An electric boiler

Abstract

An electric boiler (1, fig. 1) having a water inlet 4 and water outlet 5 connected by plural interconnected passages 23, 24, 25 containing a first heating element 10F operating at a first voltage and a second heating element 9 operating at a second voltage lower than the first voltage. The second element may be situated downstream of the first element. The first voltage may be mains voltage and the second voltage may be that of a battery, permitting a local power supply to supplement the mains supply at times of high flow rates, without the need for an inverter. The boiler may comprise an inner container surrounding the second element in a first passage 23, and an outer container surrounding the inner container and containing the first element in a second passage 24C, 24F. Wherein the outlet of the outer container 22 connects to the inlet of the inner container and the two containers share a thermally conductive wall and flow through each passage is a counter direction to that of the other. A sanitary water heating system is also claimed.

Description

The present invention relates to an electric boiler and particularly, but not exclusively, to an electric boiler suitable for heating sanitary water in domestic or commercial premises. The invention also relates to a sanitary water heating system comprising such a boiler. Generally, electric boilers have in the past tended to be predominantly used for single point of supply applications, for example electric showers, hot water supplies for single (or local) wash hand basins or similar, where it is not desired to install a traditional central emersion heater or fossil fuel boiler. This could be to avoid the expense and possible disruption associated with installing larger heating systems, or where there is a desire to ensure a reliable instantaneous supply of hot water. However, more recently electric boilers are also being used more commonly in place of the more traditional fossil fuel boiler, where they centrally provide hot water to a number of sanitary outlets and they may also form the boiler of a central heating system. These new applications for electric boilers may require more powerful boilers than those traditionally used in the examples mentioned above. In addition, for efficiency they need to be very compact, so that they have very little inertia and may thus heat the water to the desired temperature almost instantaneously. This will also minimise the heat energy remaining in the boiler once the water has stopped being drawn. This is important because such a boiler will normally be operated by a flow switch, so there is not normally an opportunity to terminate the supply of electricity to the heating element, or elements, prior to the water flow being terminated, to draw the remaining energy out of the boiler. It may also be problematic if the heating element, or elements, themselves retain large quantities of heat energy, because this could cause water in the boiler to boil after the flow through the boiler has stopped and the heating elements have been turned off. The electric boiler disclosed in patent application, publication number GB2592026A1 addresses the above problems. This provides an electric boiler where a plurality of heating elements are contained in separate flow passages, each flow passage containing a relatively small volume of water and being designed to force the water to be heated to flow at relatively high flow rates directly over the outer surface of the heating elements, minimising any boundary effect on the flow at the surfaces of the heating elements. The heating elements and passages are arranged in a coaxial configuration, such that water flowing through the boiler passes along the length of the boiler two or more times and in opposite directions, drawing water to be heated inwardly after each pass, such that the hottest water, ready to flow out of the boiler, is at the central core of the boiler with heat being conducted from the central core through the walls of an inner passage before being absorbed by the lower temperature water in the outer passage or passages. With the above disclosed boiler, there is very little inertia within the boiler, due to the relatively low volume of water stored within the boiler. Thus the boiler may provide a supply of hot water heater almost immediately on commencement of flow through the boiler. This not only has the benefit of being able to quickly provide a source of hot water on demand, without the need to have a cylinder of preheated hot water, but it may also minimise the residual energy stored within the boiler after hot water has been drawn from the boiler. The above described boiler has demonstrated that small compact boilers are feasible to replace more traditional fossil fuel fired boilers, heating relatively high flow rates of water to relatively high temperatures, but their application may be limited by the electrical power available to the boiler, particularly in domestic properties and some commercial properties in some countries, where for example in Germany only a 6kW supply may be available to a boiler, or in Italy where only a 3kW supply may be available to a boiler in some properties. It is an object of the present invention to provide a more versatile compact arrangement of electric boiler which is capable of providing an instantaneous hot water supply, suitable for a sanitary hot water supply or as a combination boiler, for the supply of both sanitary hot water and for heating the recirculating water of central heating system. According to a first aspect of the present invention there is provided an electric boiler having an inlet, an outlet and a plurality of interconnected passages for conveying water to be heated from the inlet to the outlet, the passages containing at least one first heating element arranged to operate at a first voltage and at least one second heating element arranged to operate at a second voltage lower than the first voltage. An advantage of the boiler of the above type is that it enables the boiler to operate from different power sources and preferably the at least one first heating element is arranged to be connected to a mains supply and the at least one second heating element is arranged to be connected to a battery. A significant advantage of this is that it enables power stored locally in a battery to be used to supplement the mains supply, if the mains supply is not sufficient to provide a desired flow rate of water such as for a sanitary hot water supply at a desired temperature. For example, a slow rate of supply may be acceptable when filling a hand basin, but this may not be acceptable when filling a bath or when the supply is used for a shower. A further significant advantage of having the at least one second heating element arranged to be connected to a battery, or to a supply which is at a lower voltage than the mains supply, is that it permits a local source of energy, typically energy stored in a battery, to be used to supplement the mains supply without the need to first convert that stored energy or lower voltage supply, which could for example be derived from photovoltaic (PV) cells of a solar array, to mains voltage by use of an inverter and this thus avoids the losses associated with use of an inverter. The invention also enables a property having a source of low voltage electricity, derived for example from a PV solar array, to conveniently use that electricity to supply a regularly used appliance (the boiler of the invention), which has a high demand for electrical power to provide sanitary hot water, while permitting the demand on the local supply to be limited, so as not to exceed the maximum power available from the local supply, such as a local battery, the boiler relying on the mains supply together with the battery to provide the energy required by the boiler. Another advantage of a boiler in accordance with the present invention is that it enables locally sourced renewable energy to be used in combination with electrical energy obtained from the grid, without islanding problems associated with separating locally sourced renewable energy from the grid. Preferably, the at least one first heating element is arranged to be connected to an alternating current supply with a voltage of between 220 Volts and 240 Volts or between 110 Volts and 120 Volts. These first heating elements may thus be selected to operate at between 220 and 240 Volts, the mains voltage available in many countries including most of Europe, or between 110 Volts and 120 Volts, the mains voltage in some other countries such as the US. The at least one second heating element may preferably be arranged to be connected to a battery having a voltage of between 50 Volts and 150 Volts. This is a common domestic battery voltage used for storing energy derived from PV cells of a solar array. Thus, electrical energy can be directly received by the battery from PV cells of a solar array and stored in the battery, with the battery then being directly connected to the second heating elements, without the need for any conversion in voltage. Preferably, the at least one second heating element is located within the plurality of passages downstream of the at least one first heating element, with the boiler comprising a thermally conductive inner container containing the at least one second heating element, the inner container substantially surrounding the at least one second heating element to define a first passage in which the at least one second heating element is located, the inner container having at least one inlet and an outlet, the outlet fluidly connected to the outlet of the boiler and being arranged such as to cause water received at the at least one inlet of the inner container to flow along the first passage, in close proximity to a surface of the at least one second heating element, to the inner container outlet, the boiler further comprising one or more outer containers within which the inner container is substantially located, the one or more outer containers defining a second passage extending around at least part of the inner container, the second passage containing the at least one first heating element and having at least one inlet and at least one outlet for a flow of water, wherein the at least one outlet of the one or more outer containers is connected to, or forms, the at least one inlet of the inner container and wherein the one or more outer containers are arranged such as to cause water received at the at least one inlet of the outer container to flow along the second passage, in close proximity to a surface of the at least one first heating element to the outer container outlet. By having an inner container containing the at least one second heating element, the inner container can be arranged to concentrate the flow of water over the surface of the heating element or elements, providing only a small clearance between a surface of the container and a surface of the heating element and thus a low volume space through which water is forced to flow at high flow rates, providing a high heating surface area to volume ratio. The same applies in respect of the at least one first heating element arranged in the second passage and an advantage of this is that the boiler can be designed to have a low inertia, due to the relatively low volume of water stored within the boiler. Thus the boiler may function as an almost instantaneous source of hot water heater, at least at the point where the water leaves the boiler. This not only has the benefit of being able to quickly provide a source of hot water, without the need to have a cylinder of preheated hot water, but it may also minimise the residual energy stored within the boiler after hot water has been drawn from the boiler. Another advantage of this arrangement is that, because the water flows from the first passage containing the at least one first heating element to the second passage containing the at least one second heating element, the water in the second passage, closer to the outlet will be of a higher temperature when the at least one second heating element is switched on, but any thermal energy leaving the higher temperature second passage (inner container) will be transferred to the water in the second passage (the at least one outer container) and thus the thermal energy will, initially, be retained within the system. It is preferable that the inner container and the at least one outer container share a thermally conductive wall, for then if when the flow of water has stopped and the at least one second heating element has been switched off, the internal temperature and residual energy within the at least one second heating element would be sufficient to raise the temperature of the very limited volume of water within the inner container to boiling point, the boiler may be designed to permit sufficient energy to pass through the thermally conductive wall to prevent the water in the inner container from boiling. Preferably, the first and second passages are arranged such that, in use, water in the second passage progresses along the second passage in a direction opposite to the direction in which water progresses along the first passage. The above arrangement may provide a particularly compact arrangement of boiler that is relatively inexpensive to construct. Advantageously, the boiler comprises a plurality of outer containers each in the form of a cylinder and each housing a respective first heating element, the plurality of outer containers being arranged side by side in a cylindrical pattern and connected to each other to form in their centre the inner container and the first passage, in which a second heating element is located coaxially within the plurality of first heating elements. This arrangement permits a plurality of first heating element to each be housed in a respective outer container, with each outer container arranged to minimise the volume of water stored within the container, while maximising the flow of that water across the heating surface of its respective first heating element. By connecting a plurality of such outer containers together in a cylindrical pattern, portions of the walls of the cylindrical outer containers may also form the wall of the inner container, minimising the number of components necessary to construct the boiler and thus the cost of constructing the boiler. Preferably, the boiler comprises six or more outer containers, to provide sufficient space in the centre of the cylindrical pattern for the at least one second heating element. The plurality of outer container may be surrounded by a jacket defining a third passage between the jacket and the plurality of outer containers, wherein the third passage and the second passage share a common thermally conductive wall defined by portions of the plurality of outer containers, the third passage having an inlet connected to the inlet of the boiler and a plurality of outlets forming the inlets of the respective second passage, the inlet of the third passage being at an axially opposite end to the plurality of outlets of the third passage, such that in use water flows along the second passage in an opposite direction to the flow of water in the second passage. The above arrangement permits cold water entering the boiler to be preheated in the third passage by energy lost from water in the second passage being conducted through the wall of the outer container. The boiler may be arranged such that the inlet of the boiler is a first inlet of the boiler to receive sanitary water to be heated and the outlet of the boiler is a first outlet of the boiler to provide heated sanitary water, the boiler further comprising a second outer container, outside of the jacket, defining a fourth passage between the second outer container and the jacket, the fourth passage in fluid isolation to the first, second and third passages and fluidly connecting a second inlet of the boiler to a second outlet of the boiler, which second inlet and second outlet of the boiler are arranged to be connected to a central heating system, the second outer container comprising at least one third heating element arranged to be connected to a mains supply and operated independently from the at least one first heating element. A boiler as described above is a combination boiler, able to provide both heated sanitary water and heated water for a circulatory central heating system. However, by having a second outer container through which the water of the central heating system may be cycled, energy may be transferred from the central heating water in the fourth passage to the cold water entering the third passage, permitting energy stored in the heated water of the central heating system, when available, to preheat the sanitary water entering the boiler. Two embodiments of the present invention will now be described, by way of example only, with reference to the following drawings, of which: Figure 1 is a partially exploded perspective view of the main components of a combination boiler in accordance with the present invention with the heater assembly shown partly withdrawn; Figure 2 is a perspective view of the heater assembly of Figure 1 Figure 3 is section taken along the line A-A of Figure 1; Figure 4 is a section along the line B-B in Figure 1, but with the heater assembly shown fully inserted; Figure 5 is a schematic diagram showing the components of heating system comprising the boiler of Figure 1 to 4; Figure 6 is a flow chart showing the steps performed by the control circuit of Figure 5 to the control operation of the boiler of Figure 5; Figure 7 is a flow chart of the steps performed by the control circuit to control the charging of the battery of Figure 5; Figure 8 corresponds to that of Figure 3, but shows a simplified boiler, without the ability to separately provide water for a central heating system; Figure 9 corresponds to Figure 4 but again shows the simplified boiler of Figure 8 Figure 10 is a flow chart of the components of a central heating system comprising a boiler as described with reference to Figures 8 and 9; and Figure 11 is a flow diagram of the steps performed by the control circuit of Figure 10 to control operation of the boiler of Figure 10; and Referring now to Figure 1, the main components of a boiler, indicated generally as 1 are illustrated. The term boiler in this context refers to that part of a domestic heating appliance which is the boiler, although the term “boiler” is also commonly used to refer to the appliance as a whole, which will often additionally include wiring, control circuitry, thermal insulation an outer casing and sometimes a central heating pump and expansion vessel. Boiler 1 of Figure 1 comprises a main body 2 and a heater assembly 3, (shown separately in Figure 2), with the heater assembly 3 in Figure 1 shown partly inserted into the main body 2. The main body 2 of the boiler has an inlet 4 for sanitary water to be heated, an outlet 5 for sanitary that has been heated, an inlet 6 for receiving water to be heated of a circulatory central heating system and an outlet 7 for the water of the central heating system. Referring now to Figure 2, this shows the heater assembly 3 comprising a base plate 8 to which is mounted a first heating element 9. This is designed to operate at 75 Volts and is connected to a 75 Volt supply, as described later, and has a power rating of 4 kilowatt. This first heating element is coaxially located within six further heating elements 10A to 10F, which are all identical and are hereinafter referred to as second heating elements. These are all arranged to operate at a 220 Volt mains supply and are each rated at 2 kilowatt. The heater assembly 3 contains two further heating elements 11A and 11B, which are located radially outward from the second heating elements 10E to 10F and these are identical to each other and are hereinafter referred to as third heating elements. These also are arranged to be connected to a 220 Volt mains supply, but these are each rated at 6 kilowatt. Referring now to Figure 3, a section along the line A-A of Figure 1. This shows the heating elements 9 to 11B located in the main body 2 of the boiler 1. The heating elements 11 A, 10F, 9, 10C and 11B can also be seen in Figure 4, a section along the line of B-B of Figure 1, but with the heater assembly 3 shown fully inserted into the main body 2 of the boiler 1. With reference to Figures 3 and 4, the main body 2 of the boiler is seen to comprise six inner cylinders 12E to 12F, an intermediate cylinder 13 and an outer cylinder 14 forming the outer wall of the boiler, all extending in an axial direction between (but not necessarily to) an upper end plate 15 and a lower end plate 16, seen in Figure 4. The third heating elements 11A and 11B, used for heating the water of a central heating circuit, are located in a chamber 17 formed between the intermediate cylinder 13 and outer cylinder 14, with the central heating inlet 6 and central heating outlet 7 being fluidly connected to the chamber 17, so that water of the central heating circuit may pass through chamber 17 and be heated by the third heating elements 11A and 11B. With reference to Figure 4, the sanitary water outlet 5 is seen to comprise of an outlet pipe 6 which extends through the upper end plate 15 into the boiler to support a disk shaped intermediate plate 19, on which the six inner cylinders 12A to 12F are mounted in a cylindrical pattern, only 12C and 12F of which can be seen in Figure 4. The intermediate plate 19 has six apertures 23 corresponding in position to the centre of each respective inner cylinder 12A to 12F and a central aperture 21 aligned with the first heating element 9. Each of the inner cylinders 12A to 12F, only 12C and 12F of which can be seen in Figure 4, have a cutout section 21 at their lower end, such that an inner first passage 23, defined by the space formed in the centre of the six inner cylinders 12A to 12F, is in fluid communication with the six passages 24A to 24F, defined within the respective six inner cylinders 12A to 12F, hereinafter collectively referred to as second passages. The inner cylinders 12A to 12F, together with the intermediate cylinder 13, define a third passage 25 extending between the sanitary water inlet 4 to the apertures 26 in the intermediate plate 19. In use, water drawn in through the sanitary inlet 4 passes vertically, as shown, up part of the outside wall of each the six inner cylinders 12A to 12F, absorbing any heat conducted outwardly through the six inner cylinders 12A to 12F, before it enters those same inner cylinders 12A to 12F to progress inwardly and down along the second passages, defied by those cylinders, where it is forced to pass through a low volume space in close proximity to respective heating elements 12E to 12F. The water then exits the inner cylinders 12A to 12F, through cutout sections 22, to progress inwardly again, now into the first passage 23, where the water is again forced by the low volume of the first passage into close proximity to the first, low voltage, heating element 9, before passing out of the sanitary water outlet 5. With this arrangement, when the first heating element 9 is energised, this will boost the temperature of the water in the first passage 23, with any heat energy conducted outwardly through the walls of the inner cylinders 12A to 12F being absorbed by the cooler water passing through the second passages 12A to 12F. Additional features seen only in Figure 4 are temperature sensors 26, arranged to sense the temperature of the water at the top of the first and second passages 23, 24A - 24F, only five of which can be seen in Figure 4. Referring now to Figure 5, here there is illustrated a combined central heating and sanitary hot water heating system, indicated generally as 27, comprising the boiler 1, described above with reference to Figures 1 to 4. The heating system further comprises a central heating system comprising one or more radiators 28, a pump 29 and a temperature sensor 30 detecting the temperature of water returning to the boiler 1 through central heating inlet 6. The temperature sensor 30 is connected to a control circuit 31, in the form of a printed circuit board control board, which is connected to the boiler 1, as indicated, and to each of the other components forming the heating system. These comprise a solar array, represented by photovoltaic (PV) panel 32, a fast charger 33, a battery 34, a battery power relay 35 and a flow rate sensor 38 for detecting the rate of flow of sanitary water through the boiler 1. The control circuit 31 is also connected to the mains 36 for receiving grid power. The PCB control board of the control circuit 31 comprises power relays, so that the control circuit may selectively connect the second heating elements 10E to 10F and / or the third heating elements 11A to 11B of the boiler 1 to the mains power supply 36. The control circuit 31 also, via the battery power relay 35, controls the supply of electrical energy from the battery 34 to the first heating element 9 of the boiler 1. In addition to the above, control circuit 1 receives an input from a flow rate sensor 38 on the sanitary water inlet 4, indicative of flow rate of sanitary water to be heated through the boiler 1 and receives signals from the temperature sensors 26 in the boiler 1, to enable it to control operation of the boiler 1, as will be described below with reference to Figure 6. In addition to the above, the control circuit 31 also controls the supply of energy from the PV panel 32, or fast charger 33, to the battery 34 to control charging of the battery 34, as described below with reference to Figure 7. Referring now to Figure 6, here there is illustrated the steps performed by the control circuit 31 commencing from the start 1. First the control circuit 31, at step 2, monitors whether there is a demand for hot water by determining this from the flow rate sensor 38, seen in Figure 5. If there is no demand (no flow), the sanitary water first heating element 9 and the plurality of sanitary water second heating elements 10A to 10F are turned off at step 3. Next, at step 4, the control circuit 31 determines from a program running on the PCB control board whether there is a demand for central heating. If there is no demand, then at step 5 all the first heating elements 9, second heating elements 10A to 10F and the third heating elements 11 are turned off and the process returns to the start 1. If a demand for hot water is detected at step 2, then it is determined at step 6, from the temperature sensor 30 of Figure 5, whether the central heating water returning to the boiler is above 60°C. If it is, at step 7, the central heating pump is turned on and the central heating heaters are turned off. This will cause water in the central heating system, at 60°C, to be circulated through the chamber 17 of Figure 5, so that residual energy from the central heating water will be transferred through the wall of the central cylinder 13 to the sanitary water being received at the inlet 4, as it flows through the third passage 25. Tthus the water in the central heating system will act to preheat the sanitary water prior to it entering the second passages 24A to 24F. However, if at step 6 the central heating water temperature, at the central heating inlet 6 to the boiler 1, is detected to be below 60°C, then at step 7 the circulating pump 29 will be turned off and the central heating heating elements 11A and 1B will turned off. Turning the central heating heating elements 11A and 11B off ensures that the mains power available from the mains supply 36 is available to the six second heating elements 10A to 10F for heating the sanatory water. At step 9 it is determined from the water temperature sensor 26, at the exit of the first passage 23, whether the water temperature leaving the boiler is below a predetermined set point. If it is not below a predetermined set point, that is to say it is above the predetermined set point, all the heating elements are turned off at step 10 and the process returns to the start 1. If, however, at step 9 the water temperature is detected to be below the set point, then at step 11, initially, only the mains powered second heating elements 10A to 10F are turned on. Then, if it is also determined at step 12 that the flow rate through the flow rate sensor 38 is more than ten litres per minutes and at step 13 that the battery state of charge is good, then at step 14 charging is disabled to the battery 34 and at step 15 the first heating element 9, powered by the battery 34, is turned on, so that all the sanitary water heating elements 10A to 10F and 9 are activated. At step 16 the temperature sensors 26 in the boiler 1 are monitored for any overheat. If no overheat is detected then the procedures returns to the start 1, but if a overheat condition is protected then, at step 17, all the heaters are switched off and a lock out condition established. Returning now to step 13, if the battery state of charge is not determined to be good, then the first heating element 9, powered by the battery 34, is left off and, at step 18, charging from PV panel 32 or the fast charger 33 of Figure 5 is enabled. Returning now to step 4, if here it is determined that there is a demand for central heating then the central heating pump 29 of Figure 5 is turned on at step 19 and at step 20 the temperature of the central heating water being returned to the boiler, as detected by the temperature sensor 30 of Figure 5, is determined and compared with a set point. If below the set point then the central heating, third heating elements 11A and 11B, are turned onat step 21. Alternatively, if the return water is above the set point then, at step 22, the central heating boilers are turned off. In either situation the control circuit 1 monitors at step 23 for an overheat condition and if such an overheat condition is detected all heaters are turned off at step 24 and a lock out condition established, otherwise the control circuit returns again to the start 1 of the process. As mentioned earlier, the control circuit 31 also controls charging of the battery 34 and Figure 7 is flowchart of the process adopted by the control circuit 31. From the start 1, the battery state of health is determined, at step 2, by the control circuit and if good then at step 3 it is determined whether charging is enabled and if it is then the procedure is repeated returning to the start 1. If charging is not enabled, it is determined whether charging is schedule and if at step 4 it is determined that charging is scheduled then at step 5 charging is commenced via the fast charger 33 of Figure 5. Alternatively, if charging is not scheduled, at step 4, then charging via the solar array is commenced at step 6, as represented by the PV panel 32 of Figure 5. In either case the process then returns to the start 1 and repeats. If at step 2 the battery state of health is determined not to be good, then at step 7 a charge lockout condition is established. The boiler so far described, with reference to Figures 1 to 6, is a combination boiler heating both water for a central heating system and for a sanitary hot water supply. However, the invention is equally applicable to a boiler that provides only heating for a sanitary hot water supply. Such a boiler will essentially be the same as that illustrated in Figures 1 to 4, with the exception that the central heating inlet 6, central heating outlet 7, outer cylinder 14 and third heating elements 11A and 11B will be omitted, leaving a boiler 37 having main components as illustrated in Figures 8 and 9. Figures 8 and 9 are corresponding views to those of Figures 3 to 4 and use like numerals to indicate like parts. These show the main components of a boiler 37 without the ability to provide a central heating function, but the remaining components shown in Figures 8 and 9 function in exactly the same manner as the corresponding components illustrated in Figures 3 and 4, noting that what was previously termed intermediate cylinder 13 is now an outer cylinder of the boiler. A central heating system, indicated generally as 39, which includes a boiler 37 as disclosed in Figures 8 and 9 is shown in Figure 10. This is very similar to that disclosed in Figure 5, with the exception that those components 28 to 30 associated with the central heating system have been omitted. Thus the description of the remaining components, described with reference to Figure 5 applies equally to those same components appearing in Figure 10 and in respect of which the same numerals have been used. The process adopted by the control circuit 31 of Figure 10 is very similar to the process performed by the control circuit 31 of Figure 5 and this is represented by the flow chart of Figure 11. From a comparison of the flow chart of Figure 11 with that of Figure 6, it will be realised that the process is the same with the exception that steps 4 to 8 and 19 to 24, related to or relying on a central heating system being present, have been omitted from Figure 11. The control circuit 31 of Figure 10 also performs the same battery check as is performed by the control circuit 31 of Figure 5 and this can again be seen from the flow chart of Figure 7 and the description thereof. Various embodiments of the present invention have been described above by way of example only and many alternative embodiments are possible which fall within the scope of the following claims. 19 02 24

Claims

1. An electric boiler having an inlet, an outlet and a plurality of interconnected passages for conveying water to be heated from the inlet to the outlet, the5 passages containing at least one first heating element arranged to operate at a first voltage and at least one second heating element arranged to operate at a second voltage lower than the first voltage.

2. An electric boiler as claimed in Claim 1, wherein the at least one first heating 10 element is arranged to be connected to a mains supply and wherein the at least one second heating element is arranged to be connected to a battery.

3. An electric boiler as claimed in Claim 2 wherein the at least one first heating element is arranged to be connected to an alternating current supply with a voltage15 of between 220 Volts and 240 Volts or between 110 Volts and 120 Volts.

4. An electric boiler as claimed in Claim 2 or 3, wherein the at least one second heating element is arranged to be connected to a battery having a voltage of between 50 Volts and 150 Volts.

205. An electric boiler as claimed in any one of claims 1 to 4, wherein the at least one second heating element is located within the plurality of passages downstream of the at least one first heating element.25 6. An electric boiler as claimed in Claim 5 comprising a thermally conductiveinner container containing the at least one second heating element, the inner container substantially surrounding the at least one second heating element to define a first passage in which the at least one second heating element is located, the inner container having at least one inlet and an outlet, the outlet fluidly30 connected to the outlet of the boiler and being arranged such as to cause water received at the at least one inlet of the inner container to flow along the first passage, in close proximity to a surface of the at least one second heating19 02 24element, to the inner container outlet, the boiler further comprising at least one outer container within which the inner container is substantially located, the at least one outer container defining a second passage extending around at least part of the inner container, the second passage containing the at least one first heating5 element and having at least one inlet and at least one outlet for a flow of water, wherein the at least one outlet of the at least one outer container is connected to, or forms, the at least one inlet of the inner container and wherein the at least one outer container is arranged such as to cause water received at the at least one inlet of the outer container to flow along the second passage, in close proximity to 10 a surface of the at least one first heating element, to the outer container outlet.

7. An electric boiler as claimed in Claim 6, wherein the inner container and at least one outer container share a common thermally conductive wall.15 8. An electric boiler as claimed in Claim 6 or 7, wherein the first and secondpassages are arranged such that, in use, water in the second passage progresses along the second passage in a direction opposite to the direction in which water progresses along the first passage.20 9. An electric boiler as claimed in Claim 8, comprising a plurality of outercontainers each in the form of a cylinder and each housing a respective first heating element, the plurality of outer containers being arranged side by side in a cylindrical pattern and connected to each other to form in their centre the inner container and the first passage in which a second heating element is located25 coaxially within the plurality of first heating elements.

10. An electric boiler as claimed in Claim 9 comprising at least six outer containers.30 11. An electric boiler as claimed in Claim 9 or 10, wherein the plurality of outercontainers are surrounded by a jacket defining a third passage between the jacket and the plurality of outer containers, wherein the third passage and the second19 02 24passage share a common thermally conductive wall defined by portions of the plurality of outer containers, the third passage having an inlet connected to the inlet of the boiler and a plurality of outlets forming the inlets of the respective second passage, the inlet of the third passage being at an axially opposite end to the5 plurality of outlets of the third passage such that in use water flows along the second passage in an opposite direction to the flow of water in the second passage.

12. An electric boiler as claimed in Claim 11 wherein the inlet to the boiler is a 10 first inlet of the boiler to receive sanitary water to be heated and the outlet of theboiler is a first outlet of the boiler to provide heated sanitary water, the boiler further comprising a second outer container outside of the jacket and defining a fourth passage, between the second outer container and the jacket, in fluid isolation to the first, second and third passages and fluidly connecting a second inlet of the15 boiler to a second outlet of the boiler which second inlet and second outlet are arranged to be connected to a central heating system, the second outer container comprising at least one third heating element arranged to be connected to a mains supply and operated independently from the at least one first heating element.20 13. An electric boiler as claimed in any preceding claim wherein each of the atleast one first heating element is rated at between 1,5kW and 6kW.

14. An electric boiler as claimed in any preceding claim comprising one second heating element rated at between 3kW and 10kW2515. A sanitary water heating system comprising a boiler as claimed in any preceding claim, a control circuit for controlling the boiler, a flow sensor connected to the control circuit for detecting the flow rate of water through a sanitary water supply heated by the boiler, a connection to an external mains supply, a first switch 30 controlled by the control circuit for connecting the mains supply to the at least one first heating element, a battery, a second switch controlled by the control circuit for connecting the battery to the at least one second heating element, wherein theCXIcontrol circuit is arranged to operate the first switch to connect the mains supply to the at least one first heating element on detection of a flow rate by the flow sensor and to operate the first and second switches to connect both the mains supply to the at least one first heating element and the battery to the at least one second5 heating element, on detection of a flow rate by the flow sensor which is above a predetermined flow rate.

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