WO2007068031A1

WO2007068031A1 - A circulating water heater

Info

Publication number: WO2007068031A1
Application number: PCT/AU2006/001801
Authority: WO
Inventors: Brendan Bourke; Sean Box; George Echtner
Original assignee: Rheem Australia Pty Ltd
Current assignee: Rheem Australia Pty Ltd
Priority date: 2005-12-15
Filing date: 2006-11-29
Publication date: 2007-06-21
Anticipated expiration: 2008-06-15

Abstract

The present invention provides a water heater system (100) including: a water storage tank (102) ; a heat source (114) including a water path; a pump (110) adapted to circulate water from the tank through the water path of the heat exchanger; a first temperature sensor (108) adapted to measure temperature of the water exiting from the heat exchanger; a controller (116) responsive to at least first temperature sensor (108) to control the operation of the pump (110) and a method of heating water in a water heater system.

Description

A circulating water heater

Field of the invention

[001] This invention relates to a water heater system and a method of operating the water heater system

Background of the invention

[002] The invention is suitable for use with an external unregulated heat source such as a solar collector or the heat sink of a heat pump of a refrigeration system, but can also be implemented with other heat sources, and the use of the invention need not be limited to unregulated heat sources.

[003] Any reference herein to known prior art does not, unless the contrary indication appears, constitute an admission that such prior art is commonly known by those skilled in the art to which the invention relates, at the priority date of this application.

Summary of the invention

[004] The invention provides a system and method for operating a water heating system which uses a circulating pump to draw water from one level within the tank and returns it to a second level within the tank.

[005] This can be used to control the mode of heating, the rate of reheating, the efficiency of heat transfer, and other parameters of the system.

[006] The circulating pump pumps the water to a heat source.

[007] The heat source can be external to the tank, or it can be internal.

[008] In a further embodiment, the system can include both internal and external heat sources.

[009] The operation of the pump is controlled by a controller which is responsive to one or more temperature sensors. The controller can be programmed to perform one or more different types of heating cycle. The use of an array of temperature sensors which sense the temperature of the water at different levels within the tank enable the system to be used to heat a predetermined volume of water to within a predetermined temperature range. In addition to maintaining a chosen volume of water at a desired temperature, the controller may be programmed to periodically perform a pasteurization cycle where water in the tank is maintained at or above a pasteurization temperature for a predetermined time period.

[010] The controller can be responsive to one or jnore temperature sensors. For example, the controller can be responsive to one of the sensors of the array to determine the temperature at a specified depth in the tank, and then the controller can use a heat source temperature sensor to determine the period and/or flow rate operating conditions of the water pump.

[011] According to one embodiment of the invention, there is provided a water heating system including: a water storage tank, a heat source, a water pump, one or more temperature sensors, and a controller responsive to at least one temperature sensor to control the operation of the pump.

[012] The water pump can include a pump inlet connected to draw water from near the bottom of the tank, and a pump outlet connected to feed water into the tank near the top of the tank.

[013] The heat source can include an external heat source which is external to the tank.

[014] The heat source can include an internal heat source within the tank.

[015] The temperature sensor can include an array of one or more temperature sensors located to sense the temperature of the water at one or more corresponding depths within the tank.

[016] The temperature sensor can include a heat source temperature sensor adapted to measure the temperature of the heated water leaving the heat source.

[0.17] The controller can control the duration of the pump operation

[018] The controller can control the duty cycle of the pump.

[019] The controller can control the flow rate of the pump.

[020] The water heating system can include an internal heat source and an external heat source.

[021 ] The water heating system can be adapted for top-down heating.

[022] A diffuser can be included at the hot water inlet to the tank to provide a diffused flow into the tank. [023] The system can be adapted to heat a chosen volume of water within the tank, by the provision of a plurality of temperature sensors at selected heights to measure the temperature of the water at corresponding heights within the tank.

[024] In a further embodiment, there is provided a water heater system including: a water storage tank; a heat source including a water path; a pump adapted to circulate water from the tank through the water path of the heat exchanger; a first temperature sensor adapted to measure the temperature of the water exiting from the heat exchanger; a controller controlling the rate of circulation through the water path in response to the temperature sensor.

[025] According to another embodiment there is provided a water heating system including: a water storage tank; an internal heat source; a pump adapted to circulate water from the tank through an external or internal water path; a first temperature sensor adapted to measure the temperature of water within the tank; a controller controlling the rate of circulation through the water path in response to the temperature sensor.

[026] The heat source can be a heat exchanger with .a heat transfer fluid path.

[027] The heat source can include an electric heating element.

[028] The heat source can include a gas heater.

[029] The heat source can include a solar collector.

[030] The duty cycle of the pump can be controllable.

[031 ] The pump can have a controllable flow rate.

[032] The sensors can include a strip array of temperature sensors.

[033] The controller can be responsive to one or more of the temperature sensors to control the heating process.

[034] A further temperature sensor can be provided to measure the temperature of the water entering the heat source.

[035] The controller can calculate the required circulation rate to raise the temperature of the water exiting the water path of the heat exchanger to a predetermined value.

[036] The controller can calculate the time required to raise the temperature of the water exiting the water path to a predetermined value.

[037] A heat transfer fluid pump can be provided to circulate heat transfer fluid through the heat transfer fluid path of the heat exchanger. [038] The heat transfer fluid pump can be controlled by the controller.

[039] A first valve controlled by the controller can control the flow of heat transfer fluid through the heat transfer fluid path of the heat exchanger.

[040] The system can include a heat transfer fluid by-pass path.

[041] The system can also include a heating element.

[042] The heating element can be an electric element within the tank.

[043] The heating element can be located within the tank to provide top-down heating.

[044] The invention also provides a method of heating water in a water heater system including a water storage tank, a heat source, a water pump, one or more temperature sensors, and a controller responsive to at least one temperature sensor to control the operation of the pump; the method including the steps of: sensing the temperature of water at at least one depth within the tank using one or more selected temperature sensors to sense the temperature of a predetermined volume of water within the tank; determining whether the measured temperature is within a first predetermined range or above a first predetermined threshold temperature; when the temperature is below the required value or range, initiating the operation of the pump; monitoring the temperature from the or each selected temperature sensor to determine when the temperature of the predetermined volume of water has attained a second predetermined range or threshold, switching off the pump when the temperature of the predetermined volume of water has reached the second predetermined range or threshold.

[045] The invention also provides a method of heating water in a water heater system including: a water storage tank; a heat exchanger including a heat transfer fluid path and a water path; a pump adapted to circulate water from the tank through the water path of the heat exchanger; a first temperature sensor adapted to measure the temperature of the water exiting from the heat exchanger; a controller controlling the rate of circulation through the water path in response to the temperature sensor; the method including the steps of: measuring the temperature of the water at at least one point in the tank; initiating a heating process when the temperature at at least one point in the tank falls below a corresponding threshold value; the heating process including: turning the pump on; measuring the temperature of the water exiting the heat exchanger; controlling the water flow rate based on the temperature of the water exiting the heat exchanger; turning the pump off when the temperature at at least one point in the tank rises above a corresponding threshold value. Brief description of the drawings

[046] An embodiment or embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[047] Figure 1 shows a water heating system according to a first embodiment of the invention.

[048] Figure 2 shows a water heating system according to a second embodiment of the invention.

[049] Figure 3 shows a water heating system according to a third embodiment of the invention.

[050] Figure 4 shows a water heating system according to a fourth embodiment of the invention.

[051 ] Figure 5 shows a method of operating the system of Figure 1.

[052] Figure 6 shows an alternative method of operating a system according to an embodiment of the invention which includes a heating element.

Detailed description of the embodiment or embodiments

[053] Figure 1 is a schematic illustration of a water heating system 100 which includes a storage tank 102 with the usual cold water inlet and a hot water outlet (not shown for clarity) to provide the water for use in the household or other application.

[054] Tank 102 is connected to a heat exchanger 112 via inlet pipe 104 and outlet pipe

106. Pipe 106 draws water from the lower part of tank 102 and connects to first pump 110 which is connected to the water path of heat exchanger 112. The heat exchanger 112 can be any suitable heat exchanger. A brazed plate heat exchanger (BPHE) provides efficient heat exchange. The water path of the heat exchanger 112 can include water inlet header and water outlet header (as shown at 438 and 436 of Figure 4). The water and heat transfer fluid flow through different paths in the heat exchanger, the paths being separated by one or more common walls through which heat is transferred.

[055] The heat transfer fluid can convey heat from a suitable source of heat, such as a refrigeration circuit 114 or air conditioner in which the heat transfer fluid carries the excess heat from the refrigeration or cooling equipment to the heat exchanger 112. The heat transfer fluid can be a refrigerant. [056] One or more first temperature sensors 108 sense the temperature of the water in the tank 102. Preferably there are two or more temperature sensors which measure the temperature of the water in the tank 102 at corresponding depths. The temperature sensors may be provided in the form of a flexible PCB temperature sensor strip as described in our co- pending application number PCT/AU2005/001750, the contents of which is incorporated herein by reference. The array of temperature sensors permit the temperature of the water at different heights within the tank to be measured so that the temperature of a predetermined volume of water can be monitored by a corresponding one of the sensors.

[057] The temperature sensors are connected to a controller 116. Controller 116 controls the operation of pump 110. Pump 110 can be a single speed pump or a variable speed pump. The controller can control the flow rate by controlling the pump speed where the pump is a variable speed pump. In the case where the pump does not have a controllable speed, the duty cycle can be varied by running the pump for a first predetermined period and then switching it off for a second predetermined period, and repeating this cycle as required to obtain the required average flow rate. The controller is programmed to take account of the flow-rate/heat-transfer characteristics of the heat exchanger. These characteristics may be determined empirically or calculated from the dimensions and heat transfer characteristics of the heat exchanger.

[058] For example a fast flow rate will result in a lower temperature rise per minute while a slower flow rate will provide the water with a greater increase in temperature. Where there is a large difference between the temperature of the heat transfer fluid and the temperature of the water, the water may heat more quickly to a predetermined temperature by the use of a slow flow rate. More efficient heating may be obtained by increasing the flow rate.

[059] The temperature of the water in the tank can be controlled to maintain the temperature within one of one or more preselected temperature ranges each having an upper temperature and a lower temperature, hi addition, the depth at which the temperature is measured can be selected when there are two or more tank water temperature sensors located to measure the temperature at different depths. This makes it possible to control the amount of water which is heated in relation to the demand.

[060] On the heat transfer fluid side of the heat exchanger 112, a second pump (not shown) can be arranged to circulate the heat transfer fluid'through the heat transfer fluid path of the heat exchanger 112. This pump can be controlled by controller 116. [061] When the water in the tank reaches the upper limit of its preselected operating range, the controller 116 turns off the first water pump 110 so that no further water is circulated through the heat exchanger 112 while the water remains within the selected operating temperature range.

[062] It may also be desired to stop the circulation of the heat transfer fluid through the heat exchanger 112 when the water pump 110 is turned off. This may be done, for example, to prevent overheating of the water in the water path of the heat exchanger 104. Accordingly, the controller 116 can be configured to turn the heat transfer fluid pump (not shown) off.

[063] In some cases, it may be preferred that the circulation of the heat transfer fluid is not interrupted. In this case, the heat transfer fluid can be diverted through a bypass path which diverts the heat transfer fluid around the heat exchanger 112. This is described below with reference to Figure 4.

[064] Controller 116 can be programmed by the user to monitor and control selected parameters. For example, the user can select a storage temperature within a range of temperatures. The user can also select the volume of water to be heated.

[065] In operation, the temperature sensor 108 measures the temperature at the chosen operating point at the selected depth within the tank 102 and the controller determines whether additional heating is required.

[066] Additional heat sensors, 130 and 128, can be provided to measure the temperature of the water entering and leaving the heat exchanger 112. In addition, an ambient sensor 150 and a heat transfer fluid sensor (not shown) can be provided. Accordingly, the temperature at various points in the system can be monitored and used to control the operation of the system.

[067] The controller 116 controls the operation of the pump 110.

[068] Accordingly, the controller can calculate the rate of heat transfer within the heat exchanger 112, for example, based on the flow rate of the pump and the water outlet temperature of the heat exchanger.

[069] Given that the controller has information relating to power available from the heat source, the temperature of the water in the tank at several points, the flow rate of the pump, as well as the water temperature entering and leaving the heat exchanger, the controller can thus calculate the flow rate required to raise the temperature of the water to the chosen value. [070] In an alternative, the pump can be turned on until the temperature of the chosen volume of water is within the chosen range.

[071] A diffuser can be provided at the heated water inlet 104 to minimize the turbulence. This facilitates the stratification of the water so that the desired volume to be heated is readily defined by the stratification between the hot and cold water. This is known as top- down heating.

[072] The chosen volume of water to be heated can be varied by choosing a sensor or sensors which define the new volume. Thus the system can provide variable volume top-down heating.

[073] In one mode of operation, the system can be used to provide variable volume, top- down heating. In this mode, the volume is chosen in accordance with the location of a temperature sensor in the array 108. The chosen sensor informs the controller when the temperature has fallen below a chosen operating point. The controller 116 starts the pump 110, which, in this example is assumed to be a constant speed pump. The pump draws water from the bottom of tank 102 and delivers it to the heat exchanger 112. Temperature sensor 128 monitors the temperature of the water leaving the heat exchanger 112. The controller 116 calculates the duty cycle of pump (or the speed and duration of operation of the pump if a variable speed pump were being used) to control the amount of heat required to raise the temperature of the predetermined volume of water to a second threshold higher than the temperature threshold at which the controller initiated the operation of the pump.

[074] Figure 2 illustrates a second system in which the numbers indicating similar components are numbered similarly to those in Figure 1 with the change that the prefix number is 2, while in Figure 1 the prefix number is 1. Similarly for Figures 3 and 4.

[075] In Figure 2, the external heat source 212 can be any appropriate heat source, and can be a regulated heat source, such as a gas heater or an electric heater, or it can be an unregulated heat source. The controller 216 controls pump 210 as for the embodiment described in relation to Figure 1.

[076] In Figure 3, a heater element 342 is provided in place of the external heater of the previous embodiments. The element 342 is an electrical element. This heating element is preferably located at a height above the bottom of the tank. This enables the system to be operated in element heating mode when the pump 310 is not operated, hi this mode, the pump 310 is not used, and the heating element 342 heats the water above the element in the tank. The temperature sensors 308 indicate to the control 316 when the water above the element 342 has reached the desired temperature range, and the element is turned off.

[077] However, the inclusion of the pump 310 provides a means of circulating the water so that the water in the lower portion of the tank can be heated by gradual mixing with heated water in the upper tank or near the element, enabling the whole volume of water in the tank to be heated if required. This is a form of top-down heating.

[078] The pump can be operated to circulate the water through the tank 302.

[079] The system of Figure 3 has the flexibility to operate as a stand alone water heater using the heating element 342 in either element heating or top -down heating mode to heat the water above the element 342, or in a top-down mode in which the circulating pump is used to remove cooler water from the bottom of the tank and to return it to the top of the tank where it is heated when it mixes with the water heated by the heater element 342.

[080] This enables either element heating using the internal heating element 342 or top - down heating using the heating element 342 and the circulating pump 310.

[081] In a system with an internal heating element and no pump, element heating on its own can be used only to heat the volume above the heating element 342, so that once the water above the element reaches the desired temperature range, the heating element 342 is turned off.

[082] However, when the pump is included in a system with an internal heating element, water from the bottom of the tank can be added to the top of the tank, preferably via a diffuser to reduce turbulence, so that top-down heating can proceed to any chosen level in the tank as detected by the temperature sensors 308. The returned water is added to the top of the tank at a rate which gives the desired temperature rise for a given power input of the heating element. Drawing water from the bottom of the tank and returning it at the top causes the stratification layer between the hot and cold water to descend within the tank. The rate of circulation of the pump is controlled to endeavour to retain the temperature of the selected heated volume of water within preferred operational limits. The returned water is circulated at a rate that gives the desired temperature rise for a given power input to the electric heating element and therefore the mixed water remains heated.

[083] As water is added to the top of the tank, the level of heated water is pushed downwards below the electric heating element. [084] The water continues to be circulated by the pump 310 until a predetermined volume or level of water at a predetermined temperature is reached. This is effectively top-down heating.

[085] This can be continued or discontinued at any point between where the predetermined water temperature is adjacent to the element, down to where the temperature differential between the water at the highest point and the low pick-up point is nil.

[08.6] This predetermined volume may be varied, at any time from manufacture to operation, by adjusting the output to the circulating pump in response to an input from the temperature sensors on the storage tank. This is effectively variable volume top-down heating.

[087] Figure 4 illustrates a further embodiment wherein the heat transfer fluid can be part of an independently operating refrigeration or air conditioning system 414, 418, 424, 420.

[088] Another feature of the system illustrated in Figure 4 is the provision of both the external heat source in the form of a heat exchanger 412, and an internal heating element 442.

[089] Tank 402 includes a plurality of temperature sensors 408 which measure the temperature at different depths within the tank. The temperature readings from one or more selected sensors are reported to the controller 416 which controls the pump 410 to draw water from the bottom of the tank at 440 and circulate it through the heat exchanger 412 and return it to the top of the tank at 444 when heating is required.

[090] . The heat exchanger is a brazed plate heat exchanger having a cool water inlet header 438 and a warm water outlet header 436. A temperature sensor 428 measures the temperature of the water exiting the BPHE.

[091] Then heat transfer fluid from a heat pump system such as refrigeration system 414 is pumped through the heat transfer fluid side of the heat exchanger 412 by heat transfer fluid pump 418 to provide the heat to heat the water.

[092] A bypass path is provided by a heat sink such as radiator 420. A pair of valves

422, 424 are provided to divert the heat transfer fluid away from the heat exchanger 412 under the control of the controller 416 when the water does not require heating.

[093] The provision of both an external and an internal heat source provides additional operating modes including using only the external heat source 412, or the internal heat source 442 on its own, or a combination of external and internal heat sources. [094] Means to regulate the water flow rate can also be provided so that the temperature of the water drawn off by the user is within the predetermined operating range. A controllable valve 450 can be controlled b y the controller 416 in response to the temperature of the outlet water, for example as measured by the top sensor of the sensor array 408.

[095] The operation of the embodiments will now be described with reference to the embodiment shown in the flow diagram of Figure 5.

[096] At step 504, the controller continually monitors the temperature of the water in the tank to ensure that the chosen volume of water is above a chosen threshold value. This can be determined from the reading of one or more of the temperature sensors 108 which monitor the temperature of the chosen volume of water.

[097] If the water is above the chosen temperature, the process diverts to step 522 and the water pump or the water pump and the heat transfer fluid pump are turned off.

[098] If the temperature is below the chosen range, the process moves to step 506 where the heat transfer fluid pump and the water pump are energized to circulate the water and the heat transfer fluid through the heat exchanger.

[099] With the heat transfer fluid and water flowing through the heat exchanger, the controller reads measurements of the input temperature of the water entering the heat exchanger. A measurement of the ambient air temperature may also be taken.

[0100] The controller then determines the flow rate and/or duration required to optimally raise the temperature of the water to the desired range at step 510.

[0101] At step 512, the controller runs the pump at the chosen speed for the calculated period.

[0102] The controller monitors the output temperature from the heat exchanger at 514.

[0103] At 516, the controller checks whether the temperature is within the desired range.

[0104] If the temperature is not within the desired range, an adjustment factor is calculated to recalibrate the time/duration calculation at 510.

[0105] If the temperature measured at 514 is within the desired range, the process returns to step 504 and monitoring resumes. When the stored water temperature is within the desired range, the decision step 504 turns the water pump off at 522. [0106] Figure 6 shows an alternative method of operating a system which includes a heating element such as 342 .

[0107] The controller checks the temperature/volume of the stored water at step 604. If there is sufficient hot water, the controller de-energizes the heat source and water pump at 622.

[0108] If there is insufficient hot water, the controller energizes the heat source at step

606, and monitors the temperature of the water that is directly heated by the heat source.

[0109] The controller checks whether or not the heated water temperature is above an upper threshold value. If the temperature exceeds the upper threshold value, the controller energizes the water pump at step 612 so that the water in the tank is circulated.

[0110] The controller then returns to step 604 and monitors the temperature of the stored water in the tank, and, when there is sufficient hot water, the controller changes to step 622 and turns the heating element off.

[0111] Returning to step 610, in the case where the water is not above the upper set point while the heating element is energized, the controller goes to step 614 and checks whether the water is above a lower set point. If the water temperature is not above the lower set point, the controller de-energizes the pump at step 616 so that the water above the heating element will heat more quickly.

[0112] If the water temperature is above the lower set point, the controller returns to step

604 and checks whether there is sufficient hot water. This method provides a means for ensuring that the water above the heating element can be heated quickly, and, when the water above the heating element is heated above an upper limit, the pump is turned on to heat the remainder of the water in the tank.

[0113] The water heating system of the invention can be embodied in various combinations of features and operating modes, and the following examples are provided as a non-limiting embodiments:

[0114] tank, controller, external heat source, water pump, tank temperature sensor measuring the water temperature at one or more depths - controller operates pump to raise a predetermined volume of water to a predetermined threshold;

[0115] tank, controller, external heat source, water pump, tank temperature sensor measuring the water temperature at one or more depths, heat source temperature sensor measuring the outlet temperature of the heat source - controller calculates period to run pump to raise temperature of predetermined volume of water to predetermined threshold;

[0116] tank controller, internal heat source, water pump, temperature sensor measuring the water temperature at one or more depths — controller responds to one or more tank temperature sensors to raise a predetermined volume of water to a desired threshold;

[0117] tank controller, internal heat source, external heat source, water pump, temperature sensor measuring the water temperature at one or more depths - controller responds to one or more tank temperature sensors to raise a predetermined volume of water to a desired threshold.

[0118] In the embodiments described above, the thermal characteristics of the system were determined empirically. However, if desired the equations for control of the pump can be determined theoretically. Parameters which may be used to control the operation of the system include surface area and thermal conductance of the heat exchanger, temperature differentials, water and heat transfer fluid thermal capacity, flow rates, etc.

[0119] The system of the present invention can be used to control various parameters such as the temperature of a chosen volume of water, the rate of heating and hot water recovery, pasteurization cycle, heat transfer efficiency, refrigerant cooling.

[0120] Where ever it is used, the word "comprising" is to be understood in its "open" sense, that is, in the sense of "including", and thus not limited to its "closed" sense, that is the sense of "consisting only of. A corresponding meaning is to be attributed to the corresponding words "comprise", "comprised" and "comprises" where they appear.

[0121] It will be understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text. All of these different combinations constitute various alternative aspects of the invention.

[0122] While particular embodiments of this invention have been described, it will be evident to those skilled in the art that the present invention may be embodied in other specific forms without departing from the essential characteristics thereof. The present embodiments and examples are therefore to be considered in all respects as illustrative and not restrictive, and all modifications which would be obvious to those skilled in the art are therefore intended to be embraced therein.

Claims

1. A water heating system including: a water storage tank, a heat source, a water pump, one or more temperature sensors, and a controller responsive to at least one temperature sensor to control the operation of the pump.

2. A water heating system as claimed in claim 1 , wherein the water pump includes a pump inlet connected to draw water from near the bottom of the tank, and a pump outlet connected to feed water into the tank near the top of the tank.

3. A water heating system as claimed in any one of the preceding claims, wherein the heat source includes an external heat source which is external to the tank.

4. A water heating system as claimed in any one of the preceding claims, wherein the heat source includes an internal heat source within the tank.

5. A water heating system as claimed in any one of the preceding claims, wherein said at least one temperature sensor includes an array of one or more temperature sensors located to sense the temperature of the water at one or more corresponding depths within the tank.

6. A water heating system as claimed in any one of the preceding claims, wherein said at least one temperature sensor includes a heat source temperature sensor adapted to measure the temperature of the heated water leaving the heat source.

7. A water heating system as claimed in any one of the preceding claims, wherein the controller controls the duration of the pump operation.

8. A water heating system as claimed in any one of the preceding claims, wherein the controller controls the duty cycle of the pump.

9. A water heating system as claimed in any one of the preceding claims, wherein the controller controls the flow rate of the pump.

10. A water heating system as claimed in any one of the preceding claims, including an internal heat source and an external heat source.-

11. A water heating system as claimed in any one of the preceding claims, adapted for top- down heating.

12. A water heater system including: a water storage tank; a heat source including a water path; a pump adapted to circulate water from the tank through the water path of the heat exchanger; a first temperature sensor adapted to measure the temperature of the water exiting from the heat exchanger; a controller controlling the rate of circulation through the water path in response to the temperature sensor.

13. A water heating system including: a water storage tank; an internal heat source; a pump adapted to circulate water from the tank through an external water path; a first temperature sensor adapted to measure the temperature of water within the tank; a controller controlling the rate of circulation through the water path in response to the temperature sensor.

14. A water heating system as claimed in any one of the preceding claims, wherein the heat source includes an electric heating element.

15. A water heating system as claimed in wherein the heat source includes an electric heating element.

16. A water heating system as claimed in any one of the preceding claims, wherein the heat source includes a gas heater.

17. A water heating system as claimed in any one of the preceding claims, wherein the heat source includes a solar collector.

18. A water heating system as claimed in any one of the preceding claims, wherein the duty cycle of the pump is controllable.

19. A water heating system as claimed in any one of the preceding claims, wherein the pump has a controllable flow rate.

20. A water heating system as claimed in any one of the preceding claims, wherein the water heating system includes one or more tank water temperature sensors adapted to measure the temperature of the water in the tank at one or more depths.

21. A water heating system as claimed in any one of the preceding claims, wherein the sensors include a strip array of temperature sensors.

22. A water heating system as claimed in any one of the preceding claims, wherein the controller is responsive to one or more of the temperature sensors to control the heating process.

23. A water heating system as claimed in any one of the preceding claims, including an additional temperature sensor to measure the temperature of the water outlet from the heat source.

24. A water heating system as claimed in any one of the preceding claims, including a further temperature sensor to measure the temperature of the water entering the heat source.

25. A water heating system as claimed in any one of the preceding claims, wherein the controller is adapted to calculate the required circulation rate to raise the temperature of the water exiting the water path of the heat exchanger to a predetermined value.

26. A water heating system as claimed in any one of the preceding claims, wherein the controller is adapted to calculate the time required to raise the temperature of the water exiting the water path to a predetermined value.

27. A water heating system as claimed in any one of the preceding claims, including a heat transfer fluid pump to circulate heat transfer fluid through the heat transfer fluid path of the heat exchanger.

28. A water heating system as claimed in claim 27, wherein the heat transfer fluid pump can be controlled by the controller.

29. A water heating system as claimed in any one of the preceding claims, including a first valve controlled by the controller can control the flow of heat transfer fluid through the heat transfer fluid path of the heat exchanger.

30. A water heating system as claimed in claim 29, including a heat transfer fluid by-pass path.

31. A water heating system as claimed in any one of the preceding claims, including a heating element located within the tank to provide top-down heating.

32. A method of heating water in a water heater system including a water storage tank, a heat source, a water pump, one or more temperature sensors, and a controller responsive to at least one temperature sensor to control the operation of the pump; the method including the steps of: sensing the temperature of water at at least one depth within the tank using one or more selected temperature sensors to sense the temperature of a predetermined volume of water within the tank; determining whether the measured temperature is within a first predetermined range or above a first predetermined threshold temperature; when the temperature is below the required value or range, initiating the operation of the pump; monitoring the temperature from the or each selected temperature sensor to determine when the temperature of the predetermined volume of water has attained a second predetermined range or threshold, switching off the pump when the temperature of the predetermined volume of water has reached the second predetermined range or threshold.

33. A method as claimed in claim 32, including the steps of: measuring the temperature of the water at the outlet of the heat source; calculating the time and/or the flow rate of the pump to raise the temperature of the predetermined volume of water to the predetermined threshold running the pump for the calculated period.

34. A method as claimed in claim 33, including the steps of: sensing the temperature of the predetermined volume of water at the end of the calculated period; determining the difference between the temperature of the predetermined volume of water and the predetermined temperature threshold; calculating a correction factor where the difference is greater than a permitted variance.

35. A method of heating water in a water heater system including: a water storage tank; a heat exchanger including a heat transfer fluid path and a water path; a pump adapted to circulate water from the tank through the water path of the heat exchanger; a first temperature sensor adapted to measure the temperature of the water exiting from the heat exchanger; a controller controlling the rate of circulation through the water path in response to the temperature sensor; the method including the steps of: measuring the temperature of the water at at least one point in the tank; initiating a heating process when the temperature at at least one point in the tank falls below a corresponding threshold value; the heating process including: turning the pump on; measuring the temperature of the water exiting the heat exchanger; controlling the water flow rate based on the temperature of the water exiting the heat exchanger; turning the pump off when the temperature at at least one point in the tank rises above a corresponding threshold value..

36. A water hearting system substantially as herein described with reference to the embodiments of the invention illustrated in the accompanying drawings.

37. A method of operating a water heating system substantially as herein described with reference to the embodiments of the invention illustrated in the accompanying drawings..