 is involved, it is possible to exploit to the fullest extent the heat accumulated in the hot sanitary water stored within the reservoir 33, which may be gradually fully replaced by cold water coming from the mains, thanks to the advantageous effect of water stratification allowed by the extension of the same reservoir in a prevalently vertical direction. Once the water (and heat) has been drawn from unit 5, said cold water can be heated at the predetermined storing temperature in one of the ways described hereinabove.

Claims

1. Storage boiler (1) of the combined type, comprising:

- a secondary heat exchanger (4) provided with respective first thermal exchange means for heating sanitary water supplied to said exchanger;

- a unit (5) for producing and storing hot sanitary water provided with respective second thermal exchange means for heating the sanitary water stored within said unit (5);

- a secondary hydraulic circuit (20) for the circulation of sanitary water, provided with means (27) for selectively drawing off the sanitary water heated by said secondary heat exchanger (4) and/or by said unit (5) for producing and storing hot sanitary water;
wherein said unit (5) for producing and storing hot sanitary water is arranged in said secondary hydraulic circuit (20) in parallel to the secondary heat exchanger (4);
characterised in that the second thermal exchange means provided in the unit (5) for producing and storing hot sanitary water is essentially in parallel to the first thermal exchange means provided in the secondary heat exchanger (4).

2. Boiler (1) according to claim 1, further comprising a primary heat exchanger (2) for producing hot primary water for space heating and wherein the first thermal exchange means provided in the secondary heat exchanger (4) are associated to a first hydraulic circuit (3) for the circulation of hot primary water to and from said primary heat exchanger (2).

3. Boiler (1) according to claim 2, wherein the second thermal exchange means provided in the unit (5) for producing and storing hot sanitary water are associated to a second hydraulic circuit (6) of hot primary water for the circulation of hot primary water to and from said primary heat exchanger (2) and extending in parallel to the first hydraulic circuit (3) of hot primary water.

4. Boiler (1) according to claim 2, wherein said unit (5) for producing and storing hot sanitary water is structurally independent from the primary heat exchanger (2) and said second thermal exchange means comprises a resistor.

5. Boiler (1) according to claim 2, characterised in that it comprises first valve means (9) for intercepting the hot primary water, arranged in said first hydraulic circuit (3) of hot primary water between the primary heat exchanger (2) and the secondary heat exchanger (4) for selectively diverting the hot primary water towards said secondary heat exchanger (4) and/or towards a heating plant external to boiler (1).

6. Boiler (1) according to claim 3, characterised in that it comprises second valve means (12) for intercepting the hot primary water, arranged in said second hydraulic circuit (6) of hot primary water between the primary heat exchanger (2) and the unit (5) for producing and storing hot sanitary water, for selectively diverting the hot primary water towards said secondary heat exchanger (4) and/or towards said unit (5).

7. Boiler (1) according to claims 5 and 6, wherein said second valve means (12) for intercepting the hot primary water is arranged downstream of said first valve means (9) for intercepting the hot primary water, between the first (3) and the second (6) hydraulic circuit of hot primary water.

8. Boiler (1) according to claims 5 and 6, wherein said first (9) and second (12) valve means for intercepting the hot primary water comprise a three-way diverting valve.

9. Boiler (1) according to claim 5, wherein said first valve means (9) for intercepting the hot primary water comprises a three-way mixing valve.

10. Boiler (1) according to claim 2, wherein the primary heat exchanger (2) is a gas-water exchanger comprising a condensation section and wherein the second thermal exchange means provided in the unit (5) for producing and storing hot sanitary water is associated to a second hydraulic circuit (6) of hot primary water for the circulation of hot primary water to and from the condensation section of the primary heat exchanger (2) and extending in parallel to the first hydraulic circuit (3) of hot primary water.

11. Boiler (1) according to claims 3 or 10, wherein the unit (5) for producing and storing hot sanitary water comprises a reservoir (33) and wherein the second thermal exchange means provided in said unit (5) comprises a third heat exchanger (19) in fluid communication with said second circuit (6) of hot primary water.

12. Boiler (1) according to claim 11, wherein the reservoir (33) has an elongated shape with a substantially vertical extension, and said third heat exchanger (19) is housed in the lower portion of said reservoir (33).

13. Boiler (1) according to claim 1, wherein the secondary hydraulic circuit (20) for the circulation of sanitary water comprises:

- a first section (20a) comprising a first duct (23) extending between a pipe fitting for fluid connection to the water mains and said secondary heat exchanger (4), a fluid flowpath on a sanitary water side defined in the secondary heat exchanger (4) and a second hot sanitary water delivery duct (24) extending between the secondary heat exchanger (4) and said means (27) for selectively drawing off the sanitary water heated by said secondary heat exchanger (4) and/or by said unit (5) for producing and storing hot sanitary water,

- a second section (20b) comprising a third duct (25) extending between said pipe fitting for fluid connection to the water mains and the unit (5) for producing and storing hot sanitary water, a storage area defined in said unit (5) and a fourth duct (26) extending between said unit (5) and the means (27) for selectively drawing off the sanitary water heated by the secondary heat exchanger (4) and/or by said unit (5) for producing and storing hot sanitary water.

14. Boiler (1) according to claims 1 or 13, characterised in that the means (27) for selectively drawing off sanitary water from the secondary heat exchanger (4) and/or from said unit (5) for producing and storing hot sanitary water comprises a three-way mixing valve.

Drawing