Heat pump water heater controller

Abstract

A heat pump water heater (1), including a heat pump system (6) configured to heat water in a water tank (2), and at least one heating element (25) configured to supplement the heating of the water. When operating in low ambient temperatures, the controller (20) is configured to adjust the operation of the heat pump system (6) to heat the water in the tank (2) to a control setpoint temperature which is dependent on the sensed ambient temperature, and, operate the heating element(s) (25) to further heat the water when the water temperature is below a predetermined water temperature setpoint.

Description

Heat Pump Water Heater ControllerField of the invention

[0001] The present invention relates to a heat pump water heater, and in particular, to controlling the operation of a heat pump water heater, depending upon the temperature of the ambient environment.Background of the invention

[0002] 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.

[0003] Various types of water heaters are used in residential and commercial situations, including electric, gas, solar, and heat pump water heaters.

[0004] Heat pump water heaters are often desired to be used due to their high- efficiency, as they can be around 3 times more energy efficient than other types of water heaters. Heat pump water heaters work best in hot climates, but, due to the nature of their operation, problems can occur in low temperature ambient environments.

[0005] A heat pump water heater operates by drawing in ambient heat from the surrounding air. Heat from the air is absorbed by refrigerant inside the evaporator coil and dehumidified air is exhausted. The refrigerant is then pumped through a compressor which further increases its temperature. Heat may be transferred to the water in the tank by a hot water circulation pump.

[0006] Various refrigerants may be selected for use in a heat pump water heater according to the ambient temperature variation and the setpoint to which the water is desired to be heated. For example, R134A refrigerant may typically be used to achieve a setpoint of 70°C for ambient temperature ranges of 10°C to 45°C.

[0007] However, as the ambient temperature drops lower, the maximum condensing temperature within the heat pump operating envelope lowers. Thus, maintaining a 70°C setpoint is detrimental to the longevity of the compressor, the evaporator coils, and the other heat pump components.

[0008] The current solution to overcome this is to permanently set a lower setpoint temperature to which the water in the tank is heated, or, to switch off the heat pump and use an auxiliary heating system, which may typically / optionally be installed in different levels of the storage tank, in lieu of the heat pump. Such auxiliary heating systems include electric heating elements, but these may typically consume around up to 3-4 times more power than the heat pump system, thus lowering the overall performance of the system.Summary of the invention

[0009] The present invention seeks to provide a heat pump water heater, and controller therefor, which overcomes at least some of the disadvantages of prior art heat pump water heaters.

[0010] The present invention also seeks to provide a heat pump water heater, and controller therefor, whereby the setpoint can be adjusted as a function of ambient temperature.

[0011] In one broad form, the present invention provides a heat pump water heater, including: a heat pump system configured to heat water in a water tank; a heating element configured to supplement heating of the water; and, a controller configured to: adjust the temperature to which the heat pump system heats the water in the tank depending on a sensed ambient temperature; and, operate the heating element to further heat the water when the water temperature is below a predetermined water temperature setpoint.

[0012] In a further broad form, the present invention provides a heat pump water heater, including: a heat pump system configured to heat water in a water tank; an ambient temperature sensor, configured to sense ambient temperature of a heat pump system environment; a water temperature sensor, configured to sense temperature of the water; a heating element configured to supplement heating of the water; and, a controller configured to:control the operation of the heat pump system to heat the water in the tank to a control temperature setpoint which is dependent upon the sensed ambient temperature; and, operate the heating element to further heat the water when the control temperature setpoint is below a predetermined water temperature setpoint.

[0013] Preferably, said controller is configured to reduce the control temperature setpoint of the water heater at low ambient temperatures.

[0014] Preferably, the control temperature setpoint dynamically reduces as the ambient temperature decreases.

[0015] Preferably, said controller is configured to control the operation of said heating element if said water temperature setpoint of said water heater is not reached within a predetermined time period.

[0016] Preferably, said heating element is positioned within said water tank and / or in a flow path to or from said water tank.

[0017] Preferably, said controller is configured to adjust the temperature to which the heat pump heats the water in the tank, to optimise operation of the heat pump system within an operating envelope of the heat pump.

[0018] Preferably, said controller is configured to optimise the use of the heat pump and minimise the use of the heating element to thereby minimise consumption of electricity in operating said heating element.

[0019] Preferably, the heat pump water heating system further includes a circulation pump configured to circulate hot water between the heat pump and the tank.

[0020] In a further broad form, the present invention relates to a controller for controlling the operation of a heat pump water heater to thereby heat water to a predetermined water temperature setpoint, wherein the water heater includes: a heat pump system; and, a supplementary heating element, wherein the controller is configured to: sense ambient temperature of an ambient environment of the heat pump system; determine a control temperature setpoint for operation of the heat pump system;control operation of the heat pump system, to heat the water to the control temperature setpoint; and, operate the supplementary heating element to further heat the water when the control temperature setpoint of the heat pump system is below the predetermined water temperature setpoint.

[0021] Preferably, said control temperature setpoint is reduced at low ambient temperatures.

[0022] Preferably, the controller is configured to dynamically reduce the control temperature setpoint as the ambient temperature decreases.

[0023] Preferably, the controller is further configured to control the operation of the supplementary heating element if said water temperature setpoint is not reached within a predetermined time period.

[0024] Preferably, the controller is further configured to adjust the temperature to which the heat pump heats the water in the tank, to thereby optimise operation of the heat pump system within an operating envelope of the heat pump.

[0025] Preferably, the controller is further configured to optimise the use of the heat pump and minimise the use of the heating element to thereby minimise consumption of electricity in operating said heating element.

[0026] In a further broad form, the present invention provides a method of heating water to a predetermined water temperature setpoint when using a heat pump water heater in a variable temperature ambient environment, the method including the steps of: monitoring ambient temperature of the ambient environment; adjusting the control temperature setpoint to which the heat pump water heater is operated to heat the water, dependant on the temperature of the ambient environment; monitoring water temperature of the water; and, operating a supplementary water heating element to heat the water if said water temperature is below said predetermined water temperature setpoint.

[0027] Preferably, in said adjusting step, the control temperature setpoint to which the water in the tank is heated by the heat pump water heater is reduced as a function of ambient temperature reduction.

[0028] Preferably, in said operating step, the water heating element is operated to supplement the heating of said water in said tank if said water temperature setpoint is not reached within a predetermined time period.

[0029] Preferably, in said adjusting step, the control temperature setpoint to which the heat pump water heater is operated to heat the water, is adjusted to optimise the operation of the heat pump system within an operating envelope of the heat pump.

[0030] Preferably, the method further includes the step of balancing the use of the heat pump and of the heating element to minimise consumption of electricity in operating the heating element and / or the water heater.Brief description of the drawings

[0031] The present invention will become more fully understood from the following detailed description of preferred but nonlimiting embodiments thereof, described in connection with the accompanying drawings, wherein:Figure 1 illustrates a schematic view of a heat pump water heating system in accordance with the present invention;Figure 2 illustrates a schematic diagram of an example embodiment of a typical air-to-water heat pump water heating system, in accordance with the present invention;Figure 3 illustrates a flowchart illustrating how the setpoint of the water temperature may be adjusted dependent upon the ambient temperature;Figure 4 illustrates a graphical representation of an example setpoint setback amount, dependent upon temperature, in accordance with the present invention;Figures 5(a) to 5(e) illustrate example calculations of setpoint setback temperatures according to a non-limiting embodiment of the present invention;Figure 6 illustrates a graphical representation of a further example setpoint setback amount, dependent upon temperature, in accordance with the present invention;Figures 7(a) to 7(d) illustrate further example calculations of setpoint setback temperatures according to a non-limiting embodiment of the present invention;Figure 8 illustrates a schematic view of an alternatively preferred embodiment of the present invention including heading elements to boost heating of the water heater; and,Figure 9 illustrates a flowchart illustrating an ambient setback setpoint having an auxiliary boost heater.Detailed description of preferred embodiments

[0032] In figure 1 is shown a heat pump water heater in accordance with the present invention. The water heater, generally designated by the numeral 1 , includes a water tank 2 provided with a water tank housing 3. Water is supplied to the tank 2 via inlet 4, and, after being heated, as will be described hereinbelow, is egressed via outlet 5.

[0033] Water in tank 2 is heated using a heat pump system, generally designated by the numeral 6. With the assistance of fan 9, the heat pump system 6 draws in air from the ambient environment via an air inlet 7. Heat from the ambient air is absorbed by refrigerant within the evaporator coils 8, and cooled air is expelled via the outlet 10. The refrigerant is then pumped through a compressor 11 which further increases the temperature of the refrigerant. The refrigerant is then pumped through the condenser 12, such that the heat from the refrigerant is then transferred to heat the water in the water tank 2 to a desired temperature setpoint. The cooled refrigerant is then pumped back to the evaporator coils 8, so that the cycle can repeat itself. The desired water temperature setpoint to which the water is heated may typically be monitored by a water temperature sensor or thermostat 21 , whereby the operation of the heat pump is switched on or off as needed. This heat pump operation for heating water is well known and understood to persons skilled in the art, and as such, will not be further herein described.

[0034] In an example embodiment, the present invention provides a controller 20, which controls the operation of the heat pump system 6. The controller 20 may be configured to adjust the setpoint of the water heater, that is, the temperature to which the water in the water tank 2 is heated. In particular, the controller 20 is configured to adjust the setpoint of the water in the tank 2 depending upon the temperature of the ambient environment.

[0035] In an example embodiment, the controller 20 is configured to reduce the setpoint of the water in the tank 2 in low ambient temperatures.

[0036] In a further example embodiment, the controller 20 is configured to reduce the setpoint of the water in the tank 2 as the ambient temperature decreases.

[0037] T o monitor the water temperature in the tank 2 and provide water temperature data to the controller 20, one or more water temperature sensor 21 is preferably provided. The water temperature sensor(s) 21 may be a thermistor, or any other temperature sensing device.

[0038] To monitor the ambient air temperature of the surrounding environment and provide ambient temperature data to the controller 22, an ambient temperature sensor 22 is preferably provided. The ambient temperature sensor 22 may be a thermistor or any other temperature sensing device.

[0039] In an example embodiment, the controller 20 may be configured to receive water temperature data from the water temperature sensor(s) 21 , and, ambient temperature data from the ambient temperature sensor 22. The controller 20 may then control the operation of the heat pump system to thereby adjust the setpoint temperature of the water in the water tank 2, depending upon the sensed ambient temperature.

[0040] Figure 2 shows an example embodiment of a heat pump water heater system50. The heat pump water heater system 50 includes a water tank 53 and a heat pump51. Water is circulated between the heat pump 51 and the water tank 53 via the illustrated conduits by the circulation pump 52. Figure 2 illustrates an example configuration of the various valves and temperature sensors which may optionally be included in such a heat pump water heater system.

[0041] In Figure 3 shows a flowchart which outlines an example embodiment of the general control operation of the water heater 1 by the controller 20. The controller 20 may typically be embodied utilising hardware components, a software algorithm, or, a combination of these. Persons skilled in the art will readily understand and be able to implement various forms of controller 20 configured to perform the function of the present invention.

[0042] As illustrated in figure 3, upon commencement of the operation of the water heater 1 , shown at block 30, the controller 20 monitors the ambient temperature of the surrounding environment utilising data from ambient temperature sensor 22, as shown in block 31. The controller 20 may then analyse whether the ambient temperature is below a prescribed threshold, according to predetermined design characteristics of the water heater, including the particular refrigerant used.

[0043] If the controller 20 determines that the ambient temperature is within a prescribed range or above a prescribed threshold, then the controller 20 allows the heatpump 6 to continue to operate to the predetermined setpoint temperature, as shown in block 35. The controller 20 then continues to monitor the ambient temperature, as shown in block 31 , and whilst these ambient temperature conditions prevail, the controller 20 continues to operate the heat pump 6 to heat the water in tank 2 to the predetermined setpoint temperature.

[0044] If, however, the controller 20 determines that the ambient temperature is below a prescribed threshold or range, then the controller 20 operates to reduce the setpoint temperature of the water heater 1 , as shown in block 34. The heat pump system operation is thereby adjusted so that the water in the tank 2 is then heated to the reduced setpoint as determined by the controller 20. The controller 20 then continues to monitor the ambient temperature, as shown in block 31 , and whilst the ambient temperature continues to be below the prescribed threshold or range, the controller 20 continues to operate to the reduced setpoint temperature of the water heater down to the lower limit. The lowest possible setpoint that can be reduced to is the difference between the upper limit and setback temperatures. Furthermore, if the ambient temperature is above the upper limit, the highest possible setpoint is the nominated setpoint.

[0045] In one example embodiment, the controller 20 may utilise a low ambient setpoint setback control algorithm to perform the aforementioned functions, that is, to automatically lower the setpoint to cater for low ambient temperature. As will be understood by person skilled in the art, this low ambient setpoint setback control algorithm lowers the setpoint temperature commensurate with ambient temperature.

[0046] The low ambient setback function is such that when the ambient temperature falls, the control setpoint is reduced relative to the refrigerant operating envelope. When the ambient temperature is above the upper limit, no offset will be applied. As the ambient temperature falls below the upper limit, the control setpoint will be reduced over the band from upper to lower limit and the total offset will be the Offset Delta. As the ambient temperature increases, the setpoint increases until it reaches back to normal.

[0047] By way of a non-limiting example, the following criteria may typically apply:Ambient Upper Limit = 5°C, Lower Limit = 2°C, Set point = 55°C, Offset Delta = 5°C, that is, when ambient temperature goes from 5°C down to 2°C the setpoint reduces from 55°C to 50°C.

[0048] In figure 4 is illustrated of a graphical representation of an example relationship between temperature and the ambient setpoint setback. The calculated offset may be determined by the following formula:(Ambient Upper Limit — Actual Ambient)Calculated Offset = Offset t t x(Ambient Upper Limit — Ambient Lower Limit)

[0049] The effective setback may be determined by the following formula:Effective Setback = Setpoint — Actual Offset

[0050] The functional relationship may be summarised in the following table, with example calculations being shown in Figures 5(a) to 5(e) respectively:with the limits:

[0051] In figure 6 is illustrated of a graphical representation of a further example relationship between temperature and the ambient setpoint setback. The calculated offset may be determined by the following formula:(Ambient Upper Limit — Actual Ambient) Calculated Offset = Offset At x — — ■ - - - - - - — - - - - - -(Ambient Upper Limit — Ambient Lower Limit)

[0052] The effective setback may be determined by the following formula:Effective Setback = Setpoint — Actual Offset

[0053] The functional relationship may be summarised in the following table, with example calculations being shown in Figures 7(a) to 7(d) respectively:with the limits:

[0054] It will be appreciated that the present invention provides significant advantages over prior art water heat pump heaters, including, but not limited to, effectively providing protection to the compressor from straining and operating outside its envelope in low ambient conditions, and, protecting the evaporator coils of the heat pump from icing in low ambient conditions.

[0055] In many situations, the heat pump water heater of the present invention may be embodied without the need for any auxiliary boosting, such as by using electrical elements to provide backup heating.

[0056] It will however be appreciated that in other forms of the invention, a backup heating system, such as a gas or electrical element heating system may additionally still be used, particularly in extreme low ambient temperature environments.

[0057] In figure 8 is shown a heat pump water heater 1 in accordance with an alternatively preferred example embodiment of the present invention, further incorporating a supplementary heating system to heat the water in the tank 2. The supplementary heating system includes at least one heating element 25, configured to be controlled by the controller 20 if the heat pump water heating system 6 is not able to sufficiently heat the water in the tank 2 as desired.

[0058] In this example embodiment, the water heater 1 may include the heat pump system 6, and at least one heating element 25 configured to supplement heating of the water in the tank 2. A plurality of heating elements 25 may be provided, in spaced apart locations within the water tank 2, to allow better control of the temperature of the water at varying depths within the tank 2, if desired.

[0059] The controller 20 is configured to adjust the temperature to which the heat pump system 6 heats the water in the tank 2, depending on the ambient temperature of the environment in which the heat pump system 6 is located. The temperature of the ambient environment in which the heat pump system 6 is operated may be monitored or sensed by an ambient temperature sensor 22.

[0060] The controller is thus configured to control the operation of the heat pump system 6 so that it operates within its desired operating window, ensuring longevity of the compressor and other components and the economic life of the heat pump system 6. In low temperature ambient environments, the heat pump system 6 therefore typically only heats the water in the water tank 2 to a lowered control temperature setpoint, which may be lower than the water temperature setpoint desired by the user.

[0061] The control temperature setpoint may be a linear or non-linear function of the ambient temperature and may typically dynamically reduce as the ambient temperature decreases, as herein before described and illustrated in the figures.

[0062] In these circumstances, the controller 6 preferably then operates the heating element(s) 25 to further heat the water in the tank 2 when the water temperature is below the predetermined water temperature setpoint desired by the user. The temperature of the water may be monitored or sensed by one or more water temperature sensor(s), and this data is then supplied to the controller 20.

[0063] Other components and parameters relevant to the heating of the water may additionally or alternatively be monitored and supplied to the controller 20 for controlling the operation of the water heater 1. For example, a timer may additionally or alternativelybe used to monitor the heating of the water within a prescribed period of time, so that, if the desired temperature of the water is not reached within a predetermined time period, then the controller 20 activates the operation of the auxiliary boost heating element(s) 25.

[0064] In an example operation, the controller functions may typically operate as follows.

[0065] The low ambient setback function is such that when the ambient temperature falls, the control temperature setpoint is reduced. When the ambient temperature is above the upper limit, no offset will be applied. As the ambient temperature falls below the upper limit, the control temperature setpoint will be reduced proportionally over the band from upper to lower limit and the total offset will be the Offset Delta.

[0066] If the setpoint setback is not reached within the desired criteria, for example, within an adjustable time frame (default = 180mins, adjustable from 0-300mins), then the heating element(s) 25 will be activated to supplement the heating of the water in the water tank 2.

[0067] The heating element / auxboost may typically turn off in 2 ways, when controlling sensor reaches setback setpoint (default) or when it reaches actual setpoint. The heating element / auxboost does not activate when controlling sensor reaches the active setpoint.

[0068] For example: Ambient Upper Limit = 5°C, Lower Limit = 2°C; Set point = 55°C; and, Offset Delta = 5°C. When ambient goes from 5°C down to 2°C the setpoint goes from 55°C to 50°C. When the controlling sensor has not reached the setpoint of 50°C within 180mins, the heating element / auxboost is activated. When the controlling sensor has reached the setpoint of 50°C within 180mins, the heating element / auxboost does not activate. It should be noted that the controlling sensor may typically sense the temperature of the water in the tank, and / or sense the temperature of the water entering or leaving the tank.

[0069] In figure 9 is shown a flowchart which outlines an example embodiment of the general control operation of the water heater 1 by the controller, when one or more supplementary auxiliary boost water heating element 25 is provided in the water tank 2. As shown, if and when the setpoint temperature of the water heater is reduced, as shown in block 34, the temperature of the water is then monitored, as shown in block 36, and, if the controlling sensor reaches the active setpoint within the pre-determined time period, as shown by block 37, then the heat pump 6 operates to the pre-determined setpointtemperature, as shown by block 35, and, if the controlling sensor does not reach the active setpoint within the pre-determined time period, for example, as sensed by a temperature sensor and a timer and compared with predetermined parameters, then one or more auxiliary boost water heating element 25 is then operated by the controller, to assist the heating of the water in the tank 2, as illustrated in block 38.

[0070] It will however be appreciated that the incorporation of the control system of the present invention will provide less need to utilise these backup systems, resulting in significant savings on utility costs.

[0071] In another example embodiment, the hot water heat pump can alternatively or additionally incorporate interlocking, to control independent auxiliary boosters from operating during low ambient conditions.

[0072] In another example embodiment, in situations where bulk storage is used, the top portion of the storage tanks can be banked with 70°C hot water to be delivered to users, whilst the bottom of the tank is heated to a lower temperature. In this way hot water is still delivered but the heat pump continues to work within its own envelope.

[0073] The present invention therefore seeks to maximize the use of hot water heat pumps during winter and during other low ambient temperature environments, to thereby save on electricity, by not having the need to activate auxiliary boosters / heating elements. This can typically lead to significant savings on electricity and operation costs, and achieving longer heat pump economic life. The coefficient of performance (COP) diminishes at both higher delivery temperatures and lower ambient temperatures. The setpoint set back approach addresses both of these issues and thereby maintains a higher COP during the coldest part of the day.

[0074] It will be appreciated that numerous variations and modifications may be made to the present invention, and these should be considered to be incorporated within the scope of the invention.

[0075] For example, the invention may be embodied as either a single-pass or a multi-pass heating system. That is, in one example embodiment, the invention may use a single-pass heating method, whereby the system aims to reach the highest temperature as possible, depending on ambient temperature. In this situation, temperature difference ranges may typically be from 20°C to 55°C. In an alternative example embodiment, the invention may use a multi-pass heating method, to maintain a temperature difference oftypically 5°C to 6°C between the heat pump inlet and outlet, until the unit reaches the set temperature.

[0076] For example, instead of reading direct ambient temperature, reference can be made to the compressor envelope via SST and SCT as nominated by the compressor manufacturer. Note SCT and SST are pressure measurements converted to temperature as a function of the refrigerant.

[0077] In referring to ambient temperature of an external environment, per se, it should also be understood by persons skilled in the art that the temperature within the heat pump water heater system itself could alternatively be measured and utilised by the controller. For example, the temperature at the evaporator coil, the compressor, or of the other heat pump components could be measured. As such, when referring to ambient temperature throughout this specification and claims, it will be understood that some of this terminology should be considered to incorporate measurement of the temperature of either the ambient environment external to the water heater, or, of the componentry within the water heater unit, per se.

[0078] In embodying the present invention, the heat pump may utilise a variety of refrigerants, such as, but not limited to, R1234YF, R513A, R410A, R407C, R134A, R32, R290. The selection of the refrigerant may be based on a variety of factors, including the ambient temperature conditions where the heat pump water heater is intended to be installed.

[0079] Whilst particular examples and forms of the invention have been hereinbefore described, the invention should not be considered to be limited to these. All such variations and modifications which become apparent to persons skilled in the art should be considered to be encompassed within the spirit and scope of the invention as previously herein described and as claimed hereinafter.

Claims

Claims1 . A heat pump water heater, including: a heat pump system configured to heat water in a water tank; a heating element configured to supplement heating of the water; and, a controller configured to: adjust the temperature to which the heat pump system heats the water in the tank depending on a sensed ambient temperature; and, operate the heating element to further heat the water when the water temperature is below a predetermined water temperature setpoint.

2. A heat pump water heater, including: a heat pump system configured to heat water in a water tank; an ambient temperature sensor, configured to sense ambient temperature of a heat pump system environment; a water temperature sensor, configured to sense temperature of the water; a heating element configured to supplement heating of the water; and, a controller configured to: control the operation of the heat pump system to heat the water in the tank to a control temperature setpoint which is dependent upon the sensed ambient temperature; and, operate the heating element to further heat the water when the control temperature setpoint is below a predetermined water temperature setpoint.

3. The heat pump water heater according to claim 1 or 2, wherein said controller is configured to reduce the control temperature setpoint of the water heater at low ambient temperatures.

4. The heat pump water heater according to any one of claims 1 to 3, wherein the control temperature setpoint dynamically reduces as the ambient temperature decreases.

5. The heat pump water heater according to any one of claims 1 to 4, wherein said controller is configured to control the operation of said heating element if said water temperature setpoint of said water heater is not reached within a predetermined time period.

6. The heat pump water heater according to any one of claims 1 to 5, wherein said heating element is positioned within said water tank and / or in a flow path to or from said water tank.

7. The heat pump water heater according to any one of claims 1 to 6, wherein said controller is configured to adjust the temperature to which the heat pump heats the water in the tank, to optimise operation of the heat pump system within an operating envelope of the heat pump.

8. The heat pump water heater according to any one of claims 1 to 7, wherein said controller is configured to optimise the use of the heat pump and minimise the use of the heating element to thereby minimise consumption of electricity in operating said heating element.

9. The heat pump water heating system according to any one of claims 1 to 8, further including a circulation pump configured to circulate hot water between the heat pump and the tank.

10. A controller for controlling the operation of a heat pump water heater to thereby heat water to a predetermined water temperature setpoint, wherein the water heater includes: a heat pump system; and, a supplementary heating element, wherein the controller is configured to: sense ambient temperature of an ambient environment of the heat pump system; determine a control temperature setpoint for operation of the heat pump system;control operation of the heat pump system, to heat the water to the control temperature setpoint; and, operate the supplementary heating element to further heat the water when the control temperature setpoint of the heat pump system is below the predetermined water temperature setpoint.

11. The controller according to claim 10, wherein said control temperature setpoint is reduced at low ambient temperatures.

12. The controller according to claim 10 or 11 , configured to dynamically reduce the control temperature setpoint as the ambient temperature decreases.

13. The controller according to any one of claims 10 to 12, further configured to control the operation of the supplementary heating element if said water temperature setpoint is not reached within a predetermined time period.

14. The controller according to any one of claims 10 to 13, further configured to adjust the temperature to which the heat pump heats the water in the tank, to thereby optimise operation of the heat pump system within an operating envelope of the heat pump.

15. The controller according to any one of claims 10 to 14, further configured to optimise the use of the heat pump and minimise the use of the heating element to thereby minimise consumption of electricity in operating said heating element.

16. A method of heating water to a predetermined water temperature setpoint when using a heat pump water heater in a variable temperature ambient environment, the method including the steps of: monitoring ambient temperature of the ambient environment; adjusting the control temperature setpoint to which the heat pump water heater is operated to heat the water, dependant on the temperature of the ambient environment; monitoring water temperature of the water; and,operating a supplementary water heating element to heat the water if said water temperature is below said predetermined water temperature setpoint.

17. The method according to claim 16, wherein, in said adjusting step, the control temperature setpoint to which the water in the tank is heated by the heat pump water heater is reduced as a function of ambient temperature reduction.

18. The method according to claim 16 or 17, wherein, in said operating step, the water heating element is operated to supplement the heating of said water in said tank if said water temperature setpoint is not reached within a predetermined time period.

19. The method according to any one of claims 16 to 18, wherein, in said adjusting step, the control temperature setpoint to which the heat pump water heater is operated to heat the water, is adjusted to optimise the operation of the heat pump system within an operating envelope of the heat pump.

20. The method according to any one of claims 16 to 19, wherein the method further includes the step of balancing the use of the heat pump and of the heating element to minimise consumption of electricity in operating the heating element and / or the water heater.

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