A hot water supply system

Abstract

A hot water supply system 1 comprises a thermal energy store 2 containing an energy storage fluid, with a first inlet 18, a first outlet 16, a second inlet 21, and a second outlet 20. A first heat exc

Description

The present invention relates to a hot water supply system, particularly one suitable for use in a setting where space is at a premium. The hot water supply system may be utilised, for example, in small apartments, in boats, and also in smaller workplaces having facilities for staff use such as a kitchen area or bathroom. As background, in some countries including the UK, there are a large number of small properties with gas-fired central heating, and many of these properties use what are known as combination boilers for providing heating and hot water. Combination boilers act as an instantaneous hot water heater and also as a boiler for central heating. The small properties may be apartments or small houses used as domestic dwellings or may be properties used by small businesses. For such properties, combination boilers are a popular choice because they combine a small form factor, provide a more or less immediate source of "unlimited" hot water (with 20 to 35kW output), and do not require hot water storage. Such boilers can be purchased from reputable manufacturers relatively inexpensively. Their small size and the ability to do without a hot water storage tank mean that it is generally possible to accommodate such a boiler even in a very small flat or house. Such boilers are often wall-mounted in the kitchen, or may alternatively be installed such that they stand on a floor or other horizontal surface. They may be installed within kitchen cupboards so that they are hidden from view. They are also very quick and easy to install, allowing households to get a new gas combi boiler installed inexpensively. As many countries are now turning away from using fossil fuels in favour of renewable energy sources, alternative heat sources will need to be provided in place of gas combi boilers. For larger properties, heat pumps and / or solar panels are gradually being installed in place of gas-fired boilers, however, both heat pumps and solar heating require a tank for hot water storage in order to provide hot water on demand. In a small flat, small house or small workplace with no existing hot water tank there is often not sufficient space to accommodate one and furthermore, retro-fitting alternative heating and / or hot water systems in individual flats in existing buildings can be difficult and / or too expensive for the owner. There therefore exists a need to provide a solution to the problem of finding a suitable, cost-effective replacement for gas combi boilers, particularly for settings where space is very limited, such as in smaller domestic dwellings and workplaces. According to a first aspect there is provided a hot water supply system suitable for domestic use comprising a thermal energy store, a first heat exchanger and a second heat exchanger. The thermal energy store comprises an unpressurised tank for storing an energy storage fluid. The first heat exchanger has a first inlet for receiving hot working fluid from a heat source and a first outlet for returning the cooled working fluid to the heat source after the working fluid has passed through the first heat exchanger, and has a second inlet for receiving energy storage fluid from the thermal energy store and a second outlet being an exit for the heated energy storage fluid after the energy storage fluid has passed through the first heat exchanger. The second heat exchanger has a first inlet for receiving cold water to be heated by the present system and a first outlet to permit the heated water to exit the second heat exchanger, and has a second inlet for receiving heated energy storage fluid and a second outlet for returning the cooled energy storage fluid to the thermal energy store after the energy storage fluid has passed through the second heat exchanger. The thermal energy store has a first outlet and a second outlet, a first inlet and a second inlet, and at least first to sixth fluid paths (a-f) are provided as follows: a) a first fluid path between the first outlet of the thermal energy store and the second inlet of the first heat exchanger, b) a second fluid path between the second outlet of the thermal energy store and the second inlet of the first heat exchanger, c) a third fluid path between the second outlet of the thermal energy store and the second inlet of the second heat exchanger, d) a fourth fluid path between the second outlet of the first heat exchanger and the first inlet of the thermal energy store, e) a fifth fluid path between the second outlet of the first heat exchanger and the second inlet of the second heat exchanger, f) a sixth fluid path between the second outlet of the second heat exchanger and the second inlet of the thermal energy store, and the system is configured to be switchable between a first configuration whereby the first, third, fourth and sixth fluid paths (a, c, d and f) are substantially open and the second and fifth fluid paths (b and e) are substantially closed and a second configuration whereby the second, fifth and sixth fluid paths (b, e and f) are substantially open and the first, third and fourth fluid paths (a, c and d) are substantially closed. The heat source may be, for example, a heat pump. The cold water provided to the first inlet of the second heat exchanger may be provided from a water storage tank or from the mains water supply, for example. Following heating via the second heat exchanger, this water may be provided to a local hot water outlet activated by a user. For example, if the system is used in a domestic setting, the local hot water outlet may be a tap such as a bath tap or kitchen tap. Similarly, if the system is used in a workplace, the local hot water outlet may be a kitchen tap, for example. The system may also be used on boats where hot water is required, for example for passengers to shower. For ease, the expression "domestic hot water" is used from now on to describe the heated water provided to the local hot water outlet, and it is understood that "domestic hot water" is meant to include hot water provided in workplaces for staff and / or customer use and hot water provided to boat passengers, as well as hot water provided to dwellings such as flats and houses. The above system does not require a hot water storage tank nor additional heating elements such as a coil. The system takes up approximately the same amount of space as a gas combi boiler and so can be fitted into a space vacated by an old gas combi boiler. Preferably the first inlet of the first heat exchanger and the first inlet of the second heat exchanger are pressurised inlets. The first heat exchanger is used to transfer heat from the working fluid of the heat source to the energy storage fluid and the second heat exchanger is used to transfer heat from the energy storage fluid to the water to be heated. The energy storage fluid is stored in the thermal energy store and circulates in a closed circuit as will be explained later. The energy storage fluid is prevented from mixing with the working fluid of the heat source and with the water to be heated. The energy storage fluid may be water or may alternatively be another liquid suitable for use in a domestic or similar heating system. The thermal energy store does not require a pressurised tank because the energy storage fluid is not pressurised and does not need to reach high temperatures and is at generally no more than around 40 - 65 degrees Celsius, which is sufficient to heat the domestic hot water to approximately 38 - 45 degrees Celsius. The thermal energy store is designed to accommodate energy storage fluid in the range of approximately 5-85 degrees Celsius, although the working temperature of the energy storage fluid is generally in the range of approximately 40 - 65 degrees Celsius. Furthermore, the system does not require pressure relief valves or expansion vessels. This allows the system to fit within the space constraints of a small dwelling and keeps the material and installation costs of the system low. The hot water supply system may comprise a housing. The thermal energy store, the first heat exchanger and the second heat exchanger may be located within the housing. The housing may support the thermal energy store. The housing may support the first and second heat exchangers. The housing may provide a self-contained unit that includes and supports the thermal energy store and heat exchangers. The housing may be mounted or secured to a wall or other surface as one unit, thereby reducing installation time, cost and complexity. The housing may be of any shape but is preferably of a substantially cuboid form, as this is usually space efficient and is the same shape as a usual gas combi-boiler. The housing may have six sides. The housing may comprise a top and bottom and four sides. The top of the housing is that part which, when the housing is fitted to a wall or a shelf, is the uppermost part of the housing (ie that part of the housing closest to the ceiling of the dwelling). The dimension of the housing between the top and the bottom is preferably larger than the dimension between any two opposing sides. This may provide an elongated cuboid housing, where the housing has a greater height than width or depth. The housing may have a height which is one and a half to four times the width and depth of the housing. Preferably the housing has a height which is at least twice the width and depth of the housing. The housing may have a maximum footprint of, for example, 750mm by 750mm and a maximum height of 1200mm. The housing may have maximum dimensions of, for example, a width of 600mm, a depth of 600mm and a height of 900mm, the maximum footprint of the housing in this case being 600mm by 600mm. Alternatively the housing may have maximum dimensions of, for example, a width of 500mm, a depth of 500mm and a height of 900mm. The dimensions of the housing may be, for example, 350mm x 250mm x 900mm. Alternatively the dimensions of the housing may be, for example, 350mm x 250mm x 800mm. The housing may include at least one recess or protruding part. Protruding parts of the housing may accommodate heat exchangers, pipes or other components present in the housing. Recesses in the housing may accommodate means for securing the housing to a wall or for standing on a surface. The first inlet and first outlet of the first heat exchanger and the first inlet and first outlet of the second heat exchanger are preferably located at or near the bottom of the housing. These inlets and outlets may extend outside of the housing, for example through a bottom panel of the housing. Alternatively these inlets and outlets may be located within the housing near the bottom and a bottom panel of the housing may be removable in order to access these inlets and outlets and to allow for connection to pipework. Preferably all the inlets and outlets to the present hot water supply system are located at or near the bottom of the housing. This allows the housing to be fitted in a small space as an installer only needs to access the bottom of the housing in order to connect the present system to both the heat source and the incoming cold water to be heated. The housing can also extend up to the ceiling of the room in which it is fitted if mounted on a wall as the installer does not need to access the top or sides of the housing. The housing may be configured to be mounted to a wall or to a shelf, for example, or may be fitted into a cupboard. The housing may include a support structure. The support structure may be configured to be mounted to a mounting surface such as a wall or a floor. The support structure may comprise a rear plate for mounting to a wall. Alternatively or additionally the support structure may comprise one or more feet for mounting to or resting on thefloor or a substantially horizontal surface, such as a shelf, for example. The support structure may transfer load from the thermal energy store to the wall or other surface. The thermal energy store may be an unpressurised tank. The housing itself may form some or all of the walls of the unpressurised tank or alternatively a separate unpressurised tank may be provided inside the housing. The unpressurised tank may be made from plastic, for example polypropylene. Ribs may be provided in order to stiffen the tank. The tank may take any shape but in order to use space efficiently the tank may have a substantially cuboid form. The tank may have six sides. The tank may comprise a top and bottom and four sides. The tank may have maximum dimensions of 600mm wide x 600mm depth x 900mm height. The tank may have minimum dimensions of 300mm x 250mm x 400mm. The dimensions of the tank may be, for example, 350mm x 250mm x 800mm. The tank may hold approximately 60 litres of energy storage fluid, for example water. A larger tank, eg a 90 litre tank, may be provided instead if a higher water usage in the household is foreseen. It is advantageous to provide a taller, thinner tank rather than shorter, wider tank because the energy storage fluid in the tank will stratify according to its temperature, and the strata will be more evident and more accessible in a taller, thinner tank. For example, when the heat source is activated and the system is in the first configuration, a first pump may be switched on to pump the colder energy storage fluid taken from the bottom of the tank through the first heat exchanger. The heated energy storage fluid may then be fed back into the tank. This heated energy storage fluid is warmer than the rest of the fluid at the bottom of the tank and will rise to the top of the tank, with colder fluid taking its place at the bottom. If the heat source is then deactivated, the fluid in the tank will settle into layers or strata of fluid according to the temperature of the fluid. This stratification of the energy storage fluid allows the warmer fluid at the top of the tank to be readily taken from the tank for use in the second heat exchanger to heat the incoming cold water. The domestic hot water supply system may comprise a plurality of valves, and preferably at least one valve is provided on at least each one of the second, third, fourth and fifth fluid paths (b, c, d and e). The valves may be solenoid valves, for example. A valve such as a non-return valve or a solenoid valve, for example, may be provided on the first fluid path (a). One or both of the first and second inlets of the thermal energy store may be provided with a diffuser, such as, for example, a radial diffuser. Diffusers may be provided at the inlets to the thermal energy store to diffuse energy storage fluid returningto the thermal energy store after passing through a heat exchanger. Using a diffuser can slow down the speed of the fluid entering the thermal energy store and can influence the direction in which the fluid flows. This can reduce turbulence caused by the fluid entering the thermal energy store. It is advantageous to reduce turbulence in order to aid stratification by hindering the mixing of the newly returned fluid and the fluid already in the thermal energy store. Where radial diffusers are used, the diameter of the diffuser may be a relatively large percentage of the smallest dimension of the thermal energy store. For example, the diameter of the diffuser may be in the range of approximately 25% of the smallest dimension of the thermal energy store to approximately 90% of the smallest dimension of the thermal energy store. The diffuser may have a diameter of at least 70mm. The diffuser may additionally or alternatively have a diameter of at least two times the diameter of the inlet and preferably at least three times the diameter of the inlet. Where radial or non radial diffusers are used, the diffuser may occupy approximately at least 5% of the cross sectional area of the thermal energy store, and preferably at least 10% of the cross sectional area of the thermal energy store. To aid stratification, it is preferable to provide a gap between the edge of the diffuser and the side of the thermal energy store of at least 10mm and preferably at least 20mm. The diffuser may be positioned so that fluid exiting the diffuser travels in a substantially horizontal direction, or may alternatively be positioned such that fluid exiting the diffuser travels at an angle to the horizontal direction. Such an angle may, for example, be in the range of approximately 0-30 degrees from the horizontal. Both diffusers may be provided at or near to the bottom of the thermal energy store. Alternatively the first inlet of the thermal energy store may be provided with a diffuser part way up or near to the top of the thermal energy store. In this way warmed energy storage fluid may be directly delivered to or nearer to the top part of the thermal energy store. The first and second heat exchangers may be, for example, plate heat exchangers. The heat exchangers may be located within the thermal energy store or may be located outside of the thermal energy store but within the housing. Where the heat exchangers are located within the thermal energy store the second heat exchanger may be located in an upper part of the thermal energy store. This permits the system to deliver hot water immediately upon request due to the second heat exchanger which heats the domestic water being located in the warmest part of the thermal energy store. If the heat exchangers are not located within the thermal energy store then they are preferably located within the housing underneath the thermal energy store. One or more pumps for circulating the energy storage fluid may be also located within the housing underneath the thermal energy store. Optionally one or more motors for the pumps may also be provided in this location. A manifold may be provided within or as part of the housing underneath the thermal energy store to provide fluid pathways. The second outlet of the thermal energy store may be located at an upper part of the thermal energy store. This permits the hotter energy storage fluid layer at the top part of the thermal energy store to be taken from the thermal energy store and be used to heat the incoming cold water. To conserve space, instead of the second outlet of the thermal energy store being connected to a pipe extending outwardly from the top part of the housing, an interior pipe or channel may be used to take the energy storage fluid from the second outlet of the thermal energy store down towards the heat exchangers. As the interior pipe or channel extends within or adjacent to the thermal energy store it remains within the housing and does not require extra space to be provided around the housing to accommodate it. The interior pipe may be a fixed pipe or may alternatively be a flexible pipe with a floating inlet. For example, the second outlet of the thermal energy store may be a floating inlet of a flexible pipe, with the floating inlet hole being located below the fluid surface so that the uppermost layer of the energy storage fluid can flow into the pipe even if the fluid level of the thermal energy store is not at a maximum level. Alternatively there may be an integral channel formed in the thermal energy store and having an entrance to the channel at the upper part of the thermal energy store to allow the warmer energy storage fluid layer to flow down through the channel towards the heat exchangers. In this case (as in the case of a fixed pipe) thefluid level of the thermal energy store needs to be kept above the entrance to the channel to prevent air from getting into the heat exchangers. The energy storage fluid operates in a closed loop environment and in order to help keep the quantity of the energy storage fluid constant, the thermal energy store may have a vapour impermeable seal to prevent evaporation of the energy storage fluid. A fill inlet may be provided within the system to initially fill or to top up the energy storage fluid in the thermal energy store. Advantageously the fill inlet may be provided at the bottom part of the thermal energy store (where the colder layer of energy storage fluid is found) and may be accessible from the bottom part or the lower front part of the housing to improve accessibility of the fill inlet. To save space, the fill inlet may be located near to or connect to another of the inlets or outlets of the thermal energy store. For example, the fill inlet may connect to the second inlet of the thermal energy store exterior of the thermal energy store and is configured to be closed when not in use. The thermal energy store may be clad with a heat insulating material in order to mitigate heat loss from the thermal energy store. The system may include a first pump configured to pump the energy storage fluid through the first heat exchanger when the system is operating in a first configuration. The system may include a second pump configured to pump the energy storage fluid through the second heat exchanger when the system is operating in a first configuration, and the second pump being configured to pump the energy storage fluid through both the first and second heat exchangers when the system is operating in a second configuration. The system may further include a processor configured to control the first and second pumps. The processor may also be configured to switch the configuration of the system between the first and second configurations. The processor may also control the switching on and off of the heat source. The first pump, the second pump, one or more motors and the processor may all be located underneath the thermal energy store within the housing. As some heat sources, such as heat pumps for example, tend to require a significant time to start producing heat in response to demand, perhaps 30 seconds for self-checking then some time to heat up, there may be a delay of around 1 to 2 minutes or even more between the processor activating the heat source and the heat source transferring heat to the energy storage fluid via the first heat exchanger. It is advantageous if the energy storage fluid is heated by the heat source during a low electricity tariff time, for example at night, when demand is low. This also enables most or all of the energy storage fluid in the thermal energy store to be heated and ready for use for the morning, when users are likely to want hot water for showers, etc. The heat source may also or alternatively operate to heat the energy storage fluid during the day when there is a surplus of energy available, for example when the batteries of a solar cell are full. The thermal energy store may store, for example, approximately 60 litres of energy storage fluid which may be heated by the heat source to a temperature of approximately 40 - 65 degrees Celsius. This energy storage fluid, in turn, may heat approximately 120 - 160 litres of domestic water to a temperature suitable for hand washing or showering, for example around 38 - 45 degrees Celsius. It can be seen this system allows a small 60 litre tank to provide 120 - 160 litres of hot water without requiring space for a 120 - 160 litre hot water tank. This is possible due to the ability of the system to operate in two different configurations as explained below. In the first configuration (normal configuration) the valves on the first, third and fourth fluid paths (a, c, and d) are open and the valves on the second and fifth fluid paths (b and e) are closed. The sixth fluid path (f) does not require a valve and, if one is provided, it can remain open. When the system is operating in the first configuration, the energy storage fluid may leave the thermal energy store via the first fluid path (a) and pass through the first heat exchanger where it receives heat from the working fluid of the heat pump, then it may be fed back into the thermal energy store via the fourth fluid path (d). At the same time, heated energy storage fluid may be taken from the top of the thermal energy store via the third fluid path (c) and may be passed through the second heat exchanger to heat the cold water coming from the mains (or cold water storage tank) before the (now cooler) energy storage fluid is returned to the thermal energy store via the sixth fluid path (f). The cooler energy storage fluid at the bottom of the thermal energy store may be continually pumped through the first heat exchanger so that the cooler energy storage fluid returned to the thermal energy store may continually be heated. When the system is switched to operate in the second configuration (enhanced configuration), the valves on the second and fifth fluid paths (b and e), as well as the valve on the sixth fluid path (f) if there is one, are open and the valves on the first, third and fourth fluid paths (a, c and d) are closed. When the system is operating in the second configuration, the energy storage fluid may leave the thermal energy store via the second fluid path (b) and may pass through the first heat exchanger where it receives heat from the working fluid of the heat pump. The heated energy storage fluid then may travel via the fifth fluid path (e) to the second heat exchanger and pass through the second heat exchanger to heat the cold water coming from the mains (or cold water storage tank) before the (now cooler) energy storage fluid is returned to the thermal energy store via the sixth fluid path (f). In the second configuration, the energy storage fluid that is returned to the thermal energy store is unlikely to have sufficient thermal energy left to be useful in heating the incoming cold water, and so the energy storage fluid generally does not repeat the cycle again until the thermal energy store is replenished. To replenish the energy in the thermal energy store, the system is switched back into the first configuration, and the heat source is activated and only the first pump is operated so that the energy storage fluid is cycled through the first heat exchanger only in order to heat the energy storage fluid in the thermal energy store. It can therefore be seen that the system can be switched to operate in the second configuration in situations where there is a greater need for hot water than usual, in order to provide approximately an extra 30 - 60 litres of hot water without the need to wait for the replenishment of the thermal energy store. A flow rate sensor may be provided in the hot water pipe which leads from the second heat exchanger towards one or more hot water outlets. The hot water outlets may be, for example, domestic hot water outlets such as a kitchen tap, bath tap, wash basin tap or a shower head. The domestic hot water outlets may be located in a dwelling such as a flat or small house, for example, or in a bathroom or kitchen of a workplace or on a boat. The processor, based on the hot water flow rate measured by the flow rate sensor, may determine the type of domestic hot water outlet which has been opened and use this information to determine whether to provide a start signal to the heat source or not. For example, if the flow rate determined by the flow rate sensor indicates that a shower is turned on then the processor may activate the heat source straight away in anticipation of a reasonable quantity of hot water being required. The processor may additionally or alternatively be configured to activate the heat source if the domestic hot water outlet is open for a predetermined amount of time at a particular flow rate. This permits the heat source to be activated and to start heating the energy storage fluid before the heat previously stored in the energy storage fluid has been depleted such that it is no longer able to heat the domestic hot water to a satisfactory temperature. It also permits the heat source to be activated, warm up and to start heating the energy storage fluid whilst the system is in the first configuration, so that there is no time lag for heating the energy storage fluid in the first heat exchanger when the system is switched to the second configuration. The processor may alternatively be configured to activate the heat source if the domestic hot water outlet is open for a predetermined amount of time. The predetermined amount of time may be indicative of a longer time than that generally required to wash hands, for example. This may indicate that the user is likely to want a more significant amount of hot water, for example to wash dishes or to have a shower or bath. This permits the heat source to be activated and to start heating the energy storage fluid before the heat previously stored in the energy storage fluid has been depleted such that it is no longer able to heat the domestic hot water to a satisfactory temperature. It also permits the heat source to be activated, warm up and to start heating the energy storage fluid whilst the system is in the first configuration, so that there is no time lag for heating the energy storage fluid in the first heat exchanger when the system is switched to the second configuration. The present system may comprise a plurality of sensors, at least one sensor being provided on each of a plurality of domestic hot water outlets. Each sensor may be configured to send a signal to the processor in the event of an opening of the associated domestic hot water outlet by a user. The signal ideally includes an identification of the sensor and therefore its associated domestic hot water outlet. The sensor may be or include a flow rate sensor. The processor may be configured to identify the domestic hot water outlet opened by the user and to activate the heat source if a predetermined domestic hot water outlet has been opened, such as a bath tap or shower head, for example, which may indicate that the user requires a significant amount of hot water. Again, this permits the heat source to be activated and to start heating the energy storage fluid before the heat previously stored in the energy storage fluid has been depleted such that it is no longer able to heat the domestic hot water to a satisfactory temperature. It also permits the heat source to be activated, to warm up and to start heating the energy storage fluid whilst the system is in the first configuration, so that there is no time lag for heating the energy storage fluid in the first heat exchanger when the system is switched to the second configuration. The processor may include a machine learning algorithm for monitoring and learning from the signals received from the one or more sensors and for identifying time periods when the domestic hot water usage was high and using this information to predict time periods when a user is likely to require at least a predetermined amount of hot water. Using this knowledge, the processor can ensure that the energy storage fluid is sufficiently heated before certain time periods during which a large demand for domestic hot water has been predicted. The machine learning algorithm may further monitor and learn the flow characteristics of different hot water outlets and use this to assist the processor in determining the hot water outlet that is in use. The present system may include a temperature sensor located at, in or near to the second outlet of the thermal energy store, or within the pipe or channel leading from this outlet towards the heat exchangers. This sensor may provide the processor with information relating to the temperature of the energy storage fluid in the upper part of the thermal energy store and, if the temperature drops below a predetermined temperature, the processor may activate the heat source. The processor may alternatively or additionally be configured to switch the system from the first configuration to the second configuration if the temperature falls below a predetermined temperature. The present system may include a temperature sensor located in the thermal energy store at a predetermined distance below the second outlet of the thermal energy store (ie at a predetermined distance below the inlet to the interior pipe or channel). This sensor may provide the processor with information relating to the temperature of the energy storage fluid in that part of the thermal energy store where the sensor is located, for example at a location in the middle region of the thermal energy store. The processor may use this information to switch on the heat source when the temperature drops below a predetermined level. This allows the heat source to be activated in advance of the heated energy storage fluid being depleted, so that the user does not experience a sudden drop in temperature. The present system may include a temperature sensor located in the thermal energy store near or at the bottom of the thermal energy store. This sensor may provide the processor with information relating to the temperature of the energy storage fluid at the bottom part of the thermal energy store. The processor may use this information to switch off the heat source when the temperature rises above a predetermined level. This may be particularly useful during replenishment of the thermal energy store. The present system may include a flow rate sensor located in the fluid pathway that runs from the second outlet of the thermal energy store via the second heat exchanger to the second inlet of the thermal energy store when the system is in the first configuration. This sensor may provide the processor with information relating to the flow rate of the energy storage fluid through the second heat exchanger. The processor may use this information to switch on the heat source when a predetermined quantity of energy storage fluid has passed through the second heat exchanger. This allows the heat source to be activated in advance of the heated energy storage fluid being depleted, so that the user does not experience a sudden drop in temperature. The present system may include one or more flow restrictors for restricting the flow of a fluid. For example, a flow restrictor may be located at or near to the first inlet of the second heat exchanger to regulate the flow rate of incoming cold water through the second heat exchanger, or a flow restrictor may be located between the second outlet of the thermal energy store and the second inlet of the second heat exchanger to regulate the flow of energy storage fluid through the second heat exchanger. This can help to manage the transfer of heat from the energy storage fluid to the incoming cold water and, in times where usage is more than expected, reduce the flow rate and / or temperature of the domestic hot water to thereby prolong the time that hot or warm water is available. An optional immersion coil may be provided in the thermal energy store for emergencies, for example if a heat pump fails. The present system may further comprise an instantaneous water heater in the flow path between the thermal energy store and the second heat exchanger, the instantaneous water heater being controlled by the processor. The instantaneous water heater is preferably an electric heater. Such an additional heat source can be useful for periods when the heat source is unavailable, or when the energy storage fluid of the thermal energy store is at a cooler temperature. It also provides a means to take advantage of low or negative energy tariffs or of locally generated electricity (e.g. from solar panels or a domestic wind turbine). A resilient member may be provided to help to keep the level of the fluid in the thermal energy store substantially constant. Although the energy storage fluid operates in a closed loop environment (such that the quantity of energy storage fluid should remain substantially constant), the volume of the energy storage fluid will fluctuate depending upon its temperature when its pressure is kept substantially constant. For example, for water in its liquid state between approximately zero and 100 degrees Celsius, the change in volume is approximately 5%. The resilient member may help to keep the fluid level in the thermal energy store substantially constant as the volume of the energy storage fluid varies within a temperature range of at least 5 degrees to 60 degrees Celsius without adding energy storage fluid to or removing energy storage fluid from the closed fluid circuit. The resilient member is preferably configured to yield in response to fluctuations in volume of the energy storage fluid in order to keep the pressure within the thermal energy store substantially constant and substantially equal to ambient pressure when the temperature of the energy storage fluid is in the approximate range of 5 - 85 degrees Celsius. The resilient member may be a bladder, a flexible membrane, a diaphragm, or an expansion vessel, for example. The resilient member is preferably made of a material which is fluid impermeable. The resilient member preferably is able to be squashed or to lose its shape when an external pressure is applied to it and to regain its shape when the external pressure is removed. The resilient member may be made of a material such as rubber, for example. The resilient member may be made from a natural or synthetic rubber. The resilient member may be made from neoprene. The resilient member may be fixed to the top and / or to an upper side of the thermal energy store. The resilient member may form the top or part of the top of the thermal energy store. Preferably one side of the resilient member is exposed to the energy storage fluid and the other side of the resilient member is exposed to ambient air. When the energy storage fluid is relatively cold, then the fluid level of the energy storage fluid in the thermal energy store is lower than when the energy storage fluid is heated. It is advantageous to keep the fluid level as constant as possible, particularly where the second outlet of the thermal energy store is fixed in position or is an integral channel, so that the warmest energy storage fluid at the top of the thermal energy store may be directed to the heat exchangers without the risk that the fluid level drops and air enters the heat exchangers. The resilient member may function as follows: When the energy storage fluid is relatively cold, the resilient member relaxes towards its original expanded state, taking up more room in the thermal energy store (ie reducing the volume of the thermal energy store) and causing the level of the energy storage fluid to rise to a desired level as a result. When the energy storage fluid is relatively hot, the resilient member is under greater pressure which compresses the resilient member so that it does not take up as much room in the thermal energy store (ie increasing the volume of the thermal energy store), causing the level of the energy storage fluid to drop to a desired level as a result. In this manner the level of the energy storage fluid in the thermal energy store can be kept at a generally constant level by the resilient member, regardless of the temperature fluctuations of the energy storage fluid. In use, when a user requires hot water, the domestic hot water supply system heats the domestic hot water via the second heat exchanger, using the heated energy storage fluid. For example, the system may operate as follows: The system is generally in the first (normal) configuration unless a relatively large quantity of hot water has been required already and the processor has switched the system to operate in the second (enhanced) configuration. A user may wish to use hot water in the morning when they wake up. During the night, the energy storage fluid may have been heated in anticipation of hot water usage in the morning, and all of the energy storage fluid in the thermal energy store is ideally heated and ready for use. This is achieved by switching on the heat pump during the night (which may take advantage of cheaper energy tariffs or locally stored energy) and activating the first pump so that colder energy storage fluid is taken from the bottom of the tank of the thermal energy store and pumped through the first heat exchanger. This energy storage fluid is then heated in the first heat exchanger by the working fluid of the heat pump and then the heated energy storage fluid is fed back into the tank. This heated energy storage fluid is warmer than the rest of the fluid at the bottom of the tank and will rise to the top of the tank, with colder energy storage flu id taking its place at the bottom, and being heated in its turn. Once all of the energy storage fluid in the thermal energy store has been heated, then the heat pump may be switched off. In the morning, when the user turns on a hot water tap, the system starts to operate in the first configuration and the heat pump is switched on, either immediately or with a short delay (for reasons explained below). As the heated energy storage fluid is gradually taken from the thermal energy store via the second outlet of the thermal energy store and the third fluid path (c) it is used to heat the domestic water passing through the second heat exchanger, and the preheated energy storage fluid available for use is gradually depleted. Once the temperature sensor senses that the temperature of the energy storage fluid at the second outlet of the thermal energy store has fallen below a predetermined temperature, then the system is configured to switch to the second configuration. In this second configuration, the first pump can be switched off and the flow paths of the system are switched so that the energy storage fluid leaving the thermal energy store via the second outlet is directed along the second fluid path (b) to the first heat exchanger. This energy storage fluid is warm but not sufficiently hot enough to heat the incoming cold water to a satisfactory hot water temperature. This energy storage fluid passes through the first heat exchanger where it is heated by the working fluid of the heat pump and this now sufficiently heated energy storage fluid is then directly guided to the second heat exchanger via the fifth fluid path (e) without being returned to the thermal energy store. The heated energy storage fluid is used to heat the domestic water passing through the second heat exchanger before eventually being returned to the thermal energy store via the sixth fluid path (f). This system is advantageous for the following reason. The thermal energy store may store, for example, 60 litres of energy storage fluid which may be heated by the heat pump to a temperature of approximately 40 - 65 degrees Celsius. This energy storage fluid, in turn, may heat approximately 90 - 120 litres of domestic water to a temperature suitable for showering, for example around 38 - 45 degrees Celsius, when operating in the first configuration. When the system switches to the second configuration, the energy storage fluid may heat a further approximately 30 - 60 litres of domestic water to a temperature suitable for showering. It can therefore be seen that this system allows a small 60 litre tank to provide at least 100 litres and more generally 120 - 160 litres of hot water without requiring space for a 120 - 160 litre hot water tank. Furthermore, as the tank is unpressurised, there is no need for pressure relief valves or expansion vessels, and the tank can be made out of a relatively thin plastic material, for example, instead of a more expensive material. Although the tank may have a shape such as a traditional cylindrical shape, because it is unpressurised it could instead have a shape such as a cuboid shape which can save space, particularly when the tank is fitted into a housing and / or a cupboard. The present system may also be used to supply hot water to a domestic heating circuit, for example to radiators or underfloor heating. According to a second aspect, there is provided a unit for providing a quantity of hot water suitable for washing or bathing, the unit having maximum dimensions of 750mm wide x 750mm depth x 1200mm height, the unit comprising an unpressurised tank for storing an energy storage liquid, and the unit further comprising first and second heat exchangers connected to the tank to form a closed fluid circuit for the energy storage liquid, the first heat exchanger being arranged to transfer heat from a heat source to the energy storage liquid and the second heat exchanger being arranged to transfer heat from the energy storage liquid to incoming cold water, the unit further comprising a plurality of valves located within the closed fluid circuit, and a processor for controlling the valves, the processor being configured to operate the valves such that the closed fluid circuit can be switched between a first configuration providing a first series of flow paths between the tank and the heat exchangers and a second configuration providing a second series of flow paths between the tank and the heat exchangers. The first configuration preferably provides a closed fluid circuit such that energy storage liquid can flow from a lower part of the tank through the first heat exchanger then back to the tank and from an upper part of the tank through the second heat exchanger then back to the tank. The second configuration preferably provides a closed fluid circuit such that energy storage liquid can flow from an upper part of the tank through the first heat exchanger then through the second heat exchanger then back to the tank. The processor may be configured to switch from one configuration to another depending upon an energy state of the energy storage liquid in the tank. The unit may comprise one or more temperature sensors configured to provide the processor with information relating to the energy state of the energy storage liquid at one or more locations in the tank. The unit may comprise at least one temperature sensor located in the upper part of the tank. The processor may be configured to switch the fluid circuit from the first configuration to the second configuration if the temperature of the energy storage liquid in the upper part of the tank falls below a first predetermined temperature. The processor may be configured to switch the fluid circuit from the second configuration to the first configuration if the temperature of the energy storage liquid in the upper part of the tank falls below a second predetermined temperature, the second predetermined temperature being lower than the first predetermined temperature. The unit may have maximum dimensions of 600mm wide x 600mm depth x 900mm height. The unit may comprise a pump and control arrangement to selectively circulate energy storage liquid from the tank to the first and second heat exchangers. The unit may include a first pump configured to pump the energy storage fluid through the first heat exchanger when the unit is operating in the first configuration. The system may include a second pump configured to pump the energy storage fluid through the second heat exchanger when the unit is in a first configuration, and the second pump being configured to pump the energy storage fluid through both the first and second heat exchangers when the system is operating in a second configuration. The system may further include a processor configured to control the operation of the first and second pumps. The processor may also be configured to switch the configuration of the system between the first and second configurations. The processor may also control the switching on and off of the heat source. The unit may comprise a housing. The first pump, the second pump, one or more motors and the processor may all be located underneath the thermal energy store within the housing. The unpressurised tank may have further features as described above. The tank may be a thermal energy store for storing an energy storage liquid such as water, for example. The tank may be located within the housing. The housing itself may form some or all of the walls of the unpressurised tank or alternatively a separate unpressurised tank may be provided inside the housing. The housing may have a substantially cuboid form. The housing may comprise a top and bottom and four sides. The top of the housing is that part which, when the housing is fitted to a wall or a shelf, is the uppermost part of the housing (ie that part of the housing closest to the ceiling). The dimension of the housing between the top and the bottom is preferably larger than the dimension between any two opposing sides. The housing may include at least one recess or protruding part as mentioned above. An elongated cuboid housing may be provided, where the housing has a greater height than width or depth. The housing may have a height which is one and a half to four times the width and depth of the housing. Preferably the housing has a height which is at least twice the width and depth of the housing. The housing may have maximum dimensions of 750mm wide x 750mm depth x 1200mm height. The housing may have maximum dimensions of 600mm wide x 600mm depth x 900mm height. The housing may have a width of 500mm, a depth of 500mm and a height of 900mm. The dimensions of the housing may be, for example, 350mm x 250mm x 900mm. Alternatively the dimensions of the housing may be, for example, 350mm x 250mm x 800mm. The unit may be configured to be mounted to a wall or to a shelf, for example, or may be fitted into a cupboard. The unit may include a support structure. The support structure may be configured to be mounted to a mounting surface such as a wall or a floor. The support structure may comprise a rear plate for mounting to a wall. Alternatively or additionally the support structure may comprise one or more feet for mounting to or resting on the floor or a substantially horizontal surface, such as a shelf, for example. Preferably all the inlets and outlets to the unit are located substantially at the bottom of the unit. This allows the unit to be fitted in a small space as an installer only needs to access the bottom of the unit in order to connect the unit to both the heat source and the incoming cold water to be heated. The unit can also extend up to the ceiling of the room in which it is fitted if mounted on a wall as the installer does not need to access the top or sides of the unit. The unit may comprise two pressurised inlets and two outlets for connecting the first and second heat exchangers to the heat source and the incoming cold water respectively. The first and second heat exchangers may be, for example, plate heat exchangers. The heat exchangers may be located within the tank or may be located outside of the tank but within the housing. Where the heat exchangers are located within the tank the second heat exchanger may be located in an upper part of the tank. If the heat exchangers are not located within the tank then they are preferably located within the housing and are preferably positioned underneath the tank. A manifold may be provided underneath the tank. One or more pumps for circulating the liquid in the tank through the heat exchangers may also be located within the housing and positioned underneath the tank. Optionally one or more motors for the pumps may also be located within the housing and positioned underneath the tank. The unit may further include a processor for controlling the one or more pumps. Where a manifold is provided, the first and second heat exchangers and the fluid circuit may form an integral part of the manifold. The manifold may be provided within the housing. A plurality of solenoid valves may be provided in the manifold, and a processor may control the valves in the manifold to select the flow path for the energy storage liquid. The processor may be configured to control the opening and closing of the valves to thereby switch the fluid circuit between the first configuration and the second configuration. The liquid in the tank operates in a closed loop environment and in order to help keep the quantity of the liquid in the tank constant, the tank may have a vapour impermeable seal to prevent evaporation of the liquid. A fill inlet may be provided within the unit to initially fill or to top up the liquid in the tank. Advantageously the fill inlet may be provided at the bottom part of the tank and may be accessible from the bottom part or the lower front part of the unit to improve accessibility of the fill inlet. To save space, the fill inlet may be located near to or connect to another of the inlets or outlets of the tank exterior of the tank but within the housing. The unit may include cladding, the cladding being formed from a heat insulating material in order to mitigate heat loss from the tank. The unit may be configured such that energy storage liquid is taken from the upper part of the tank to heat incoming cold water. By taking energy storage liquid from an upper part of the tank as previously explained, the hotter layer of energy storage liquid in the top part of the tank can be used in the second heat exchanger to heat incoming cold water for domestic or similar use. To conserve space, instead of the second outlet being connected to a pipe extending outwardly from the top part of the tank, an interior pipe or channel may be used to take the liquid from the upper part of the tank to the outlet of the tank which is located at the bottom of the tank. As the interior pipe or channel extends within the tank it does not require extra space to be provided around the tank to accommodate it. As previously mentioned, the second outlet of the tank may comprise one of the following: a rigid pipe located in the interior of the tank and having an inlet at an upper part of the tank, a flexible pipe with a floating inlet, the floating inlet being configured such that, when energy storage liquid is present in the tank, the inlet to the flexible pipe floats just below the liquid surface, or an integral channel formed in the tank and having an inlet to the channel at the upper part of the tank. The unit may include a first temperature sensor located in the upper part of the tank at or near the inlet to the pipe or channel. Alternatively the first temperature sensor may be located within the pipe or channel. This sensor may provide the processor with information relating to the temperature of the energy storage liquid in the upper part of the tank. The processor may be configured to switch the fluid circuit from the first configuration to the second configuration if a predetermined temperature is detected. For example, if the temperature of the liquid near the top of the tank drops to a predetermined temperature, then the fluid circuit is switched from the first configuration (where the energy storage liquid is taken from the upper part of the tank directly to the second heat exchanger to heat the incoming cold water) to the second configuration (where the energy storage liquid is taken from the upper part of the tank to the first heat exchanger to itself be heated before flowing to the second heat exchanger to heat the incoming cold water). A resilient member as previously described may be provided to help to keep the level of fluid in the tank substantially constant. The resilient member may be fixed to the top and / or to an upper side of the tank. The resilient member may form the top or part of the top of the tank. Preferably one side of the resilient member is exposed to the liquid in the tank and the other side of the resilient member is exposed to ambient air. The unit may include a second temperature sensor located in the upper or middle part of the tank at a predetermined distance below the inlet to the interior pipe or channel. The second temperature sensor may provide the processor with information relating to the temperature of the energy storage liquid in that part of the tank where the second temperature sensor is located. The processor may be configured to use this information to switch on the heat source when the processor determines that the temperature has dropped below a predetermined level. This allows the heat source to be activated in advance of the heated energy storage liquid in the tank being depleted, so that the user does not experience a sudden drop in temperature. The unit may include a flow rate sensor located in the fluid pathway between the tank and the second heat exchanger. This flow rate sensor may provide the processor with information relating to the flow rate of the energy storage liquid through the second heat exchanger. The processor may use this information to switch on the heat source when a predetermined quantity of energy storage liquid has passed through the second heat exchanger. This allows the heat source to be activated in advance of the heated energy storage liquid being depleted, so that the user does not experience a sudden drop in temperature. The unit may include at least one flow restrictor for restricting the flow of a fluid. For example, a flow restrictor may be used to regulate the flow rate of incoming cold water from the mains or cold water storage through the heat exchanger, and / or a flow restrictor may be used to regulate the flow of hotter energy storage liquid from the upper part of the tank to the second heat exchanger. This can help to manage the transfer of heat from the liquid stored in the tank to the incoming cold water and, in times where usage is more than expected, reduce the flow rate and / or temperature of the domestic hot water to thereby prolong the time that hot or warm water is available. An optional immersion coil may be provided in the tank for emergencies, for example if the heat pump fails. The unit may also include an instantaneous water heater in the flow path between the tank and the second heat exchanger, the instantaneous water heater being controlled by the processor. The instantaneous water heater is preferably an electric heater. Such an additional heat source can be useful for periods when the heat source is unavailable, or when the liquid in the tank is at a cooler temperature. It also provides a means to take advantage of low or negative energy tariffs or of locally generated electricity (e.g. from solar panels or a domestic wind turbine). The unit may also be used to supply hot water to a domestic heating circuit, for example to radiators or underfloor heating. The unit may be part of a hot water supply system as described above and may incorporate features of the hot water supply system as described above. According to a third aspect, there is provided a domestic water heating appliance integrated within a housing comprising: an unpressurised storage tank containing an energy storage fluid, a first heat exchanger with connections for connection to a pressurised heating circuit including a remote heat source with a power on lag time, a second heat exchanger with connections to a domestic hot water circuit, a pump and valve arrangement for selectively circulating the energy storage fluid in a closed fluid circuit through the heat exchangers and the storage tank in dependence on the operating mode, the tank shape and connections being arranged to promote stratification of the energy storage fluid in the tank, and a processor for detecting a demand for domestic hot water and for controlling the pump and valve arrangement and for detecting a state of operation of the remote heat source and / or for signalling a demand for heat, the processor being arranged to operate the heating appliance in at least a first mode wherein the pump and valve arrangement is in a first configuration and in at least a second mode wherein the pump and valve arrangement is in a second configuration. The first mode is preferably for heating the domestic hot water in the second heat exchanger when the remote heat source is active, and the first mode may comprise: a) determining whether hot water is being demanded at a domestic hot water outlet, and when it is determined that hot water is being demanded at the domestic hot water outlet, then b) detecting a state of operation of the remote heat source and, if the remote heat source is not active, signalling a demand for heat, c) determining whether the temperature of the energy storage fluid in an upper part of the storage tank is at least at a first predetermined temperature, and if so, then d) controlling the appliance to pump energy storage fluid from an upper part of the storage tank via a circulation pump through third fluid path (c) to the second heat exchanger, e) using the energy storage fluid to heat the domestic hot water in the second heat exchanger, f) controlling the appliance to pump the used energy storage fluid from the second heat exchanger via sixth fluid path (f) to return to a lower part of the storage tank, g) determining whether there is continued demand for hot water and, if so, then repeating steps c) to f) above, and h) whilst repeating steps c) to f) above, controlling the appliance to pump energy storage fluid from the storage tank via a first circulation pump through first fluid path (a) to the first heat exchanger, i) heating the energy storage fluid in the first heat exchanger, j) controlling the appliance to pump the heated energy storage fluid from the first heat exchanger via fourth fluid path (d) back to the storage tank. The second mode may be for heating the domestic hot water in the second heat exchanger when the remote heat source is active and the temperature of the energy storage fluid in an upper part of the storage tank is below a first predetermined temperature, and the second mode may comprise: a) determining whether hot water is being demanded at a domestic hot water outlet, and when it is determined that hot water is being demanded at the domestic hot water outlet, then b) detecting a state of operation of the remote heat source and, if the remote heat source is active, c) determining whether the temperature of the energy storage fluid in the storage tank is lower than the first predetermined temperature but at least at a second predetermined temperature, and if so, then d) controlling the appliance to pump energy storage fluid from an upper part of the storage tank via a circulation pump through second fluid path (b) to the first heat exchanger, e) heating the energy storage fluid in the first heat exchanger, f) controlling the appliance to pump the heated energy storage fluid from the first heat exchanger via fifth fluid path (e) to the second heat exchanger, g) using the energy storage fluid to heat the domestic hot water in the second heat exchanger, h) controlling the appliance to pump the used energy storage fluid from the second heat exchanger via sixth fluid path (f) to return to a lower part of the storage tank, i) determining whether hot water is being demanded at a domestic hot water outlet and, if so, then repeating steps c) to h) above. The processor may be further arranged to operate the heating appliance in a third mode, the third mode may be a tank replenishment mode, and the pump and valve arrangement may be in a third configuration. The third mode may be for replenishing the storage tank with hot energy storage fluid heated by the first heat exchanger, and the third mode may comprise: a) determining whether hot water is being demanded at a domestic hot water outlet, and when it is determined that no hot water is being demanded at the domestic hot water outlet, then: b) detecting a state of operation of the remote heat source and, if the remote heat source is not active, signalling a demand for heat, c) when it is determined that the remote heat source is active, controlling the appliance to pump energy storage fluid from the storage tank via a circulation pump through first fluid path (a) to the first heat exchanger, d) heating the energy storage fluid in the first heat exchanger, e) controlling the appliance to pump the heated energy storage fluid from the first heat exchanger via fourth fluid path (d) back to the storage tank, f) determining whether the temperature of the heated energy storage fluid in the storage tank has reached a predetermined temperature, and g) if the temperature has not reached the predetermined temperature, repeating steps b) to f) above. The processor may be further arranged to operate the heating appliance in a fourth mode, the fourth mode may be a low hot water usage mode, and the pump and valve arrangement may be in a fourth configuration. The fourth mode may be for heating the domestic hot water in the second heat exchanger when the remote heat source is not active, and the fourth mode may comprise: a) determining whether hot water is being demanded at a domestic hot water outlet, and when it is determined that hot water is being demanded at the domestic hot water outlet, then: b) detecting a state of operation of the remote heat source and, if the remote heat source is not active, c) determining whether the temperature of the energy storage fluid in the storage tank is at least at a first predetermined temperature, and if so, then d) controlling the appliance to pump energy storage fluid from an upper part of the storage tank via a circulation pump through third fluid path (c) to the second heat exchanger, e) using the energy storage fluid to heat the domestic hot water in the second heat exchanger, f) controlling the appliance to pump the used energy storage fluid from the second heat exchanger via sixth fluid path (f) to return to a lower part of the storage tank, g) determining whether (i) the temperature of the heated energy storage fluid in a middle region of the storage tank has fallen to a predetermined temperature lower than the first predetermined temperature, or (ii) a predetermined amount of time has passed during a continuous demand for hot water, or (ill) a predetermined amount of heated energy storage fluid has been used during a continuous demand for hot water, or (iv) hot water has been demanded from a predetermined particular hot water outlet, and, h) if none of (i) to (iv) apply, then repeating steps a) to g) above. The fourth mode permits the appliance to be used to heat water if only a small amount of hot water is required. For example, if a user is washing their hands then it is generally not worth switching on a heat pump with its considerable time lag to provide heat as the user will only require a small amount of hot water. This also helps to prevent excessive amounts of switching on and off of the heat source, which is important as it is not advisable to subject some heat sources to being switched on and off multiple times in quick succession. If any of the conditions (i) to (iv) above apply, then the processor is preferably arranged to switch operation of the heating appliance from the fourth mode to the first mode. By identifying if there has been or is likely to be a substantial demand for water, the heat source can be switched on in a timely manner to ensure that the water temperature remains sufficiently warm and does not substantially fluctuate when the user requires hot water. When the appliance operates in the first mode, the remote heat source is preferably activated if it is not already active and the temperature of the energy storage fluid in an upper part of the storage tank is preferably determined to be at least at a first predetermined temperature. The first mode is a "normal" mode of operation of the appliance. In the first mode, the hotter energy storage fluid from the top part of the storage tank is used to heat the incoming cold water whilst at the same time the colder energy storage fluid from the bottom part of the storage tank is heated in the first heat exchanger before being returned to the storage tank. This permits the heat source to operate at a higher coefficient of performance as it is the coldest energy storage fluid in the storage tank that is pumped through the first heat exchanger to be heated. Furthermore, whilst the incoming cold water is being heated by the hotter energy storage fluid from the storage tank, the coolest energy storage fluid at the bottom of the storage tank is continuously being heated and returned to the storage tank, thereby replenishing heat energy in the storage tank. However, if the appliance operates for a reasonable amount of time in the first mode, for example if a user is taking a bath or if two or more people take showers after one another, then eventually the temperature of the energy storage fluid in an upper part of the storage tank may fall below the first predetermined temperature. Once this occurs the processor is preferably arranged to switch operation of the heating appliance from the first mode to the second mode. In the second mode (the "enhanced" mode"), the configuration of the flow paths in the appliance has preferably been changed by the processor so that instead of the energy storage fluid from the upper part of the tank being pumped directly to the second heat exchanger, the energy storage fluid from the top of the tank passes through the first heat exchanger to heat it up further before it is pumped through the second heat exchanger to heat the incoming cold water. This may allow the (now cooler) energy storage fluid in the storage tank to be heated to a more desirable temperature (for example similar to the first predetermined temperature) before it is used to heat the incoming cold water. Once the (now cooler) energy storage fluid in the storage tank has been exhausted then the temperature of the energy storage fluid in the upper part of the storage tank may fall below the second predetermined temperature and the storage tank will need to be replenished before it can be used again. To replenish the storage tank, the processor preferably operates the heating appliance in the third mode. Once the energy storage fluid in the storage tank is heated to a satisfactory temperature again, then the processor may instruct the heat source to switch off and the heating appliance may again be used (for example in the fourth mode, at least initially) when hot water is again required. At times when cheap electricity is available or there is a surplus of energy available, the appliance may operate in the third mode to replenish the heat stored in the tank. The appliance may include features as described in the first and second embodiments above. The appliance may comprise a manifold in fluid connection with the storage tank. The processor may control the manifold to select an internal flowpath corresponding to a particular operating mode of the appliance. The processor may control the pump and valve arrangement to change the configuration of the flow paths of the appliance by controlling the on / off state of the valves and the on / off state of the pumps in dependence upon the operating mode. The appliance may comprise a first circulation pump configured to pump energy storage fluid through the first heat exchanger when the appliance is being operated in the first mode, a second circulation pump configured to pump the energy storage fluid through the second heat exchanger when the appliance is being operated in the first mode and the second circulation pump may be configured to pump the energy storage fluid through both the first and second heat exchangers when the appliance is being operated in the second mode. The first circulation pump may be configured to pump energy storage fluid through the first heat exchanger when the appliance is being operated in the third mode. The second circulation pump may be configured to pump energy storage fluid through the second heat exchanger when the appliance is being operated in the fourth mode. The appliance may comprise a plurality of solenoid valves that are controlled by the processor to provide the appliance with a plurality of flow paths in dependence upon the operating mode. The appliance may also be used to supply hot water to a domestic heating circuit, for example to radiators or underfloor heating. The appliance may be part of a hot water supply system as described above and may incorporate features of the hot water supply system as described above. According to a fourth aspect, there is provided a method of controlling an appliance for providing a quantity of hot water suitable for domestic washing or bathing, the appliance comprising: a thermal energy store for storing substantially unpressurised energy storage fluid, the appliance further being provided with a first heat exchanger that is coupled between the appliance and a heat source and a second heat exchanger that is coupled between the appliance and the incoming cold water to be heated, the first and second heat exchangers being connected to the thermal energy store to form a closed fluid circuit for the energy storage fluid, the appliance further comprising at least one circulation pump for circulating the energy storage fluid through the heat exchangers and a plurality of valves for providing different flow paths for the energy storage fluid within the appliance, the thermal energy store having a first outlet from the lower part of the thermal energy store and a second outlet from the upper part of the thermal energy store, and a first inlet and a second inlet, and wherein at least first to sixth fluid paths (a-f) are provided as follows: a) a first fluid path between the first outlet of the thermal energy store and the inlet of the first heat exchanger, b) a second fluid path between the second outlet of the thermal energy store and the inlet of the first heat exchanger, c) a third fluid path between the second outlet of the thermal energy store and the inlet of the second heat exchanger, d) a fourth fluid path between the outlet of the first heat exchanger and the first inlet of the thermal energy store, e) a fifth fluid path between the outlet of the first heat exchanger and the inlet of the second heat exchanger, f) a sixth fluid path between the outlet of the second heat exchanger and the second inlet of the thermal energy store, a flow rate sensor to detect the flow rate of the hot water to one or more hot water outlets when a hot water outlet is opened, a processor including a memory operatively coupled to the processor, the memory storing information and instructions for the processor, the processor being configured to receive information from one or more sensors and to compare the information received from the one or more sensors with information pre-stored in the memory, the processor being configured to control the circulation pump and the valves, the method further comprising: detecting the opening of a hot water outlet, detecting whether the heat source is active or not, detecting the temperature of the energy storage fluid in the upper part of the thermal energy store, deciding whether to activate the heat source if it is not active, selecting one of a plurality of operating modes of the appliance, operating the valves to configure the flow paths within the appliance corresponding to the selected operating mode, and activating the circulation pump. If the detected temperature of the energy storage fluid in the upper part of the thermal energy store is at least a predetermined first temperature, the processor is preferably arranged to operate the appliance in a first configuration whereby the first, third, fourth and sixth fluid paths (a, c, d and f) are substantially open and the second and fifth fluid paths (b and e) are substantially closed. If the detected temperature of the energy storage fluid in the upper part of the thermal energy store is lower than the predetermined first temperature but is at least a predetermined second temperature, the processor is preferably arranged to operate the appliance in a second configuration whereby second, fifth and sixth fluid paths (b, e and f) are substantially open and the first, third and fourth fluid paths (a, c and d) are substantially closed. If the detected temperature of the energy storage fluid in the upper part of the thermal energy store is lower than the predetermined second temperature, the processor is preferably arranged to operate the appliance in an energy replenishment mode comprising: a) detecting a state of operation of the heat source and, if the heat source is not active, signalling a demand for heat, b) when it is determined that the heat source is active, controlling the appliance to pump energy storage fluid from the thermal energy store via the circulation pump through the first fluid path (a) to the first heat exchanger, c) heating the energy storage fluid in the first heat exchanger, d) controlling the appliance to pump the heated energy storage fluid from the first heat exchanger via the fourth fluid path (d) back to the storage tank, e) determining whether the temperature of the heated energy storage fluid in the thermal energy store has reached a predetermined temperature, and f) if the temperature has not reached the predetermined temperature, repeating steps a) to e) above. The processor, based on the information provided by the flow rate sensor, may be configured to identify the hot water outlet which has been opened from its flow characteristic. The processor may use this information to determine whether to activate the heat source or not. The processor may be configured to activate the heat source if a hot water outlet is open for a predetermined amount of time at a particular flow rate. The processor may be configured to activate the heat source if the temperature of the energy storage fluid in a middle region of the thermal energy store has fallen to a predetermined temperature lower than the first predetermined temperature. The appliance may comprise a first circulation pump configured to pump energy storage fluid through the first heat exchanger when the appliance is operating in the first configuration and a second circulation pump configured to pump the energy storage fluid through the second heat exchanger when the appliance is operating in the first configuration and through both the second and first heat exchanger when the appliance is operating in the second configuration. The processor may be configured to control the first and second circulation pumps. A flow restrictor may be provided in the incoming cold water pipe, the processor being configured to control the flow restrictor to regulate the flow rate of incoming cold water through the second heat exchanger. A flow restrictor may be provided in the closed fluid circuit, the flow restrictor being located at or between the second outlet from the thermal energy store and the inlet of the second heat exchanger, the processor being configured to control the flow restrictor to regulate the flow rate of the energy storage fluid through the second heat exchanger. The processor may be provided with a machine learning algorithm and may be configured to use the machine learning algorithm to predict time periods when hot water usage is likely to be high. The processor may be configured to activate the heat source in advance of time periods predicted to have high hot water usage. An instantaneous water heater may be provided in the flow path between the thermal energy store and the second heat exchanger. The instantaneous water heater may be controlled by the processor and the processor may be configured to activate the instantaneous water heater in the event that the thermal energy store is depleted and hot water is still required immediately. A controller for a heating appliance comprising a processor and non-volatile memory storing instructions for causing the processor to perform the method is also provided. A computer program product comprising a computer readable medium or data packet comprising instructions to cause a processor to perform the method is also provided. The present invention will now be described by way of example only with reference to the following drawings: Figure 1 shows a schematic diagram of a water heating unit or appliance forming part of a domestic hot water supply system operating in a first mode in accordance with the present invention. Figure 2 shows a schematic diagram of a water heating unit or appliance forming part of a domestic hot water supply system operating in a second mode in accordance with the present invention. Figure 3 shows a more detailed schematic diagram of a water heating unit or appliance forming part of a domestic hot water supply system in accordance with the present invention Figure 4 shows an isometric view of a thermal energy store of a water heating unit or appliance forming part of a domestic hot water supply system in accordance with the present invention. Figure 5a shows a cross section through a thermal energy store having a resilient member, the thermal energy store being part of a water heating unit or appliance forming part of a domestic hot water supply system in accordance with the present invention. Figure 5b shows the thermal energy store of figure 5a when pressure acts on the resilient member. Figure 5c shows a top view of the thermal energy store of figures 5a and 5b. Figure 6 shows a schematic diagram of a domestic hot water supply system operating in a first mode in accordance with the present invention. Figure 7 shows a schematic diagram of a domestic hot water supply system operating in a second mode in accordance with the present invention. Figure 8 shows a schematic diagram of a domestic hot water supply in accordance with the present invention integrated into a dwelling. Figure 9 shows a flow chart indicating potential operation of the water heating unit or appliance. Figure 10 shows test results of an appliance in accordance with the present invention. Figure 1 shows a water heating unit or appliance 1 forming part of a domestic hot water supply system operating in a first mode ("normal" mode) and including a thermal energy store 2, a first heat exchanger 3 and a second heat exchanger 4. The first heat exchanger 3 exchanges heat between a working fluid in a heat pump (not shown) and an energy storage fluid of the thermal energy store 2. The second fluid exchanger 4 exchanges heat between the energy storage fluid and the domestic water to be heated. In this first mode, the heating unit has a first configuration as shown in figure 1. The first heat exchanger 3 has a first pressurised inlet 5 for receiving hot working fluid from the heat pump and a second unpressurised inlet 7 for receiving colder energy storage fluid from the bottom part of the thermal energy store 2. The heat exchanger 3 also has a first outlet 6 for sending the working fluid back to the heat pump once heat exchange has taken place and a second outlet 8 for returning the heated energy storage fluid back to the thermal energy store 2. A pump 9 is provided for pumping the energy storage fluid through the first heat exchanger 3. The second heat exchanger 4 has a first pressurised inlet 10 for receiving cold domestic water from the mains or a cold water tank. The heat exchanger 4 also has a second unpressurised inlet 12 for receiving heated energy storage fluid from the upper part of the thermal energy store 2. The heat exchanger 4 also has a first outlet 11 for providing domestic hot water to domestic hot water outlets such as taps, and a second outlet 13 for returning the cooled energy storage fluid back to the thermal energy store 2 once heat exchange has taken place. A pump 14 is provided for pumping the energy storage fluid through the second heat exchanger 4. In the first mode, the energy storage fluid circulates in a closed fluid system which includes the thermal energy store 2, the first and second heat exchangers 3, 4 and the fluid connections between them such as pipes, inlets and outlets. The energy storage fluid provided to the first heat exchanger 3 is drawn from the bottom part 15 of the thermal energy store 2 via outlet 16. The coldest energy storage fluid will be found at or near to the bottom 15 of the thermal energy store 2 and the hottest energy storage fluid will be found at or near to the top 17 of the thermal energy store 2. The colder energy storage fluid from the bottom part 15 of the thermal energy store 2 is pumped through the first heat exchanger 3 by pump 9 when the heat pump is switched on. In the first mode, the heat pump is switched on. The energy storage fluid is heated by the working fluid of the heat pump passing through the heat exchanger 3. The heated energy storage fluid is then returned to the thermal energy store via inlet 18. The heated energy storage fluid is less dense than the cooler energy storage fluid in the thermal energy store 2 and so will rise to the upper part 19 of the thermal energy store. During a demand for domestic hot water, for example when a tap in the building is turned on, then the pump 14 is activated to pump energy storage fluid from the upper part 19 of the thermal energy store 2 via outlet 20 to the second heat exchanger 4. As outlet 20 is located at the upper part 19 of the thermal energy store 2 the energy storage fluid taken via outlet 20 is the hottest energy storage fluid in the thermal energy store. This hot energy storage fluid exchanges its heat with the cold domestic water in the second heat exchanger 4 and heated domestic water exits the heat exchanger 4. The cooled energy storage fluid exits the heat exchanger 4 and is returned to the bottom part 15 of the thermal energy store 2 via inlet 21. The cooler energy storage fluid is then drawn from the bottom part 15 of the thermal energy store 2 via outlet 16 and is provided to the first heat exchanger 3 and the cycle continues. Figure 2 shows a water heating unit or appliance 1 forming part of a domestic hot water supply system operating in a second mode ("enhanced" mode). Like in the first configuration, the heating unit includes the thermal energy store 2, the first heat exchanger 3 and the second heat exchanger 4. The first heat exchanger 3 exchanges heat between a working fluid in a heat pump (not shown) and an energy storage fluid of the thermal energy store 2. The second fluid exchanger 4 exchanges heat between the energy storage fluid and the domestic water to be heated. In this second mode, the heating unit has a second configuration as shown in figure 2. The first heat exchanger 3 has a first pressurised inlet 5 for receiving hot working fluid from the heat pump and a second unpressurised inlet 7 for receiving warmer energy storage fluid from upper part of the thermal energy store 2. The heat exchanger 3 also has a first outlet 6 for returning the working fluid back to the heat pump once heat exchange has taken place and a second outlet 8 for sending the heated energy storage fluid to the second heat exchanger 4. The second heat exchanger 4 has a first pressurised inlet 10 for receiving cold domestic water from the mains or a cold water tank. The heat exchanger 4 also has a second unpressurised inlet 12 for receiving heated energy storage fluid from the first heat exchanger 3. The heat exchanger 4 also has a first outlet 11 for providing domestic hot water to the domestic hot water outlets such as taps, and a second outlet 13 for sending the cooled energy storage fluid back to the thermal energy store 2 once heat exchange has taken place. A pump 14 is provided for pumping the energy storage fluid from the upper part of the thermal energy store through thefirst heat exchanger 3 then through the second heat exchanger4 then back to the thermal energy store 2. In the second mode, the heat pump is already switched on and operating to heat the energy storage fluid passing through the first heat exchanger 3. The energy storage fluid circulates in a closed fluid system which includes the thermal energy store 2, the first and second heat exchangers 3, 4 and the fluid connections between them such as pipes, inlets and outlets. However, the configuration of this closed fluid system of the second mode is different from the configuration of the closed fluid circuit of the first mode. In the second mode, the energy storage fluid provided to the first heat exchanger 3 is not drawn from the outlet 16 at the bottom of the thermal energy store but is instead drawn from the outlet 20 at the upper part of the thermal energy store, where the warmer energy storage fluid will be found. When the heating device is operating in the second mode, the energy storage fluid at the upper part of the thermal energy store will not generally be as hot as it is in the first mode. In the first mode, the energy storage fluid at the upper part of the thermal energy store is generally in the range of approximately 45 - 60 degrees Celsius. Once the hotter energy storage fluid has been used up and the processor detects that the temperature at the upper part of the thermal energy store has fallen below a first predetermined temperature, the processor switches the system to operate in the second mode. In the second mode, the energy storage fluid at the upper part of the thermal energy store is generally in the range of approximately 33 - 35 degrees Celsius. In the second mode, this warm energy storage fluid is pumped through the first heat exchanger 3 by pump 14. The energy storage fluid is heated to a temperature of approximately 45 - 50 degrees by the working fluid of the heat pump passing through the heat exchanger 3. The heated energy storage fluid is then sent directly to the second inlet 12 of the second heat exchanger 4 to heat the incoming cold water. Upon exiting the second heat exchanger via second outlet 13 the energy storage fluid is returned to the thermal energy store via inlet 21. Once the energy storage fluid at the upper part of the thermal energy store falls below approximately 30 degrees Celsius then it is not readily able to be heated to a temperature sufficient for heating the incoming cold water to a reasonable hot water temperature of approximately 40 degrees, and the domestic hot water will run cooler. At this point it may be desirable to switch off the domestic hot water outlets and allow the thermal energy store to recharge. The thermal energy store is recharged by operating the heating unit in the third mode ("replenishment" mode). In this third mode, the heating unit has a third configuration similar to the first configuration shown in figure 1 and the heat pump is switched on. However, the pump 14 is not activated and energy storage fluid does not flow from the second outlet 20 of the thermal energy store to the second heat exchanger 4. Pump 9 is switched on and energy storage fluid flows from the bottom part 15 of the thermal energy store via outlet 16 to the first heat exchanger 3. The energy storage fluid is heated by the working fluid of the heat pump passing through the heat exchanger 3. The heated energy storage fluid is then sent back to the thermal energy store 2 via inlet 18. The newly heated energy storage fluid is less dense that the cooler energy storage fluid in the thermal energy store 2 and so will rise to the upper part 19 of the thermal energy store. Cooler energy storage fluid near the bottom of the thermal energy store will continue to be taken via outlet 16 to be heated in the first heat exchanger 3 and returned to the thermal energy store. This may continue until the thermal energy store is completely recharged, and the temperature of the energy storage fluid in the thermal energy store is approximately 45 - 60 degrees Celsius. Temperature sensors may be provided at various locations including the upper part, the middle part and the bottom part of the thermal energy store to monitor the temperature of the energy storage fluid in different locations within the thermal energy store. The thermal energy store may be recharged, for example, when there is an excess of energy available, eg from solar panels, or when energy prices are low, eg at night. The thermal energy store may also be recharged during usage of domestic hot water (by operating the heating unit in the first mode) or in anticipation of domestic hot water requirements. The heat pump may be used to heat most or all of the energy storage fluid in anticipation of a requirement for hot water. For example when energy prices are low at night the heating system may operate in the third mode to heat most or all of the energy storage fluid ready for the morning. As another example, the processor may activate the heat pump if there is excess energy, eg from solar panels, during the afternoon and operate the heating system in the third mode to heat the energy storage fluid ready for evening use when the residents of the building return home from work. If most or all of the energy storage fluid in the thermal energy store is heated up then sufficient hot water may be provided to meet the needs of a small one or two person household. For example, if the thermal energy store is a 60 litre tank and the energy storage fluid in the tank is water, then the thermal energy store should be able to provide at least 90 litres of water at 40 degrees Celsius, which is sufficient hot water for a small bath or two normal showers. For a small one or two person household, this may be sufficient hot water for the morning or evening and the processor can activate the heat pump and operate the heating system in the third mode during cheap or excess energy periods to heat up the energy storage fluid ready for use again. If the system is recharged and ready for use but the heat pump has been switched off and / or if only a small amount of hot water is required, the heating unit may be operated in a fourth mode ("low usage" or "eco" mode). As heat pumps take time to activate, it can take several minutes before a heat pump can provide domestic hot water once activated. The present system can be operated in the fourth mode during the time it takes the heat pump to warm up, in order to provide instant hot water to a hot water outlet. The fourth mode allows the hotter energy storage fluid to be taken from the upper part of the thermal energy store and to be used to heat the domestic water for a couple of minutes or so until the heat pump is working efficiently and can heat up the energy storage fluid again, at which point the system can be switched to operate in the first mode. This system permits hot water to be rapidly available to the user without waiting for the heat pump to commence effective operation before hot water is available. In other words the preheated energy storage fluid can be used to heat domestic hot water during the time between the user turning on a hot water tap and the heat pump being activated and being able to provide domestic hot water. In this fourth mode, the heating unit has a fourth configuration similarto the first configuration shown in figure 1 but the heat pump is either switched off or is switched on but in a warming up phase and not yet actively able to heat the energy storage fluid. The pump 9 is not activated and energy storage fluid does not flow from the thermal energy store to the first heat exchangers. Like for the first mode, during a demand for domestic hot water, the pump 14 is activated to pump energy storage fluid from the upper part 19 of the thermal energy store 2 via outlet 20 to the second heat exchanger 4 to heat the domestic hot water. The fourth mode can also be used in situations where the system detects that only a small amount of hot water is required. For example, sensors may be provided on the hot water outlets and the processor may be able to identify which hot water outlet is associated with a particular sensor. The processor can then identify the hot water outlet that has been opened and decide whether this is likely to lead to a high demand for hot water (for example in the case of a shower or bath) or a low demand for hot water (for example in the case of a bathroom sink tap for handwashing). If the processor identifies a high demand for hot water then it can activate the heat pump immediately and switch to the first mode once the heat pump is ready to heat the energy storage fluid. Alternatively if the processor identifies a low demand for hot water then it can choose to not activate the heat pump and to keep the system operating in the fourth mode. The processor may operate the system in the fourth mode with the heat pump switched off until a predetermined condition has been met. For example, once a predetermined amount of time has passed the processor may activate the heat pump and prepare to switch to the first mode. Alternatively the processor may activate the heat pump when a predetermined amount of energy storage fluid has been taken from the upper part of thermal energy store, or when a temperature of the energy storage fluid in the middle part of the tank drops below a predetermined temperature. Figure 3 shows the flow paths of the heating appliance in more detail. Figure 3 shows the thermal energy store 2, the first heat exchanger 3 and the second heat exchanger 4 of a water heating unit or appliance 1 forming part of a domestic hot water supply system. The first heat exchanger 3 exchanges heat between a working fluid in a heat pump (not shown) and an energy storage fluid of the thermal energy store 2. The second fluid exchanger 4 exchanges heat between the energy storage fluid and the domestic water to be heated. The heating appliance includes a plurality of fluid flow paths, which may be formed within a manifold located beneath the thermal energy store 2. The first heat exchanger 3 has a first pressurised inlet 5 for receiving hot working fluid from the heat pump and a second unpressurised inlet 7 for receiving energy storage fluid from the thermal energy store 2. The heat exchanger 3 also has a first outlet 6 for sending the working fluid back to the heat pump once heat exchange has taken place and a second outlet 8 for the heated energy storage fluid to exit the heat exchanger. A pump 9 is provided for pumping the energy storage fluid through the first heat exchanger 3. The second heat exchanger 4 has a first pressurised inlet 10 for receiving cold domestic water from the mains or a cold water tank. The heat exchanger 4 also has a second unpressurised inlet 12 for receiving heated energy storage fluid. The heat exchanger 4 also has a first outlet 11 for providing domestic hot water to domestic hot water outlets such as taps, and a second outlet 13 for returning the cooled energy storage fluid back to the thermal energy store 2 once heat exchange with the domestic water has taken place. A pump 14 is provided for pumping the energy storage fluid through the second heat exchanger 4. Valves 56-60 are provided to configure different fluid circuits within the appliance to allow the energy storage fluid to flow in different fluid paths dependent upon the mode of operation of the appliance. The valves are controlled by the processor. In this example, valves 56, 57, 58 and 59 are solenoid valves and valve 60 is a non-return valve. When the system is operating in the first mode, the valves 56 and 57 are open and valves 58 and 59 are closed. Both pumps 9 and 14 are switched on. The pumps are controlled by the processor. Hot energy storage fluid is pumped from the upper part 19 of the thermal energy store through outlet 20 by pump 14 and passes through open valve 57 to enter the second heat exchanger 4 via second inlet 12. After heat exchange has taken place, the cooler energy storage fluid leaves the second heat exchanger via second outlet 13 and is returned to the thermal energy store 2 via inlet 21. A diffuser 26 may be provided at the inlet 21 to reduce turbulence caused by the incoming fluid and to help to maintain the stratification of the fluid in the thermal energy store. At the same time, cooler energy storage fluid from the bottom part 15 of the thermal energy store is pumped through outlet 16 via non-return valve 60 to enter the first heat exchanger 3 via inlet 7. After heat exchange has taken place, the warmer energy storage fluid leaves the first heat exchanger via the outlet 8 and passes through the open valve 56 before returning to the thermal energy store 2 via inlet 18. A diffuser 25 may be provided at the inlet 18 to reduce turbulence caused by the incoming fluid and to help maintain the stratification of the fluid in the thermal energy store. While the system is operating in the first mode, the cooler energy storage fluid from the bottom part of the tank is being heated, but if the hot water outlet is open for a longer period of time, for example 15 minutes, then the hottest energy storage fluid in the thermal energy store may become depleted and be replaced by warm fluid in the range of approximately 33 - 35 degrees Celsius. When the processor detects that the hot fluid in the upper part of the thermal energy store has been depleted it switches the mode of operation to the second mode. When the system is switched to the second mode, the valves 56 and 57 are closed and the valves 58 and 59 are opened. Pump 9 is switched off leaving only pump 14 activated. The warm thermal energy fluid (at approximately 33 - 35 degrees Celsius) is pumped from the upper part 19 of the thermal energy store through outlet 20 by pump 14 and then passes through open valve 58 to enter the first heat exchanger 3 via inlet 7. After heat exchange has taken place, the thermal energy fluid is at a higher temperature of approximately 45 - 50 degrees Celsius and is now able to heat the domestic hot water. This thermal energy fluid exits the first heat exchanger via outlet 8 and then passes through open valve 59 to enter the second heat exchanger 4 via inlet 12. After heat exchange has taken place, the cooled energy storage fluid leaves the second heat exchanger and is returned to the thermal energy store via inlet 21. After the energy storage fluid in the thermal energy store has been circulated through the heat exchangers once in the second mode, the thermal energy store will be depleted. Before further hot water can be provided, the thermal energy store needs to be replenished with heat energy by operating the system in the third mode. To do this, the processor switches the configuration of the fluid circuit so that the valves 56 and 57 are opened and the valves 58 and 59 are closed. Pump 14 is switched off and pump 9 is activated. When the energy in the thermal energy store is replenished, the pump 9 and the heat pump are switched off. Figure 4 shows a thermal energy store 2. In this example the thermal energy store is a plastic unpressurised tank 24, but other suitable materials could be used for the tank. The tank does not need to hold pressurised energy storage fluid and therefore does not need to be designed as a pressure tank. This means that the tank can be made, for example, from a relatively cheap plastic material and can have relatively thin walls. The tank also does not need to be of a cylindrical form but instead can be any shape. For space efficiency, a cuboid shape may be desirable. The tank shown in this example is a cuboid shaped tank filled to a fill level 22 with an energy storage fluid 23. In this example the energy storage fluid is water, but other suitable fluids may be used instead. The tank 24 includes an inlet 18 from the first heat exchanger 3 (not shown in this figure) and an outlet 16 to the first heat exchanger 3. The tank further includes an inlet 21 from the second heat exchanger 4 (not shown in this figure) and an outlet 20 from the upper part of the tank to the heat exchangers. Diffusers 25, 26 are provided in the tank 24. The inlet 18 is in fluid connection with diffuser 25 and the inlet 21 is in fluid connection with the diffuser 26. In this example diffusers 25 and 26 are radial diffusers but other diffusers may be used. The outlet 20 to the heat exchangers 3, 4 is provided at the upper part 19 of the tank 24 in order to take the hotter water from the upper part of the tank rather than the colder water at the bottom of the tank. Water entering outlet 20 is guided downwards towards the heat exchangers via pipe 27. Pipe T1 extends from the upper part 19 of the tank 24 through the bottom 15 of the tank 24 and is located within the tank 24 in order to save space. This permits water to be taken from the upper part 19 of the tank 24 without requiring pipes to be located external to the tank 24 other than at the bottom 15. This allows the tank 24 to have a cuboid shape without pipes or other elements extending outside of the tank 24 other than at the bottom 15, thereby permitting the tank 24 to be installed in a small space whilst enabling easy access for installation. In use, when the heat pump is switched on and the appliance is operating in the first mode or the third mode, colder water from the bottom 15 of the tank 24 is fed to the first heat exchanger 3 via outlet 16. This water is heated in heat exchanger 3 and is then returned to the tank via tank inlet 18. Diffuser 25 is in fluid connection with the inlet 18 and the heated water returning to the tank via inlet 18 is fed into diffuser 25, which acts to slow the flow speed of the water entering the tank and to change its direction. In this example, radial diffuser 25 is substantially disc shaped with holes 28 around its periphery. In this example, the diffuser 25 is located near the bottom 15 of the tank and is positioned such that the disc is substantially flat or inclined at a small angle with respect to the bottom of the tank. Water entering the diffuser must turn by approximately 90 degrees in order to exit the diffuser via a hole 28. Water leaving hole 28 moves in a direction approximately parallel or inclined at a small angle with respect to the bottom of the tank and has a low speed. This reduces the turbulence caused by water returning to the tank which in turn minimises mixing and thereby maintains the stratification of the water in the tank in accordance with its temperature, with the hotter water being at the top and the colder water at the bottom. If desired, the diffuser 25 may be located above diffuser 26 rather than the diffusers being located side by side. This has the advantage that a larger diffuser may be used and the hotter water returning to the tank can be introduced at a higher level within the tank than the colder water returning to the tank via diffuser 26. When a domestic hot water outlet such as a tap is turned on and the appliance is operating in the first or fourth mode, the hot water from the upper part 19 of the tank 24 is fed via outlet 20 and pipe T1 to the second heat exchanger 4. After passing through the heat exchanger 4 to heat up the domestic water supply, the cooled water is returned to the tank via inlet 21. Diffuser 26 is in fluid connection with the inlet 21 and the heated water returning to the tank via inlet 21 is fed into diffuser 26, which acts to slow the flow speed of the water entering the tank and to change its direction. In this example, radial diffuser 26 is substantially disc shaped with holes 28 around its periphery. The diffuser 25 is located near the bottom 15 of the tank and is positioned such that the disc is substantially flat or inclined at a small angle with respect to the bottom of the tank. Water entering the diffuser must turn by approximately 90 degrees in order to exit the diffuser via a hole 28. Water leaving hole 28 moves in a direction approximately parallel or inclined at a small angle with respect to the bottom of the tank and has a low speed. This reduces the turbulence caused by the colder water returning to the tank which in turn minimises mixing and thereby maintains the stratification of the water in the tank in accordance with its temperature, with the hotter water being at the top and the colder water at the bottom. In this example, the pipe T1 is a rigid plastic or metal pipe but other materials could be used. Instead of a rigid pipe, a flexible pipe could be used instead. A flexible pipe having an inlet near the top of the flexible pipe (corresponding to outlet 20) configured to float just below the surfaceof the water would be less sensitive to fluctuations in the water level in the tank. Alternatively, instead of a pipe, the tank may have an integral channel built into the side of the tank with an inlet to the channel (corresponding to outlet 20) located near the top of the tank. Figures 5a - 5c show a thermal energy store 2. In this example, the thermal energy store is a plastic unpressurised tank 24, but other suitable materials could be used for the tank. The tank is filled to a fill level 22 with an energy storage fluid 23. In this example the energy storage fluid is water, but other suitable fluids may be used instead. The tank 24 has the same inlets and outlets as those described with respect to figure 4, but in figures 5a - 5c only the outlet 20 and pipe T1 are shown for simplicity. The tank 24 includes a resilient member 29. The resilient member 29 helps to keep the fluid level in the tank 24 substantially constant, regardless of the temperature of the water in the tank 24. In the example shown in figures 5a - 5c the resilient member is a bladder or expansion vessel. The resilient member 29 is made of a material which is fluid impermeable and is able to be squashed or to lose its shape when an external pressure is applied to it and to regain its shape when the external pressure is removed. In this example, the resilient member is made of rubber, although other suitable materials may be used instead. In the example shown in figures 5a - 5c the resilient member 29 is attached to the top 30 of the tank 24. The top of the tank has a hole 31 in it to accommodate the resilient member, which extends into the tank via the hole 31. The resilient member may be attached to the top of the tank by fixing means such as screws or adhesive or the resilient member and the top of the tank may be configured such that, when the resilient member is placed in position in the tank, there is an interference fit between the tank and the resilient member keeping the resilient member in place. In this example, a fluid impermeable seal 32 is provided between the resilient member and the top of the tank, but this is an optional feature. It can be seen that one side of the resilient member is exposed to the water 23 in the tank (this is the surface identified as 33 in figures 5a and 5b) and the other side of the resilient member (this is the surface identified as 34 in figures 5a and 5b) is exposed to ambient air. As the water 23 in the tank 24 is generally within a closed fluid circuit the same quantity of water is generally present in the fluid circuit at all times. However, it will be appreciated that, as the water 23 heats up, it increases in volume. In order to keep the fluid level in the tank relatively constant regardless of the temperature of the water 23, the resilient member 29 is used. In figure 5a the water 23 in the tank 24 is cold. The pressure on the surface 33 of the resilient member from the cold water 23 is not sufficient to deform or squash the resilient member and so the resilient member remains in its original expanded state. In this state the resilient member takes up its maximum volume in the tank with the result that the water 23 remains above the tank outlet 20 even when the water 23 is cold. As the water 23 is heated up, the resilient member 29 is subject to increasing pressure from the water 23 which compresses the resilient member so that its volume decreases. In this manner as the water in the tank is heated, its volume increases whilst the volume of the resilient member decreases. This enables the level of the fluid in the tank to be kept relatively constant. This can be seen in figure 5b which shows the tank 24 when the water 23 is heated. It can be seen that the fill level 22 of the tank is similar to that of figure 5a and that the water 23 remains above the tank outlet 20. As the water cools again, the resilient member gradually relaxes towards its original expanded state, thereby keeping the level of the fluid in the tank relatively constant. The tank 24 may be clad with a heat insulating material or insulating jacket (not shown). The jacket is optional and may be used with any tank 24 or thermal energy store 2 such as those described in these examples in order to preserve the heat in the energy storage fluid 23. Figure 6 shows a domestic hot water supply system 100 arranged in a first configuration and including a heating unit or appliance similar to that shown in figures 1 to 3 including a thermal energy store 2, a first heat exchanger 3 and a second heat exchanger 4. The first heat exchanger 3 exchanges heat between a working fluid in a heat pump 35 and an energy storage fluid 23 of the thermal energy store 2. The second fluid exchanger 4 exchanges heat between the energy storage fluid 23 and the domestic water to be heated. This process is described in more detail above with respect to figure 1. The appliance includes a housing 40 which contains the thermal energy store 2, the heat exchangers 3,4 and a processor 36. The processor sends signals to and receives signals from various elements of the hot water supply system and controls its operation. The processor 36 also communicates with the processor 37 of the heat pump 35. In this example, a flow rate sensor 43 is provided at the first outlet 11 of the second heat exchanger 4 to measure the flow rate of the domestic hot water exiting the second heat exchanger. The flow rate sensor 43 could instead be located at the first inlet 10 to the second heat exchanger 4. A flow rate sensor 55 is provided in pipe T1 to measure the flow rate of the energy storage fluid 23 through the second heat exchanger. A temperature sensor 41 is provided at the first outlet 11 of the second heat exchanger 4 to measure the temperature of the domestic hot water exiting the second heat exchanger. A flow restrictor 42 may optionally be placed at the first inlet 10 or first outlet 11 of the second heat exchanger 4 to limit the flow of domestic hot water from the second heat exchanger 4 to the domestic hot water outlets such as taps or shower heads. A temperature sensor 38 is located in the thermal energy store 2 at approximately the same height as the outlet 20. The temperature sensor 38 measures the temperature of the energy storage fluid 23 at the upper part of the thermal energy store. A further temperature sensor 39 may be provided at a location within the thermal energy store which is lower than the temperature sensor 38, for example at approximately a third of the distance between the temperature sensor 38 and the bottom 15 of the thermal energy store. A further temperature sensor 54 may be provided at the bottom part 15 of the thermal energy store. Further temperature sensors may be provided in the thermal energy store, if desired. The processor 36 receives data from the temperature sensors 38, 39, 41 and 54 and from the flow rate sensors 43 and 55. The processor 36 controls the operation of the pumps 9 and 14 and flow restrictor 42. When a domestic hot water outlet is opened, for example a tap, the flow rate sensor 43 detects this event and sends a signal to the processor 36. The processor 36 activates the pump 14 to pump heated energy storage fluid from the outlet 20 of the thermal energy store 2 through the second heat exchanger 4 to heat the domestic hot water. The flow rate sensor 43 monitors the domestic hot water flow rate, and sends this information to the processor 36. The processor 36 may be configured to send a signal to the processor 37 of the heat pump 35 to activate the heat pump as soon as the flow rate sensor 43 detects that a domestic hot water outlet has been opened. Alternatively, the processor 36 may delay activating the heat pump 35 and instead operate the heating appliance in the fourth mode for a predetermined time period, for example 30 seconds. If, during this time, the flow rate sensor 43 detects that the domestic hot water outlet has been closed again then the processor 36 does not activate the heat pump. This prevents the heat pump from being activated too frequently, for example each time a person washes their hands. Instead the energy storage fluid 23 stored in the thermal energy store 2 can be used to heat the domestic hot water for low hot water use activities such as hand washing. The processor 36 may use the information received from the flow rate sensor 43 to determine the type of domestic hot water outlet that has been opened. For example, the flow rate may be different for bathroom taps, the kitchen tap and a shower head, with a shower head having a larger flow rate. The flow rate characteristics of each outlet may also be different, such as the rate of change of flow profile when an outlet is opened. The processor 36 may activate the heat pump 35 immediately if it determines, for example, that the domestic hot water outlet that has been opened is a shower head, as it would be expected that a shower would take several minutes and use a significant amount of hot water in comparison to washing hands. This allows the heat pump 35 to immediately start to reheat the energy storage fluid 23 in the thermal energy store 2 to ensure that the domestic hot water should not run out before the shower is finished and / or to prevent the user from experiencing a colder temperature in the shower for a short time until the energy storage fluid 23 has been sufficiently reheated. When the heat pump is activated and able to provide hot working fluid to the first heat exchanger, the processor 36 also activates pump 9 to pump energy storage fluid from the thermal energy store 2 through the first heat exchanger 3 to heat the energy storage fluid. The appliance is then operating in the first mode, and the colder energy storage fluid at the bottom part of the tank is being heated at the same time as the warmer energy storage fluid at the upper part of the tank is being used to heat the domestic water. The thermal energy store 2 may be provided with a further temperature sensor 39 located below the outlet 20 of the thermal energy store. The temperature sensor 39 is also monitored by the processor 36 and can give an early warning if the heated energy storage fluid 23 in the thermal energy store is starting to run low. As mentioned before, the energy storage fluid 23 in the thermal energy store 2 will stratify according to its temperature, with the colder fluid being at the bottom of the thermal energy store. As hot energy storage fluid is depleted, the level of the colder fluid will rise and the temperature sensor 39 will sense this temperature change in the energy storage fluid. The processor 36 can then activate the heat pump and switch the operation of the appliance from the fourth mode to the first mode before the heated energy storage fluid runs out, for example. The flow rate sensor 55 provided in pipe T1 may also be monitored by the processor and, if the processor determines that a predetermined amount of energy storage fluid has been used since the tank was last replenished with heat, the processor may then activate the heat pump and switch the operation of the appliance from the fourth mode to the first mode (or alternatively to the third mode if no further hot water is immediately demanded but may be anticipated in the near future). The processor 36 also monitors the temperature of the energy storage fluid 23 near the outlet 20 of the thermal energy store 2 via temperature sensor 38. If the temperature of the energy storage fluid 23 falls below a predetermined value, the processor 36 may activate the heat pump if it is not already switched on, particularly if it is at a cheap electricity tariff time or if there is excess energy stored, for example, in a solar cell and / or the processor anticipates a demand for hot water in the near future. The appliance can then be operated in the third mode, in order to replenish the heat energy in the thermal energy store before hot water is demanded. During operation in the third mode, the processor 36 monitors the temperature of the energy storage fluid in the thermal energy store, particularly atthe bottom part of the thermal energy store, and, when the temperature in the thermal energy store reaches a predetermined temperature, it switches off the heat pump as the thermal energy store should now be replenished and ready for use. When the appliance is operating in the first mode and there is a continuing demand for hot water, if the processor 36 determines that the temperature of the energy storage fluid 23 at the outlet 20 of the thermal energy store is below a predetermined value (first predetermined value) then the processor 36 can switch the operation of the appliance from the first mode to the second mode. During the time that a domestic hot water outlet is open, the temperature sensor 41 monitors the temperature of the domestic hot water and sends this information to the processor 36. If the temperature of the domestic hot water starts to decrease, the processor 36 may activate the flow restrictor 42 to reduce flow of the domestic hot water. The processor 36 may also slow down the pump 14 to reduce the flow of energy storage fluid 23 into the second heat exchanger 4 or may activate a flow restrictor (not shown) in the pipe 27. Figure 7 shows the domestic hot water supply system 100 of Figure 6 in a second configuration when it is operating in the second mode. In this mode the heat pump is active and the processor has switched off pump 9 whilst pump 14 continues to operate. The processor has reconfigured the fluid circuit by changing the open / closed state of the valves 56 - 59 such that valves 58 and 59 are now open and valves 56 and 57 are now closed. In this mode the warmer energy storage fluid taken from the upper part of the thermal energy store via outlet 20 is pumped directly to the first heat exchanger 3 where it is heated to a temperature sufficient to heat the domestic water. The heated energy storage fluid then flows to the second heat exchanger 4 to heat the domestic water. After heat exchange has taken place with the domestic water, the (now cooler) energy storage fluid is returned to the thermal energy store. The appliance is only able to operate effectively in the second mode until the warmer energy storage fluid in the thermal energy store is exhausted. This is because, in the second mode, the appliance heats the energy storage fluid from the upper part of the thermal energy store which has been prewarmed during the previous operation in the first mode and therefore the heat exchanger 3 is not able to heat the cooler energy storage fluid at the bottom part of the thermal energy store whilst operating in the second mode. Once the cooler energy storage fluid layer reaches the outlet 20 no further hot water can be provided until the thermal energy store has been recharged, unless an instantaneous water heater is, for example, positioned in the flow path between the thermal energy store and the second heat exchanger. During the time that a domestic hot water outlet is open when the appliance is operating in the second mode, one or more of the temperature sensors 38, 39, 41 are operational and send temperature data to the processor 36. If the temperature at the upper part of the thermal energy store or the temperature of the domestic hot water starts to decrease, the processor 36 may activate the flow restrictor 42 to reduce flow of the domestic hot water. The processor 36 may also slow down the pump 14 to reduce the flow of energy storage fluid 23 into the second heat exchanger 4 or may activate a flow restrictor (not shown) in the pipe 27. When the processor determines that the temperature of the energy storage fluid 23 at the outlet 20 of the thermal energy store has fallen below a predetermined value (second predetermined value) then the processor 36 can switch the operation of the appliance from the second mode to the third mode as soon as the hot water outlet is closed in order to recharge the energy in the thermal energy store. Figure 8 shows a domestic hot water supply system 1 similar to that described in figures 6 and 7. In this embodiment, the thermal energy store 2, the heat exchangers 3, 4 and the processor 36 are all contained within a housing 40. The elements contained within the housing are not shown in detail in figure 8 but are shown and described in previous figures. A heat pump 35 is located outside of the housing but provides working fluid to the first heat exchanger 3 located within the housing 40 via pipe 44 and the working fluid is returned to the heat pump via pipe 45. Domestic water from the mains supply or from a cold water tank is provided to the second heat exchanger 4 via pipe 46. The heated domestic water exits the second heat exchanger 4 via hot water pipe 47. The heated water in pipe 47 travels to a domestic hot water outlet upon demand, ie when the domestic hot water outlet is opened. In this example, there are three domestic hot water outlets, namely shower head 48, kitchen sink tap 49 and wash basin tap 50. Flow rate sensors 51, 52 and 53 are located respectively at the hot water outlets 50, 49 and 48. The flow rate sensors 51, 52 and 53 notify the processor 36 when hot water flows from pipe 47 through one or more of the hot water outlets 50,49,48. Each sensor is identifiable by the processor, so the processor can distinguish between the opening of the kitchen sink tap 49 and the opening of the wash basin tap 50. In use, if the processor receives a signal from flow rate sensor 51 the processor knows that hot water is flowing out of the wash basin tap 50 and may assume that the amount of hot water that will be used will be fairly low. The processor may be configured not to activate the heat pump in the event of use of hot water via the wash basin tap 50, except in particular circumstances. Such circumstances may be, for example, if the wash basin tap 50 is left open for a predetermined amount of time, or the heated energy storage fluid in the thermal energy store is already depleted due to previous use of domestic hot water. Similarly, if the shower head 48 is activated then the flow rate sensor 53 will send a signal to the processor 36 and the processor will recognise that the domestic hot water flow is via the shower head. The processor may be configured to switch on the heat pump as soon as the shower is turned on, in order to ensure that there is sufficient hot water for the user to take a shower. It can be seen that it may be advantageous for the processor to identify the hot water outlet that is operated in order to determine whether (and / or how soon after the hot water outlet is opened) the heat pump should be activated. In this example, flow rate sensors 51,52 and 53 are described, however, other sensors may be used instead, such as motion sensors located on taps that send a signal to the processor if a tap is turned by a user. Figure 9 shows a flow chart providing an example of how the appliance may operate. In figure 9, when a hot water outlet is opened, the appliance is first operated in the fourth mode. Once a predetermined condition is met, the processor switches the mode of operation of the appliance to the first mode. A predetermined condition may be, for example, when the temperature of the energy storage fluid in a middle part of the thermal energy store falls below a predetermined temperature. A different precondition may be, for example, when a domestic hot water outlet has been opened for a predetermined amount of time, or when a particular domestic hot water outlet has been opened. Once a precondition has been met, the processor activates the heat source before switching to the first mode. The appliance continues to operate in the first mode until either the domestic hot water outlet is closed or until the temperature at the upper part or second outlet of the thermal energy store falls below a first predetermined temperature. When this occurs, if the domestic hot water outlet is still open then the processor switches the mode of operation of the appliance to the second mode. If the domestic hot water outlet is closed, then the processor switches the mode of operation to the third mode in order to recharge the thermal energy store. If the domestic hot water outlet remains open, then the appliance continues to operate in the second mode until hot water is no longer demanded or until the temperature at the upper part or second outlet of the thermal energy store falls below a second predetermined temperature (which is lower than the first predetermined temperature). Once this occurs, the processor waits until the domestic hot water outlet is closed before switching the mode of operation to the third mode. Once the energy storage fluid in the thermal energy store has been reheated to a predetermined temperature, then the processor switches off the heat source and the pump 9 and waits for a domestic hot water outlet to be opened again before operating the appliance in the fourth mode and the cycle continues. Example In tests, an energy storage fluid (water) in a 60 litre tank was heated to a temperature of 60 degrees Celsius by a 6kW heat pump. Incoming domestic cold water at 11 degrees Celsius was heated by the appliance to a temperature of 42 degrees Celsius and over 150 litres of domestic hot water at 42 degrees Celsius was provided over an operating period of no less than 20 minutes. Test results are shown in Figure 10. Various parameters are shown in the graphs against time in seconds (the x axis). The top graph shows temperature in degrees Celsius (y axis) against time in seconds (x axis) for at various locations including the bottom part of the tank and the domestic hot water outflow from the second heat exchanger. In the above examples, the thermal energy store may be a plastic tank, or made be made from other suitable materials, such as materials which are fluid impermeable and which preferably have thermal insulation properties. The tank may also be provided with a jacket or cladding in order to prevent thermal energy loss. The thermal energy store may be provided in the form of a tank located within a housing or may alternatively may be a tank which itself forms part of the housing of the hot water supply system. The energy storage fluid may be water or another fluid suitable for use in a domestic 5 system. The above embodiments are to be understood as illustrative examples. Further embodiments, aspects or examples are envisaged. It is to be understood that any feature described in relation to any one embodiment, aspect or example may be used alone, or in combination with other features described, 10 and may also be used in combination with one or more features of any other of the embodiments, aspects or examples, or any combination of any other of the embodiments, aspects or examples. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

Claims

1. A hot water supply system suitable for domestic use comprisinga thermal energy store, the thermal energy store comprising an unpressurised tank for storing an energy storage fluid;a first heat exchanger having a first inlet for receiving hot working fluid from a heat source and a first outlet for returning the cooled working fluid to the heat source after the working fluid has passed through the first heat exchanger, and having a second inlet for receiving energy storage fluid from the thermal energy store and a second outlet being an exit for the heated energy storage fluid after the energy storage fluid has passed through the first heat exchanger;a second heat exchanger having a first inlet for receiving cold water to be heated by the present system and a first outlet to permit the heated water to exit the second heat exchanger, and having a second inlet for receiving heated energy storage fluid and a second outlet for returning the cooled energy storage fluid to the thermal energy store after the energy storage fluid has passed through the second heat exchanger;the thermal energy store havinga first outlet and a second outlet,a first inlet and a second inlet,wherein at least first to sixth fluid paths are provided as follows:a) a first fluid path between the first outlet of the thermal energy store and the second inlet of the first heat exchanger,b) a second fluid path between the second outlet of the thermal energy store and the second inlet of the first heat exchanger,c) a third fluid path between the second outlet of the thermal energy store and the second inlet of the second heat exchanger,d) a fourth fluid path between the second outlet of the first heat exchanger and the first inlet of the thermal energy store,e) a fifth fluid path between the second outlet of the first heat exchanger and the second inlet of the second heat exchanger,f) a sixth fluid path between the second outlet of the second heat exchanger and the second inlet of the thermal energy store,and whereinthe system is configured to be switchable between a first configuration whereby the first, third, fourth and sixth fluid paths are substantially open and the second and fifth fluid paths are substantially closed and a second configuration whereby the second, fifth and sixth fluid paths are substantially open and the first, third and fourth fluid paths are substantially closed.

2. A hot water supply system in accordance with claim 1 wherein the system further comprises a first pump configured to pump energy storage fluid through the first heat exchanger when the system is operating in the first configuration, a second pump configured to pump the energy storage fluid through the second heat exchanger when the system is operating in the first configuration and through both the second and first heat exchanger when the system is operating in the second configuration, and a processor configured to control the first and second pumps.

3. A hot water supply system in accordance with claim 1 or claim 2 further comprising a plurality of valves, at least one valve being provided on at least each one of the second, third, fourth and fifth fluid paths and wherein the processor is configured to control the valves and to thereby switch the system between a first configuration and a second configuration.

4. A hot water supply system in accordance with claim 3 wherein the valves are solenoid valves.

5. A hot water supply system in accordance with any of the preceding claims wherein the first inlet of the first heat exchanger and the first inlet of the second heat exchanger are pressurised inlets.

6. A hot water supply system in accordance with any of the preceding claims wherein the thermal energy store, the first heat exchanger and the second heat exchanger are located within and supported by a housing, the housing being substantially cuboid.

7. A hot water supply system in accordance with claim 6 wherein the housing comprises a top and bottom and four sides and wherein the dimension of the housing between the top and the bottom is larger than the dimension between any two opposing sides.

8. A hot water supply system in accordance with claim 7, the housing having a maximum footprint of 600mm by 600mm and a maximum height of 900mm.

9. A hot water supply system in accordance with claim 7 or claim 8 wherein the first inlet and first outlet of the first heat exchanger and the first inlet and first outlet of the second heat exchanger are located at or near the bottom of the housing.

10. A hot water supply system in accordance with any of claims 6 to 9 wherein the housing is configured to be mounted to a wall or shelf.

11. A hot water supply system in accordance with any of the preceding claims wherein the first and second inlets of the thermal energy store are each provided with a diffuser.

12. A hot water supply system in accordance with any of the preceding claims wherein the second outlet of the thermal energy store is located at an upper part of the thermal energy store.

13. A hot water supply system in accordance with any of the preceding claims wherein the second outlet of the thermal energy store comprises one of the following:a rigid pipe located in the interior of the thermal energy store and having an inlet at an upper part of the thermal energy store,a flexible pipe located in the interior of the thermal energy store and having a floating inlet, the floating inlet being configured such that, when energy storage liquid is present in the thermal energy store, the inlet to the flexible pipe floats just below the liquid surface, or an integral channel formed in the thermal energy store and having an inlet to the channel at the upper part of the thermal energy store.

14. A hot water supply system in accordance with claim 13, further comprising a temperature sensor located in the thermal energy store at a predetermined distance below the inlet to the interior pipe or channel.

15. A hot water supply system in accordance with claim 14 wherein the temperature sensor is configured to provide a processor with information relating to the temperature of the energy storage fluid in that part of the thermal energy store where the temperature sensor is locatedand wherein the processor is configured to switch on the heat source when it determines that the temperature drops below a predetermined level.

16. A hot water supply system in accordance with any of claims 13 to 15 wherein a temperature sensor is provided at, in or near to the second outlet of the thermal energy store, or within the pipe or channel leading from this outlet towards the heat exchangers.

17. A hot water supply system in accordance with claim 16 wherein the temperature sensor is configured to provide a processor with information relating to the temperature of the energy storage fluid in the upper part of the thermal energy store, and wherein the processor is configured to switch the system from the first configuration to the second configuration if the temperature falls below a predetermined temperature.

18. A hot water supply system in accordance with any of the preceding claims wherein the thermal energy store comprises a resilient member, the resilient member being arranged to keep the level of the fluid in the thermal energy store substantially constant.

19. A hot water supply system in accordance with any of the preceding claims wherein the first and second heat exchangers are located beneath the thermal energy store.

20. A hot water supply system in accordance with any of the preceding claims wherein the thermal energy store is clad with a heat insulating material.

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