WO2011001179A2

WO2011001179A2 - Water heating system

Info

Publication number: WO2011001179A2
Application number: PCT/GB2010/051084
Authority: WO
Inventors: Duncan Alexander Bennett
Original assignee: Duncan Alexander Bennett
Priority date: 2009-06-30
Filing date: 2010-06-30
Publication date: 2011-01-06
Also published as: US20140003801A1; CA2839810A1; GB0911314D0; WO2011001179A3

Abstract

Disclosed is a water heating system comprising: a hot water storage tank having a first port located in a lower region of the tank and a second port located in an upper region of the tank; a water flow pathway extending from the first port through a heating chamber to the second port; electrical heating means (for example a heating element) arranged to heat water within the heating chamber, when required, so that water within the water heating system will circulate from the first port to the second port through the heating chamber, by convection, when water is not drawn from the water heating system; a water supply outlet communicating with the water flow pathway intermediate the heating chamber and the second port; and a check valve arranged to restrict the flow of water out of the hot water storage tank by way of the second port when water is drawn from the water heating system through the water supply outlet. Therefore, when water is drawn from the system, the flow of water from the tank to the water supply outlet is restricted, and a much higher proportion of water passing through the water supply outlet has flowed directly from the heating chamber. Since water received at the water supply outlet directly from the heating chamber will have been more recently in contact with the electrical heating means than water received from the hot water storage tank via the second port, less electrical heating energy is required in order that water at a given temperature may be drawn from the system, or water drawn from the system will be at a higher temperature, than water drawn from a system lacking a check valve. Therefore, the system of the present invention is more efficient than a system lacking a check valve.

Description

WATER HEATING SYSTEM Field of the invention The invention relates to the field of water heating systems. Background to the invention Hot water systems for buildings (such as dwellings or commercial establishments) typically comprise a hot water storage tank, from which hot water is distributed via pipe work, to one or more outlets such as taps, showers and the like. The hot water storage tanks of such systems are enclosed, insulated tanks comprising an immersion heater and a thermostat device to regulate the operation of the immersion heater according to the detected temperature of the water in the tank. An electrical immersion heater comprises an electrical heating element, for example a U- shaped heating element, and is operable to regulate the temperature of the entire contents of the tank. The disadvantage of arrangements of this type is that the capacity of the hot water storage tank determines the maximum volume of hot water available to be drawn from the system at a given time, and that once the system is replenished with unheated water (usually from a mains supply), re-heating the entire volume of water in the tank may take several hours. A number of devices have been proposed to address this problem. For example, it is known to provide a water heating system without a hot water storage tank, in which water is heated on demand by rapidly heating a section of pipe work. Systems of this type, such as "combination boilers" which provide hot water for central heating systems and to hot water outlets, are able to provide a limited flow of hot water to outlets. Therefore, while they are suitable for small scale applications, there remains a need for water heating systems, such as for larger buildings, which are capable of supplying hot water at higher flow rates for periods of time. Modifications to systems comprising immersion heaters have been proposed. For example, GB 2,275,325 discloses a hot water storage tank having a modified immersion heater, comprising a heating element located within an elongate heating chamber, wherein water is drawn from the bottom of the tank into the heating chamber, and thus into contact with the heating element, and returned to the top of the tank through pipe work connected between the top of the heating chamber and the top of the tank. Water is circulated by convection and, if water is allowed to circulate for long enough, the temperature of the water in the system reaches equilibrium. However, since the heating element can heat the relatively small volume of water within the heating chamber more rapidly than it would be possible for the same heating element to heat the whole tank, a given flow rate of water may be drawn from the system directly from downstream of the heating chamber at a required outlet temperature, providing that the temperature of the water entering the heating chamber is sufficiently high to be raised to the required outlet temperature during the time taken for water to flow through the casing at that flow rate. Accordingly, the modified immersion heater system of GB 2,275,325 can supply water at a required outlet temperature from a hot water storage tank in which water is at a lower temperature than a conventional immersion heater system and is therefore more efficient than conventional immersion heater systems. WO 2004/081463 discloses a similar system wherein water is also circulated within a modified immersion heater, around external pipe work connected to the encased immersion heater, and back into the top of the tank (or to outlets, when required). However systems of this type suffer from a number of disadvantages. When water is drawn from the system downstream of the heating chamber, water also flows from the top of the tank. As water from the tank is typically at a lower temperature, the efficiency of the system is compromised. Additionally, due to the low rate of convectional flow through the heating chamber, significant mixing also occurs in lower region of the tank between water in the tank and water which has already been drawn into the casing, again reducing the efficiency of the system. Further, hot water storage tanks are required to be re-filled when water is drawn from the system. Therefore, hot water systems additionally comprise an inlet to the tank for unheated water (typically connected directly to the mains water supply). A problem of known systems comprising modified immersion heaters of the aforesaid type is for a current of unheated water from the inlet (which is typically towards the base of the tank) to be drawn directly towards the inlet of the heating chamber when hot water is being drawn from the system, such that the temperature of water entering the casing is lower than the average temperature of the water in the tank, which significantly impairs the performance of the system. Therefore, there remains a need for a water heating system comprising a hot water storage tank, with improved efficiency in comparison to known systems. Summary of the invention According to a first aspect of the present invention, there is provided a water heating system comprising: a hot water storage tank having a first port located in a lower region of the tank and a second port located in an upper region of the tank; a water flow pathway extending from the first port through a heating chamber to the second port; electrical heating means (for example one or more heating elements) arranged to heat water within the heating chamber, when required, so that water within the water heating system will circulate from the first port to the second port through the heating chamber, by convection, when water is not drawn from the water heating system; a water supply outlet communicating with the water flow pathway intermediate the heating chamber and the second port; and a check valve arranged to restrict the flow of water out of the hot water storage tank by way of the second port when water is drawn from the water heating system through the water supply outlet. Therefore, when water is drawn from the system, the flow of water from the tank to the water supply outlet is restricted, and all, or at least the substantial majority of water passing through the water supply outlet has flowed directly from the heating chamber rather than directly from the hot water storage tank. Since all, or at least the substantial majority of water received at the water supply outlet directly from the heating chamber will have been more recently in contact with the electrical heating means than water received from the hot water storage tank via the second port, less electrical heating energy is required for water be drawn from the system at a given temperature to, or water drawn from the system will be at a higher temperature, in comparison to a system lacking a check valve. Therefore, the system of the present invention is more efficient than a system lacking a check valve. Preferably the water flow pathway is located within, or at least partly within, the hot water storage tank. In some embodiments, the first port and the heating chamber (which may comprise the first port) are located within the hot water storage tank and are secured to the hot water storage tank by a sealing arrangement around the fluid pathway, and the water supply outlet is external to the hot water storage tank. The water flow pathway may extend through a conduit including the electrical heating element and adapted to be retrofitted to the hot water storage tank. In some embodiments, the water flow pathway extends through a conduit including the electrical heating element and external to the hot water storage tank. A water flow pathway extending through a conduit including the electrical heating element and located external to the hot water storage tank can be connected to a water heating system comprising a hot water storage tank which is not adapted in such a way that the water flow pathway can be readily installed within, or partly within, the hot water storage tank. For example, hot water storage tanks to which a hot water system of the present invention is to be retrofitted may not be equipped with apertures suitable for the water flow pathway to be installed, and thus an external water flow pathway enables the retrofitting of such hot water storage tanks. In some embodiments the check valve is located at the second port. Preferably the check valve is operable between an open position, in which water is permitted to flow within the water flow pathway between the first port and the second port, and a closed position, in which the flow of water out of the hot water storage tank by way of the second port is restricted, responsive to a pressure differential across the check valve. In some embodiments, when water is drawn from the water heating system through the water supply outlet, the pressure of water at the water supply outlet is less than the pressure of water in the hot water storage tank, such that the pressure differential across the check valve causes the check valve to be in a closed position, and when water is not being drawn from the water heating system and water within the heating chamber is being heated by the electrical heating means, the pressure of water within the heating chamber is greater than the pressure of water in the hot water storage tank, such that the pressure differential across the check valve causes the check valve to be in an open position and water within the water heating system will circulate from the first port to the second port through the heating chamber, by convection. The water heating system may comprise more than one heating chamber. In this case, the electrical heating means may comprise one or more heating elements operable to heat the more than one heating chamber. The water flow pathway may be branched. For example a first heating chamber and a supplementary heating chamber may each be connected by way of a conduit to the second port and the water supply outlet but connected to separate first ports, each of which is located in a lower region of the tank. In some embodiments, the water heating system has a first mode for heating water during circulation from the first port to the second port through the heating chamber, by convection, while water is not drawn from the water heating system and a second, higher power, mode for heating water when water is drawn from the water heating system through the water supply outlet. It may be that the system comprises a supplementary heating chamber and water is heated in the supplementary heating chamber in the second mode, but not the first mode. Typically, the water heating system comprises a sensor, such as a flow sensor, for determining whether water is being drawn from the water heating system through the water supply output and is operable to switch from the first mode to the second mode responsive to the detection of water flow by the sensor. Further optional features of a water heating system having said first and second modes correspond to the optional features discussed in relation to the third aspect below. Preferably the check valve comprises a moveable member, moveable between a closed position in which the moveable member forms a cooperative fit with an aperture in the water flow pathway (which may, for example, be the second port) and an open position. Preferably the moveable member has an aperture, or a plurality of apertures therethrough to provide a restricted path for water to pass through the moveable member. Alternatively, or in addition, in a closed position, the moveable member and the aperture in the water flow pathway define an aperture, to provide a restricted path for water to flow around the moveable member. In some embodiments the or each aperture has a capillary dimension. Known check valve arrangements which are sufficiently sensitive to change to an open position under the influence of the convectional flow of water through a water flow pathway are prone to leak when water is drawn from the hot water system. Consequently, water drawn from the hot water system comprises a mixture of water which has flowed through a heating chamber directly to the outlet, and water (which is typically at a lower temperature) drawn directly from the hot water storage tank. Leakage occurs when the pressure differential across a check valve is insufficient to create a seal between the moveable member and an aperture in the water flow pathway, or when the pressure differential, and thus the initial flow rate, is too great and prevents the formation of a cooperative fit between the moveable member and the aperture in the water flow pathway. In either case, water flows across the valve and holds the valve in an open position and the check valve thus fails to function adequately. An aperture or apertures in or around the moveable member permits a small flow of water when the check valve is in a closed position. The small flow of water through the aperture generates a venturi effect which acts so as to force the moveable member into a closed position. In some embodiments the water heating system is a domestic water heating system. The water heating system may be a commercial water heating system, for example for commercial establishments such as shops, hotels, gymnasiums and the like. In some embodiments the water heating system is a potable water heating system. In some embodiments the water heating system is a central heating system for a building. Preferably the electrical heating means is elongate. Preferably the electrical heating means is an elongate heating element (for example a heating element in the form of an elongate loop). In some embodiments the electrical heating element is U-shaped. Preferably the heating chamber is elongate. An elongate electrical heating means (such as a heating element formed as an elongate loop) may be formed and arranged to fit closely within an elongate heating chamber, thereby minimising the internal volume of the heating chamber. Thus, in use, a minimum volume of water is required to be heated at one time by the electrical heating means, such that the temperature of the water in the heating chamber may be increased rapidly. Preferably the electrical heating means comprises a plurality of heating elements (which may be elongate, or U-shaped). For example, the heating chamber may be generally cylindrical in shape and the plurality of heating elements may be disposed in a generally circularly symmetric arrangement around the periphery of the interior of the heating chamber. In some embodiments the heating chamber further comprises a space filling member. Preferably, in embodiments having one or more heating elements formed in the shape of an elongate loop (for example, one or more U-shaped heating elements), the space filling member is positioned within the loops of one or more heating elements. Typically, electrical heating means such as heating elements are formed as loops, for example U-shaped, comprising electrically conducting material. The lifetime of a heating element is compromised if the radius of curvature of any portion of the heating element is too small. Therefore, in order to have an acceptable operational lifetime for use in a water heating system (typically several years), a heating element formed as a loop, for example a U-shaped heating element, must be above a minimum width (typically in the region of 70mm for domestic applications). It is advantageous for the volume of water in the heating chamber to be minimized, in order to maximise the maximum rate of heating. A space filling member positioned in the heating chamber within the loop of one or more heating elements reduces the volume of water within the heating chamber, such that the temperature of the said water may be increased more rapidly, as compared to a heating chamber lacking a space filling member. In some embodiments, the space filling member is generally cylindrical, or prismatic, and positioned between the loops of a plurality of heating elements arranged, for example, within a generally cylindrical heating chamber. The water flow pathway within the heating chamber of such an arrangement is generally annular. Preferably the water heating system further comprises an inlet means, operable to admit a diffuse flow of unheated water into a lower region of the hot water storage tank. The inlet means preferably comprises at least one perforated pipe. Alternatively, or in addition, the inlet means may comprise a mesh or other open pored material. The inlet means may, for example, comprise a perforated pipe in a lower region of the hot water storage tank, or may comprise a plurality of pipes (each of which may be perforated) in a lower region of the hot water storage tank. In use, water is drawn from the hot water system, by way of the water supply outlet. It is desirable for water flowing through the water supply outlet to be at a high temperature. Therefore, since the temperature of water leaving the heating chamber is higher if the temperature of water entering the water heating chamber is higher, it is desirable for the temperature of the water entering the water flow pathway through the first port to be as high as possible. When sufficient water is drawn from the hot water system, water must be admitted by way of the inlet means. Consequently, in some situations, water is simultaneously drawn from the water heating system by way of the water supply outlet and admitted to the water heating system by way of the inlet means. A diffuse flow of unheated water admitted to the hot water storage tank by way of the inlet means is more effectively mixed with water already in the tank (which is typically at a higher temperature than the unheated water). Therefore, when water is simultaneously drawn from the water heating system by way of the water supply outlet and admitted to the water heating system by way of the inlet means, the temperature of the water entering the water flow pathway through the first port is higher (as compared to a water heating system having an inlet means admitting non-diffuse flow of unheated water, such as an inlet pipe). In some embodiments, the cross sectional area of the first port is less than the cross sectional area of the water flow pathway adjacent to the first port. Optionally the cross sectional area of the first port is less than the cross sectional area of the water flow pathway in the heating chamber, and/or less than the cross sectional area of other regions of the water flow pathway. A first port with a restricted cross sectional area (in relation to the adjacent portion of the water flow pathway) advantageously reduces mixing, by diffusion, between water having entered the water flow pathway and water in the hot water storage tank. Such mixing introduces water from the water flow pathway (which is typically partially heated) into the hot water storage tank, and water from the hot water storage tank (which is typically at a lower temperature than water in the water flow pathway) into the water flow pathway. Thus, this "backflow" mixing will result in cooling of the water in the water flow pathway. In order to heat water to a given temperature, therefore, the reduction of such backflow mixing, by restriction of the cross sectional area of the first port, increases the efficiency of the water heating system. Preferably the cross sectional area of the first port is approximately equal to, or less than, 50% of the cross sectional area of the water flow pathway adjacent the first port. In some embodiments, the cross sectional area of the first port is approximately equal to, or less than, 30%, or approximately equal to, or less than, 15% of the cross sectional area of the water flow pathway adjacent the first port. The upper region may, for example, be the top half, the top 25%, the top 10% or the very to of the water storage tank. The lower region may, for example, be the bottom 75%, the bottom half, or the bottom 25% of the water storage tank. According to a second aspect of the present invention, there is provided a water heating system comprising: a hot water storage tank having a first port located in a lower region of the tank and a second port located in an upper region of the tank; a water flow pathway extending from the first port through a heating chamber to the second port; at least one electrical heating element formed with a loop (which may be elongate, or U-shaped) and arranged to heat water within the heating chamber, when required, so that water within the water heating system will circulate from the first port to the second port through the heating chamber, by convection, when water is not drawn from the water heating system; a water supply outlet communicating with the water flow pathway intermediate the heating chamber and the second port; and a space filling member positioned within the loop of the each said heating element. The lifetime of a heating element formed with a loop is compromised if the radius of curvature of any portion of the heating element is too great. Therefore, in order to have an acceptable operational lifetime for use in a water heating system (typically several years), a heating element formed with a loop, for example a U-shaped heating element, must be above a minimum width (typically in the region of 70mm for domestic applications). It is advantageous for the volume of water in the heating chamber to be minimized. A water heating system having a space filling member positioned in the heating chamber within the loop of one or more heating elements reduces the volume of water within the heating chamber, such that the temperature of the said water may be increased more rapidly, as compared to a water heating system having a heating chamber lacking a space filling member. Preferably the water heating system comprises a plurality of heating elements (which may be elongate, or U-shaped) and the space filling member is positioned within the loops of the plurality of heating elements. For example, the heating chamber may be generally cylindrical in shape and the plurality of heating elements may be disposed in a generally circularly symmetric arrangement around the periphery of the interior of the heating chamber, extending around the space filling member. In some embodiments, the space filling member is generally cylindrical, or prismatic, and positioned between the loops of a plurality of heating elements arranged, for example, within a generally cylindrical heating chamber. The water flow pathway within the heating chamber of such an arrangement is generally annular. Further preferred and optional features of the second aspect correspond to preferred and optional features of the water heating system according to the first aspect. According to a third aspect of the present invention, there is provided a water heating system comprising: a hot water storage tank having a first port located in a lower region of the tank and a second port located in an upper region of the tank; a water flow pathway extending from the first port through a heating chamber to the second port; electrical heating means (for example one or more heating elements) arranged to heat water within the heating chamber, when required, so that water within the water heating system will circulate from the first port to the second port through the heating chamber, by convection, when water is not drawn from the water heating system; and a water supply outlet communicating with the water flow pathway intermediate the heating chamber and the second port; wherein the water heating system has a first mode for heating water during circulation from the first port to the second port through the heating chamber, by convection, while water is not drawn from the water heating system and a second, higher power, mode for heating water when water is drawn from the water heating system through the water supply outlet. Thus, higher power output may be employed when delivering water to the water supply output than for ongoing maintenance of the temperature of water within the tank. Thus, water may be delivered to the water supply output at a greater temperature than the temperature to which water is heated in the hot water storage tank when water is not being delivered to the water supply output. It may be less efficient to use higher power output for both the purpose of the delivery of water and maintenance of the temperature of water within the tank. For example, if only the second, higher power, mode was employed, it may be necessary for the electrical heating means to be switched on and off rapidly to avoid overheating water during circulation from the first port to the second port, reducing working lifetime. It may be that the power used to heat water in the second mode, while water is drawn from the water heating system, could cause boiling or other undesirable effects if the same amount of power was used when circulating water from the first port to the second port when water is not being drawn from the water heating system through the water supply outlet. Typically, the power output by the electrical heating means in the second mode is at least 150%, and preferably at least 200%, of the power output by the electrical heating means in the first mode. It may be that the electrical heating means comprises one or more electrical heating elements which are used to heat water in the second mode, but not in the first mode. The water heating system may comprise more than one heating chamber. In this case, the electrical heating means may comprise one or more heating elements operable to heat the more than one heating chamber. The water flow pathway may be branched. For example, the more than one heating chamber may comprise a first heating chamber and a supplementary heating chamber each connected by way of a conduit to the second port and the water supply outlet but connected to separate first ports, each of which is located in a lower region of the tank. It may be that the system comprises a said supplementary heating chamber and water is heated in the supplementary heating chamber, by one or more electrical heating elements, in the second mode, but not the first mode. The supplementary heating chamber may have a greater volume than the said first heating chamber. Each heating chamber may be optimised for its respective function. In some embodiments, each heating chamber is smaller than would have been required to deliver the same heating effect using a single heating chamber. Conduits extending from the first and supplementary heating chambers to water supply outlet may meet at a generally T-shaped connector so that water from the supplementary heating chamber can flow directly through the connector towards the water supply outlet when water is drawn through the water supply outlet. This is especially helpful when the supplementary heating chamber is larger and/or operable to output more power to heat water than the first heating chamber. A conduit extending to the second port is typically connected between the said T-shaped connector and the first heating chamber to minimise water flow through the supplementary heating chamber when water is heated in the first heating chamber, but not the second heating chamber. Typically, the water heating system comprises a sensor, such as a flow sensor, for determining whether water is being drawn from the water heating system through the water supply output and is operable to switch from the first mode to the second mode responsive to the detection of water flow by the sensor. Thus, the water heating system is typically operable to switch automatically between the first and second mode depending on whether water is drawn from the water heating system through the water supply outlet. In some embodiments, the or each low power heating chamber is in communication with the water supply outlet, such that water from the or each low power heating chamber circulates by convention to the second port, when water is not drawn from the system, and water from the or each low power heating chamber and the or each high power heating chamber flows to the water supply outlet when water is drawn from the system. The water supply outlet may communicate with at least one said water flow pathway intermediate the heating chamber and a said second port. In some embodiments a check valve is arranged to restrict the flow of water out of the hot water storage tank by way of the second port when water is drawn from the water heating system through the water supply outlet, in which case the water heating system is also a water heating system according to the first aspect of the invention. Further preferred and optional features of the third aspect correspond to preferred and optional features of the water heating system according to the first and second aspects. Description of the Drawings An example embodiment of the present invention will now be illustrated with reference to the following Figures in which: Figure 1 shows a schematic cross sectional view of a water heating system according to the present invention. Figure 2 shows a cross sectional side view of a heating chamber. Figure 3 shows a cross sectional plan view of a heating chamber. Figure 4 shows a cross sectional side view of a check valve in a closed position. Figure 5 shows a cross sectional side view of a check valve in an open position; and Figure 6 shows a schematic view of a heating system have two heating chambers. Detailed Description of an Example Embodiment Figure 1 shows a water heating system 1 , comprising a hot water storage tank 3, shrouded in insulation 4. The tank is provided with three openings. Through an opening 5 in the base of the tank passes a cold water inlet pipe 7. The inlet pipe has perforated section 9 within the tank. At opening 11 in the top of the tank is installed a cylindrical heating chamber 13, secured to the tank by lockring 15. Leads 17 connect to a heating element within the heating chamber, as shown in Figures 2 and 3 and described in further detail below. The heating chamber has an inlet port 19 and is in fluid communication with pipe 21 , which leads through opening 23 in the tank. Outlet pipe 25 connects to pipe 21 intermediate the heating chamber and opening 23. The outlet pipe connects to one or more outlets 26 (which may be taps or showers or other outlets typically found on building hot water systems) operable to regulate the flow of water out of the hot water system. Also connected to pipe 21 , in line with pressure release pipe 27 is pressure release valve 28 which is operable to permit a flow of water when the pressure within the water heating system is above a predetermined threshold. Alternatively, or in addition, a pressure release valve (not shown) may be positioned on the hot water storage tank. At the end of pipe 21 in the tank is a port 29, comprising a check valve 31. Thus, a water flow pathway extends from inlet port 19, through the heating chamber 13, pipe 21 and to port 29. Figure 2 shows a cross sectional view of the heating chamber 13. The heating chamber is cylindrical. Passing through the top plate 32 are leads 17, electrically connected to heating elements 34. Also extending through the top plate is pipe 21. Heating elements 34 are U-shaped and arranged around space filler 36 (composed of a heat resistant material such as a heat resistant plastics, metal or ceramic material) and the assembly is positioned along the central axis of the heating chamber, such that an annular channel 38 extends around the heating elements and space filler. Inlet port 19 extends from the side of the heating chamber, close to the base 39, and has a smaller cross sectional area than the area of annular channel 38, and of the heating chamber immediately adjacent to inlet port 19. The top plate, and therefore the heating chamber and all components therein, are secured to opening 11 in the hot water storage tank. Opening 11 is of a standard configuration for hot water storage tanks of the type found in water heating systems (and typically immersion heaters are installed therein). Opening 11 comprises an externally threaded cylindrical portion 40. The heating chamber passes through the opening 1 1 and the top plate, which has an outer diameter equal to the outer diameter of the cylindrical portion 40, and thus greater than the inner diameter of cylindrical portion 40, rests on the upper end of cylindrical portion 40. An o-ring (not shown) is positioned between the top plate and the cylindrical portion. Lockring 15, which is provided with a circular aperture 16 and an internally threaded portion 41 , is threadably secured to cylindrical portion 40. The diameter of the circular aperture is smaller than the diameter of the top plate and thus the top plate is secured between the cylindrical portion 40 of the opening 1 1 and the lockring. The leads 17 and pipe 21 extend through the circular aperture 16 of the lockring. Figure 3 shows a plan cross sectional view of the heating chamber through A. In the example shown, four heating elements 34A, 34B, 34C and 34D are positioned around the space filler 36. Annular channel 38 extends around the heating elements and space filler. Figure 4 shows a cross sectional view of port 29 at opening 23 in the hot water storage tank. All features are circularly symmetric about B. Pipe 21 and port 29 are provided with flanges 44,45 and are secured to an externally threaded cylindrical portion of opening 23, with a lock ring 42 in a manner similar to the arrangement described in respect of Figure 2, above. Check valve 31 is positioned at port 29. The check valve comprises a moveable member 52 which is generally cup-shaped, with an aperture 54 through the centre. The moveable member is provided with a conical first sealing surface 56 formed so as to provide a seal against a second conical sealing surface 58 on the inner surface of the valve casing 59. The lower portion of the valve casing comprises an open framework and the water flow pathway through the valve casing comprises holes defined by the framework (not show). A guide element 60 is supported within the valve casing and is formed so as to slideably receive the moveable member, as shown in Figure 5. In use, the hot water storage tank is filled with cold water from cold water inlet pipe 7. When the outlets 26 are closed, and electrical current is flowing through the leads 17 (which are typically connected to a mains electrical supply) and the heating elements 34, water flows by convection through inlet port 19, into the heating chamber 13, past the heating elements 34 through the annular channel 38, out of the heating chamber through pipe 21 and back into the hot water storage tank through port 29 and check valve 31. As shown in Figure 4, water flowing into the tank through pipe 21 at port 29 (in the direction C) impinges the top surface 50 of the moveable member 52, forcing the moveable member in the direction C until the moveable member abuts the base of the valve casing and the check valve is in an open position, as shown in Figure 5. Water is then able to flow between the conical sealing surfaces 56, 58 and past the moveable member and through the apertures defined by the framework of the valve casing into the hot water storage tank. Water entering the hot water storage tank through port 29 is at a higher temperature than water entering the heating chamber through inlet port 19. The restricted cross sectional area of inlet port 19 (in comparison to the cross sectional area of the adjacent heating chamber) restricts diffusion of water from the heating chamber back into the tank, which would otherwise lower the temperature of the water in the heating chamber. Thus, since a small volume of water (equal to the volume of water in the heating chamber) is heated at any one time, the rate at which the temperature of the water in the heating chamber rises is considerably greater than if heat energy was introduced to the volume of water in the entire system at the same rate (as would be the case for a water heating system equipped with an immersion heater but lacking a heating chamber). Furthermore, the provision of the space filler between the loops of the heating elements further reduces the volume of water within the heating chamber and thereby further increases the rate at which the temperature of water within the heating chamber increases. Initially therefore, a temperature gradient is established within the hot water storage tank as hot water entering the tank through port 29 mixes with cooler water present in the tank. The hot water storage tank is typically equipped with a thermostat, which regulates power to the heating elements (not shown). Left indefinitely to circulate, the temperature of water in the tank would eventually equilibrate to a temperature determined by the thermostat setting. One or more outlets 26 can be opened to draw water from the tank. Water may be drawn from the tank when the heating elements are switched on, or switched off, but typically, it will be required for water drawn from the tank to be at as high a temperature as possible, and therefore the heating elements will be switched on. When one or more outlets are opened, water flows, under the influence of a pressure differential between the outlet pipe 25 and the tank, along pipe 21 and through outlet pipe 25. The pressure differential also places the check valve 31 into a closed position, such that water flows through the outlet pipe along pipe 21 directly from the heating chamber, and not from the tank through port 29. The pressure differential between the tank and the outlet pipe initially results in a flow of water in the direction D through port 29. The force of the flow of water against the lower surface 51 of the moveable member moves the moveable member in the direction D until the conical sealing surface 56 abuts the conical sealing surface 58 and a seal is formed. Thereafter, a small flow of water passes along aperture 54 and the force of the resulting venturi effect maintains the seal and holds the check valve in the closed position, as shown in Figure 4. The flow of water through the capillary is negligible in comparison to the flow of water through the outlet valve. When water is drawn from the tank through the outlet pipe, and equal volume of water is admitted to the tank through the cold water inlet pipe 7. The flow of water through the perforations of the perforated section 9 of the inlet pipe diffuse the flow of cold water into the tank, and thereby assists mixing between the heated or partially heated water in the tank, and the cold water admitted to the tank. In known water heating systems comprising a simple cold water inlet pipe that does not diffuse the flow of water admitted to the tank, the is a tendency for cold water to flow directly from the inlet pipe towards the inlet port 19 (when one or more outlets are open). Such a flow will tend to lower the temperature of the water passing through the inlet port and so also lower the temperature of the water flowing through the outlet pipe. Diffusing the flow of cold water into the tank therefore increases the temperature of the hot water drawn from the tank. The heating elements are operable, within the time taken from water to flow through the annular channel when one or more outlets are open, to heat water to a predetermined upper temperature, from at or above a corresponding lower temperature. If, as is typically the case, the temperature of the water in the tank is maintained above the lower temperature, a mixture of cold water from the inlet pipe and the heated or partially heated water in the tank may be at a temperature at or above the lower temperature. Therefore, by mixing water in the tank with water from the inlet pipe, the water heating system is able to provide a volume of water at a predetermined upper temperature that is greater than the volume of water in the tank. The perforated portion of the cold water inlet pipe more effectively mixes cold water from the inlet pipe with heated or partially heated water in the tank. Thus, the more effectively this mixing is achieved, the higher the temperature of water flowing through the inlet port, and the greater the total volume of water above the predetermined upper temperature that may be drawn from the water heating system at one time. Figure 6 and shows a water heating system 102 according to the third aspect of the invention, comprising a low power heating chamber 1 13 (functioning as a first heating chamber) and a high power heating chamber 114 (functioning as a second heating system), retrofitted to a hot water storage tank 103. The tank has an inlet pipe 107 comprising a perforated section 109, similar to the arrangement discussed above in relation to water heating system 1. Each of the heating chambers 1 13, 1 14 comprise U-shaped electrical heating elements (respectively a low power, e.g. 3kW, heating element and a high power, e.g. 12 kW, heating element or preferably two commonly controlled 6 kW heating elements), and a space filler. Inlet ports 1 19 and 120 extend from a lower part of the tank to the heating chamber. Outlet pipe 125 extends from the upper part of the low power heating chamber, to pipe 121 which is in communication with the top of the tank. Thus, a water flow pathway extends from the inlet port 119, through heating chamber 113, along pipes 125 and 121 , and into the tank. A non- return valve is positioned at the distal end of pipe 121 , to prevent water from circulating along pipe 121 and back into the heating chamber, although heating systems according to the third aspect of the invention may be provided without such a valve. A pressure release valve 128 is also positioned in line with pipe 125. A second path for water to flow extends from inlet port 120, through the high power heating chamber, pipe 130, outlet pipe 132 and to outlets 126. A further pressure release valve 127 is provided in line with pipe 130. The high power chamber is operable to heat water to a higher temperature and at a greater flow rate, than the low power chamber, by virtue of higher power heating elements and a water flow pathway having a greater cross sectional area. Water heating system 102 is provided with connector pipe 134 between pipes 125 and 132. In use, when water is not being drawn from the system through one or more of the outlets 126, the low power heater functions at its maximum rated power (and thus at its optimum efficiency) for all, or for some of the time. Water circulates by convection from inlet 1 19 to port 129. Water is heated to maintain a predetermined temperature. When there is a demand for hot water from one or more of the outlets, water is drawn through inlet 120 and through the high power heating chamber, which is operable to provide heat at a greater rate than the low power heating chamber. Water also passes through the inlet 119 and so water heated in either chamber is supplied to the outlet. The water level in the tank is maintained by water entering the tank through the inlet pipe 107. Thus, the high power heating chamber is not required to operate at a lower power, or to continuously be switched on and off, when water is not being drawn from the tank through the outlets. Further modifications and variations may be made within the scope of the invention herein disclosed.

Claims

Claims 1. A water heating system comprising: a hot water storage tank having a first port located in a lower region of the tank and a second port located in an upper region of the tank; a water flow pathway extending from the first port through a heating chamber to the second port; electrical heating means arranged to heat water within the heating chamber, when required, so that water within the water heating system will circulate from the first port to the second port through the heating chamber, by convection, when water is not drawn from the water heating system; a water supply outlet communicating with the water flow pathway intermediate the heating chamber and the second port; and a check valve arranged to restrict the flow of water out of the hot water storage tank by way of the second port when water is drawn from the water heating system through the water supply outlet.

2. A water heating system according to claim 1 , wherein the water flow pathway is located at least partially within the hot water storage tank.

3. A water heating system according to claim 1 , wherein the water flow pathway extends through a conduit including the electrical heating element and external to the hot water storage tank.

4. A water heating system according to any one preceding claim, wherein the check valve is located at the second port.

5. A water heating system according to any one preceding claim, wherein the check valve is operable between an open position, in which water is permitted to flow within the water flow pathway between the first port and the second port, and a closed position, in which the flow of water out of the hot water storage tank by way of the second port is restricted, responsive to a pressure differential across the check valve.

6. A water heating system according to any one preceding claim, wherein the check valve comprises a moveable member, moveable between a closed position in which the moveable member forms a cooperative fit with an aperture in the water flow pathway and an open position.

7. A water heating system according to claim 6, wherein the moveable member comprises at least one aperture.

8. A water heating system according to any one preceding claim, wherein the electrical heating means comprises an elongate U-shaped heating element.

9. A water heating system according to any one preceding claim, wherein the electrical heating means comprises a plurality of heating elements, disposed in a generally circularly symmetric arrangement around the periphery of the interior of the heating chamber.

10. A water heating system according to any one preceding claim, wherein the heating chamber further comprises a space filling member.

1 1. A water heating system according to claim 10, wherein the space filling member is positioned within the elongate loops of one or more heating elements.

12. A water heating system according to claim 1 1 , wherein the space filling member is generally cylindrical or prismatic, and positioned between the loops of a plurality of heating elements arranged, within a generally cylindrical heating chamber and wherein the water flow pathway within the heating chamber is generally annular.

13. A water heating system according to any one preceding claim comprising an inlet means, operable to admit a diffuse flow of unheated water into a lower region of the hot water storage tank.

14. A water heating system according to claim 13, wherein inlet means comprises at least one perforated pipe.

15. A water heating system according to claims 13 or 14, wherein the inlet means comprises mesh or other open pored material.

16. A water heating system according any one preceding claim, wherein the cross sectional area of the first port is less than the cross sectional area of the water flow pathway adjacent to the first port.

17. A water heating system according to claim 16, wherein the cross sectional area of the first port is less than the cross sectional area of the water flow pathway in the heating chamber.

18. A water heating system according to claim 16 or claim 17, wherein the cross sectional area of the first port is less than 15% of the cross sectional area of the water flow pathway adjacent the first port.

19. A water heating system according to any one preceding claim, having a first mode for heating water during circulation from the first port to the second port through the heating chamber, by convection, while water is not drawn from the water heating system and a second, higher power, mode for heating water when water is drawn from the water heating system through the water supply outlet. It may be that the system comprises a supplementary heating chamber and water is heated in the supplementary heating chamber in the second mode, but not the first mode. Typically, the water heating system comprises a sensor, such as a flow sensor, for determining whether water is being drawn from the water heating system through the water supply output and is operable to switch from the first mode to the second mode responsive to the detection of water flow by the sensor. Further optional features of a water heating system having said first and second modes correspond to the optional features discussed in relation to the third aspect below.

20. A domestic water heating system according to any one preceding claim.

21. A potable water heating system according to any one preceding claim.

22. A water heating system comprising: a hot water storage tank having a first port located in a lower region of the tank and a second port located in an upper region of the tank; a water flow pathway extending from the first port through a heating chamber to the second port; electrical heating means arranged to heat water within the heating chamber, when required, so that water within the water heating system will circulate from the first port to the second port through the heating chamber, by convection, when water is not drawn from the water heating system; and a water supply outlet communicating with the water flow pathway intermediate the heating chamber and the second port; wherein the water heating system has a first mode for heating water during circulation from the first port to the second port through the heating chamber, by convection, while water is not drawn from the water heating system and a second, higher power, mode for heating water when water is drawn from the water heating system through the water supply outlet.

23. A water heating system according to claim 22, wherein the power output by the electrical heating means in the second mode is at least 150%, of the power output by the electrical heating means in the first mode.

24. A water heating system according to claim 22 or claim 23, wherein the electrical heating means comprises one or more electrical heating elements which are used to heat water in the second mode, but not in the first mode.

25. A water heating system according to any one of claims 22 to 24, wherein the water heating system comprises more than one heating chamber.

26. A water heating system according to claim 25, wherein the more than one heating chamber comprises a first heating chamber and a supplementary heating chamber, each connected by way of a conduit to the second port and the water supply outlet but connected to separate first ports, each of which is located in a lower region of the tank, and wherein water is heated in the supplementary heating chamber, by one or more electrical heating elements, in the second mode, but not the first mode.

27. A water heating system according to any one of claims 22 to 26, wherein the water heating system comprises a sensor for determining whether water is being drawn from the water heating system through the water supply output and is operable to switch automatically between the first and second mode depending on whether water is drawn from the water heating system through the water supply outlet, as determined by the sensor.

28. A water heating system according to any one of claims 22 to 27, wherein a check valve is arranged to restrict the flow of water out of the hot water storage tank by way of the second port when water is drawn from the water heating system through the water supply outlet.