A hot water supply system

Abstract

A domestic water heating appliance 1 has an unpressurised storage tank 2 containing an energy storage fluid, a first heat exchanger 3 for connection to a pressurised heating circuit, a second 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 housing with means for securing the housing to a wall or for standing on a surface, the housing containing and supporting within it a thermal energy store, a first heat exchanger and a second heat exchanger. The thermal energy store comprises an unpressurised tank for storing between 40 litres and 290 litres of an energy storage fluid. Preferably the thermal energy store is configured to store between 40 litres and 120 litres of the 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 for returning the heated energy storage fluid to the thermal energy store 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 energy storage fluid from the thermal energy store 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 for providing energy storage fluid to the first heat exchanger, a first inlet for receiving heated energy storage fluid from the first heat exchanger, a second outlet for providing energy storage fluid to the second heat exchanger, and a second inlet for receiving cooled energy storage fluid from the second heat exchanger. The energy storage fluid is contained within a closed fluid circuit within the housing and is prevented from mixing with the working fluid of the heat source and with the water to be heated. The system includes an arrangement for accommodating variation in volume of the energy storage fluid within a temperature range of at least 5 degrees to 60 degrees Celsius. The housing provides a self-contained unit that includes and supports the thermal energy store and heat exchangers. The housing can be mounted or secured to a wall or other surface as one unit. 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 follows: The energy storage fluid leaves the thermal energy store and passes through the first heat exchanger where it receives heat from the working fluid of the heat source, then it is fed back into the thermal energy store. Heated energy storage fluid is taken from the thermal energy store and 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. The energy storage fluid is now ready to repeat the cycle again. 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 have 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 provides an elongated 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 a maximum width of 600mm, a maximum depth of 600mm and a maximum 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 the floor 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 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 300mm x 400mm. 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 a first pump is 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 is then 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. 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 returning to 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 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 to the second inlet of the second heat exchanger in order to heat the incoming cold water. 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 into the second heat exchanger. In this case (as in the case of a fixed pipe) the fluid level of the thermal energy store needs to be kept above the entrance to the channel to prevent air from getting into the second heat exchanger. 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, a second pump configured to pump the energy storage fluid through the second heat exchanger, and a processor configured to control the first and second pumps. 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 90 - 120 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 90 - 120 litres of hot water without requiring space for a 90 - 120 litre hot water tank. A flow rate sensor may be provided in the domestic hot water pipe which leads from the second heat exchanger towards one or more domestic hot water outlets. Domestic hot water outlets may be, for example, 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 characteristic 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. 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. 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. 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 to the second heat exchanger. This sensor may provide the processor with information relating to the temperature of the energy storage fluid in the top part of the thermal energy store and, if the temperature drops below a predetermined level, the processor may activate the heat source. 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. 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. The processor may use this information to switch on the heat source when the temperature drops below a predetermined level at a particular position in the thermal energy store. 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 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. 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). The arrangement for accommodating variation in volume of the energy storage fluid may comprise a resilient member arranged to help to keep the fluid level of the thermal energy store substantially constant. 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. 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 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 fill 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 fill 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 second heat exchanger without the risk that the fill level drops and air enters the heat exchanger. 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 fill 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 fill level of the energy storage fluid to drop to a desired level as a result. In this manner the fill 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. 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 tank for storing an energy storage liquid for use in a domestic hot water supply system, wherein the tank is unpressurised, is of a substantially cuboid form, has maximum dimensions of 600mm wide x 600mm depth x 900mm height, has minimum dimensions of 300mm x 300mm x 400mm, and comprises at least two outlets and at least two inlets and all of the inlets and outlets of the tank are located substantially at the bottom of the tank. The tank may be used in a hot water supply system such as that previously described. The tank may be a thermal energy store for storing an energy storage medium. The energy storage medium may be a liquid such as water, for example. The tank may be made from plastic, for example polypropylene. Ribs may be provided in order to stiffen the tank. The tank may comprise a top and bottom and four sides. The tank may comprise at least two outlets and at least two inlets. All of the inlets and outlets of the tank are preferably located substantially at the bottom of the tank. This allows the tank to be fitted in a small space as an installer only needs to access the bottom of the tank in order to connect the tank to pipework. The tank 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 tank. The tank may be configured to be mounted to a wall or to a shelf, for example, or may be fitted into a cupboard. The tank may include a support structure 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. It is advantageous to provide a taller, thinner tank rather than shorter, wider tank because the liquid in the tank will stratify according to its temperature, and the strata will be more evident and more accessible in a taller, thinner tank. This stratification of energy storage liquid allows the warmer liquid at the top of the tank to be readily taken from the tank. The warmer liquid may be used, for example, in a heat exchanger to heat incoming cold water from a domestic supply. The inlets and outlets of the tank are preferably positioned and configured to promote stratification of energy storage liquid in the tank. One or both of the inlets of the tank may be provided with a diffuser, such as, for example, a radial diffuser. Diffusers may be provided at the inlets to the tank to diffuse liquid entering the tank, thereby slowing down the speed of the liquid entering the tank and influencing the direction in which the liquid flows. This can reduce turbulence caused by the liquid entering the tank. It is advantageous to reduce turbulence in order to aid stratification by hindering the mixing of the newly returned liquid and the liquid 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 tank. For example, the diameter of the diffuser may be in the range of approximately 25% of the smallest dimension of the tank to approximately 90% of the smallest dimension of the tank. 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 tank, and preferably at least 10% of the cross sectional area of the tank. To aid stratification, it is preferable to provide a gap between the edge of the diffuser and the side of the tank of at least 10mm and preferably at least 20mm. The diffuser may be positioned so that liquid exiting the diffuser travels in a substantially horizontal direction, or may alternatively be positioned such that liquid 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 tank. Alternatively one of the inlets of the tank may be provided with a diffuser part way up or near to the top of the tank so that warmer liquid may be directly delivered to or nearer to the top part of the tank. One of the outlets of the tank may be configured to take liquid from an upper part of the tank. This permits the hotter layer of liquid in the top part of the tank to be taken from the tank for use, for example, to heat incoming cold water for domestic use. An interior pipe may be provided in the tank or a channel may be formed in the tank, the interior pipe or channel extending from the outlet at the bottom of the tank to the upper part of the tank and having an inlet at the upper part of the tank such that liquid may flow from the upper part of the tank to the outlet 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. The interior pipe may be a fixed pipe having an inlet or may alternatively be a flexible pipe with a floating inlet. For example, a flexible pipe having a floating inlet may be provided, the floating inlet being configured such that, when energy storage liquid is present in the tank, the inlet floats just below the liquid surface. This permits the uppermost layer of the liquid to flow into the pipe even if the fill level of the tank is not at a maximum level. Alternatively there may be an integral channel formed in the tank and having an inlet to the channel at the upper part of the tank to allow the warmer liquid at the upper part of the tank to flow down through the channel to the outlet at the bottom of the tank. In this case (as in the case of a fixed pipe) the fluid level in the tank needs to be kept above the entrance to the channel to prevent air from getting into the system. The tank may include a vapour impermeable seal to prevent evaporation of the liquid in the tank. A fill inlet may be provided 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 (where the colder layer of liquid is found) and may be accessible from the bottom of the tank 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. For example, the fill inlet may connect to an inlet of the tank exterior of the tank and is configured to be closed when not in use. The tank may be clad with a heat insulating material in order to mitigate heat loss from the tank. The tank may include a temperature sensor located in the upper part of the tank at or near to the inlet to the pipe or channel leading to the outlet from the upper part of the tank. Alternatively the temperature sensor may be located within the pipe or channel leading to the outlet from the upper part of the tank. This sensor may provide the hot water system with information relating to the temperature of the liquid at the top part of the tank. The tank may include a further temperature sensor located in the tank at a predetermined distance below the inlet to the interior pipe or channel. The tank may include a further temperature sensor located in the tank at or near to the bottom of the tank. An optional immersion coil may be provided in the tank for emergencies, for example if the heat pump fails. A resilient member as previously described may be provided to help to keep the fluid level in the tank substantially constant. The resilient member may be configured to yield in response to fluctuations in volume of the energy storage liquid in order to keep the pressure within the thermal energy store substantially constant. The resilient member may be configured to keep the level of the energy storage liquid in the tank substantially constant as the volume of the energy storage liquid varies within a temperature range of at least 5 degrees to 60 degrees Celsius. 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 resilient member may be configured to deform when pressure is applied to it and to regain its shape when the pressure is removed. The tank may be provided with a housing. The tank may be at least partly contained within the housing. The housing may support the tank. The tank may be provided with a manifold located under the tank, the tank inlets and outlets being housed in the manifold. The housing may be provided with a support structure configured to be mounted to a mounting surface such as a wall or a floor. The tank may have maximum dimensions of, for example, a width of 500mm, a depth of 500mm and a height of 800mm. The tank may hold approximately 60 litres of liquid, for example water. A larger tank, eg a 90 litre tank, may be provided instead. According to a third aspect, there is provided a tank for storing an energy storage liquid for circulating in a closed fluid circuit having at least one heat exchanger and being suitable for a domestic hot water supply system, wherein the tank is unpressurised, is of a substantially cuboid form, has maximum dimensions of 600mm wide x 600mm depth x 900mm height and minimum dimensions of 300mm x 300mm x 400mm, and wherein the tank comprises an arrangement configured to keep the level of the energy storage liquid in the tank substantially constant as the volume of the energy storage liquid varies within a temperature range of at least 5 degrees to 60 degrees Celsius without adding or removing fluid from the closed fluid circuit. The arrangement configured to keep the level of the energy storage liquid in the tank substantially constant may comprise a resilient member configured to yield in response to fluctuations in volume of the energy storage liquid in order to keep the pressure within the thermal energy store substantially constant. The resilient member may be as described above. The tank may have further features as described above. The tank may be part of a hot water supply system for providing a quantity of hot water suitable for washing or bathing. According to a fourth aspect, there is provided a tank for storing an energy storage liquid such as water for circulating in a closed fluid circuit and being suitable for use in a domestic hot water supply system, the tank being unpressurised and comprising at least one temperature sensor and a liquid level sensor arranged to sense a range of fluid levels as the temperature of the energy storage liquid in the tank varies and to signal a need to top up or a potential leakage condition based on monitoring a series of measurements giving a liquid level indication and temperature measurements over time. The tank may have maximum dimensions of 600mm wide x 600mm depth x 900mm height. The tank may have minimum dimensions of 300mm wide x 300mm wide x 400mm height. The tank may have a substantially cuboid form. The tank may have further features as described above. The tank may be part of a hot water supply system for providing a quantity of hot water suitable for washing or bathing. The liquid level sensor may be a float type sensor. A temperature sensor may be integrated with or arranged on the liquid level sensor in order to measure the temperature of the liquid near its surface. Additionally or alternatively one or more temperature sensors as previously described may be provided in the tank. The tank may include a vapour impermeable seal to prevent evaporation of the liquid in the tank. A fill inlet may be provided to initially fill or to top up the liquid in the tank. The fill inlet may be provided at the bottom part of the tank and may be accessible from the bottom of the tank 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. For example, the fill inlet may connect to an inlet of the tank exterior of the tank and is configured to be closed when not in use. Although the energy storage liquid in the tank is contained within a closed fluid circuit, it will be appreciated that the liquid may gradually evaporate over a prolonged time period or may gradually or suddenly decrease if a leak develops in the fluid circuit. In pressurised tanks, a pressure sensor is usually provided to monitor the pressure in the tank and signal if there is a sudden drop in pressure or if the pressure decreases below a predetermined level which may indicate a leak. As the present tank is unpressurised, there will not be a significant drop in pressure if a leak occurs and so another means of leak detection is required, particularly for gradual leakage over a long time period. As previously mentioned, the energy storage fluid used to heat the domestic water is taken from the upper part of the tank via an outlet. It is therefore important that the level of fluid in the tank is maintained at a level sufficient to allow the fluid to enter the outlet. The fluid in the tank will change in volume as a function of its temperature, with colder fluid requiring less volume and therefore having a lower fluid level in the tank than the same quantity of warmer fluid. The tank may comprise an arrangement configured to keep the level of the energy storage liquid in the tank substantially constant as the volume of the energy storage liquid varies within a temperature range of at least 5 degrees to 60 degrees Celsius without adding or removing energy storage fluid from the closed fluid circuit. This arrangement may comprise a resilient member configured to yield in response to fluctuations in volume of the energy storage liquid in order to keep the pressure within the tank substantially constant. The resilient member may be as described above. If such an arrangement for keeping the fluid level in the tank substantially constant is used, then it may be even more difficult to determine in a timely manner if there is a gradual leak of fluid. Although there may still be sufficient fluid to flow through the outlet and around the closed fluid circuit, a reduction in the overall amount of fluid in the tank can result in inefficiencies in the system and this can result in a noticeable drop in performance of the system if the system operates at suboptimal efficiency for several months or longer. A simple fill level sensor is able to check whether the outlet remains below the fluid level in the tank at all times, but it is not able to check whether fluid has been lost from the closed fluid circuit unless the amount lost is very significant. This is due to the volume of the fluid changing as it is heated and cooled. In order to determine whether fluid is gradually being lost from the circuit, it is desirable to monitor and record the fluid level in the tank as the temperature of the fluid varies over time, in order to provide a series of measurements of fluid level at particular temperatures and to continue to monitor the fluid level and temperature of the fluid and to compare the fluid level at a particular temperature with the earlier recorded fluid levels at that particular temperature to check if the fluid level remains constant at a particular temperature over a period of time. If the fluid in the circuit is found to be decreasing, then the sensor can alert the user and the user can top up the fluid in the circuit to maintain efficiency of the system and / or investigate if there is a leak if the fluid loss occurred in a relatively short time span. The tank may be clad with a heat insulating material in order to mitigate heat loss from the tank. The tank may be at least partly contained within a housing. The housing may be as described above. According to a fifth 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 a housing with means for securing the housing to a wall or for standing on a surface, the housing containing and supporting within it: an unpressurised tank for storing an energy storage liquid, 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, and a pump and control arrangement to selectively circulate energy storage liquid from the tank to the first and second heat exchangers. The unit may have maximum dimensions of 600mm wide x 600mm depth x 900mm height. The housing may have a substantially cuboid form. The housing may include at least one recess or protruding part as mentioned above. 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 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 be 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 maximum dimensions of 600mm wide x 600mm depth x 900mm height. The housing may have maximum dimensions of a width of 500mm, a depth of 500mm and a height of 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. The support structure may transfer load to a wall from the tank. 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 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. 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 unit. The tank may comprise two unpressurised inlets and two outlets including: a first outlet configured to provide energy storage liquid from the lower part of the tank to the first heat exchanger, a first inlet configured to receive heated energy storage liquid from the first heat exchanger, a second outlet configured to provide energy storage liquid from the upper part of the tank to the second heat exchanger, and a second inlet configured to receive cooled energy storage liquid from the second heat exchanger, and wherein the inlets and outlets of the tank are all located substantially at the bottom of the tank. By taking liquid from an upper part of the tank, the hotter layer of liquid in the top part of the tank can be used in the second heat exchanger to heat incoming cold water for domestic 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. 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 information relating to the temperature of the liquid in the top part of the tank. This information may be provided to the processor. A resilient member as previously described may be provided to help to keep the fluid level 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 middle or upper 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 in that part of the tank has dropped below a predetermined level. This allows the heat source to be activated in advance of the heated liquid in the tank fluid 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 that runs from the second outlet of the tank via the second heat exchanger to the second inlet of the tank. This 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 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. According to a sixth 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 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 arrangement and for detecting a state of operation of the remote heat source and / or for signalling a demand for heat, the processor arranged to operate at least a first mode for circulating energy storage fluid from an upper part of the tank through the second heat exchanger to return to a lower part of the tank to provide heated domestic water in the absence of a demand for heat signal or absence of detection of active operation of the remote heat source for at least a first period of time, and a second mode for heating domestic hot water during active operation of the remote heat source, the second mode comprising circulating energy storage fluid from an upper part of the tank through the second heat exchanger to return to a lower part of the tank and simultaneously circulating energy storage fluid from a lower part of the tank through the first heat exchanger to return to the tank in order to replenish heat stored in the storage tank. The processor may be arranged to operate a third mode for replenishing heat stored in the storage tank, the third mode comprising: determining whether there is a demand for domestic hot water, and when it is determined that no hot water is being demanded, then detecting a state of operation of the remote heat source and, if the remote heat source is not active, signalling a demand for heat, and when it is determined that the remote heat source is active, circulating energy storage fluid from a lower part of the tank through the first heat exchanger to return to the tank in order to replenish heat stored in the storage tank. The processor may be configured to determine if any of (i) to (iv) below apply: (i) the temperature of the heated energy storage fluid in a middle region of the storage tank has fallen to a 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. The processor may be arranged to switch operation of the heating appliance from the first mode to the second mode if any of (i) to (iv) above apply. The first mode permits the appliance to be used to heat water without activating the remote heat source 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, such as heat pumps for example, to being switched on and off multiple times in quick succession. By identifying if there has been or is likely to be a substantial demand for water, the mode can be switched from the first mode to the second mode and 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. In the second 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. 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 elongate shape of the storage tank, the position and configuration of its inlets and outlets and the resulting stratification of the fluid in the tank allow the appliance to operate efficiently. 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 be of a substantially cuboid form. The housing may includes at least one recess or protruding part as described above. The first and second heat exchangers and the pump arrangement may be located within the housing and underneath the tank. All the inlets and outlets to the appliance may be located at or near the bottom of the housing. The remote heat source may be a heat pump, for example. The domestic water heating appliance may comprise a resilient member configured to keep the fluid level of the tank substantially constant. The resilient member may be configured to keep the level of the energy storage liquid in the tank substantially constant as the volume of the energy storage liquid varies within a temperature range of at least 5 degrees to 60 degrees Celsius. The resilient member may include features as previously described. The housing of the domestic water heating appliance may comprise a support structure configured to be mounted to a mounting surface such as a wall or a floor as previously described. The tank may be clad with a heat insulating material in order to mitigate heat loss from the tank. The tank may have features as described previously. The tank preferably comprises an outlet configured to provide energy storage fluid from the upper part of the tank to the second heat exchanger, and the outlet 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. One or more temperature sensors may be located in the tank. Preferably a temperature sensor is located in the tank at a predetermined distance below the inlet to the interior pipe or channel. The temperature sensor may be configured to provide the processor with information relating to the temperature of the energy storage fluid in that part of the tank where the temperature sensor is located. The processor may be configured to switch on the remote heat source when it determines that the temperature in that part of the tank drops below a predetermined level. A flow rate sensor may be provided in the outlet configured to provide energy storage fluid from the upper part of the tank to the second heat exchanger. The flow rate sensor may be configured to provide the processor with information relating to the flow rate of the energy storage fluid in the outlet. The processor may be configured to switch on the remote heat source when it determines that a predetermined amount of energy storage fluid from the tank has flowed through the outlet. The processor may be additionally or alternatively configured to identify a domestic hot water outlet opened by a user, using information received from a flow rate sensor positioned in a domestic hot water pipe which leads from the second heat exchanger towards one or more domestic hot water outlets for example, and to switch on the remote heat source if a predetermined domestic hot water outlet has been opened. The processor may be configured to detect a demand for domestic hot water when a domestic hot water outlet is opened and to switch on the remote heat source when it determines that a predetermined amount of time has passed since an opening of a domestic hot water outlet. The pump arrangement preferably comprises a first pump configured to pump the energy storage fluid through the first heat exchanger and a second pump configured to pump the energy storage fluid through the second heat exchanger. The processor may be configured to detect a demand for domestic hot water when a hot water outlet is opened and to switch on the second pump to circulate energy storage fluid from an upper part of the tank through the second heat exchanger to provide domestic hot water. The processor may be configured to switch on the first pump to circulate energy storage fluid through the first heat exchanger when it determines that the remote heat source is switched on. 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. According to a seventh aspect, there is provided a method of controlling a heating appliance for providing a quantity of hot water suitable for domestic washing or bathing, the heating appliance comprising: a thermal energy store for storing substantially unpressurised energy storage fluid, the heating appliance further being provided with a first heat exchanger that is coupled between the heating appliance and a heat source and a second heat exchanger that is coupled between the heating 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 liquid, the heating appliance further comprising a first pump for circulating the energy storage fluid through the first heat exchanger and a second pump for circulating the energy storage fluid through the second heat exchanger, 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 first and second pumps, the method further comprising detecting the opening of a hot water outlet, activating the second pump, using the flow rate information provided by the flow rate sensor and at least one other factor to decide whether the heat source should be activated, and wherein the processor is configured to instruct the heat source to activate if the processor decides that the heat source should be activated. 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 additionally or alternatively be configured to activate the heat source if the hot water outlet is open for a predetermined amount of time at a particular flow rate. A sensor may be provided for each of a plurality of hot water outlets, each sensor being configured to send an identification signal to the processor in the event that its associated hot water outlet is opened. The processor may be configured to identify the hot water outlet and to activate the heat source if a predetermined hot water outlet has been opened. The heating appliance may further comprise a first temperature sensor in the thermal energy store. The method may further comprise: determining the temperature of the energy storage fluid in the thermal energy store, and comparing the temperature with a pre-stored target temperature, and wherein the processor is configured to instruct the heat source to activate if the pre-stored target temperature is higher than the determined temperature of the energy storage fluid. Alternatively the heating appliance may further comprise a first temperature sensor in the closed fluid circuit, the first temperature sensor being located at or between an outlet from the thermal energy store and an inlet of the second heat exchanger, and a second temperature sensor located in the thermal energy store at a predetermined location in the upper or middle part of the thermal energy store. The method may further comprise: determining the temperature of the energy storage fluid at the location of the first temperature sensor, determining the temperature of the energy storage fluid at the location of the second temperature sensor, and comparing the temperatures with a pre-stored target temperature, and wherein the processor is configured to instruct the heat source to activate if the pre-stored target temperature is higher than the determined temperature of the energy storage fluid at the location of either or both of the first temperature sensor or the second temperature sensor. The heating appliance may comprise a first flow restrictor in the incoming cold water pipe. The processor may be configured to control the flow restrictor to regulate the flow rate of incoming cold water through the second heat exchanger. Additionally or alternatively the heating appliance may comprise a second flow restrictor in the closed fluid circuit, the flow restrictor being located at or between an outlet from the thermal energy store and an inlet of the second heat exchanger. The processor may be configured to control the flow restrictor to regulate the flow rate of the energy storage fluid through the second heat exchanger. When a hot water outlet is opened, the processor may: receive information relating to the flow rate of the hot water and the temperature of the energy storage fluid in the thermal energy store from the sensors, compare the information received from the sensors with information pre-stored in the memory, identify the hot water outlet which has been opened from its flow characteristic or from an identification signal, use the detected flow rate information, the detected temperature information and information pertaining to the identified hot water outlet to decide (i) if the heat source should be activated, and (ii) if the flow rate of incoming cold water through the second heat exchanger and / or the flow rate of the energy storage fluid through the second heat exchanger should be regulated, and send instructions to activate the heat source and / or restrict the flow rate of incoming cold water through the second heat exchanger and / or restrict the flow rate of the energy storage fluid through the second heat exchanger if the processor decides that such action should be taken. 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 pump in advance of time periods predicted to have high water usage. The heating appliance may comprise an instantaneous water heater located in the flow path between the thermal energy store and the second heat exchanger. The instantaneous water heater may be controlled by the processor. The processor may be configured to activate the instantaneous water heater in the event that the heat source is unable to be activated. A temperature sensor may be provided in the closed fluid circuit. The temperature sensor may be located at or between an outlet from the thermal energy store and an inlet of the second heat exchanger. The processor may be configured to receive information from the temperature sensor and may be configured to compare the temperature detected by the temperature sensor with a prestored target temperature and to activate the instantaneous water heater if the detected temperature is lower than the target temperature. A controller for a heating appliance comprising a processor and non-volatile memory storing instructions for causing the processor to perform the method may be provided. A computer program product comprising a computer readable medium or data packet comprising instructions to cause a processor to perform the method may be 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 hot water supply system in accordance with the present invention. Figure 2 shows an isometric view of a thermal energy store of a hot water supply system in accordance with the present invention. Figure 3a shows a cross section through a thermal energy store having a resilient member, the thermal energy store being part of a hot water supply system in accordance with the present invention. Figure 3b shows the thermal energy store of figure 3a when pressure acts on the resilient member. Figure 3c shows a top view of the thermal energy store of figures 3a and 3b. Figure 4 shows a schematic diagram of a hot water supply system in accordance with the present invention. Figure 5 shows a schematic diagram of a hot water supply system in accordance with the present invention integrated into a dwelling. Figure 1 shows a hot water supply system 1 suitable for domestic use 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 domestic water to be heated. In this example, 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 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 sending 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 thermal energy store 2. 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 through the second heat exchanger 4. 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 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. This colder energy storage fluid is pumped through the first heat exchanger 3 by pump 9 when the heat pump is switched on. The heat pump may be activated, for example, when there is an excess of energy available, eg from solar panels, or when energy prices are low, eg at night. The heat pump may also be activated during usage of domestic hot water or in anticipation of domestic hot water requirements. 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 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. When domestic hot water is required, 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 amongst 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 fed back to the bottom 15 of the thermal energy store 2 via inlet 21 and the cycle begins again. 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 heat pump may operate to heat most or all of the energy storage fluid ready for the morning. As another example, the heat pump may use excess energy, eg from solar panels, during the afternoon 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 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 2 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 approximately 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 heat pump can be operated again during cheap or excess energy periods to heat up the energy storage fluid ready for use. The heat pump may also be activated during a time of increased demand for domestic hot water. As heat pumps take time to activate, it can take several minutes before the activated heat pump can provide domestic hot water. The present system 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. 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. Figure 2 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 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 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 to the second heat exchanger. 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 second heat exchanger 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 second heat exchanger 4 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, 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 case, 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, 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 case, 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 surface of 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 3a - 3c 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 2, but in Figures 3a - 3d 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 fill level 22 of the tank 24 substantially constant, regardless of the temperature of the water in the tank 24. In the example shown in figures 3a - 3c the resilient member is a bladder, diaphragm, 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 3a - 3c 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 3a and 3b) and the other side of the resilient member (this is the surface identified as 34 in figures 3a and 3b) 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 22 in the tank relatively constant regardless of the temperature of the water 23, the resilient member 29 is used. In figure 3a 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 fluid level 22 in the tank to be kept relatively constant. This can be seen in figure 3b which shows the tank 24 when the water 23 is heated. It can be seen that the fluid level 22 of the tank is similar to that of figure 3a 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 fluid level of the tank relatively constant. The tank 24 may be clad with a heat insulating material or insulating jacket (not shown). Such cladding 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. The tank may include one or more temperature sensors, such as the temperature sensors 38, 39 that can be seen in figure 4. The tank may include a liquid level sensor, such as liquid level sensor 70 that can be seen in figure 4. The liquid level sensor 70 senses a range of fluid levels as the temperature of the energy storage liquid in the tank varies. The liquid level sensor 70 may include an integral temperature sensor or additionally or alternatively the temperature may be monitored by one or more other temperature sensors in the tank, such as temperature sensor 38 for example. The liquid level sensor may be a float type sensor, for example a continuous level float sensor and may comprise a float and guide rod arrangement. The liquid level sensor may include its own processor and memory and may also include its own alarm for alerting a user to a potential leakage problem. Alternatively the liquid level sensor may be connected to the processor of the domestic hot water supply system, as shown in figure 4. Figure 4 shows a domestic hot water supply system 1 similar to that shown in figures 1 and 2 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 system 1 also includes a housing 40 which contains the thermal energy store 2, the heat exchangers 3, 4 and a processor 36. The processor sends and receives signals to 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 temperature sensor 41 is also 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. Further temperature sensors may be provided in the thermal energy store, for example near the bottom of the thermal energy store, if desired. The processor 36 receives data from the temperature sensors 38, 39 and 41 and from the flow rate sensor 43 as well as the liquid level sensor 70. 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. In another configuration, the processor 36 may delay activating the heat pump 35 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. In another configuration, 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 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, 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. 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 heat pump if it has not already been activated and the processor 36 may also activate the flow restrictor 42 to reduce flow of the domestic hot water. 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 dips below a predetermined value, the processor 36 may activate the heat pump if it is not already switched on. 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 and may also activate the flow restrictor 42. 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 previously mentioned, 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 before the heated energy storage fluid runs out. 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 and may also activate the flow restrictor 42. The processor 36 monitors and stores the information received from the liquid level sensor 70 and the information received from the temperature sensors. The processor compares the information received from both the liquid level sensor and a temperature sensor at a particular time with the information received from the liquid level sensor at one or more previous times at that particular temperature to check if the fluid level remains constant at a particular temperature over a period of time. If the fluid in the circuit is found to be decreasing, then the processor alerts the user. Figure 5 shows a domestic hot water supply system 1 similar to that described in figure 4. 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 5 but are shown and described in figure 4. 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. In the above examples, the thermal energy store may be a plastic tank, or it may 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 tank is unpressurised and, including in use, the interior of the tank remains at approximately atmospheric pressure. 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 system. The above embodiment is to be understood as an illustrative example. 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, 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 domestic water heating appliance integrated within a housing comprisingan 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 arrangement for selectively circulating the energy storage fluid in a closed fluid circuit through the heat exchangers and the storage tank in dependence on an operating mode, wherein the pump arrangement comprises a first pump configured to pump the energy storage fluid from a lower part of the tank through the first heat exchanger to return to the tank and a second pump configured to pump the energy storage fluid from an upper part of the tank through the second heat exchanger to return to a lower part of the tank , anda processor for detecting a demand for domestic hot water and for controlling the pump arrangement and for detecting a state of operation of the remote heat source and / or for signalling a demand for heat, the processor being configured to switch on the first pump to circulate energy storage fluid through the first heat exchanger when it determines that the remote heat source is switched on and to detect a demand for domestic hot water when a hot water outlet is opened and to switch on the second pump to circulate energy storage fluid from an upper part of the tank through the second heat exchanger to provide domestic hot water when a demand for domestic hot water is detected, the processor further being arranged to operate at leasta first operating mode for circulating energy storage fluid from an upper part of the tank through the second heat exchanger to return to a lower part of the tank to provide heated domestic water in the absence of a demand for heat signal or absence of detection of active operation of the remote heat source for at least a first period of time, a second operating mode for heating domestic hot water during active operation of the remote heat source, the second mode comprising circulating energy storage fluid from an upper part of the tank through the second heat exchanger to return to a lower part of the tank and simultaneously circulating energy storage fluid from a lower part of the tank through the first heat exchanger to return to the tank in order to replenish heat stored in the energy storage fluid stored in the storage tank, anda third operating mode for replenishing heat stored in the storage tank, the third mode comprising determining whether there is a demand for domestic hot water, and when it is determined that no hot water is being demanded, then detecting a state of operation of the remote heat source and, if the remote heat source is not active, signalling a demand for heat, and when it is determined that the remote heat source is active, circulating energy storage fluid from a lower part of the tank through the first heat exchanger to return to the tank in order to replenish heat stored in the storage tank.

2. A domestic water heating appliance in accordance with claim 1, whereinthe processor is configured to determine if any of (i) to (iv) below apply:(i) the temperature of the heated energy storage fluid in a middle region of the storage tank has fallen to a predetermined temperature, or(ii) a predetermined amount of time has passed during a continuous demand for hot water, or(iii) 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.

3. A domestic water heating appliance in accordance with claim 2, wherein the processor is arranged to switch operation of the heating appliance from the first mode to the second mode if any of (i) to (iv) apply.

4. A domestic water heating appliance in accordance with any one of the preceding claims, wherein the housing has maximum dimensions of 750mm wide x 750mm depth x 1200mm height.

5. A domestic water heating appliance in accordance with claim 4, wherein the housing has maximum dimensions of 600mm wide x 600mm depth x 900mm height.

6. A domestic water heating appliance in accordance with any one of the preceding claims, wherein the housing is of a substantially cuboid form.

7. A domestic water heating appliance in accordance with any one of the preceding claims, wherein the housing includes at least one recess or protruding part.

8. A domestic water heating appliance in accordance with any one of the preceding claims, wherein all fluid inlets and outlets of the appliance are located at or near the bottom of the housing.

9. A domestic water heating appliance in accordance with any one of the preceding claims, wherein the remote heat source is a heat pump.

10. A domestic water heating appliance in accordance with any one of the preceding claims, wherein the domestic water heating appliance comprises a resilient member configured to keep the level of the energy storage liquid in the tank substantially constant as the volume of the energy storage liquid varies within a temperature range of at least 5 degrees to 60 degrees Celsius.

11. A domestic water heating appliance in accordance with any one of the preceding claims, wherein the housing comprises a support structure configured to be mounted to a mounting surface such as a wall or a floor.

12. A domestic water heating appliance in accordance with any one of the preceding claims, wherein the tank is clad with a heat insulating material in order to mitigate heat loss from the tank.

13. A domestic water heating appliance in accordance with any one of the preceding claims, wherein the tank comprises an outlet configured to provide energy storage fluid from an upper part of the tank to the second heat exchanger, and wherein the outlet comprises 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, oran integral channel formed in the tank and having an inlet to the channel at the upper part of the tank.

14. A domestic water heating appliance in accordance with claim 13, wherein a temperature sensor is provided in the tank at a predetermined distance below the inlet to the interior pipe or channel.

15. A domestic water heating appliance in accordance with claim 14, wherein the temperature sensor is configured to provide the processor with information relating to the temperature of the energy storage fluid in that part of the tank where the temperature sensor is located and wherein the processor is configured to switch on the remote heat source when it determines that the temperature drops below a predetermined level.

16. A domestic water heating appliance in accordance with any one of claims 13 to 15, wherein a flow rate sensor is provided in the outlet configured to provide energy storage fluid from the upper part of the tank to the second heat exchanger and wherein the flow rate sensor is configured to provide the processor with information relating to the flow rate of the energy storage fluid in the outlet and wherein the processor is configured to switch on the remote heat source when it determines that a predetermined amount of energy storage fluid from the tank has flowed through the outlet.

17. A domestic water heating appliance in accordance with any one of the preceding claims, wherein the processor is configured to detect a demand for domestic hot water when a domestic hot water outlet is opened and to identify the domestic hot water outlet opened by auser and to signal a demand for heat to the remote heat source if a predetermined domestic hot water outlet has been opened.

18. A domestic water heating appliance in accordance with any one of the preceding claims, wherein the processor is configured to detect a demand for domestic hot water when a domestic hot water outlet is opened and to signal a demand for heat to the remote heat source when it determines that a predetermined amount of time has passed since an opening of a domestic hot water outlet.

19. A domestic water heating appliance in accordance with any of the preceding claims wherein the processor is provided with a machine learning algorithm and is configured to use the machine learning algorithm to predict time periods when hot water usage is likely to be high.s

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