WO2025052159A1

WO2025052159A1 - Electric water heater

Info

Publication number: WO2025052159A1
Application number: PCT/IB2023/058774
Authority: WO
Inventors: Giovanni Landro; Amedeo Roncallo
Original assignee: Irc Srl
Current assignee: Irc Srl
Priority date: 2023-09-05
Filing date: 2023-09-05
Publication date: 2025-03-13
Anticipated expiration: 2026-03-05

Abstract

The present invention concerns an electric storage water heater preferably, but not necessarily, for domestic use and characterised by two pre-heating (100) and maintaining and delivery (101) containers connected to each other by a suitable solenoid valve. Said first container (100) being characterised by a smaller volume V0 and used to rapidly heat hot water with which to progressively fill said second container (101) characterised by a larger volume V1. Further, the above-mentioned containers (100) and (101 ) are respectively characterised by a bundle or plurality of heating resistors (R01, R02....R0n) where n≥2 and (R11, R12....R1n) where n≥1. This configuration is particularly effective for the rapid and homogeneous heating of water. In particular, this distribution is particularly effective within said first pre-heating container (101), since a plurality of heating resistors that can be activated separately or in sequence makes it possible to avoid all those problems attributable to the low thermal conductivity of the water and to speed up the heating of the water, further facilitating its rapid transfer to the second storage and delivery container (101). This second container (101) can thus deliver water at a pre-set temperature since it is not subject, unlike a traditional water heater, to compensatory and unfavourable additions of cold water from the water mains.

Description

Electric water heater

Technical field

This invention concerns an improved and cost-effective system for domestic hot water production.

More precisely, the proposed system relates to an electric water heater preferably but not necessarily for domestic or recreational use (e.g. for boats, campers and the like). This electric water heater is particularly cost-effective in terms of energy efficiency, space-saving for the same amount of domestic hot water delivered, and further improving delivery speed and flow stability.

Background Art

Devices known as water heaters are sometimes used alternatively and separately from boilers for heating and domestic hot water production.

These water heaters can be of different types: gas, electric, instantaneous, storage, open chamber, sealed chamber, biofuel and heat pump. Furthermore, these devices can be distinguished between instantaneous water heaters in which domestic hot water is produced instantaneously at the moment it is required and storage water heaters, i.e. characterised by a tank and which need to pre-heat the water inside before delivering it to fixtures.

Electric storage water heaters are decidedly outdated devices that for many years have maintained a considerable popularity, especially in those contexts where it is not possible or convenient to install a gas water heater or, more generally, an instantaneous generation water heater. This spread of electric storage water heaters continues today, however, in step with the progressive electrification that is affecting many aspects of contemporary society from mobility to transport to air conditioning, etc. Electric water heaters typically consist of a tank into which cold domestic water is fed and then heated and maintained at a constant temperature, typically between thirty-five and sixty degrees. This type of product is therefore based on the so-called Joule effect: a resistor in the boiler generates heat thanks to the power supply. The generated heat heats the water inside the tank and the water temperature can be regulated by a thermostat used to keep it constant. These devices are characterised by their low average purchase costs and, as they do not provide instantaneous heating, they require very low power, making them perfectly suitable for other areas in addition to the typical domestic application, including use in boats, campers, caravans, housing modules, mobile stations, etc. Due to their popularity and notoriety, electric storage water heaters have, over time, been the subject of numerous patents and in particular patents aimed at reducing their energy consumption. These include patents GB 2 508 865 A (Reeves Jonathan David [GB]) 18 June 2014 (2014-06-18) and WO 2009/009804 A2 (Erasmus Maria Christina Gertru [ZA] ; Erasmus Teuns [ZA] ). A further interesting approach is provided by patent IT102010901873498 (Improved water and energy-saving device for electric water heaters in the name of Giuseppe MOSCATO;

20 September 2010); this application having subsequently been converted into the utility model 202013902201879. This patent was an evolution of an earlier patent by the same author with the number W01993IT00142. According to these inventions, a conventional electric water heater is equipped with an 'economiser' system, which regulates the cold water supply to the water heater tank in order to achieve the following main advantages:

1 ) Adding less water than the total capacity of the tank, thus allowing a limited volume of liquid to be heated, with the advantage of having at least an initial amount of hot water available in a shorter time;

2) Closing the cold water supply during the delivery of domestic hot water to the fixtures, avoiding mixing with cold water supplies, i.e. allowing the full use of the hot water available in the storage tank.

Although these basic concepts and the methodological approach are interesting, these inventions are not adequately exploitable in practice because:

• the volume of the boiler remains unchanged, thus being under-utilised and disadvantageous in terms of space occupied for the same amount of hot water produced; • added water quantity control, obtained by a control unit, implies an input of the quantity of water regulated by the user and automated only on the basis of projections and forecasts of the expected average consumption.

In addition, it must be ensured that the water input is such that the storage level is always above the heating resistor in order to avoid dangerous overheating.

In addition to the aforementioned inconveniences, reasonable doubts also arise as to the dynamic functioning of such a system as illustrated in the said patent and the attached Figs. 2,3,4: According to Fig.2, i.e. in normal operating condition of the water heater, duct B fills with water. Subsequently, and as shown in Fig.3, valve X opens, allowing cold water to enter the boiler. Now if water enters, as in a traditional boiler, above the highest point of duct B, that duct (which was already full of water previously) will remain reasonably full of water. This is the heating condition for the volume of water you have decided to add. Additionally, this system does not prevent the subsequent mixing of hot and cold water. For example, if one decides to let in only 20 litres and then later wants to let in another 10, the latter would mix to the detriment of the intended economiser function.

At some point, the fixtures require hot water and go to Fig.4. Valve X in this circumstance is turned and all water contained in pipe B flows out of the system. Once the tank has been emptied, valve X is closed again, thus returning to the previous conditions of Fig.2 or Fig.3. Inside the tank, however, there is air, which is not vented except in the traditional way provided by all boilers, i.e. through the hot supply once the tap is turned on: hot water comes out for a while with its noises and cessations.

Electric water heaters with storage are subject to a number of inconveniences compared to instantaneous generation water heaters, some of which are understandably inherent to the very structure and mode of use of these appliances, in particular: • a typically increasing footprint as the number of expected potential users increases; this footprint depends mainly on the increased capacity of the storage tank as a function of the required production autonomy;

• a water pre-heating time before the first delivery or alternatively the delivery after emptying; this time being proportional to the above-mentioned capacity; • limited but virtually constant energy consumption to keep the water in the tank at the desired temperature;

• an inconstant delivery in terms of temperature as the tank is emptied with the need for users to correct and adjust the mixing in order to maintain a constant and pleasant domestic water temperature.

These certainly diverse inconveniences in terms of their effects are, however, related to each other, in fact:

• a large tank capacity makes it possible to satisfy a greater number of users and provide (at least in an initial phase) a reasonably stable delivery of domestic hot water at temperature; this high capacity, however, considerably penalises both space requirements and pre-heating time;

• On the contrary, a reduced tank capacity allows a rapid pre-heating of an adequate amount of water to satisfy at least an initial delivery (e.g. a shower) and further reduces the footprint of the device, but this is at the expense of a reduced autonomy and an inconstant delivery, i.e. characterised by a rapidly decreasing flow temperature due to the compensatory inflow of cold water.

In this context, it can be observed that the aforementioned construction choices (large or small tank with associated inconvenience) are both influenced and affected by the progressive emptying and filling of the tank. Typically, an electric water heater with storage is in fact connected - in direct connection and at mains pressure - between a cold water line and at least one or more plumbing fixtures. In this way, in the ordinary operation of the water heater, every single hot water delivery will be automatically compensated for by the addition into the storage tank of an equivalent amount of cold water from the water mains.

This mechanism makes it possible to keep the water heater constantly at full load, but at the same time causes the volume of water it contains to cool down as a result of the mixing of the cold water supplied and the pre-heated hot water already present. Now, the incidence of these cold water additions is obviously minimal and almost imperceptible in the case of small withdrawals, and in these cases the small gradient is not even perceived by the user as the water tends to thermally stratify inside the boiler. In this way, the limited cold contribution does not, at least initially, mix with the preponderant hot portion and the water heater continues to deliver domestic hot water at the required temperature. Even after mixing, the modest thermal gradient is then detected by the thermostat and compensated for by briefly switching on the heating resistor, which quickly brings the contained volume back to the required temperature. In the case of small, repeated withdrawals, this repetition of heating cycles at every even minimal consumption of hot water is obviously disadvantageous in energy terms, but the inconvenience connected to the continuous supply of cold water from the water mains is much more significant in the case of large withdrawals (typically for a large number of users). In this situation, the mixing of cold inputs with the pre-existing hot ones causes a rapid temperature variation that the heating resistor, even if activated, cannot compensate for in a short time (unless resistors of considerable power are installed that are obviously not adequate for the purpose and above all do not perform well in terms of efficiency and thermal transmissibility). This leads to a progressive cooling of the overall volume and results in an undesirable and rapid drop in temperature of the delivered water, which forces the user to compensate by annoying progressive mixing in order to maintain an adequate domestic hot water temperature. Even more seriously, the progressive and rapid emptying of the tank, which cannot be promptly compensated for by the heating resistor, causes a kind of waste in terms of adequate use of the hot water originally produced and stored at the initial temperature. In fact, a portion of this initial energy - originally produced at a high temperature - is eventually progressively 'wasted' in dilution to only partially and inadequately heat the water remaining in the boiler when the final temperature falls below the useful value to ensure adequate comfort for users. In this condition, the boiler is in fact filled with water that is certainly warmer than the mains water but at a temperature not sufficient for immediate use. Moreover, being at full load, it takes a considerable time to bring the entire water mass back to an acceptable comfortable temperature. Finally, it should be noted that if the same quantity of water originally produced by the boiler were not mixed progressively with cold, this quantity would reasonably be consumed more efficiently, being mixed only by the user at their leisure and, above all, consumed at least until it is completely finished without any loss of performance and comfort. Disclosure of invention

This patent intends to overcome the aforementioned criticalities by implementing a new electric storage water heater that enables the production and delivery of an equal amount of domestic hot water that has been pre-heated to the required temperature and, in particular, enables the full delivery of this domestic hot water through a constant temperature flow.

A further objective of the proposed patent is to build an electric water heater with storage characterised by a reduced footprint for the same amount of hot water produced and delivered, by this definition meaning a regular flow of water at a temperature above a predefined comfort threshold.

A further objective of the proposed patent is to build a water heater which, if necessary, allows partial emptying of the storage tank and which regulates, possibly limiting, the supply of cold water during the loading and unloading phases in those situations where this supply may be penalising in terms of energy yield and comfort (wait times, variable temperature flow, etc.).

A further objective of the proposed patent is to build an electric storage water heater which, with the same maximum electrical power input, allows the progressive heating of increasing portions of domestic hot water and, in particular, which makes available in a short time at least an initial quantity of hot water necessary for the expected average satisfaction of a user at the required temperature.

A further objective of the proposed patent is to build an electric storage water heater which, with the same maximum installed electrical power and after the tank has been emptied, can more quickly heat a quantity of domestic hot water useful for satisfying at least one user at the required temperature. A further aim of the proposed patent is to build an electric water heater with storage that allows the heating processes to be managed separately and more efficiently than those of maintaining the temperature of the domestic hot water produced and, in particular, facilitates and optimises the transmissibility of heat to water by overcoming the known limits due to the low thermal conductivity of this element. The above-mentioned objectives and other purposes, which will become clearer below, are achieved by the electric storage water heater according to the attached claims. The aforementioned objectives and other purposes are also achieved by a procedure for the use of said electric water heater according to the attached method claims. In an advantageous implementation, according to the invention, the electric water heater comprises a tank divided into two sections connected to each other by a suitable solenoid valve. The first section is connected to the water mains (cold water) through a solenoid valve that controls the supply and forms the so-called pre-heating volume. This section is reasonably characterised by a smaller volume and a bundle or plurality of heating resistors that replace the classic heating resistor of high specific power used in traditional water heaters. This configuration is particularly effective for the rapid and homogenous heating of small portions of water that are subsequently and progressively fed into the second section through the aforementioned connection valve. In particular, the distribution of a plurality of separately activatable heating resistors in this reduced volume makes it possible to avoid all those problems attributable to the low thermal conductivity of water and to speed up and even standardise the heating of a small quantity of water by facilitating its rapid transfer to a second tank section connected in cascade. This second section of the tank constitutes the so-called maintaining and delivery volume, i.e. that portion of the tank dedicated to the storage and delivery to the fixtures of domestic hot water produced at the required temperature. This second section is reasonably characterised by a larger volume and by one or more low-power heating resistors dedicated solely to maintaining the water at the set temperature since it is not subject, unlike a traditional water heater, to compensatory and unfavourable cold water supplies from the water mains. According to the proposed system, this maintaining and delivery volume is then progressively filled by successive contributions of hot water produced at the required temperature in the pre-heating volume. The same maintaining and delivery volume in its current use is progressively emptied by delivering domestic hot water at a constant temperature to the fixtures; this delivered water being compensated in volume with an equivalent amount of air entering through a vent and does not cause a significant thermal gradient (which could in any case be compensated with very little energy by the low power heating resistors present). In an advantageous implementation of the proposed invention, the aforementioned dynamics of production, maintenance and delivery to the fixtures as well as the coordination between the two foreseen heating and maintenance-delivery volumes is automated and controlled by temperature sensors, level sensors, solenoid valves and pumps; these devices being appropriately controlled by a control unit. On the basis of the operating states and the data collected by the various sensors, this control unit drives the actuators involved in the heating and delivery process, i.e. the aforementioned heating resistors (heating resistor bundle/series located in the pre-heating volume and traditional maintaining resistor contained in the maintaining volume). In particular, the control unit plays a crucial role in optimising heat exchange and facilitating the rapid and homogeneous heating of domestic water added into the pre-heating volume. In fact, the control unit drives and appropriately coordinates a plurality (at least two) of electric heating resistors equally spaced out in a reticular manner inside said pre-heating volume. This arrangement not only (and predictably) makes it possible to increase the exchange surface area and general transmissibility at the same power output, but further enables the typical problem of uneven and localised heating in the vicinity of the heating resistor that characterises traditional water heaters to be overcome by an alternating activation system controlled by said control unit. In fact, it is well known that the low thermal conductivity of water typically determines a strong rise in temperature of only the portion of water located near the heating resistors, thus decreasing the transferability of the heat supplied locally and slowing down the overall process, since it is necessary to wait for this heat concentrated near the traditional heating resistor to spread / propagate progressively to the adjacent portions of water. In order to overcome this problem and increase the temperature differential between the heating resistor and the surrounding water, the heating resistors in the pre-heating and maintaining volume, according to the proposed invention, are at least two or more in number, each heating resistor being sized at the maximum rating plate value. These heating resistors, suitably spaced and distributed in the pre-heating volume, are then alternately switched on/off by the control unit so that, with the same maximum installed power, the generated heat is distributed volumetrically more evenly. Additionally, this heat is not only distributed in an obviously improved manner, but also delivered by the heating resistors more efficiently and quickly, since alternating the heating resistors prevents the aforementioned excessive overheating of the water in the vicinity of the heating resistors due to low thermal conductivity. The alternating use between several resistors allows the unused resistor(s) to dissipate heat during the period of non-use and thus reduce the differential with the surrounding water. This allows it to be used more effectively and advantageously in the next activation cycle.

Thanks to the aforementioned automation and control system, the dual heating and storage tank structure and the effective use of several heating resistors in an alternating manner, the electric water heater according to the invention thus enables:

• Delivering water at a constant temperature to fixtures through the storage and delivery tank;

• Benefiting quickly from at least a portion of the hot water produced by the pre-heating volume; this portion is particularly useful as it considerably reduces the waiting time for the first withdrawal and the time required to restore at least a first deliverable quantity in the event of a complete emptying of the storage tank.

• Obtaining advantages in terms of system performance during both water heating and maintenance. Additionally, delivering water at a constant outlet temperature also reduces water wastage by the user who does not have to change the hot/cold water mix.

Brief description of drawings

Further features and advantages of the proposed technical solution will be more evident in the following description of a preferred, but not exclusive, form of implementation, represented in the 3 attached drawing plates where: • Fig.1 illustrates the general scheme of the proposed solution with evidence of the main constituent elements;

• Fig.2 illustrates the loading phase of cold water from the mains into the pre-heating volume; • Fig. 3 illustrates the discharge phase of heated water from the pre-heating volume to the maintaining and delivery volume;

• Fig.4 illustrates the constant-temperature delivery of domestic hot water to fixtures;

• Fig.5 illustrates the back up and filling phase in case of early emptying of the maintaining volume;

• Fig.6 illustrates the maintaining condition with the system at full load.

It should be noted that the figures attached to this application are one of the possible forms of implementation of the system, in order to better understand its advantages and described characteristics. These forms of execution are therefore to be understood as purely illustrative and not restrictive of the inventive concept, i.e. building an electric water heater with storage that overcomes the problems referred to in the known art, in particular by enabling the delivery of domestic hot water at a constant temperature, reducing the size of the system and the wait time for the use of said hot water.

Best mode for carrying out the invention

With reference to the enclosed tables of drawings and particularly Fig.1, an example of the electric storage water heater according to the proposed invention is illustrated in schematic form. In particular, the structure of the water heater is shown, which consists of the following main elements:

• a first container (100) having a capacity VO; said first container VO being used as a pre-heating volume and connected to the water network by a first solenoid valve Ev1 ; said first container being provided with a volumetrically distributed plurality of heating resistors R01, R02....R0n (where n>2) of a thermostat TO, of a vent to allow the entry and exit of air and further being provided with two level switches LOA, LOB used to detect the filling (LOA) and emptying (LOB) conditions of the container itself; • a second container (101) having a capacity V1>V0; said second container (101) being used as a maintaining and delivery volume and being connected to said first container

(100) by a second solenoid valve Ev2 and being, as anticipated, characterised by a capacity V1 greater than said first volume V1 ; said second container (101 ) being used for maintaining in temperature and delivering to users the domestic hot water produced and cyclically received by said first container (100). Said second container

(101) being equipped with a second heating resistor R1 (or alternatively by a series of heating resistors R11, R12....R1n) of lower power than the aforementioned R01, R02....R0n and, said second container (101) being further provided with a thermostat T1 , with a vent to allow the entry and exit of air and provided with two level switches L1A, L1B used to detect the filling (L1A) and emptying (L1B) conditions of said container;

• a P1 pump preferably operating with a pressure switch; this P1 pump being used to convey the domestic hot water stored in the second container (101), i.e. the storage and delivery volume, to the fixtures;

• a CU control unit connected to the aforementioned TO and T1 thermostats, Ev1 and Ev2 solenoid valves, LOA, LOB, L1A, L1B level switches, P1 pump; R01 ,R02,....,R0n resistors and R11, R12.R12 resistors.... R1n ; this CU control unit being used to coordinate the loading, heating, temperature maintenance and delivery phases of domestic hot water produced by the electric water heater.

With reference to the attached drawing tables and particularly Fig.2, the initial heating phase controlled by the CU controller is illustrated. During this phase, mains water is fed into the pre-heating tank through the EV1 solenoid valve. Heating takes place by individually or alternately activating the heating resistors R01 ,R02,....,R0n. During the ignition phase, the delta t with the surrounding water will in fact be maximum. As soon as the temperature of the water surrounding a resistor (e.g. R01) approaches that of the resistor itself, the control unit will deactivate it and activate another resistor (e.g. R02) characterised by a better temperature differential. In this way, the first resistor used will stop heating and allow the heat in the surrounding water to spread to the surrounding areas, while the maximum power will be delivered by another resistor with a better thermal differential to the surrounding water. Repeating this cycle produces a significantly faster and more effective heating of the domestic hot water volume contained in the two pre-heating volumes VO and maintenance volume V1.

With reference to the attached drawing tables and particularly Fig.3, the transfer of domestic hot water from the pre-heating volume VO to the maintaining volume V1 is illustrated. This phase takes place when the hot water contained in the pre-heating volume reaches the desired temperature and is carried out by the EV1 solenoid valve, which is controlled by the control unit so that the domestic hot water flows into the V1 maintaining volume. This makes it possible to store domestic hot water in this volume and at the same time to feed new water for heating from the water mains back into the pre-heating volume VO. This transfer operation is, of course, facilitated by the vents that compensate for inlet and outlet flows. These heating and transfer phases are of course repeated until the maintaining and delivery volume is completely filled. With reference to the attached drawing tables and particularly Fig.4, an example of delivery to fixtures is illustrated. In this condition, the domestic hot water delivered from the storage volume is only compensated by the pre-heating volume if it is already at the desired temperature. If it is not yet at temperature, the solenoid valve EV1 interrupts the connection between the two containers (100) and (101), preventing cold or otherwise not yet at temperature water from flowing down into the storage and delivery volume, causing a negative temperature gradient. In this situation, the delivered water is then temporarily compensated by air (through the vent). The maintaining and delivery volume is then progressively emptied while at the same time the pre-heating volume is activated (via the heating resistors R01, R02, ....,R0n) to quickly restore its quantity of water and only make it available/transfer it when the required delivery temperature is reached. With reference to the attached drawing tables and particularly Fig.5, an example of delivery to fixtures characterised by high demand is illustrated. In this case, the system requires the second storage and delivery container (101) to be almost emptied without being adequately compensated by added water. In this case, the controller-driven system ensures that the water contained in the maintaining and delivery volume V1 is not compensated for by inadequate additions while the pre-heating tank (100) is used to replenish an adequate amount of water to the required temperature as quickly as possible.

With reference to the accompanying drawing tables and particularly Fig.6, on the other hand, a situation of delivery to fixtures characterised by little or no demand is illustrated, Under these conditions, both volumes VO and V1 are characterised by being at full load with domestic hot water at the desired temperature. Under these conditions, the system cyclically uses the maintaining resistors R11, R12....R1 n to compensate for any heat loss generated in the maintaining and delivery tank. These losses are compensated for with little energy expenditure as the power, dynamics and timing involved in this process do not require the same solutions applied in the heating phase where it is necessary to heat very quickly and efficiently.

Industrial applicability

The advantages of the invention according to the proposed invention patent are many and obvious, both in terms of efficiency and in terms of improving the electric water heaters currently used for domestic hot water production. The proposed solution makes it possible to build a water heater that is more efficient from an energy point of view, faster and more constant from the point of view of domestic hot water flow delivery, almost half the size, and, finally, particularly suitable for quickly restoring an initial quantity of water to the desired temperature from conditions of emptying due to excessive demand and large delivery flows. While the invention is susceptible to various modifications and alternative constructions, some preferred forms of construction have been shown in the drawings and described in detail in the previously illustrated example. It should be understood, however, that there is no intention to limit the invention to the specific form of construction illustrated, but, on the contrary, it is intended to cover all modifications, alternative constructions, and equivalents that fall within the scope of the invention as defined in the annexed claims. The use of "for example", "etc.", "or" indicates non-exclusive alternatives without limitation unless otherwise indicated. The use of 'includes' means 'includes, but not limited to' unless otherwise indicated.

In particular, the invention may be built with technical equivalents, materials or supplementary devices suitable for the purpose and scope of application.

Conformation and sizing of parts and manufactured products may duly vary, but remain consistent with the proposed solution.

By way of example and not limited to, it should be noted that possible modifications to contain and heat fluids other than hot water for sanitary purposes by the proposed invention may be included. Furthermore, the two containers (100) and (101) can be constructively integrated and, in particular, the lower-volume container (101) can be incorporated inside the container (100) while retaining the same operating system and connections. It should also be noted that the device and the technique employed lend themselves to any liquid without loss of performance, resulting, as mentioned above, particularly convenient in those contexts where the medium to be heated is characterised by low thermal conductivity. In fact, the proposed invention can be easily adapted with minor structural and technical changes to obtain an industrially applicable boiler for various product and application areas.

That said, the necessary modifications to the system including adjustments and sizing appropriate to the individual installation will be easily deduced by a suitably educated technician in the field without going outside the scope of protection of the claimed patent. This without invalidating or circumventing the inventive core of the invention presented in the example. Additionally, the system can be adapted and further equipped with systems and accessories that are commonly used in the application field and are also available to a specialised technician.

Claims

1. Energy-efficient electric storage water heater comprising:

• a first pre-heating container (100) having capacity VO; said first container (100) being connected to the water mains by a first solenoid valve (EvO); said first container being provided with a volumetrically distributed and spaced out plurality of high power heating resistors (R01, R02....R0n) wherein n>2; said container (101) being further provided with at least one thermostat (TO), at least one air vent and at least two level switches (LOA), (LOB) used to detect the filling and emptying conditions of said first pre-heating container (100);

• a second maintaining and delivery container (101) having a capacity V1>V0; said second container (101) being connected to said first pre-heating container (100) by a second solenoid valve (Ev1) and being provided with a volumetrically distributed plurality of low power heating resistors (R11, R12....R1 n) wherein n>1; said second maintaining and delivery container (101) being further provided with at least one thermostat (T1), at least one air vent and at least two level switches L1 A, L1 B used to detect the filling and emptying conditions of said maintaining and delivery container (101);

• a pump P1 connected to said second maintaining and delivery container (101); this pump being used to convey water to the fixtures;

• a control unit (CU) connected to said thermostats (TO) and (T1); to said solenoid valves (EvO) and (Ev); to said level switches (LOA), (LOB), (L1A), (L1B); to said pump (P1); to said high-power heating resistors (R01 ,R02,....,R0n) and to said heating resistors (R11, R12,R12....R1 n); said control unit (CU) being used to progressively heat a volume V0 of water introduced through said solenoid valve (EVO) into said pre-heating container (100) and, upon reaching a temperature set with said thermostat (TO), to transfer said water through said solenoid valve (Ev1) to said maintaining and delivery container (101); said progressive heating process being repeated until reaching the capacity V1 of said second maintaining and delivery container (101).

2. Electric water heater according to claim 1) wherein said control unit (CU) interrupts, by the solenoid valve (EV1), the flow between said first pre-heating container (100) and said second maintaining and delivery container (101) when the pump (P1) is activated to deliver water to the fixtures and the temperature in said first pre-heating container (100) has not yet reached the temperature set by the thermostat (TO).

3. Electric water heater according to claim 1) wherein said control unit (CU) during the process of heating the container (100) sequentially and selectively activates at least one of said high-power heating resistors (R01,R02,....,R0n); said at least one selected heating resistor being characterised by the maximum thermal gradient with respect to the surrounding water; said sequential activation being repeated until the temperature (To) is reached.

4. Electric water heater according to claim 1) wherein said control unit (CU) during the heating process of said second maintaining and delivery container (101 ) sequentially and selectively activates at least one of said low-power maintaining resistors (R11, R12,....R1n); said at least one selected resistor being characterised by the maximum thermal gradient with respect to the surrounding water; said sequential activation being repeated until the temperature T1 is reached.

5. Electric water heater according to claim 1) wherein said first pre-heating container (100) is incorporated within said second maintaining and delivery container (101).