Summary of the invention [002] When reference is made in this specification to subjecting the water to heating by the supplementary water heater, this is to be taken to mean that the water is heated by the supplementary water heater whenever the temperature of the water is below a temperature to which the supplementary water heater is configured for heating the water.

[003] The present invention provides a water sanitising process to be performed in a water heating system which includes: a water storage means, ar solar water heater for heating water in the storage means, and a supplementary water heater for heating the water, the system having a sanitation mode of operation whereby when the system determines that a predetermined heating condition is met, the water is subjected to heating by the supplementary water heater, the process including the sanitation initiation steps of : determining a time period Pw that has passed since the water last satisfied a. predetermined sanitation-satisfaction criterion of the system; then comparing Pw with a predetermined maximum allowable period Pw. max; then if Pw is equal to or greater than Pw. max, placing the system in the sanitation mode of operation; then determining whether the predetermined heating condition is met; and then if the predetermined heating condition is met, subjecting the water to heating by the supplementary water heater.

[004] The sanitation-satisfaction criterion can be that heating of the water, that has been effected by the supplementary water heater, is terminated. Then, the termination of the heating of the water can be effected by the deactivation of the supplementary water heater.

[005] Alternatively the sanitation-satisfaction criterion can be that said water heating system is activated.

[006] When the system is in the sanitation mode of operation, a first said predetermined heating condition can be where the temperature Tw of the water is less than a predetermined minimum sanitation temperature Ts. min, the process including the steps, of : determining Tw; then comparing Tw with Ts. min; then if Tw is less than Ts. min, determining that the first predetermined heating condition is satisfied; and then if it is determined that the first predetermined heating condition is satisfied, subjecting the water to heating by the supplementary water heater.

[007] When the system is in the sanitation mode of operation, a second said predetermined heating condition can be where Tw is greater than or equal to Ts. min and where Tw has been greater than or equal to Ts. min for a time period Ps which is less than a predetermined minimum sanitation time period Ps. min, the process including the steps, after the step of comparing Tw with Ts. min, and if Tw is equal to or greater than Ts. min, of : comparing Ps with Ps. min; then if Ps is less than Ps. min, determining that the second predetermined heating condition is satisfied; and then if it is determined that the second predetermined heating condition is satisfied, subjecting the water to heating by the supplementary water heater.

[008] The step of placing the system in the sanitation mode of operation can include the step of setting Ps to zero.

[009] The step of placing the system in the sanitation mode of operation can include activating the supplementary water heater.

[010] The water sanitising process can includes the further sanitation initiation steps, after the step of comparing Pw with the predetermined maximum allowable period Pw. max, and if Pw is less than Pw. max, of : determining whether the water satisfies a. predetermined test parameter; and then if the water satisfies the predetermined test parameter, placing the system in the sanitation mode of operation.

[011] The predetermined test parameter can include that Pw is greater than a first predetermined proportion of Pw. max and that Tw is equal to or greater than a first predetermined test temperature. Then, preferably, the first predetermined proportion of Pw. max is substantially two-thirds of Pw. max and the first predetermined test temperature is substantially 35oC.

[001] The test parameter can further include that Pw is equal to or less than the first predetermined proportion, that Pw is greater than a second predetermined proportion of Pw. max and that Tw is equal to or greater than a second predetermined test temperature. Then, preferably, the second predetermined proportion of Pw. max is substantially one-third of Pw. max and the second predetermined test temperature is substantially 45oC.

[013] The test parameter further can include that Pw is equal to or less than the second predetermined proportion, that Pw is less than a third predetermined proportion of Pw. max, and that Tw is equal to or greater than a third predetermined test temperature. Then, according to one preferred embodiment, the third predetermined proportion of Pw. max is equal to the second predetermined proportion and the third predetermined test temperature is substantially 55oC, while according to another preferred embodiment the third predetermined proportion of Pw. max is substantially one-third of Pw. max and the third predetermined test temperature is substantially 55oC.

[014] The test parameter can further include that Pw is equal to or less than the first predetermined proportion, that Pw is equal to or less than a plurality of further predetermined proportions of Pw. max, that Pw is greater than a. yet further predetermined proportion of Pw. max and that Tw is equal to or greater than a further predetermined test temperature corresponding to said yet further predetermined proportion.

[015] The water sanitising process can include the sanitation initiation step, after the step of determining whether the water satisfies the predetermined test parameter, and if the water does not satisfy the predetermined test parameter, of repeating the sanitation initiation steps.

[016] The water sanitising process can include the step of exiting the sanitation mode of operation, if the system is in the sanitation mode of operation, after the step of determining whether the predetermined heating condition is met, and if the predetermined heating condition is not met. The step of exiting the sanitation mode of operation can include deactivating the supplementary water heater.

[017] The predetermined sanitation-satisfaction criterion can satisfied substantially when the step of deactivating the supplementary water heater is carried out. The process can include the step, when the predetermined sanitation-satisfaction criterion of the system is satisfied, of setting Pw to zero.

[018] The present invention also provides a water heating system including : a solar water heater having a collector, a tank and a heat exchanger to heat water in said tank with heat derived from solar energy; a heating means in said tank; and a sanitising control system connected to said heating means, the control system being configured to effect a sanitising process in which the water is heated by the heating means to a. sanitising temperature and maintained at said sanitising temperature for a predetermined sanitising period if said control system detects that a predetermined time period has elapsed since the water heating system was switched on or since the water was last maintained at said sanitising temperature for said predetermined sanitising period.

[019] The water heating system can include an instantaneous water heater configured for receiving water from said tank so as to heat the water from said tank to a desired temperature if said water is below the desired temperature. Preferably, said instantaneous water heater is a gas heater or an electric heater.

[020] The heating means can be a heat exchanger which transfers heat to said water from combustion gases. In another preferred embodiment, said heating means is an electric element.

[021] The sanitising process can be the process described in paragraph [002] above.

Brief description of the drawings [022] 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 : [023] Figure 1 is is a schematic perspective view of a first water heating system for carrying out a process according to an embodiment of the invention; [024] Figure 2 is a schematic perspective view of an alternative water heating system; and [025] Figure 3 is a block diagram representing a solar water heating control process.

Detailed description of the embodiment or embodiments [026] Referring to Figure 1, there is shown a first water heating system 10 which includes sloping solar water heating panels 12, a solar heater water storage tank 14 and an instantaneous gas water heater 16.

[027] The panels 12 have tubes 18 which are interconnected at a lower end by header pipes 20 and at an upper end by header pipes 22. The header pipes 20 are interconnected to each other, and are connected to the tank 14 by a pipe 26. The header pipes 22 are also interconnected to each other, and are connected to the tank 14 by a pipe 24. A further pipe 28 leads from the tank 14 to the water heater 16. The water heater 16 is connected to further piping (not shown) leading to locations where water in the system 10 will ultimately be used, such as domestic taps.

[028] Also included is a supplementary electric water heating element 30 disposed within the tank 14.

[029] The system 10 is controlled by a system controller 32 (shown diagrammatically in Figure 1).

[030] In normal operations, in a preferred embodiment, the system 10 is configured to operate as a thermosiphon system in which the panels 12 are oriented at an angle, with the header pipes 20 being disposed at a lower end as foreshadowed above, and the header pipes 22 being disposed at an upper end also as foreshadowed above. It will be appreciated that the tank 14 is disposed adjacent the upper end of the panels 12.

[031] Water in the tubes 18 is heated by the sun radiating onto the panels 12, which causes the water in these tubes to move upwards, by the process of natural convection, towards the header pipes 22. This water then passes via the pipe 24 into the tank 14. At the same time, water in the tank 14, which is at a cooler temperature due to not having just been heated in the panels 12, passes from the tank 14 via the pipe 26 into the header pipes 20. The system 10 thus circulates the water using the thermosiphon process just described. If desired, a pump can be used to circulate the water.

[032] When water is required for use by a user, who, for example, opens a tap, the water is drawn from the tank 14, via the pipe 28, and via the water heater 16 to the above- mentioned piping (not shown) leading to the tap in question. The gas heater 16 is configured to further heat the water from the tank 14 if it is not already sufficiently hot due to the heating effects of the panels 12.

[033] If desired, in another embodiment to that described above, the water in the tank 14 can be heated by a heat exchanger (not shown) arranged inside or outside the tank 14, with the heat exchanger being in fluid communication with the panels 12, whereby a heat transfer fluid is supplied to absorb heat from the panels 12 and to transfer that heat to the water in the tank 14. The heat transfer fluid can circulate by thermosiphoning means, or a pump can be present in the heat transfer fluid circuit to circulate the heat transfer fluid therein.

[034] The function of the electric element 30 is described below.

[035] Referring now to Figure 2, the system 40 shown includes a pair of solar panels 42 having tubes 44 extending between header pipes 46 and 48 which interconnect the respective ends of the tubes. The system 40 also includes a water tank 50 and a gas powered water heater 52.

[036] The header pipes 46 are interconnected to each other, and to a pipe 54 which leads to a port of a pump 56 which is at a position near to the lower end of the tank 50.

[037] Similarly, the header pipes 48 are interconnected to each other and are connected to a pipe 58 which leads to another port of the pump 56. A third port of the pump opens into the tank 50 at 57.

[038] A further pipe 60 leads from a position near the upper end of the tank 50 to the gas heater 52. As in the case of Figure 1, piping (not shown) leads from the gas heater 52 to positions where a user may use water from the system 40, such as domestic taps.

[039] The system 40 is also provided with a supplementary electric water element 62 in the tank 50.

[040] In normal operation, water is circulated between the tank 50 and the panels 42 by means of the pump 56. This is achieved by the pump forcing the water along the pipe 54 into the header pipes 46 so that the water then passes along the tubes 44 into the header pipes 42, whereafter it passes along the pipe 58 back to the pump 56.

[041] If desired, in another embodiment (not shown), the system 40 can be provided with a heat exchanger similar to that referred to above in relation to the system 10, which is arranged inside or outside the tank 50, with the heat exchanger being in fluid communication with the panels 42, whereby a heat transfer fluid is supplied to absorb heat from the panels 42 and to transfer that heat to the water in the tank 50. The heat transfer fluid can circulate by thermosiphoning means, or a pump can be present in the heat transfer fluid circuit to circulate the heat transfer fluid therein.

[042] The process by which water in the system 40 is used is similar to that described above in relation to the system 10.

[043] As in the case of the supplementary electric element 30 of the system 10, the use of the supplementary electric element 62 of the system 40 is described below.

[044] The system 40 also includes a controller 64 similar to the controller 32 of the system 10.

[045] In general terms, the controllers 32 and 64 operate so as to effect a sanitising process in which the water in the respective tank 14 or 50 is heated by the supplementary electric element 30 or 62 to a sanitising temperature, and maintained at the sanitising temperature for a predetermined sanitising period, if a certain condition is met. This condition is that the controller 32 or 64 detects that a predetermined time period has elapsed since the water heating system 10 or 40 was switched on, or since the water was last maintained at the sanitising temperature for the predetermined sanitising period.

[046] Reference is now made to the control process, generally designated 70, represented by the block diagram of Figure 3. Although the process 70 is described below in relation to both the systems 10 and 40, it is to be understood that the process 70 applies respectively (separately) to the systems 10 or 40.

[047] The nature of the process 70, which is described in detail below, can be summarised as follows. If the time period Pw that has passed since the water was last successfully sanitised equals or exceeds a certain maximum period of Pw. max (30 days), then the conditions for undesirable organisms and bacteria to develop in the water are considered as being ripe, and the process 70 will move into a sanitation mode of operation of the respective system 10 or 40, to sanitise the water.

[048] Even if the period Pw is less than Pw. max, then in certain circumstances the process 70 will move into the sanitation mode of operation of the system 10 or 40, while in other circumstances it will not but will instead continue to cycle until such time as Pw does equal or exceed Pw. max, or until another criterion for entering the sanitation mode of operation is met. Each such other criterion for entering the sanitation mode involves the time period Pw exceeding a certain respective predetermined proportion of Pw. max and the temperature Tw of the water being at, or above, a certain respective predetermined test temperature corresponding to that respective predetermined proportion.

[049] If the requirements for each respective predetermined proportion of Pw. max and its corresponding test temperature are met, then the sanitation mode of operation is entered, but if not, then the cycle continues.

[050] Essentially, the philosophy is that, even if the time period Pw is less than Pw. max, if the period Pw exceeds the relevant predetermined proportion of Pw. max, and the temperature is sufficiently high, then the amount of energy which is required to be added by the supplementary electric element 30 or 62, to raise the temperature Tw of the water to the minimum sanitation temperature Ts. min, will not be so great as to negatively affect the efficiency of the system 10 or 40 to an excessive extent, and it will thus be worthwhile entering the sanitation mode of operation at that stage. On the other hand, where these selected criteria are not met, then it will not be worth entering the sanitation mode of operation, and the process 70 will continue to cycle until the conditions for entering the sanitation mode are met.

[051] Essentially, the process 70 is for determining whether water in the systems 10 or 40 meets criteria for these systems to be placed in their respective sanitation modes of operation as described below. Also as described below, these sanitation modes of operation are for sanitising water in the systems 10 or 40, for the purpose of killing organisms such as the legionella bacteria.

[052] When reference is made to water in the system, this is to be understood to be the water contained in the pipes, tubes and tanks of the systems 10 or 40 as described above, excluding, in the system 10, the pipe 28, gas heater 16 and piping (not shown) leading from this gas heater 16 to positions where the water is used, and, in respect of the system 40, excluding the pipes 60, gas heater 52, and piping (not shown) leading from the gas heater 52 to the positions where the water is used. It will thus be appreciated that the pipes, tubes and tanks of the systems 10 and 40 in which the water in these systems is disposed, constitute storage means of the respective systems 10 and 40.

[053] In the block diagram of Figure 3 there are shown diamond shaped blocks and rectangular blocks. The diamond shaped blocks represent determinations to be made (that is, questions to be answered) by the respective controller 32 or 64. The point at which the controller 32 or 64 arrives at the determination to be made is represented by the vertical line joining the relevant diamond shaped block. If the answer to the relevant question represented by the block is a"yes"answer, then the progress of the process 70 is represented by the line extending to the right of the respective block. If the answer is a"no"answer, then the process 70 continues as represented by the line extending to the left of the respective block.

[054] It will be assumed that, at the start of the process 70, the relevant supplementary electric element 30 or 62 of the respective system 10 or 40 to which the process 70 relates, is in a deactivated condition.

[055] The process 70 can be considered as starting at, and proceeding from, the position 73 on line 72. The first question arrived at by the process 70 is represented by the block 74 and is as follows: Is the relevant supplementary electric element 30 or 64 in an activated state? The answer at the start of the process 70, as mentioned above, is"no".

Therefore the process 70 proceeds to the left along the line 76 to the next question, represented by the block 78. This question is: Is the time period Pw that has passed, since the water in the system 10 or 40 last complied with that system's so-called sanitation-satisfaction criteria, greater or equal to the maximum allowable time period Pw. max. (In other words, has the time period Pw that has passed, since sanitation of the water was last completed, greater or equal to Pw. max ? ) Pw. max is the time period, either set by water regulator authorities or by scientific studies, as the maximum period the water can stand before it requires sanitation. This, typically, can be 30 days or any appropriate length of time.

[056] If the time period Pw that has passed is greater or equal to the maximum allowable time period Pw. max, then the process 70 proceeds along the line 80 to the block 82.

The block 82 represents the steps of activating the supplementary electric element 30 or 62.

The block 82 also represents the step of setting to zero a time period Ps which represents the period of time for which the water in the system 10 or 40 has exceeded a temperature required for sanitation to be carried out, that is, the so-called minimum sanitation temperature Ts. min of the respective system, as described in more detail below.

[057] The step represented by the block 82 also represents the point at which the system 10 or 40 enters its so-called sanitation mode of operation, which is essentially the mode that the system 10 or 40 is in when its supplementary electric element 30 or 62 is activated or energised.

[058] After the step represented by the block 82, the process 70 continues along the line 84 which, as can be seen, joins up with the line 72. Accordingly, the question represented by the block 74 is again asked as part of the process 70. As the supplementary electric element 30 or 62 has just been activated as represented by the block 82, the answer to the question represented by the block 74 will now be"yes", so that the process 70 will now continue to the right, along the line 86. The line 86 leads to a further question represented by the block 88, which is: is the temperature Tw of the water in the system 10 or 40 equal to or greater than the above-mentioned predetermined minimum sanitation temperature Ts. min? In the preferred embodiment, Ts. min is 60°C.

[059] As the supplementary electric element 30 or 62 has only just been activated as represented by the block 82, it is likely that the answer to this question will be"no". In this event, the process 70 will proceed to the left along the line 90 to the step represented by the block 92. The step represented by the block 92 is to once again set Ps, being the time period for which the temperature Tw of the water has equalled or exceeded the minimum sanitation temperature Ts. min, to zero. This is because the temperature Tw indeed does not equal or exceed Ts. min.

[060] After the step represented by the block 92, the process 70 continues along the line 94 and then along the line 96 to once again join the line 84 which in turn again joins the line 72.

[061] The answer to the question represented by the block 74-that is: is the supplementary electric element 30 or 62 in an activated state?-is still"yes", so that the process 70 again continues to the right along the line 86. In other words, the process 70 is still in the sanitation mode of operation of the system 10 or 40.

[062] As long as the temperature Tw of the water is below the minimum sanitation temperature Ts. min, the answer to the question represented by the block 88 will be"no", so that the process 70 will continue to the left as represented by the line 90 to the step represented by the block 92, that is, to continue resetting Ps to zero and then to continue along the lines 94,96, 84 and 72 to the question represented by the block 74.

[063] It will be appreciated that during this cycle of operation, the supplementary electric element 30 or 62 is in an activated condition so that the water is continually being heated by this element so that ultimately, after sufficient such cycles, the temperature Tw of the water will equal or exceed the minimum sanitation temperature Ts. min. When this occurs, then the answer to the question represented by the block 88 will be"yes"so that the process 70 will proceed to the right as represented by the line 98 to the question represented by the block 100.

[064] This question is whether Ps (being the time period for which the temperature Tw of the water has exceeded Ts. min) is equal to or greater than a particular predetermined minimum time period for successful sanitation, Ps. min. The question, as represented by the block 100, is based on the requirement that, for the water to be successfully sanitised, its temperature Tw must not merely equal or exceed Ts. min, but must have so equalled or exceeded it for a minimum period of Ps. min.

[065] If the answer to this question, represented by the block 100, is"no", then the process 70 proceeds to the left as represented by the line 102, followed by the lines 96,84 and 72, to once again arrive at the question represented by the block 74.

[066] At this time, the supplementary electric element 30 or 62 is in an activated condition so that the process 70 again proceeds to the right as represented by the line 86.

Accordingly, in this event, it will be appreciated that the water is still being subjected to heating by the supplementary water element 30 or 62 with the process 70 still being in the sanitation mode of operation of the system 10 or 40.

[067] It will be recalled, as mentioned above, that, even if the temperature Tw of the water is equal to or exceeds the minimum sanitation temperature Ts. min, then this does not necessarily mean that sanitation of the water should be terminated. That is, the question represented by the block 100 itself represents yet a further condition to be satisfied before sanitation can be terminated, this condition being whether the water has been maintained at a sufficiently high temperature (that is, with Tw being equal to or greater than Ts. min) for a sufficient length of time (that is, with Ps being equal to or greater than Ps. min). Thus the cycle continues until such time as the temperature Tw of the water has been equal to or greater than Ts. min for a period of time Ps which is equal to or greater than Ps. min.

[068] Once this occurs, then the answer to the question represented by the block 100 will be"yes"so that the process 70 will proceed to the right as represented by the line 104, to the following step represented by the block 106. This step as represented by the block 106 is to deactivate the supplementary electric element 30 or 62 and also to set to zero Pw which, as described above, is the time period since the water in the system 10 or 40 last complied with the relevant sanitation-satisfaction criterion. The sanitation-satisfaction criterion is, in effect, the point at which sanitation of the water has been successfully completed, which is represented by the step of deactivating the supplementary electric element 30 or 62 as represented by the block 106. Thus, it will be appreciated that, at the point at which the sanitation-satisfaction criterion is complied with (ie. when the supplementary electric element 30 or 62 is deactivated), the time that has passed since this event is zero, which is the reason that Pw is set to zero.

[069] The deactivating of the supplementary electric element 30 or 62 not only represents the sanitation-satisfaction criterion being complied with, but also represents the system 10 or 40 being removed from (ie. exiting) its sanitation mode of operation.

[070] Accordingly, once the step represented by the block 106 has been carried out, the process 70 continues as represented by the lines 96,84 and 72, to once again arrive at the question represented by the block 74. At this point, as the supplementary electric element 30 or 62 is in a deactivated condition (as represented by the step identified by the block 106), the answer to the question represented by the block 74 is once again"no", so that the process 70 proceeds to the left as represented by the line 76 to the question represented by the block 78.

[071] As mentioned above, the question represented by the block 78 is whether the time period Pw, being the period since the water last complied with the sanitation-satisfaction criterion, equals or exceeds a maximum predetermined time period Pw. max. It will be recalled that when Pw did exceed Pw. max, this marked the point at which the process 70 entered the sanitation mode of operation of the system 10 or 40. In the present stage of the process 70, as the step represented by the block 106 (that is, deactivating the supplementary electric element 30 or 62 and setting Pw to zero) has only just occurred, the time period Pw will not yet equal or exceed the maximum allowable time period Pw. max, so that the answer to the question represented by the block 78 will be"no".

[072] The fact that the time period Pw since sanitation was last successfully completed does not equal or exceed the maximum allowable period Pw. max, does not necessarily mean that the water will not be subjected to the sanitation mode of operation of the system 10 or 40.

Indeed, there are further criteria which need to be addressed in determining whether or not the sanitation mode of operation of the system 10 or 40 is to be entered.

[073] The first such criterion is the question represented by the block 110. This is the question as to whether the time period Pw exceeds a first predetermined proportion, being two thirds, of Pw. max. If the answer to that question is"yes", then the process 70 continues to the right as represented by the line 112, to the further question represented by the block 114. This question is whether the temperature Tw of the water is equal to or exceeds a first test temperature, being 35°C. If the water satisfies these criteria (that is, if the answer to the questions represented by the blocks 110 and 114 are both"yes"), then the process 70 proceeds as represented by the lines 116 and 80, to the step represented by the block 82, followed by the lines 84 and 72. As the step represented by the block 82 involves activating the supplementary element 30 or 62, the carrying out of this step at this stage therefore also involves the system 10 or 40 again entering its sanitation mode of operation, as described above.

[074] On the other hand, if the answer to the question represented by the block 110 is "yes"but the answer to the question represented by the block 114 (that is, whether the temperature Tw of the water is equal to or greater than 35°C) is"no", then the process 70 proceeds to the left as represented by the line 118, followed by the line 84, and then the line 72, to once again arrive at the question represented by the block 74. It will be appreciated that, in this case, the step represented by the block 82, that is, the activating of the supplementary electric element 30 or 62, will be circumvented so that, when the question represented by the block 74 is arrived at, the supplementary electric element will be in a deactivated state. Thus the answer to that question will be"no", and the process 70 will again proceed to the left as represented by the line 76 to the question represented by the block 78.

[075] If the question represented by the block 110, that is, whether the time period Pw is greater than two thirds of Pw. max, is"no", then the process 70 proceeds to the left as represented by the line 120, to the further question represented by the block 122.

[076] This question is whether the time period Pw exceeds a second predetermined proportion, being one third, of Pw. max, in which event the process 70 proceeds to the right as represented by the line 124, to the question represented by the block 126.

[077] This is the question as to whether the temperature Tw of the water is equal to or exceeds a second test temperature, being 45°C. If the answer to the questions represented by the blocks 122 and 126 are both"yes", then the process 70 proceeds as represented by the line 128 followed again by the line 80 to the step represented by the block 82, and then as represented by the lines 84 and 72, to the question represented by the block 74. The carrying out of the step represented by the block 82, in this case, again marks the point at which the system 10 or 40 enters its sanitation mode of operation.

[078] If the answer to the question represented by the block 126, that is whether Tw is equal to or exceeds 45°C, is"no", then the process 70 proceeds to the left as represented by the line 130, followed by the line 118, and then lines 84 and 72 to again arrive at the question represented by the block 74. Thus, in this event as well, the step represented by the block 82, of activating the supplementary electric element 30 or 62, is circumvented so that the process proceeds again as represented by the lines 76 to the question represented by the block 78, thus continuing the cycle with the system 10 or 40 not being in its sanitation mode of operation.

[079] If the answer to the question represented by the block 122 is"no", that is, if the time period Pw is not greater than one third of Pw. max, then the process 70 proceeds to the left, as represented by the line 132, to the question represented by the block 134. This question is whether the time period Pw is less than one third of the Pw. max. If the answer to this question is"yes", then the process 70 proceeds to the right as represented by the line 136, to the question represented by the block 138.

[080] The question represented by the block 138 is whether the temperature Tw of the water equals or exceeds a. third test temperature, being 55°C. If the answer to this question is "yes", then the process 70 proceeds to the right as represented by the line 140, followed by the line 80, to the step represented by the block 82, which is to activate the supplementary electric element 30 or 62, and to set Ps to zero, and then to proceed as represented by the lines 84 and 72 to the question represented by the block 74. In this event, again, as the supplementary electric element 30 or 62 will be in an activated condition, the answer to the question represented by the block 74 will be"yes"so that the process 70 will proceed to the right as represented by the line 86, that is, in the sanitation mode of operation of the system 10 or 40.

[081] If the answer to the question represented by the block 134 (that is, whether the time period Pw is less than one third of Pw. max) is"no", then the process 70 proceeds to the left as represented by the line 142 followed by the line 118 and then the lines 84 and 72 to the question represented by the block 74. Similarly, if the answer to the question represented by the block 138 (that is, whether the temperature Tw of the water equals or exceeds 55°C) is "no", then the process 70 proceeds as represented by the line 144 followed by the line 118 and then the lines 84 and 72 to the question represented by the block 74. In both these cases, the step represented by the block 82 (that is, activating the supplementary electric element 30 or 62) is circumvented so that the answer to the question represented by the block 74 will be"no", and the process 70 will proceed to the left as represented by the line 76 to the question represented by the block 78.

[082] It will thus be appreciated that the block diagram of Figure 3 represents two identifiable areas. The first area is represented by the blocks 78,110, 114,122, 126,142 and 138, which is that part of the process 70 which determines whether or not the sanitation mode of operation of the system 10 or 40 is to be entered. The second area of the block diagram is represented by the blocks 88,92, 100 and 106, being the part of the process 70 within the sanitation mode of operation of the system 10 or 40, and which is adapted for determining whether the process 70 is to remain in this sanitation mode of operation, or to exit the sanitation mode of operation.

[083] The embodiments thus provide the means to monitor a solar water heating system and to sanitise the water. This is achieved, first, by monitoring the system to determine whether a sufficient period of time has elapsed for there to be significant risk of undesirable organisms, such as legionella bacteria, to have developed in the continually warm conditions resulting from the solar nature of the system. Then, the water is subjected to the type of heating conditions that are suitable for killing such organisms. At the same time, the system is adapted to make use of at least some of the heat provided by the solar features of the system itself (such as the solar heat-absorbing panels), so that the added heat energy required to effect sanitising is less than it would be in the absence of the solar features.

[084] While particular embodiments are described above, it will be appreciated that the invention can be embodied in many other forms, as will be understood by a person skilled in the art. For example, the system can be adapted with separate processes (that would be represented in separate block diagrams, unlike the diagram of Figure 3) for the two"areas"of the diagram referred to above. It will be recalled that the first such area is for determining whether or not the sanitation mode of operation of the relevant system needs to be entered. The second area is for determining whether the process is to remain in the sanitation mode of operation, or to exit the sanitation mode of operation. In other words, the aspects of control associated with these two areas might be carried out separately, with means for communicating with each other so that the system"knows"whether and when to switch from one control process to the other, that is, when the sanitation mode of operation is entered or exited.

[085] Alternatively, the system can be adapted so as not to enter the sanitation mode of operation at all in cases where the nature of heating provided by the solar features themselves are sufficient to achieve suitable sanitation of the water. Such a case may be where there are hot sunny weather conditions which are suitable for maintaining the water at a temperature equal to or exceeding Tw. max for a period equal to or exceeding Pw. max.

[086] The process embodiment represented in Figure 3 is configured to enter the sanitation mode of operation when certain specific relationships exist between elapsed time and water temperature (eg. Pw being greater than two-thirds of Pw. max and Tw being equal to or greater than 35°C). In Figure 3, three such relationship are provided, as represented by the blocks 110 and 114, 122 and 126, and 134 and 138, each being conditional on the preceding relationship. However, in other embodiments, further such relationships (i. e"levels"of interrogation) can be provided so as to refine the process, whereby the controller will operate with a"fuzzy logic"so as to take advantage of the solar energy to thereby reduce the amount of heat needed to get the water up to sanitising temperatures. For example, in various embodiments, there may be between 1 and 100 further such relationships, each being a relationship between elapsed time (in the sense of a predetermined proportion of Pw. max) and a corresponding temperature. Thus each such relationship will constitute a further level of interrogation, whereby, if the parameters of a particular relationship are satisfied, the process will enter the sanitation mode of operation.

[087] While an electric element 30 or 62 is indicated above, a gas burner can be utilised instead. This may require a modified tank design, or a flued tank to be used.

[088] 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.

[089] 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.

[090] 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.