Field of the Invention The present invention relates to a method of operating a water chiller. The invention has been primarily developed in relation to a combined water chiller and boiling water heater unit and will be described hereinafter with reference to this application. However, it will be appreciated that the invention is not limited to this particular field of use and also suitable for use in a stand alone water chiller unit.

Background of the Invention A known combined water chiller and boiling water heater unit provides instant boiling and instant chilled water from a single tap. The tap is typically mounted on a bench top or on a sink and the heater and chiller are housed together in a module, commonly in a cupboard under the sink. The unit includes a boiling water storage tank as well as a chilled water storage tank. An electronic controller controls both the boiling and chiller units. The chiller unit has a complete refrigeration circuit which includes a compressor, a condenser, a fan and an evaporator. The chiller unit also has a chilled water tank with the evaporator (ie. cooling coil) and level and temperature sensors therein. As per any refrigeration plant, to achieve cooling, heat must be removed. The refrigeration process involves the refrigerant being compressed through the compressor. This compression also raises the temperature of the refrigerant. The refrigerant then passes through a heat exchanger, known as a condenser, which cools the refrigerant. Thereafter the refrigerant passes through an evaporator which allows the refrigerant to expand causing the refrigerant to cool rapidly. This evaporator is submerged in the chilled water tank. As the water is hotter than the refrigerant, heat is removed from the water and passed into the refrigerant through the evaporator coils. The refrigerant then passes through the compressor again and the cycle starts over. The heat is removed through a heat exchanger condenser that is force air cooled via an electric fan. The air is thus the cooling medium and its temperature rises. The effect of all of this is that the ambient air temperature within the unit, and within the cupboard, rises. Cupboards are often not well ventilated and as such the temperature of ambient air in the cupboard can rise noticeably. It follows that the hotter the ambient air, the less efficient the cooling process. It should also be noted that water contracts in size as it cools to a temperature of 4 0C. As the water cools from 4 to 0 0C it expands again until the water is completely frozen. As water freezes the temperature remains constant at 0 0C until all the water has frozen and thereafter the temperature will continue to drop. It is important that the water in the unit's chilled water tank is not allowed to freeze, as this can cause the tank to rupture. A known approach to this issue has involved shutting down the compressor and indicating a fault if the compressor has been running continuously for one hour with no water being drawn off from the unit. This is based on the assumptions that, under normal circumstances, a compressor will normally only run for 5 to 10 minutes at a time if no new water is being introduced to the tank and that one hour continuous running is insufficient to completely freeze all of the water in the tank. Units of this type suffer from the problem that: if the cupboard is not sufficiently ventilated; the incoming ambient water temperature is high; and the unit is required to work continuously due to the demands of chilled water, then this can result in the ambient air temperature inside the cupboard becoming so high that the chiller is only able to reduce the water temperature to about 7 or 8°C. Accordingly, the rate at which the unit is attempting to dissipate heat is the same rate at which the unit is absorbing heat. A state of equilibrium is thus reached and no further cooling of the water occurs. As a result, even if water is not being drawn off, the compressor runs continuously and upon reaching an hour the unit indicates a fault and shuts down. This results in a service call being required, which is both a cost and a source of dissatisfaction to the user.

Object of the Invention It is the object of the present invention to substantially overcome at least ameliorate one or more of the above prior art deficiencies.

Summary of the Invention Accordingly, in a first aspect, the present invention provides a method of operating a water chiller having: a water tank, a cold water tap, a condenser and a fan, the method including the following steps: (a) monitoring a first predetermined time period since the cold water tap has been activated and if the first time period has not been reached then the chiller is said to be operating in normal mode and the method includes returning to step (a) or if the first time period has been reached then the chiller is said to be operating in protection mode and the method includes proceeding to step (b); (b) determining if the compressor is on or off and if the compressor is off then proceeding to step (c) or if the compressor is on then proceeding to step (e); 5 (c) determining if a predetermined high set point temperature for the water in the tank has been reached and if the high set point has been reached then proceeding to step (d) or if the high set point has not been reached then returning to step (b); (d) turning the compressor on then returning to step (b); (e) determining if a predetermined low set point temperature for the water I0 in the tank has been reached and if the low set point has been reached then proceeding to step (f) and if the low set point has not been reached then proceeding to step (g); (f) turning the compressor off, turning the fan on for a second predetermined time period and then returning to step (b); (g) determining if a third predetermined time period has elapsed since the is chiller entered the protection mode and if the third time period has been reached then proceeding to step (h) and if the third time period has not been reached then returning to step (b); (h) determining if a fourth predetermined time period has elapsed and if the fourth time period has been reached then proceeding to step (i) or if the fourth time period 0 has not been reached then returning to step (b); (i) measuring the temperature of the water in the chiller at least three times at intervals of a fifth predetermined time period and calculating an first average temperature then proceeding to step (j); (j) waiting for a sixth predetermined time period another then measuring 5 the temperature of the water in the chiller and calculating a second average temperature for the last at least three measurements; (k) comparing the first and second average temperatures and if the first average temperature is less than the second average temperature then returning to step (b) or ifthe first temperature is equal to or more than the second average temperature then o proceeding to step (1); (1) turning the compressor off and keeping the fan on then proceeding to step (m); and (m) waiting for a seventh predetermined period of time before returning to step (b) wherein if the cold water tap is activated when the chiller is in the protection mode then the chiller is altered to the normal mode. The first, second, third, fourth, fifth, sixth and seventh predetermined time periods are preferably approximately 30, 5, 30, 5, 5, 5 and 30 minutes respectively. The low set point and the high set point temperatures are preferably approximately 4.7 0C and 10.00C respectively. Step (i) preferably includes measuring the temperature of the water in the chiller three times before calculating an average.

Brief Description of the Drawings An embodiment of the invention will now be described, by way of an example only, with reference to the accompanying drawings in which: Fig. 1 is a schematic diagram of the components of a water heater in accordance with an embodiment of the invention. Fig. 2 is a logic diagram associated with an embodiment of a method for operating a heater according to the invention; and Fig. 3 is a further logic diagram associated with the method set out in Fig. 2.

Detailed Description of the Preferred Embodiments Referring firstly to Fig. 1, there is shown an embodiment of a water chiller 10 in accordance with an embodiment of the present invention. The chiller 10 forms a part of a combined boiling water heater and instant chilled water unit but the components of the boiling water heater are not shown for the sake of clarity. The chiller 10 includes an insulated water tank 12 which has chilling evaporator coils 14 and a temperature sensor (not shown) therein. The tank 12 also has a cold water tap (not shown) to enable users to draw water from the tank 12. The chiller 10 also includes a PCB controller (not shown), a compressor 16, a condenser and fan 18, a filter dryer 20 and a capillary tube 22. The components of the chiller 10 are arranged to operate as per a normal refrigeration cycle. Turning now to Fig. 2, there is shown a logic diagram associated with the initial steps of operating the chiller 10 in a manner which avoids freezing of water in the tank 12. When the chiller 10 is operating and there is no potential risk of the water in the tank 12 freezing, it is said to be operating in a 'normal' mode, as indicated at Step 30 of Fig. 2. The controller continuously checks the length of time since the cold water tap was last operated, as shown in Step 32. As shown in Step 30, if the cold tap has been operated within the preceding 30 minutes, the unit continues to operate in the normal mode. As shown in Step 34, if the cold tap has not been operated for 30 minutes or more then the chiller 10 switches to a 'protection' mode that has safeguards against freezing the water in the tank 12, as will be described below. However, as indicated at Step 36, if during any time the chiller 10 is operating in the protection mode and the cold tap is operated, it reverts to the normal mode, at Step 32, and the controller again begins checking for periods where the cold tap has not operated for 30 minutes. The logic steps associated with the protection mode will now be described in relation to Fig. 3. As indicated at Step 40, the protection mode initially involves checking whether or not the compressor 16 is on. If the compressor 16 is not on, then the controller, as per Step 42, checks whether the water high set point temperature of 10.0 0C of the water in the tank 12 has been reached. If the water temperature is at or above 10.0 0C the compressor 16 is turned on, as indicated by Step 44, and the controller returns to Step 40 and checks whether or not the compressor 16 is on. If the temperature of the water in the tank 12 has not reached 10.0 0C then the compressor 16 remains off and the controller returns to checking whether or not the compressor is on at Step 40. If the compressor 16 is on then, as indicated at Step 46, the controller checks as to whether or not the water in the tank 12 has reached the low set point temperature of 4.7°C. If this is the case then, as indicated at Step 48, the compressor 16 is turned off and the fan is operated for a further 5 minutes to remove the heat soak that occurs. The controller then returns to Step 40 and continues to check whether or not the compressor 16 is on. If the temperature of the water in the tank 12 has not reached the low set point of 4.7°C then, as indicated in Step 50, the controller checks whether or not 30 minutes has elapsed since entering the protection mode. If not, the controller returns to Step 40. If yes, then the controller proceeds to Step 52 and waits for a further 5 minutes before, at Step 54, it measures the temperature of the water in the tank 12. A new reading of the water temperature is taken every subsequent 5 minutes and, after three readings, an average of those three readings is calculated. After a further five minutes another reading is taken and the new moving average is compared to the previous average, as indicated at Step 56. If the new average is less than the previous average it means that the chiller 10 is still chilling down and the normal operation continues. However, if the new average is the same or higher than the previous average it means that no more cooling is occurring. There are two main reasons that could lead to this occurring. The first is that the ambient temperature of the air has reached a point in which the chiller 10 is operating at equilibrium and is to not to take any further heat out of the water. The second reason may be a failure of the controller or the temperature sensor probe. Normally, as previously described in relation to Step 48, when the water is cooled to 4.70C the compressor 16 is turned off. The cooling fan 18 then continues to run for a further 5 minutes to remove the heat soak that occurs. If however the controller or the temperature sensor probe fails, the water may be continued to be cooled down past the 4.7 0C set point. If this occurs then the water starts to freeze and 0 0C and remains at this temperature until all of the water is frozen. However, the chiller 10 will recognise that the temperature is not dropping and will shut down the compressor 16 and turn on the fan 18, as indicated at Step 58. As indicated at Step 60, prior to a complete freeze of the water in the tank 12, the fan 18 is kept running for 30 minutes after the compressor 16 has been shut down in order to clear any residual ambient heat in the chiller 10 and the cupboard. Thereafter, the chiller 10 returns to the normal mode of operation. Typically, the excess heat mode described above may occur after the chiller 10 has been operating continuously during the day and at the end of the day when the users have departed, the cupboard is too hot for the chilled water's lower set point temperature to be reached. The method described above advantageously allows the heat in the cupboard to be dissipated in the evening and allows the chiller 10 to cool down to the point where the next day it is ready to function in the normal operating mode. Further, the controller also includes provision to supply a warning message if the overheating protection mode regularly occurs to indicate that the cupboard ventilation is inadequate and needs attention. Although the invention has been described with reference to a preferred embodiment, it will be appreciated for those skilled in the art that the invention may be embodied in many other forms.