Central heating and hot water delivery system

Abstract

A central heating and hot water system comprising primary device 11 that stores heat in a primary core (100, fig 2). A secondary device 12 is configured to store and supply heat to a hot water circuit

Description

Significant efforts and progress have been made to decarbonise the heating and supply of hot water in the domestic environment due to the development of low-carbon replacements for fossil-fuel combi boilers. Thermal storage devices have been developed to store heat, typically originating from electrical power, for on-demand use, such as dry core storage boilers, direct electric hot water cylinders and heat batteries using phase change materials (“PCMs”). Electrical storage boilers convert electrical energy into heat using electrical heating elements or resistive heating elements and store the heat in a core material, or storage medium, located in a core. In dry core storage boilers, for example, heat is usually transferred from the core material by a fan or pump driving a transfer fluid, such as air, between the core and a heat exchanger in a closed loop. The heat exchanger transfers the heat to a fluid in a domestic heating and / or hot water system. One such electrical storage boiler is disclosed in WO2021037865A1. Electrical storage boilers may be dry core storage boilers, solid core devices, dry core devices, thermal storage boilers, dry core storage boilers or zero emission boilers. Typically, electrical storage boilers consume electrical power at times of low demand (or excess generation) across an electricity grid or network, such as during the night, when it has a lower cost. Increasingly this can occur at any time of day due to the increase in generation from renewable sources. Hybrid central heating and hot water systems have been implemented to utilise the benefits of different types of devices. In particular, known systems use electrical storage boilers to meet central heating requirements and separate PCM heat batteries or direct electric hot water cylinders (such as immersion heaters) to meet hot water requirements. Electrical storage boilers are well-suited to space heating demands due to their high energy storage density. PCM batteries are well-suited for hot water applications due to the potential for high heat power output, fast response times and low standing heat loss. Direct electric hot water cylinders are suitable for hot water applications due to their high heat power output and relative simplicity of operation and installation. However, such systems are sub-optimal as the individual thermal stores have to be sized to meet the full demand for central heating and hot water respectively. This is particularly acute in summer months when there is no central heating demand such that the electrical storage boiler is underutilised. The separate PCM heat battery or direct electric hot water cylinder must be specified to meet the full hot water demand, leading to higher - 2 -costs, and the lower energy density of PCM heat batteries means that it is likely that more charging has to occur during peak-rate tariff periods, therefore increasing the cost of providing hot water. Known systems have used electrical storage boilers to supply heated water to recharge secondary thermal stores, including PCM heat batteries. The secondary thermal store will itself call for heat from the electrical storage boiler when required. However, such a system can result in a slow response to recharging the secondary thermal store and inefficiencies in the timing of recharging using the electrical storage boiler rather than via an electrical immersion element in the secondary thermal store. SUMMARY Objects of the present disclosure include providing hybrid central heating and hot water systems with high energy storage capacity, high heat power output for hot water, fast response, minimal standing heat loss and relatively small system volume. A further object includes providing a system having two thermal storage devices, each capable of storing heat for a prolonged period of time, in which heat can be exchanged between the thermal storage devices in order to improve overall efficiency of the system. Further objects include improved methods of operating such systems. The present invention provides systems and methods in accordance with the claims. The present disclosure generally provides a central heating and hot water delivery system comprising: a hot water circuit extending between a cold water inlet and a hot water outlet; a primary device configured to store heat in a primary core; a secondary device configured to store heat therein and to supply heat to the hot water circuit between the cold water inlet and the hot water outlet; a primary fluid circuit configured to selectively circulate fluid between the primary and secondary devices such that heat from the primary core is transferred to the secondary device; and at least one controller. The hot water circuit may comprise a hot water sensor system located between the cold water inlet and the hot water outlet. The primary fluid circuit may comprise a primary sensor system. The present disclosure provides that the at least one controller is configured to: receive hot water demand data from the hot water sensor system; determine from the hot water demand data if water demand in the hot water circuit has been initialised and, if initialised, operate the primary fluid circuit to transfer heated fluid from the primary device to the secondary device. The present disclosure further provides a method comprising, by the controller: receiving hot water demand data from the hot water sensor system; determining from the hot water demand data if water demand in the hot water circuit has been initialised and, if initialised or in response to the determination that water demand has been initialised, operating the primary fluid circuit to transfer heated fluid from the primary device to the secondary device. By initialising recharging of the secondary device as soon as hot water is demanded, and thus generally trying to maintain the secondary device at full charge, standing heat loss from the entire system can be reduced. In particular, the standing loss of heat (i.e. rate of heat loss) of the primary device is preferably greater than the standing loss of heat of the secondary device, particularly if the former has a greater heat storage capacity than the latter and / or the former comprises an electrical storage boiler and the latter comprises a PCM battery or hot water cylinder. Therefore, by immediately transferring heat to the secondary device, rather than waiting for it to reach a certain low charge level, overall standing heat loss from the system is reduced and efficiency is improved. Furthermore, recharging response time is improved because recharging starts before the state of charge of the secondary device has reached a minimum level. Power output performance of the secondary device is also improved by immediately charging the secondary device. The heat storage capacity of the primary device is preferably greater than the heat storage capacity of the secondary device. Thus, the higher thermal storage capacity of a primary device is used to recharge or supply energy to the secondary device, which provides heat to a hot water circuit. By utilising the primary core to recharge the secondary device, the secondary device can be relatively smaller in capacity to meet a certain hot water demand and the primary device can still be utilised to improve system efficiency even when no central heating is required, thereby reducing overall capital cost. The additional power input and storage capacity of the primary device can be utilised to take advantage of low-cost electricity / off-peak tariff periods when these are available, thereby reducing the operating cost of providing hot water as well as space heating. In embodiments the present disclosure provides that the at least one controller is configured to: determine a secondary state of charge of the secondary device, optionally based upon data received from the secondary device, hot water sensor system and / or primary sensor system; monitor the determined secondary state of charge over a first extended time period, optionally being 24 or fewer hours; determine the heat consumed by the secondary device and / or hot water circuit in the first extended time period; operate the primary device, optionally after expiry of the first extended time period and / or in an off-peak period, such that the primary core stores an amount of heat based upon the heat consumed by the secondary device and / or hot water circuit in the first extended time period. The amount of heat stored may be determined based upon (a) at least the determined amount of heat consumed by the secondary device and / or hot water circuit in the first extended time period; and / or (b) an amount of heat up to a predetermined level above the determined amount of heat consumed by the secondary device and / or hot water circuit in the first extended time period. The amount of heat stored may be determined as per (a) or (b), less or subtracting (c) an amount of heat equivalent to the useful heat -4-storage capacity of the secondary device, or an amount of heat that the secondary device is to be supplied separately to from the primary device, such as by being charged directly via its alternative charging device. The present disclosure provides a method comprising, by the controller: determining the secondary state of charge; monitoring the determined secondary state of charge over a first extended time period; determining the heat consumed by the secondary device and / or hot water circuit in the first extended time period; and operating the primary device such that the primary core stores an amount of heat based upon the heat consumed by the secondary device and / or hot water circuit in the first extended time period.. In particular, the total hot water energy consumption per day is therefore calculated in real time and demand is predicted before the next off-peak charge period of the primary device. This therefore minimises standing loss from the ZEB, since it does not overcharge as compared to the expected demand. In embodiments the present disclosure provides that the at least one controller is configured to: operate the primary fluid circuit to transfer heated fluid from the primary device to the secondary device; operate the primary fluid circuit to stop transferring heated fluid from the primary device to the secondary device, optionally based upon a preliminary determination that a secondary state of charge of the secondary device has reached a maximum charge; and after a repeat time period, again operate the primary fluid circuit to transfer heated fluid from the primary device to the secondary device. The present disclosure provides a method comprising, by the controller: operating the primary fluid circuit to transfer heated fluid from the primary device to the secondary device; operating the primary fluid circuit to stop transferring heated fluid from primary device to the secondary device; and after a repeat time period, again operating the primary fluid circuit to transfer heated fluid from the primary device to the secondary device. Due to limited thermal conductivity of the secondary device, it can take some time for heat to dissipate from primary fluid circuit into the secondary device. Therefore, it can appear that the secondary device has reached the maximum secondary state of charge, when in fact the heat has not yet fully dissipated throughout the secondary device. Therefore, by giving the heat time to homogenise and then charging the secondary device again, it is possible to increase the total heat stored and increase the power output when the secondary device is discharged. In embodiments the present disclosure that the primary device comprises a heat exchanger for exchanging heat from the primary core to the primary fluid circuit, wherein the primary fluid circuit comprises a return temperature sensor upstream of the heat exchanger between the secondary device and the heat exchanger and / or a flow temperature sensor downstream of the heat exchanger between the heat exchanger and the secondary device. The at least one controller may be configured to: receive return and flow temperature data from the return and flow temperature sensors respectively; and operate the primary fluid circuit to stop transferring heated fluid from the primary device to the secondary device if the return temperature data is within a predetermined range of the flow temperature data. The present disclosure provides a method comprising, by the controller: receiving return and flow temperature data from the return and flow temperature sensors respectively; and operating the primary fluid circuit to stop transferring heated fluid from the primary device to the secondary device if the return temperature data is within a predetermined range of the flow temperature data. When the secondary device approaches maximum secondary state of charge, no more heat can be extracted by the secondary device from the primary fluid circuit and thus the return temperature to the primary device approaches the flow temperature from the primary device. Utilising such a determination to stop charging of the secondary device may be more accurate than stopping charging based upon determining that a maximum secondary state of charge has been reached, since this may be distorted and incorrect due to slow dissipation of heat in the secondary device. In embodiments the present disclosure provides that the at least one controller is configured to: determine a temperature of fluid in the primary fluid circuit based upon data received from the primary sensor system; and if water demand has been initialised, operate the primary fluid circuit to transfer heated fluid from the primary device to the secondary device if the temperature of fluid in the primary fluid circuit exceeds a heat-preserving temperature. The present disclosure provides a method comprising, by the controller: determining a temperature of fluid in the primary fluid circuit based upon data received from the primary sensor system; and if water demand has been initialised, operating the primary fluid circuit to transfer heated fluid from the primary device to the secondary device if the temperature of fluid in the primary fluid circuit exceeds a heat-preserving temperature. Therefore, when a hot water demand is initialised, a primary pump of the primary fluid circuit circulates fluid therearound whilst a diverter valve remains open to a central heating circuit and closed to the secondary device. The diverter valve is only operated to divert flow to charge the secondary device once the temperature of the primary fluid circuit is above the threshold heat-preserving temperature. This prevents heat being extracted from the secondary device into the central heating circuit until heat is being delivered by the primary device and subsequently delivered at sufficient temperature to the secondary device. In embodiments the present disclosure provides that the secondary device may comprise an alternative charging device for storing heat in the secondary device and the at least one controller is configured to: operate in a primary lower power input mode, by default, in which heat is transferred from the primary device to the secondary device such that storing of heat for the hot water circuit is prioritised; operate in a secondary lower power input mode, if predicted hot water demand over an upcoming time period is anticipated to be less than or equal to the useful storage capacity of the secondary device, or less than or equal to the charge obtainable from available off-peak period(s), in which the alternative charging device is operated to charge the secondary device during off-peak periods; operate in a secondary high power input mode, if predicted hot water demand over an upcoming time period is anticipated to be more than the useful storage capacity of the secondary device, or more than the charge obtainable from available off-peak period(s), in which the primary core is charged and the alternative charging device charges the secondary device; and / or operate in a primary high power input mode, if a predicted space heating demand for an upcoming time period exceeds the useful capacity of the primary device, in which the primary device charges the secondary device to a minimum state of charge threshold, such that beyond this threshold, storage of heat in the primary device is prioritised over transfer of such heat to the secondary device. A secondary peak charging mode may be implemented if hot water demand is not met by any of the preceding steps. A primary peak charging mode may be implemented if hot water and / or space heating demand is not met by any of the preceding steps. The present disclosure provides a method comprising, by the controller: operating in a primary lower power input mode, by default, in which heat is transferred from the primary device to the secondary device such that storing of heat for the hot water circuit is prioritised; operating in a secondary lower power input mode, if predicted hot water demand over an upcoming time period is anticipated to be less than or equal to the useful storage capacity of the secondary device, or less than or equal to the charge obtainable from available off-peak period(s), in which the alternative charging device is operated to charge the secondary device during off-peak periods; operating in a secondary high power input mode, if predicted hot water demand over an upcoming time period is anticipated to be more than the useful storage capacity of the secondary device, or more than the charge obtainable from available off-peak period(s), in which the primary core is charged and the alternative charging device charges the secondary device; and / or operating in a primary high power input mode, if a predicted space heating demand for an upcoming time period exceeds the useful capacity of the primary device, in which the primary device charges the secondary device to a minimum state of charge threshold, such that beyond this threshold, storage of heat in the primary device is prioritised over transfer of such heat to the secondary device. A secondary peak charging mode may be implemented if hot water demand is not met by any of the preceding steps. A primary peak charging mode may be implemented if hot water and / or space heating demand is not met by any of the preceding steps. By charging the primary and secondary devices as much as possible during off-peak periods, excess power capacity from the power grid can be received by the system when available. Hence overall efficiency of the power grid and system can be improved. Using the primary device to store heat, even when the central heating is not required, means that heat can be generated from electricity during the off-peak periods for later use during the peak periods by the secondary device. In embodiments the present disclosure provides that the hot water sensor system is located in the hot water circuit separately to the secondary device and the primary sensor system is located in the primary fluid circuit separately to the secondary device. The at least one controller may be configured to: determine the heat stored in the secondary device at the start of a time period; determine the heat stored in the secondary device over the time period based upon data received from the primary sensor system; determine the heat released from the secondary device over the time period based upon data received from the hot water sensor system; and determine the secondary state of charge of the secondary device based upon the heat stored in the secondary device at the start of a time period, the heat stored in the secondary device over the time period and the heat released from secondary device over the time period. The present disclosure provides a method comprising, by the controller: determining the heat stored in the secondary device at the start of the time period; determining the heat stored in the secondary device over the time period based upon data received from the primary sensor system; determining the heat released from the secondary device over the time period based upon data received from the hot water sensor system; and determining the secondary state of charge of the secondary device based upon the heat stored in the secondary device at the start of a time period, the heat stored in the secondary device over the time period and the heat released from secondary device over the time period. In prior art systems the state of charge of the secondary device is determined utilising sensors integrated therein. However, by determining the state of charge using sensors external to the secondary device, the state of charge determined can be more accurate, since joule counting can be performed and the state of charged modelled based on an overall heat balance. In particular, if the secondary device comprises a PCM store, existing internal sensors (include battery pressure and temperature) may not provide a particularly accurate measure of state of charge because the secondary device operates as an isothermal process (i.e. latent heat causing a change in phase). In the case of hot water cylinders, typically temperature is measured using a single sensor, often on the outside of the cylinder, which is also inaccurate due to water mixing effects within the cylinder and lag time. In embodiments the present disclosure provides the hot water circuit comprising, between the cold water inlet and hot water outlet, a supplementary heater. The at least one controller is configured to: receive a delay response time period, optionally from a user via a personal computing device; receive a setpoint temperature indicative of a demanded temperature of hot water exiting the hot water outlet; operate the secondary device to supply heat to the hot water circuit based upon the setpoint temperature; and operate the supplementary heater to supply heat to the hot water circuit to achieve the setpoint temperature, after expiry of the delay response time period starting from when the secondary device is initially operated to supply heat to the hot water circuit. The present disclosure provides a method of operating such a system comprising, by the controller: receiving the delay response time period and setpoint temperature; operating the secondary device to supply heat to the hot water circuit based upon the setpoint temperature; waiting for the expiry of the delay response time period starting from when the secondary device is initially operated to supply heat to the hot water circuit; and subsequently operating the supplementary heater to supply heat to the hot water circuit to achieve the setpoint temperature. The hot water circuit may comprise at least one downstream temperature sensor and the at least one controller may monitor a downstream temperature based upon data received from the at least one downstream temperature sensor and control the secondary device and supplementary heater so as to raise the temperature at the at least one downstream temperature sensor to the setpoint temperature. If the delay response time period is above a delay response threshold, the at least one controller may operate, during the delay response time period, the primary device and primary fluid circuit so as to supply heat from the primary core to the secondary device. The at least one controller may be configured to receive the delay response time period from an external computing system configured to receive the delay response time period as a user input. If the delay response time period is below a delay response threshold, the at least one controller may operate the supplementary heater before and / or at the same time as operating the primary device and primary fluid circuit to supply heat from the primary core to the secondary device. There is therefore a user selectable delay in response time (e.g. via an application on a personal computing device) based on a performance preference. If the user wishes to improve efficiency and reduce energy consumption, they can introduce a delay in response time to allow the stored heat from the primary and secondary devices to respond first, avoiding / minimising the use of electricity by the supplementary heater. Conversely, if the user requires a faster response regardless of electricity consumption, then the supplementary heater can operate immediately upon detection of a hot water demand, filling in the time before heat is supplied from the primary and secondary devices. In embodiments the present disclosure provides a central heating and hot water delivery system comprising: a hot water circuit extending between a cold water inlet and a hot water outlet, a primary device configured to store heat in a primary core; a secondary device configured to store heat therein and to supply heat to the hot water circuit between the cold water inlet and the hot water outlet; the hot water circuit comprises a cold water take-off line leading from the cold water inlet to between the secondary device and hot water outlet; and an active modulating valve configured to mix cold water from the cold water take-off line with hot water exiting the secondary device. The system further comprises at least one controller configured to operate the active modulating valve to control fluid flow therethrough. The at least one controller may be configured to receive a hot water demand, including a setpoint temperature, and operate the active modulating valve to control the temperature of water exiting through the hot water outlet. Such a system may comprise the primary fluid circuit as described herein, and the present disclosure provides a method of operating such a system including the steps performed by the at least one controller. The system of the present disclosure may be supplied as four main component systems, namely (a) the primary device, (b) the secondary device, (c) the primary fluid circuit and hot water circuit together (collectively "the hot water module”) and (d) the at least one controller. This allows for integration with existing products and may be useful for retrofit installations. However, the system can still operate with two or more of (a) to (d) combined into a single device. The present disclosure further provides a at least one controller configured to operate a device according to any one of the preceding methods. The primary device may comprise a fluid system for extracting heat from the primary core, at least one heating element for heating the core, at least one heat exchanger connected to the primary fluid circuit and / or at least one bypass arrangement. The at least one controller may control the fluid system, at least one heating element and / or at least one bypass arrangement to control heat supplied to the primary fluid circuit. The system of the present disclosure is preferably a domestic system. The primary fluid circuit, fluid system and hot water circuit disclosed herein are generally configured to circulate fluid in one direction during normal use and thus the terms “upstream” and “downstream” should be interpreted based upon the normal use direction. Furthermore, in the present disclosure the term “charge” refers to storing heat in the primary or secondary device and “discharge” refers to releasing or extracting heat from the primary or secondary device. The methods and controller operations described herein may be implemented as a computer program comprising program instructions to operate the controller, which is a computer. The computer program may be stored on a computer-readable medium. The controller may include a processor or processors (e.g., local, virtual, or cloud-based) such as a Central Processing unit (CPU), and / or a single or a collection of Graphics Processing Units (GPUs). The processor may execute logic in the form of a software program. The controller may include a memory including volatile and non-volatile storage medium. A computer-readable medium may be included to store the logic or program instructions. The different parts of the system may be connected using a network (e.g. wireless networks and wired networks). The computer system may include one or more interfaces. The computer system may contain a suitable operating system such as UNIX, Windows (RTM) or may be a Container host, for example. The present disclosure provides a non-transitory computer-readable storage medium including program code which when executed by at least one processor causes operations comprising the methods of the present disclosure. It should be noted that any feature described above may be used with any particular aspect or embodiment of the disclosure. BRIEF DESCRIPTION OF THE DRAWINGS By way of example only, embodiments of methods and devices in accordance with the present disclosure are now described with reference to, and as shown in, the accompanying drawings, in which: Figure 1 is a schematic view of an embodiment of a system in accordance with the present disclosure; Figure 2 is a cross-sectional view of an embodiment of a primary device of the present disclosure; Figure 3 is a schematic view of a further embodiment of a system in accordance with the present disclosure, in which features are the same as in Figure 1 unless otherwise indicated; Figure 4 is a flowchart illustrating the general operation of the system and the method of the present disclosure; Figure 5 is a flowchart illustrating determining a state of charge of a secondary device in accordance with the present disclosure; Figure 6 is a flowchart illustrating initiating charging of a secondary device in accordance with the present disclosure; Figure 7 is a flowchart illustrating implementing different modes in accordance with the present disclosure; and Figure 8 is a flowchart illustrating the implementation of one or more operations in order to optimise operation of the primary and secondary devices of the present disclosure. DETAILED DESCRIPTION As illustrated in the exemplary embodiment of Figure 1, the present disclosure relates to a central heating and hot water system 10 comprising primary and secondary devices 11, 12. The primary and secondary devices 11,12 store heat therein, may be referred to as thermal storage devices and can supply heat non-instantaneously as compared to when they initially receive and store such heat. In particular, the primary and secondary devices 11, 12 are configured to receive heat or energy and store heat therein - 11 -for delayed release. The time delay between receipt of energy and release of heat may be up to a plurality of hours or days. The system 10 comprises a primary fluid circuit 20 extending between the primary and secondary devices 11, 12 for circulating fluid therebetween and a hot water circuit 50 connected to, for receiving heat from, the secondary device 12. The primary fluid circuit 20 and hot water circuit 50 each comprise at least one pipe, conduit, valve, expansion vessel, pressure sensor, flowrate sensor, drain and / or vent. The primary device The primary device 11 stores heat in a primary core 100, which may be solid and / or dry. The primary device 11 may have a higher maximum heat capacity than the secondary device 12 and may have at least twice the maximum heat capacity of the secondary device 12. The primary device 11 may be an electrical storage boiler, dry core storage boiler, solid core device, dry core device, thermal storage boiler, dry core storage boiler or zero emission boiler. Figure 2 illustrates a particular embodiment of the primary device 11, which may comprise an electrical storage boiler, particularly a dry core storage boiler, as illustrated. The primary device 11 comprises a primary core 100, or heat storage medium. The primary core 100 comprises a medium capable of storing thermal energy, principally by sensible heat storage (or antiferromagnetic and eutectoid transition effects), for extraction at a later time. The primary core 100 may comprise a phase change material or may comprise a metal, such as at least one of an iron oxide and / or a ferrous metal or iron alloy. The iron oxide may comprise magnetite (Fe3O4), hematite (Fe2O3), wustite (FeO) and / or any other suitable iron oxide. Further details of suitable cores 100 may be found in at least WO2021037865A1. The primary device 11 may further comprise a core housing 130 wherein the primary core 100 is disposed within the core housing 130. The primary device 11 may comprise a fluid system 103 extending through the primary core 100 and a base 101 for heating a transfer fluid in the primary core 100 and for circulating the heated transfer fluid between the primary core 100 and the base 101. The primary device 11 further comprises at least one heating element 15a, 15b, 15c, which may (each) comprise an electric resistive heating element, for heating the primary core 100. As illustrated in Figure 1, the primary device 11 may comprise an array of heating elements 15a, 15b, 15c distributed across the primary core 100. The primary device 11 is connected to the primary fluid circuit 20 and the fluid system 103 is configured for extracting the heat from the transfer fluid for supply to the primary fluid circuit 20. The fluid system 103 defines a fluid circulation circuit for the transfer - 12 -fluid to circulate between the primary core 100 and the base 101. The fluid system 103 is preferably a closed loop, constant volume system. The transfer fluid is preferably air. The fluid system 103 may comprise a heat exchanger 133 and a fan 135, which may be mounted inside the base 101. The heat exchanger 133 may be located downstream of the primary core 100 and upstream of the fan 135. The heat exchanger 133 is connected to the fluid system and the primary fluid circuit 20 and is configured to extract heat from the heated transfer fluid and transfer the heat to the primary fluid circuit 20. The fan 135 is configured to direct the transfer fluid around the fluid system 103. The fan 135 may comprise a fan inlet (not shown) for receiving cooler transfer fluid from the heat exchanger 133 and may comprise a fan outlet 137 out of which it drives transfer fluid towards the fluid system 103 and the primary core 100. The fan 135 may be driven by a fan motor 145, which may be variable to drive the fan 135 to provide a variable flowrate and thus control the power output from the primary core 100. The primary device 11 may further comprise a bypass arrangement 160 for enabling transfer fluid exiting the fan 135 to bypass the primary core 100 and be directed to the heat exchanger 133. A suitable bypass arrangement 160 is described further in WO2021037865A1. The bypass arrangement 160 comprises a bypass actuator (not shown) for controlling the opening of the bypass arrangement 160. The secondary device The secondary device 12 may comprise a secondary core in which heat is stored. The secondary device 12 may comprise an indirect device in which heat is stored in the secondary core and released to the hot water circuit 50 via an indirect coil. The secondary core may be charged and receive heat from the primary fluid circuit 20, such as by receiving heated fluid from the primary fluid circuit 20. In particular, the secondary device 12 may comprise a water cylinder (the water of the hot water circuit 50 forming the core) with an indirect coil therein. The secondary device may or may not comprise a PCM battery, in which the secondary core may comprise a PCM, and the change of phase may be utilised as a means of storing heat. The secondary device 12 may comprise an alternative charging device 12a, such as an immersion heater in the case of a water cylinder or an electrical element in the case of a PCM battery, for increasing the heat stored therein in addition to or alternatively to heat supplied from the primary fluid circuit 20. The primary fluid circuit The primary fluid circuit 20 is connected to the primary and secondary devices 11, 12 and is configured to direct heated fluid from the primary device 11 to the secondary device 12. The fluid in the primary fluid circuit 20 is preferably water. The primary fluid circuit 20 comprises a central heating circuit 21 for circulating fluid between the primary device 11 and at least one radiator 22 for heating a domestic space. The central heating circuit 21 may also be connected to the secondary device 12 by the primary fluid circuit 20 for receiving fluid therefrom. The primary fluid circuit 20 may comprise at least one device temperature sensor Tr, Tf and / or at least one primary flowrate sensor Fz for determining the temperature and / or flowrate of fluid flowing through the primary fluid circuit 20 in the primary device 11. The at least one device temperature sensor TR, TF may comprise return and flow temperature sensors Tr, Tf located in the return (i.e. upstream or colder) and flow (i.e. downstream or warmer) lines of the primary fluid circuit 20 in or adjacent to the primary device 11, particularly upstream and downstream of the heat exchanger 133. The at least one device temperature sensor TR, TF, at least one primary flowrate sensor Fz and conduits of the primary fluid circuit 20 within the primary device 11 may form part of the primary device 11, with the rest of the primary fluid circuit 20 being connected thereto during installation. Hence such components may be considered to be part of the primary device 11 and / or primary fluid circuit 20. The primary fluid circuit 20 may comprise primary device flow and / or return valves 23, 24 connected downstream and / or upstream respectively of the primary device 11 for controlling flow through the primary device 11. The primary fluid circuit 20 comprises a primary pump 25 for circulating fluid therearound, which may be located downstream of the primary device 11 and upstream of the secondary device 12 and / or the central heating circuit 21. The primary pump 25 controls the rate of flow of fluid and heat through the primary fluid circuit 20, including between the primary and secondary devices 11,12 and to the central heating circuit 21. The primary fluid circuit 20 may comprise a primary temperature sensor TD for determining the temperature of fluid in the primary fluid circuit 20 between the primary device 11, optionally also the primary pump 25, and the secondary device 12. The primary temperature sensor TD is preferably located upstream of the automatic bypass valve 30 and diverter valve 32, where there is water flow and hence the ability to detect water temperature even when there is no flow around the central heating system 21 or the secondary device 12. The primary fluid circuit 20 comprises a diverter valve 32 for controlling fluid flow, particularly heated fluid flow, from the primary device 11 to the secondary device 12 and / or the central heating circuit 21. The diverter valve 32 may be connected therebetween, upstream of the secondary device 12 and central heating circuit 21 and downstream of the primary device 11. Thus, the diverter valve 32 controls the ratio of heat supplied from the primary device 11 to the secondary device 12 and / or the central heating circuit 21. The diverter valve 32 may be a 3-port diverter valve or a 3-port mid-position valve or one or more 2-port zone valves. The primary fluid circuit 20 may comprise secondary device return and / or flow valves 33, 34 connected downstream and / or upstream respectively of the secondary device 12 for controlling flow through the secondary device 12. The secondary device flow valve 34 may be connected between the diverter valve 32 and the secondary device 12. The secondary device return valve 33 may be connected between the secondary device 12 and the primary device 11 and / or primary device return valve 24. The central heating circuit 21 may extend from the diverter valve 32, optionally through a central heating flow valve 36, through the at least one radiator or other heat emitter 22, optionally through a central heating return valve 37 and back to the rest of the primary fluid circuit 20, towards the primary device 11. In particular, downstream of secondary device return and central heating return valves 33, 37, a return flow line 40 from the secondary device 12 meets a return flow line 41 from the central heating circuit 21 at a junction 42, from which a final return flow line 43 extends to the primary device 11 via the primary return valve 24. The primary fluid circuit 20 may also comprise an air vent 26, a drain 27, an expansion vessel 28, a pressure safety valve 29, an automatic bypass valve 30 and a filter 38. The system 10 may comprise a refill loop 31, comprising at least one refill valve 31a, 31 b and / or refill check valve 31 c, for supplying water from the hot water circuit 50 to the primary fluid circuit 20. The primary fluid circuit 20 thus comprises a primary sensor system 39 comprising the at least one device temperature sensor TR, TF, at least one primary flowrate sensor Fz and / or primary temperature sensor TD. The primary sensor system 39 is located in the primary fluid circuit 20 separately to the secondary device 12 and may be at least partially located in and / or partially form part of the primary device 11. The hot water circuit The hot water circuit 50 is configured for receiving cold water, heating the water, and supplying the heated water for direct use, such as by being dispensed via taps or faucets in a domestic environment. The hot water circuit 50 may be a potable or drinkable water circuit for supplying clean and safe to drink heated water. The hot water circuit 50 extends from a cold water inlet 51, through the secondary device 12 and to a hot water outlet 52. The term “cold water” refers to generally unheated water supplied directly from a mains water supply, whilst the term “hot water” refers to the water output from the hot water circuit 50, comprising the cold water heated therein to a higher temperature. The cold water inlet 51 may be connected to the mains water supply - 15-and the hot water outlet 52 may be connected to one or more hot water dispensers, such as tap(s). The hot water circuit 50 may comprise an inlet valve unit 53 for controlling flow and / or pressure from the cold water inlet 51 to the secondary device 12. The inlet valve unit 53 may comprise a safety valve 53a, a check valve 53b and / or an inlet control valve 53c, the latter being for controlling pressure. The illustrated hot water circuit 50 further comprises a hot water circuit flowrate sensor Fi, a water inlet temperature sensor T1 and / or a water pressure sensor Pi. The hot water circuit flowrate sensor Fi and water pressure sensor Pi are configured to determine the pressure and / or flowrate of water flowing through the hot water circuit 50 and may be located between the cold water inlet 51 and secondary device 12 as illustrated or may be located between the hot water outlet 52 and secondary device 12. The water inlet temperature sensor Ti is located between the cold water inlet 51 and secondary device 12, such as between the inlet valve unit 53 and the secondary device 12. The refill loop 31 may extend from the hot water circuit 50 from downstream of the cold water inlet 51, optionally also the inlet valve unit 53, and upstream of the secondary device 12 to the primary fluid circuit 20 downstream of the secondary device 12 and upstream of the primary device 11. The refill loop 31 may thus supply, preferably potable, water to the primary fluid circuit 20 from the cold water inlet 51 of the hot water circuit 50. The hot water circuit 50 may further comprise a supplementary heater 55 located between the secondary device 12 and the hot water outlet 52. The supplementary heater 55 may be a direct electric heating element for directly heating the water passing therethrough. For example, the supplementary heater 55 may provide additional heating to water exiting the secondary device 12 in order to raise its temperature to a demanded temperature, which may be necessary if the secondary device 12 does not have sufficient charge or heat stored therein to reach the demanded temperature. The hot water circuit 50 may also comprise heater inlet and outlet temperature sensors T2 and T3 located, preferably immediately, upstream, and downstream of the supplementary heater 55 for determining the temperature of water entering and exiting the supplementary heater 55 respectively. As illustrated, the hot water circuit 50 may also comprise an expansion vessel or shock arrestor 60, a flow control unit 61 located between the cold water inlet 51 and / or secondary device 12 and / or supplementary heater 55 and the hot water outlet 52; and a water outlet temperature sensor T4 located between the mixing valve 58 and hot water outlet 52 for measuring the temperature of the hot water exiting the hot water circuit 50. The flow control unit 61 may comprise at least one valve, pump and / or temperature-based variable flow restrictor for controlling the rate of flow through the hot water circuit 50. The hot water circuit 50 may further comprise a mixing valve 58 for mixing hot water exiting the secondary device 12, and also exiting the supplementary heater 55 if present, with cold water from the cold water inlet 51 and supplying such mixed water to the hot water outlet 52. The hot water circuit 50 may thus comprise a cold water take-off line 59 leading from the cold water inlet 51 to between the secondary device 12 and hot water outlet 52, preferably to between the supplementary heater 55 and flow control unit 61 as illustrated. The mixing valve 58 may be a self-acting valve, may be a 3-port thermostatic mixing valve and / or may be an active modulating valve. As illustrated in Figure 3, the mixing valve 58 may alternatively comprise an actively controlled, optionally 2-port, modulating valve 58 located in the cold water take-off line 59 for modulating the supply of cold water to the hot water. The hot water circuit 50 thus comprises a hot water sensor system 54, comprising the hot water circuit flowrate sensor Fi, the water inlet temperature sensor Ti, the water outlet temperature sensor T4, optionally the heater inlet and / or outlet temperature sensors T2 and T3 and optionally the water pressure sensor Pi. The hot water sensor system 54 is located in the hot water circuit 50 separately to the secondary device 12. In other words, the elements of the hot water sensor system 54 do not form part of the secondary device 12 and are instead connected to the secondary device 12 during installation, such as via conduits or pipes. The control system The system 10 further comprises a control system 70 comprising at least one controller 71 in communication with the primary sensor system 39, the hot water sensor system 54 (such as the sensors TR, TF, TD, Ti, T2, T3, T4, Pi, Fi, Fz) for receiving signal(s) or data (such as indicative of temperature, pressure and / or flow) therefrom and in communication with the valves 23, 24, 30, 31 a, 31 b, 32, 33, 34, 36, 37, 53a, 53c, 58 for control thereof. The at least one controller 71 is also in communication with the primary and secondary devices 11, 12, and supplementary heater 55 if present, for receiving sensor information, including electrical power consumption data, therefrom and for controlling the operation thereof. The at least one controller 71 is also in communication with the primary pump 25 and flow control unit 61 for controlling flow around the primary fluid circuit 20 and the hot water circuit 50. Although not illustrated, the control system 70 comprises wired or wireless communication means between such components of the system 10. The control system 70 and at least one controller 71 are configured to perform the methods of the present disclosure. The at least one controller 71 comprises at least one processor configured to perform operations based upon the instructions. The at least one controller 71 comprises at least one memory, which may store instructions or algorithms in the form of data for operation by the at least one processor. The control system 70 may comprise a communication component 72 for enabling communication between the at least one controller 71 and an external computing system 80, which may be considered part of the system 10, via a wired or wireless network 81 (such as wireless interface with the Internet, ethernet, fibre optic, satellite communication network, broadband communication network, cellular, Bluetooth). The external computing system 80 may comprise a personal computing device 82 that has communications capabilities (e.g., for communicating with the internet or other communications network such as a cellular network), such as at least one of a smartphone, laptop computer or tablet computer, and / or may comprise an external server system 83. In particular, the external computing system 80 may comprise a personal computing device 82 having an application thereon providing a user interface 84 (such as via a display of the personal computing device 82). The personal computing device 82 may communicate directly with the at least one controller 71, such as via Bluetooth or wireless, or may communicate indirectly with the at least one controller 71, such as via the external server system 83. Through the user interface 84, a user may be able to enter commands for controlling the at least one controller 71 and thus system 10. Through the user interface 84, a user may be able to visualise sensor and other data output from the at least one controller 71. In the embodiment of Figure 1 the at least one controller 71 comprises an external controller 71 located separately to the primary and secondary devices 11, 12. The external controller 71 and / or control system 70 (including communication component 72) may be incorporated into the packaging of the hot water module unit (i.e. the primary fluid circuit 20 and hot water circuit 20). The at least one controller 71 also comprises integrated controllers 71 in each of the primary and secondary devices 11, 12. Alternatively, the at least one controller 71 does not include a controller separate to the primary and secondary devices 11, 12, with the main control disclosed herein being performed by the controller 71 of the primary device 11. The at least one controller 71 may further be embodied in the external computing system 80, such as the personal computing device 82 and / or external server system 83. Operations of sequential steps disclosed herein may be implemented by different controllers 71. Therefore, unless indicated to the contrary, the references herein to operations by the at least one controller 71 may be considered to be references to operations by an external controller 71 (if present), by the external computing system 80, an integrated controller 71 of the primary device 11 and / or an integrated controller 71 of the secondary device 12. Thus, in the primary device 11, the at least one controller 71 is connected to the at least one heating element 15a, 15b, 15c for controlling heating of the primary core 100 and is connected to the fan 135 and / or bypass arrangement 160 for controlling the flow of transfer fluid around the fluid system 103. The at least one controller 71 may therefore control the at least one heating element 15a, 15b, 15c, fluid system 103 and / or bypass arrangement 160 and such control may be based upon the output from at least one temperature sensor TR, TF and / or flowrate sensor Fz within the primary device 11. The at least one controller 71 may also monitor the electrical power consumption of the at least one heating element 15a, 15b, 15c and / or the frequency of the electricity grid to which it is connected and use the resulting data, optionally also electricity tariff, local atmospheric weather and / or grid carbon intensity data, to control the time of heating of the primary core 100. In the secondary device 12, the at least one controller 71 may control operation thereof, such as by controlling the heating via the alternative charging device 12a. The at least one controller 71 may receive control signals from the secondary device 12, such as indications of the state of charge (i.e. the current amount of stored heat as compared to the maximum amount of stored heat) and temperature measurements. System heating and charging operation Figure 4 is a flowchart illustrating an embodiment of the operation of the system 10 in accordance with the present disclosure. In order to charge the primary device 11, the at least one controller 71 operates the system 10 such that heat is generated in the primary device 11 (step 400). The at least one controller 71 controls the power supply to supply electrical energy to the heating elements 15a, 15b, 15c. The heating elements 15a, 15b, 15c convert the electrical energy to heat, which is then stored in the primary core 100 for later extraction by the fluid system 103. In order to provide heat to the primary fluid circuit 20, the at least one controller 71 operates the primary device 11 such that heat is released or extracted from the primary device 11 (step 401). The at least one controller 71 operates the fan 135 to circulate transfer fluid around the fluid system 103. The fan 135 directs cooler transfer fluid from the heat exchanger 133 to the primary core 100 where it receives heat. The heated transfer fluid is then driven into the heat exchanger 133, through which heat is transferred to the primary fluid circuit 20. The at least one controller 71 may control the power output of, and extraction of heat from, the primary device 11, such as by varying the speed of the fan 135 and / or controlling the bypass arrangement 160, to control the transfer fluid flowrate through the primary core 100. In particular, the at least one controller 71 may determine or receive a desired heat power output and / or water temperature in the primary fluid circuit 20 and operate the fan 135 to at least one of a plurality of fan speeds (e.g. selecting one speed from a continuous range of speeds) and / or bypass arrangement 160 based upon the desired heat power output and / or desired water temperature. In order to provide central heating, the at least one controller 71 may operate the primary fluid circuit 20 to circulate fluid between the primary device 11 and the at least one radiator 22 for providing space heating (steps 402-404). In particular, the at least one - 19-controller 71 operates the primary pump 25 to circulate fluid heated by the primary device 11, such as fluid receiving heat in the heat exchanger 133, through the primary fluid circuit 20. The at least one controller 71 operates the diverter valve 32 so as to direct fluid to the central heating circuit 21 and not the secondary device 12. The at least one controller 71 may also open the valves 23, 24, 36, 37 and shut the valves 33, 34. Fluid is therefore substantially continuously circulated around the primary fluid circuit 20 and central heating circuit 21. The heat is then released in the at least one radiator 22. In order to charge the secondary device 12, the at least one controller 71 may operate the system 10 such that heated fluid in the primary fluid circuit 20 is circulated through the secondary device 12 for supplying heat thereto for storage (steps 402, 403, 405). The at least one controller 71 operates the primary pump 25 to circulate fluid heated by the primary device 11, such as fluid receiving heat in the heat exchanger 133, through the primary fluid circuit 20. The at least one controller 71 operates the diverter valve 32 so as to direct fluid to the secondary device 12 and not the central heating circuit 21. The at least one controller 71 may also open the valves 23, 24, 33, 34 and shut valves 36, 37. Fluid is therefore substantially continuously circulated around the primary fluid circuit 20 between the primary and secondary devices 11, 12. The secondary device 12 receives such heat and stores it for use, such as in a water cylinder or in a PCM battery. In order to supply both central heating and charging of the secondary device 12, the at least one controller 71 operates the diverter valve 32 so as to direct fluid to both the secondary device 12 and the central heating circuit 21 (steps 402-405). The at least one controller 71 may also open the valves 23, 24, 33, 34, 36, 37. Fluid is therefore substantially continuously circulated around the primary fluid circuit 20 between the primary device 11, the secondary device 12 and the central heating circuit 21. The secondary device 12 is therefore recharged and the at least one radiator 22 provides space heating. It will be appreciated that heat is provided to the primary fluid circuit 20 from the primary device 11, and the primary device may be charged, whilst central heating is provided and the secondary device 12 is charged. The secondary device 12 can also be charged or receive heat from the alternative charging device 12a separately or simultaneously to heat being received from the primary fluid circuit 20. The secondary device 12 provides heat to the water in the hot water circuit 50 when hot water is demanded from the hot water circuit 50 (step 406). In particular, a faucet or tap downstream of the hot water outlet 52 may be opened such that, due to water pressure from the mains supply at the cold water inlet 51, water flows through the hot water circuit 50. Heat is transferred from the secondary device 12 into the water in the hot water circuit 50 either passively (i.e. without initialisation by the at least one controller 71) or actively (i.e. with active operation of the secondary device 12 by the at least one controller 71). The at least one controller 71 may determine that hot water is demanded based upon flowrate -20-data received from the hot water circuit flowrate sensor Fi and / or temperature data from at least one downstream temperature sensor T2, T3, T4 indicating that water is flowing through the hot water circuit 50. The inlet valve unit 53 and / or mixing valve 58 may be open, whether due to the at least one controller 71 actively opening them upon initialisation of the hot water demand or due to the at least one controller 71 normally maintaining them open (i.e. when hot water is demanded and when it is not demanded). The at least one controller 71 may operate the flow control unit 61 so as to control the flow of water through the hot water circuit 50. Whilst hot water is demanded, the at least one controller 71 may operate the mixing valve 58 so as to mix cold water, which has not passed through the secondary device 12, with hot water, which has passed through the secondary device 12, so as to control the temperature of the hot water exiting the hot water outlet 52 (step 407). In the embodiment of Figure 1, such bypassed cold water passes through the cold water take-off line 59. In particular, the at least one controller 71 may receive or store a setpoint hot water temperature and determine a hot water temperature of hot water at or before the hot water outlet 52 based upon data received from the water outlet temperature sensor T4. The at least one controller 71 may compare the setpoint hot water temperature with the determined hot water temperature and, if the determined hot water temperature is above the setpoint hot water temperature, operate the mixing valve 58 so as to add cold water to the hot water and reduce the temperature of the hot water at or before the hot water outlet 52 to the setpoint hot water temperature. The setpoint hot water temperature may be set by a user via the user interface 84 of the personal computing device 82. When hot water is demanded, the at least one controller 71 may also operate the supplementary heater 55 to supply heat to the water downstream of the secondary device 12 in the hot water circuit 50 (step 408). In particular, the at least one controller 71 may receive or store a setpoint hot water temperature and may determine the downstream temperature of water exiting the secondary device 12, such as based upon data from at least one downstream temperature sensors T2, T3, T4. The at least one controller 71 may compare the downstream temperature and the setpoint temperature and, if the downstream temperature is below the setpoint temperature, operate the supplementary heater 55 to provide heat to the water in the hot water circuit 50. The at least one controller 71 may also determine the temperature of water exiting the supplementary heater 55, such as based upon data from at least one of the temperature sensors T3, T4. The at least one controller 71 may utilise the determined temperature of water exiting the supplementary heater 55 in a feedback loop to control the heat output of the supplementary heater 55. Thus, if the temperature of water exiting the secondary device 12 is below the setpoint temperature, the at least one controller 71 may - 21 -increase the heat output of the supplementary heater 55 until temperature of water exiting the secondary device 12 is at the setpoint temperature. The process may be substantially continuous and may comprise combinations of some or all of steps 400-408 operating simultaneously or non-simultaneously. As indicated at step 409, charging of the primary and secondary devices 11,12 may be continuously controlled and thus optimised, as set out below. Determining the state of charge of the secondary device The at least one controller 71 is configured to determine or receive the secondary state of charge of the secondary device 12. The secondary device 12 may comprise internal sensors and a controller 71 thereof may output a secondary state of charge value to other controllers 71, such as an external controller 71 or a controller 71 of the primary device 11. However, as described above in the Summary, the secondary state of charge value output by the secondary device 12 may not be suitable for control of the system 10. Thus the at least one controller 71 also determines the secondary state of charge of the secondary device 12 utilising components outside of the secondary device 12, particularly components of the primary fluid circuit 20 and hot water circuit 50. The secondary state of charge is determined via a joule counting method or algorithm, in which the heat released from, and stored in, the secondary device 12 is tracked over a time period such that a current secondary state of charge can be determined. The time period tracked may continuously change, such as by being the previous second, hour, or day of operation of the secondary device 12. The method, illustrated in Figure 5, of determining the secondary state of charge, as implemented by the at least one controller 71, generally comprises determining the secondary state of charge (SoC) based upon (step 506): • Determining the heat stored in the secondary device 12 at the start of a time period (Qo), which may be stored on the at least one controller 71 (particularly the memory thereof) and continuously updated as the secondary state of charge is continuously updated (steps 500, 501); • Determining the heat stored in the secondary device 12 (Qusefui heat,in) over the time period based upon data received from the primary sensor system 39, by determining the amount of heat received in the primary fluid circuit 20 from the primary device 11 over the time period (steps 502, 503); and • Determining the heat released from the secondary device 12 (Qusefui heat,out) over the time period based upon data received from the hot water sensor system, by determining the amount of heat added to the water in the hot water circuit 50 passing through the secondary device 12 over the time period (steps 504, 505). Such a determination may be illustrated by the following equation: Qi 1 SoC [kWh] = = —_ 3600 3600 heat,in + Q electric al,in) useful he at,out + Qheatloss) + Qo \ r0 / The heat released (Qusefui heat,out) is determined based upon data received from the hot water sensor system 54. In particular, the heat released may be determined based upon the data output from the water inlet temperature sensor Ti, from at least one downstream temperature sensor T2, T3, T4 located downstream of the secondary device 12 and / or from the hot water circuit flowrate sensor Fi. The amount of heat added to the hot water circuit 50 may be determined based upon flow rate data received from the hot water circuit flowrate sensor Fi, cold water temperature data received from the water inlet temperature sensor Ti and / or hot water temperature data received from the at least one temperature sensor T2, T3, T4downstream of the secondary device 12. The determination of the state of charge may also be based upon the heat loss (Qheatl0Ss) from the secondary device 12, which may be estimated. The amount of heat received in the primary fluid circuit 20 from the primary device 11 is determined based upon temperature and flowrate data received from return and / or flow temperature sensors TR, TF and at least one primary flowrate sensor Fz. The at least one controller 71 determines the heat received over a time period by determining the amount of heat added to the water passing through the heat exchanger 133 over the time period. This amount of heat added is determined based upon temperature and / or flowrate data received from the return and / or flow temperature sensors TR, TF and at least one primary flowrate sensor Fz. When the primary fluid circuit 20 provides heat to the secondary device 12, the heat stored in the secondary device 12 is determined based upon the heat received in the primary fluid circuit 20 from the primary device 11 and optionally based upon estimated heat losses between the primary and secondary devices 11, 12. However, when the primary fluid circuit 20 supplies heat to both the central heating circuit 21 and the secondary device 12, some heat will be extracted in the central heating circuit 21 and some in the secondary device 13. Therefore, the heat stored in the secondary device 12 may also be determined based upon the proportion or amount of fluid directed to the secondary device 12 by the diverter valve 32 over the time period. The at least one controller 71 may thus monitor the status of the diverter valve 32, particularly the ratio of fluid directed between the secondary device 12 and central heating circuit 21. If the diverter valve 32 comprises an open / closed diverter valve 32 directing all fluid to the central heating circuit 21 or to the secondary device 12, the at least one controller 71 may monitor the amount of time the diverter valve 32 spends in each of its two possible statuses. The at least one controller 71 may also determine the heat stored in the secondary device 12 over the time period based upon the energy supplied by the alternative charging device 12a to the secondary device 12 over the time period electrical,™) ■ The energy supplied by the alternative charging device 12a may be determined based upon the monitoring of electrical power consumed by the alternative charging device 12a whilst it is charging the secondary device 12. The primary device 11 may be subsequently operated based upon the determined state of charge of the secondary device 12 (step 507), as discussed hereinbelow. Timina and initiation of charging of secondary device The secondary state of charge reduces from a maximum secondary state of charge of the secondary device 12 when heat is drawn out of the secondary device 12 as water flows around the hot water circuit 50 when demanded. The present disclosure therefore provides improved and efficient operations of the at least one controller 71 and methods for returning the secondary device 12 to its maximum secondary state of charge. The at least one controller 71 may determine whether to charge the secondary device 12 based upon the determined secondary state of charge. In particular, if the at least one controller 71 determines that the secondary state of charge has fallen below a secondary state of charge threshold, the at least one controller 71 may charge the secondary device 12 using heat directed from the primary device 11 via the primary fluid circuit and / or by the alternative charging device 12a. The at least one controller 71 may charge the secondary device 12 by determining whether hot water has been demanded in the hot water circuit 50, as illustrated in Figure 6. In particular, the at least one controller receives hot water demand data from the hot water sensor system 54 (step 601), determines from the hot water demand data if water demand in the hot water circuit 50 has been initialised (step 602) and, if initialised, operates the primary fluid circuit 20, such as by operating the diverter valve 32 and / or primary pump 25 accordingly, to transfer heated fluid from the primary device 11 to the secondary device 12 (step 603). In other words, the initialisation of hot water demand results in an immediate response of operating the primary fluid circuit 20 to charge the secondary device using heat from the primary device. Therefore, charging of the secondary device 12 occurs immediately after an initial demand of hot water and the secondary device 12 is maintained at or within a predetermined range of the maximum secondary state of charge (step 604). Standing losses of the system 10 can therefore be reduced, as discussed in the Summary. The hot water demand data is indicative of water being demanded from the hot water circuit 50. Flow through the hot water circuit 50 occurs due to water pressure at the cold water inlet 51 and such flow is apparent in the water demand data generated from the hot water sensor system 54. In particular, the at least one controller 71 may receive hot water flowrate data from the water circuit flowrate sensor Fi and determine that water demand has initialised based upon the hot water flowrate exceeding a hot water flowrate threshold. The at least one controller may also receive water outlet temperature data from the at least one downstream temperature sensor T2, T3, T4and determine that water demand has initialised based upon the water outlet temperature exceeding a water outlet temperature threshold. Since the water flow will result in heat being extracted from the secondary device 12, the temperature of the water in the hot water circuit 50 will increase and this increase is detected via the at least one downstream temperature sensor T2, T3, T4. Once water demand in the hot water circuit 50 has been initialised such that the secondary device 12 is to be / is being recharged using the heat from the primary core 100, the at least one controller 71 selectively operates the primary device 11 to release heat into the primary fluid circuit 20 and operates the primary pump 25 and / or diverter valve 32 to direct the heated fluid to the secondary device 12. Otherwise, if the primary device 11 is already providing heat to the central heating circuit 21, the diverter valve 32 may be operated to start directing the heat to the secondary device 12 in order to recharge the secondary device 12. The primary device 11 may not charge the secondary device 12 if a primary state of charge of the primary device 11 is too low to provide sufficient heat to the secondary device 12 and / or if the heat stored in the primary device 11 is to be maintained for heating the central heating circuit 21. The primary state of charge of the primary device 11 may be determined by estimating the amount of heat provided to the primary device 11 over a time period, such as by power metering the power supplied to the at least one heating element 15a, 15b, 15c, as compared to estimating the amount of heat released from the primary device 11 over the time period, such as using the data from the at least one device temperature sensor TR, TF and / or at least one primary flowrate sensor Fz. The at least one controller 71 may not charge the secondary device 12 using the primary device 11 and may charge the secondary device 12 by the alternative charging device 12a if the primary state of charge is below a primary state of charge lower threshold. The primary state of charge lower threshold may be that at which the primary device 11 has insufficient heat to supply to charge the secondary device 12 or has insufficient heat to meet an anticipated upcoming heat demand by the central heating circuit 21. Charging of the secondary device 12 may also be repeatedly started and stopped to provide time for the heat to dissipate therein during charging (step 605). Therefore, the at least one controller 71 may be configured to (a) operate the primary fluid circuit 20 to transfer heated fluid from the primary device 11 to the secondary device 12 and / or operate the alternative charging device 12a to charge the secondary device 12, (b) operate the primary fluid circuit 20 to stop transferring heated fluid from primary device 11 to the -25-secondary device 12 and / or stop operating the alternative charging device 12a and (c) after a repeat time period, again operating the primary fluid circuit to transfer heated fluid from the primary device 11 to the secondary device 12. Step (b) may occur once the at least one controller 71 has made a preliminary (and possibly incorrect) determination that the secondary state of charge has reached the maximum secondary state of charge. Steps (a) to (c) may be repeated a predetermined number of times and / or may be repeated until the at least one controller 71 has determined that the secondary state of charge has definitely reached the maximum secondary state of charge, such as via the return temperature method discussed below. Charging of the secondary device 12 by the direction of flow thereto from the primary device 11 may also not start until the at least one controller 71 has determined that the temperature in the primary fluid circuit 20 is high enough to supply sufficient heat to charge the secondary device 12. Thus the at least one controller 71 may be configured to determine a temperature of fluid in the primary fluid circuit 20 based upon data received from the primary sensor system 39. In particular, such temperature may be determined from the primary temperature sensor TD, since this measurement accounts for any heat losses in the pipework resulting from the primary device 11 being located at a substantial distance from the primary pump 25 and secondary device 12. By having the primary temperature sensor Td located upstream of the automatic bypass valve 30, there is always flow past the primary temperature sensor Td even when there is no flow in the central heating circuit 21. This allows the temperature of the fluid in the primary fluid circuit 20 to be detected even when there is no space heating demand. The primary fluid circuit 20, particularly the diverter valve 32 thereof, is operated to transfer heated fluid from the primary device 11 to the secondary device 12 if the temperature of fluid in the primary fluid circuit 20 exceeds a heat-preserving temperature. During or prior to the temperature determination, the at least one controller 71 may operate the primary pump 25 to circulate fluid around the primary fluid circuit 20 so as to distribute heat therearound and allow heat to be released into the fluid by the primary device 11. Ceasing charging of the secondary device The at least one controller 71 may stop charging of the secondary device 12 (step 606 in Figure 6) by the primary device 11 and / or alternative charging device 12a once it has determined that the secondary state of charge has reached the maximum secondary state of charge, being the maximum heat capacity of the secondary device 12. Alternatively or additionally, the at least one controller 71 may stop charging of the secondary device 12 by the primary device 11 by determining if the temperature of fluid entering the primary device 11 is the same or close to the temperature of fluid leaving the primary device 11. The at least one controller 71 is configured to receive return and flow temperature data from the return and flow temperature sensors TR, TF respectively and operate the primary fluid circuit 20, such as the primary pump 25 and / or diverter valve 32, to stop transferring heated fluid from the primary device 11 to the secondary device 12 if the return temperature is within a predetermined range of the flow temperature. Charge Optimisation over a longer time period The charging of the primary and secondary devices 11,12 can also be optimised over a longer time period, such as over days, months, and seasons. Furthermore, the charging can be optimised to take account of off-peak periods, when grid demand is lower and there can be excess power capacity (and electricity costs are lower for the consumer, typically at night), and peak periods, in which the demand for electricity is higher (and electricity costs are higher for the consumer, typically in the day). In embodiments, the at least one controller 71 may charge the primary device 11 during the off-peak period(s) and not during the peak period(s), preferably because the primary device 11 has such a high heat storage capacity. The at least one controller 71 may then charge via heat transfer the secondary device 12 a plurality of times during the peak period(s) so as to have sufficient charge for supplying hot water, due to the secondary device 12 having a lower heat storage capacity than the primary device 11. The at least one controller 71 may implement one or more different modes 701-706 indicative of the total heat demand of the system 10, as illustrated in Figure 7. The order illustrated in Figure 7 may be the merit order between the different modes. The merit order may generally be: Using excess heat from the primary device 11 to heat the hot water circuit 50 (mode 701), the excess heat being that which is not reserved for central heating demands e.g. in a primary high power input mode 704; - Off-peak charging of the secondary device 12 when that is sufficient for anticipated hot water demand in an upcoming time period (mode 702); - Off-peak charging of the primary device 11 when required to meet anticipated hot water and / or space heating demand in an upcoming time period (mode 703); Limiting transfer of heat from the primary device 11 to the secondary device 12 when anticipated space heating demand in an upcoming time period is relatively high, such that the primary device 11 needs to store a certain amount of heat to meet such demand (mode 704) Peak charging of the secondary device 12 when off-peak charging is insufficient to meet hot water demand (mode 705); and / or Peak charging of the primary device 11 when off-peak charging and secondary peak charging is insufficient to meet space heating and / or hot water demand (mode 706). The mode 701-706 determination disclosed herein may be made based upon a predicted heat demand of the system 10 (i.e. hot water and / or central space heating) in an upcoming time, peak and / or off-peak period. The predicted heat demand may be determined from historic heat demands monitored and stored by the at least one controller 71. The at least one controller 71 may determine the total charging heat obtainable from a known duration and power input to the system 10 during the off-peak period(s). In a primary low power input mode (701), which may be the default operation by the at least one controller 71, storing heat for hot water is prioritised over space heating and excess heat (i.e. heat not anticipated to be reserved for space heating) from the primary device 11 is transferred to the secondary device 12. The secondary device 12 is retained at full charge or other specified target threshold charge. In particular, the at least one controller 71 operates the primary fluid circuit 20 and primary device 11 to direct heat from the primary core 100 to the secondary device 12 when the secondary state of charge of the secondary device 12 is below a maximum secondary state of charge or other predetermined secondary state of charge. Such charging may be initialised via any of the methods described above, such as by determining the secondary state of charge or based upon a detected water demand. The fluid or part thereof is not directed to the central heating circuit 21. A secondary lower power input mode (702) may be implemented in place of the primary lower power input mode if the hot water demand is predicted to be relatively low in an upcoming time period, and the hot water demand cannot be satisfied in the primary low power input mode. In particular, this mode may be implemented if predicted hot water demand over an upcoming time period is anticipated to be less than or equal to the useful storage capacity of the secondary device 12, or less than or equal to the charge obtainable from available off-peak period(s). In this mode, the alternative charging device 12a is operated to charge the secondary device 12 during off-peak periods. This avoids heat losses resulting from the transfer of heat from the primary to the secondary device 11, 12. A secondary high power input mode (703) may be implemented if the conditions for the secondary lower power input mode are not met, i.e. the hot water demand over an upcoming time period is anticipated to be relatively high. Both of the primary and secondary devices 11,12 charge in the available off-peak period(s) to satisfy predicted hot water demand, maximising use of available off-peak electricity for providing hot water. In particular, the primary core 100 is heated by the at least one heating element 15a, 15b, 15c whilst the alternative charging device 12a charges the secondary device 12. A primary high power input mode (704) may be implemented if the space heating demand is predicted to be relatively high in an upcoming time period. In particular, the mode may be implemented if a predicted space heating demand for an upcoming time period exceeds the useful capacity of the primary device 11, which may be the available capacity of the primary device 11, less any reserved capacity for charging the secondary device 12 for hot water demand, and / or the predicted space heating demand for an upcoming time period exceeds the charge of the primary device 11 obtainable from available off-peak period(s). In this mode, the primary device 11 may only charge the secondary device 12 to a minimum state of charge threshold, which may be user controllable. The primary and secondary devices 11, 12 are charged in off-peak periods. Therefore, stored heat may be retained in the primary device 11 and available charge power across both devices 11,12 may be maximised during the off-peak period(s). A secondary peak charging mode (705) may be implemented if hot water demand is sufficiently high, such that off-peak charging is insufficient to meet demand, and is not met by any of the preceding modes. In particular, the alternative charging device 12a is operated to charge the secondary device 12 in peak periods, such that sufficient heat can be provided as necessary to the hot water circuit 50. A primary peak charging mode (706) may be implemented if hot water and / or space heating demand is sufficiently high, such that off-peak charging and secondary peak charging is insufficient to meet demand, and is not met by any of the preceding modes. In particular, the primary device 11 is charged by the primary core 100 being heated by the at least one heating element 15a, 15b, 15c in peak periods. In addition, the system 10 may be operable so as to avoid discharging all heat from the primary device 11 when it is not efficient to do so. In particular, in a high power input mode the at least one controller 71 may be operable to implement a charge profile over successive peak and off-peak periods. In the charge profile, during and towards the end of the or each peak period, referred to as an end period and for example being the final 25% of the peak period, the at least one controller 71 may operate the primary fluid circuit 20 so as to prevent heat being transferred from the primary device 11 to the secondary device 12 even if the secondary state of charge is below a maximum secondary state of charge (step 710). The end period may be determined based on a calculation of charging time required for the secondary device 12 (for example based on the heat required [kWh] divided by the input charge power available [kW]) as a proportion of the peak period. The at least one controller 71 may do so based upon a command received at the user interface 84 of the personal computing device 82. Therefore, the primary device 11 may not discharge all of its heat into the secondary device 12 just before the off-peak period starts. Subsequently, during the off-peak period, the at least one controller 71 charges the primary core 100 and operates the alternative charging device 12a to charge the secondary device 12, thereby -29-allowing maximum use of power charging capacity during the off-peak period. During the off-peak period, the at least one controller 71 may operate the primary fluid circuit 20 to prevent heat being transferred from the primary device 11 to the secondary device 12, such as by not running the primary pump 25. By setting the minimum state of charge threshold, which may be user-controllable, of the secondary device 12, the primary device 11 only needs to be charged sufficiently to meet this minimum state of charge threshold. This may be implemented particularly if anticipated hot water demand in an extended time period is low or unlikely. This allows for the overall standing loss to be minimised further, since less energy is stored in the primary device 11 during the upcoming time period for charging the secondary device 12. In embodiments, particularly in the case of dynamic tariffs (or other dynamic signal to indicate spare / constrained capacity on the grid) the at least one controller 71 operates to optimise charging of the primary and secondary devices 11,12 respectively (electrically and / or by heat transfer) according to an optimisation algorithm using as inputs the tariff (or other grid capacity / carbon intensity signal); the predicted heat and hot water demand profiles; the maximum state of charge of the devices 11, 12; the heat loss of the devices 11,12 depending on state of charge (which may be estimated or modelled); pipework heat loss for transferring heat between the devices 11,12 (which may be estimated or modelled); the power capacity of the devices 11, 12; import capacity limit of the property in which the system 10 is installed; self-generated solar PV predicted excess generation where installed; and / or minimum state of charge requirements of the devices 11, 12. Over a long time period of several days or more, the determination of the state of charge of the secondary device 12 can be used to control charging of the primary device 11 by ensuring that the primary device 11 has enough heat stored therein for charging the secondary device 12 when necessary. This may be particularly useful in the secondary high power input mode when the heat in the primary device 11 is used to supplement charging of the secondary device 12. The at least one controller 71 therefore monitors the determined secondary state of charge over a first extended time period, which may be 24 or fewer hours and / or the peak period. It then determines the heat consumed by the secondary device 12 and / or hot water circuit 50 in the first extended time period based upon such monitoring. The primary device 11 is then recharged based upon the determined heat consumption via the at least one heating element 15a, 15b, 15c. The recharging may occur after expiry of the first extended time period and / or in the off-peak period(s). The recharging may be such that the primary core 100 stores an amount of heat based upon the heat consumed by the secondary device 12 and / or hot water circuit 50 in the first extended time period. In particular, the amount of heat stored may be based upon (a) at least the amount of heat consumed by the secondary device 12 and / or hot water circuit 50 in the first extended time period, aiming to ensure that there is sufficient heat stored in the primary device 11 for a subsequent extended time period. The recharging may be based upon (b) the primary core 100 storing an amount of heat up to a predetermined level above the amount of heat consumed by the secondary device 12 and / or hot water circuit 50 in the first extended time period, aiming to provide some leeway for the subsequent extended time period having a higher than anticipated heat consumption. The amount of heat stored may account for the heat stored in the secondary device 12 such that the heat stored in the primary device 11 is less than (a) and (b). In particular, the amount of heat stored in the primary device 11 may be determined by subtracting, from (a) or (b), an amount of heat equivalent to the heat storage capacity of the secondary device 12 (i.e. the maximum secondary state of charge), or an amount of heat that the secondary device 12 is to be charged directly via its alternative charging device 12a. In embodiments, the at least one controller 71 may charge the primary device 11 only towards the end of the off-peak period, such as within the final 25% of the off-peak period or start charging the primary device 11 during the off-peak period after the start thereof such that the primary device 11 reaches full charge at the end of the off-peak period. This minimises standing losses, as opposed to charging from the start of the off-peak period and unnecessarily maintaining a high state of charge. Operation of the supplementary heater The at least one controller 71 may operate the supplementary heater 55 to heat water in the hot water circuit 50 when such additional heating is required. In particular, the supplementary heater 55 may be operated if the hot water demand exceeds the available heat power output of the secondary device and / or if the primary state of charge is below a primary state of charge lower threshold and the secondary state of charge is below a secondary state of charge lower threshold, each threshold indicating a minimum or depleted state of charge of the respective primary and secondary devices 11, 12. However, the supplementary heater 55 may consume relatively high amounts of power at peak times and thus may have high carbon usage, also increasing costs for the consumer. Therefore, in the present disclosure, a user may be able to enter commands on the user interface 84 so as to control when the supplementary heater 55 operates. The at least one controller 71 is therefore configured to monitor a downstream temperature based upon data received from the at least one downstream temperature sensor T2, T3, T4. The supplementary heater 55 is operated to supply heat to the hot water circuit 50 if the downstream temperature is below a setpoint temperature for a delay response time period. The setpoint temperature is the temperature demanded of the hot water circuit 50 and may be set via a command at the user interface 84. Thus, the supplementary heater 55 only operates if the temperature has been insufficient for a reasonable period of time, namely the response time period, to account for heat to be otherwise generated and circulated around the system 10. In particular, during the delay response time period, heat is supplied from the secondary device 12 to the hot water circuit 50. Thus, the delay response time period allows for the secondary device 12 to start working and start supplying heat to the hot water circuit 50 before the supplementary heater 55 starts operating. The delay response time period may be set to be sufficiently long that the secondary device receives heat from and is charged by the primary device before the supplementary heater 55 can operate. In particular, if the delay response time period is above a delay response threshold, the at least one controller 71 operates, during the delay response time period, the primary device 11 and primary fluid circuit 20 so as to supply heat from the primary core 100 to the secondary device 12. The delay response time period may also be set to be very short or zero so that heat can be supplied from the supplementary heater 55 immediately, such as if a user wants hot water to be delivered at sufficient temperature immediately without waiting for delays in the heat release and charging of the secondary device 12. Therefore, if the delay response time period is below a delay response threshold, the at least one controller 71 operates the supplementary heater 55 before or at the same time as operating the primary device 11 and primary fluid circuit 20 to supply heat from the primary core 100 to the secondary device 12. The at least one controller 71 is configured to receive the delay response time period from the external computing system 80. The external computing system 80 is configured to receive the delay response time period as a user input, such as to the user interface 84 of the personal computing device 82. Operation by a user on the application of the personal computing device As will be appreciated from the above, the user can control the operation of the system 10 by entering commands to the personal computing device 82. The user may be able to set all aforementioned setpoints and thresholds so as to control the system 10 as required. Furthermore, output from all sensors of the system 10 and control status of all components may be communicated to the application for viewing by the user on the personal computing device 82. As a result, the user can operate the system 10 within a single operating environment. In a particular embodiment, in which the system 10 is as in Figure 3 with an active modulating valve 58, optionally located in the cold water take-off line 59, the user can control the temperature of hot water leaving the hot water outlet 52. The at least one controller 71 may thus be configured to receive hot water demand data, including a setpoint temperature, and operate the active modulating valve 58 to control the temperature of water exiting through the hot water outlet 52. In particular, the setpoint temperature may be set by a user on the personal computing device 82, such as via an application thereof. For example, if the active modulating valve 58 comprises a 2-port valve located in the cold water take-off line 59, the at least one controller 71 may modulate the opening of the valve 58 so as to control the flow of cold water into the hot water prior to such water exiting the hot water outlet 52. The at least one controller 71 may control the valve 58 based upon the at least one downstream temperature sensor T2, T3, T4. Combination of operations As illustrated in Figure 8, the different operations disclosed herein may be implemented in the same system 10 and method in order to optimise the operation of the system 10 and particularly of the devices 11, 12. Therefore, in order to achieve such optimisation (step 801), the at least one controller 71 is configured to perform one or more of the following steps, as detailed hereinabove: • Initiate charging of the secondary device 12 from the primary device 11 as soon as hot water is demanded (802); • Control charging of the primary device 11 based upon state of charge of the secondary device 12 (803); • Only charge the secondary device 12 from the primary fluid circuit 20 if the temperature therein is above a heat-preserving temperature (804); • Stop charging the secondary device 12 if a return temperature to the primary device 11 is within a predetermined range of a flow temperature from the primary device (805); • Determine the state of charge of the secondary device 12 using external sensors (806); • Operate the system in different modes depending upon whether there is high or low power input requirement during off-peak or peak periods (807); • Repeatedly stop and restart charging of secondary device 12 to allow heat to dissipate (808); and / or • Operate the supplementary heater 55 based upon a user definable delay response time period (809).

Claims

1. A central heating and hot water delivery system comprising:a hot water circuit extending between a cold water inlet and a hot water outlet;a primary device configured to store heat in a primary core;a secondary device configured to store heat therein and to supply heat to the hot water circuit between the cold water inlet and the hot water outlet;a primary fluid circuit configured to selectively circulate fluid between the primary and secondary devices such that heat from the primary core is transferred to the secondary device; andat least one controller configured to:determine a secondary state of charge of the secondary device;monitor the determined secondary state of charge over a first extended time period;determine the heat consumed by the secondary device and / or hot water circuit in the first extended time period; andoperate the primary device such that the primary core stores an amount of heat based upon the heat consumed by the secondary device and / or hot water circuit in the first extended time period.

2. The system of claim 1 wherein the amount of heat stored in the primary core is based upon:(a) at least the determined amount of heat consumed by the secondary device and / or hot water circuit in the first extended time period;(b) an amount of heat up to a predetermined level above the determined amount of heat consumed by the secondary device and / or hot water circuit in the first extended time period; and / or(c) an amount of heat equivalent to the heat storage capacity of the secondary device, or an amount of heat that the secondary device is to be supplied separately to from the primary device.

3. The system of any preceding claim wherein the hot water circuit comprises a hot water sensor system located between a cold water inlet and a hot water outlet, and the primary fluid circuit comprises a primary sensor system, wherein the secondary state of charge is determined based upon data received from the hot water sensor system and / or primary sensor system.

4. The system of claim 3 wherein the hot water sensor system is located in the hot water circuit separately to the secondary device and the primary sensor system is located in the primary fluid circuit separately to the secondary device.

5. The system of claim 3 or claim 4 wherein the at least one controller is configured to determine the secondary state of charge by:determining the heat stored in the secondary device at the start of a first time period;determining the heat stored in the secondary device over the first time period based upon data received from the primary sensor system;determining the heat released from the secondary device over the first time period based upon data received from the hot water sensor system; anddetermining the secondary state of charge of the secondary device based upon the heat stored in the secondary device at the start of the first time period, the heat stored in the secondary device over the first time period and the heat released from secondary device over the first time period.

6. The system of claim 5 wherein the at least one controller is configured to, based upon the determined state of charge, operate the primary device to release heat into the primary fluid circuit and operate the primary fluid circuit to direct heat to the secondary device for charging the secondary device.

7. The system of claim 5 or claim 6 wherein secondary device comprises an alternative charging device for storing heat in the secondary device and the at least one controller is configured to determine the heat stored in the secondary device over the time period also based upon the heat added by the alternative charging device to the secondary device over the time period.

8. The system of any one of claims 3 to 7 wherein the primary sensor system comprises: a primary flowrate sensor for determining a flowrate of fluid through the primary fluid circuit;a return temperature sensor for determining a return temperature of fluid returning to the primary device from the secondary device; and / ora flow temperature sensor for determining a flow temperature of fluid flowing from the primary device to the secondary device, wherein:the primary flowrate sensor, flow temperature sensor and / or return temperature sensor are located in the primary fluid circuit separately to the secondary device; andthe at least one controller is optionally configured to determine the heat stored in the secondary device over the time period based upon data received from the flow temperature sensor, return temperature sensor and / or flow temperature sensor.

9. The system of any one of claims 3 to 8 wherein the primary sensor system is at least partially located within and / or is part of the primary device.

10. The system of any one of claims 3 to 9 wherein the hot water sensor system comprises:a hot water circuit flowrate sensor for determining a flowrate of water through the hot water circuit;a water inlet temperature sensor for determining a water inlet temperature of water flowing into the hot water circuit; and / ora downstream temperature sensor located downstream of the secondary device for determining a downstream temperature of water exiting the secondary device, wherein: the hot water circuit flowrate sensor, water inlet temperature sensor and downstream temperature sensor are located in the hot water circuit separately to the secondary device; andthe at least one controller is optionally configured to determine the heat released from the secondary device over the time period based upon data received from the hot water circuit flowrate sensor, water inlet temperature sensor and / or downstream temperature sensor.

11. The system of any one of claims 5 to 10 wherein the primary fluid circuit comprises a diverter valve for selectively directing fluid from the primary device to at least one of the secondary device and a central heating circuit, wherein the at least one controller is configured to determine the heat stored in the secondary device based upon the proportion or amount of fluid directed to the secondary device by the diverter valve over the time period.

12. The system of claim 11 wherein the proportion of fluid is determined by monitoring the status of the diverter valve over the time period.

13. The system of any preceding claim wherein the at least one controller is configured to: operate in a primary lower power input mode, by default, in which heat is transferred from the primary device to the secondary device such that storing of heat for the hot water circuit is prioritised;operate in a secondary lower power input mode, if predicted hot water demand over an upcoming time period is anticipated to be less than or equal to the useful storage capacity of the secondary device, or less than or equal to the charge obtainable from available off-peak period(s), in which the alternative charging device is operated to charge the secondary device during off-peak period(s);operate in a secondary high power input mode, if predicted hot water demand over an upcoming period is anticipated to be more than the useful storage capacity of the secondary device, or more than the charge obtainable from available off-peak period(s), in which the primary core is charged and the alternative charging device charges the secondary device;operate in a primary high power input mode, if a predicted space heating demand for an upcoming period exceeds a useful storage capacity of the primary device, in which the primary device charges the secondary device to a minimum state of charge threshold, such that storage of heat in the primary device is prioritised over transfer of such heat to the secondary device;operate in a secondary peak charging mode in which the alternative charging device is operated to charge the secondary device during peak period(s); and / oroperate in a primary peak charging mode in which the primary core is heated during peak period(s).

14. The system of any one of claims 3 to 13 wherein the at least one controller is configured to:determine a temperature of fluid in the primary fluid circuit based upon data received from the primary sensor system; andif water demand has been initialised, operate the primary fluid circuit to transfer heated fluid from the primary device to the secondary device if the temperature of fluid in the primary fluid circuit exceeds a heat-preserving temperature.

15. The system of any one of claims 3 to 14 wherein the primary sensor system comprises: a return temperature sensor upstream of the primary core between the secondary device and the primary core; anda flow temperature sensor downstream of the primary core between the primary core and the secondary device,wherein the at least one controller is configured to:receive return and flow temperature data from the return and flow temperature sensors respectively; andoperate the primary fluid circuit to stop transferring heated fluid from the primary device to the secondary device if the return temperature data is within a predetermined range of the flow temperature data.

16. The system of any preceding claim wherein the at least one controller is configured to: operate the primary fluid circuit to transfer heated fluid from the primary device to the secondary device;operate the primary fluid circuit to stop transferring heated fluid from primary device to the secondary device, optionally based upon a preliminary determination that a secondary state of charge of the secondary device has reached a maximum charge; andafter a repeat time period, again operate the primary fluid circuit to transfer heated fluid from the primary device to the secondary device.

17. The system of any preceding claim wherein the primary device is an electrical storage boiler storing heat in the primary core, the electrical storage boiler comprising a heat exchanger and the primary fluid circuit is configured to selectively circulate fluid between the heat exchanger of the electrical storage boiler and the secondary device such that heat from the primary core can be transferred to the secondary device.

18. The system of any preceding claim wherein the primary device comprises at least one heating element for supplying heat to the primary core, wherein the at least one controller is configured to operate the primary device such that the primary core stores heat by operating the at least one heating element to supply heat to the primary core.

19. The system of any preceding claim wherein the hot water circuit comprises a supplementary heater located in the hot water circuit between the secondary device and the hot water outlet, wherein the at least one controller is configured to:receive a delay response time period;receive a setpoint temperature indicative of a demanded temperature of hot water exiting the hot water outlet;operate the secondary device to supply heat to the hot water circuit based upon the setpoint temperature; andoperate the supplementary heater to supply heat to the hot water circuit to achieve the setpoint temperature, after expiry of the delay response time period starting from when the secondary device is initially operated to supply heat to the hot water circuit.

20. A method of operating the central heating and hot water delivery system of any preceding claim, the method comprising, by the controller:determining the secondary state of charge;monitoring the determined secondary state of charge over a first extended time period;determining the heat consumed by the secondary device and / or hot water circuit in5 the first extended time period; andoperating the primary device such that the primary core stores an amount of heat based upon the heat consumed by the secondary device and / or hot water circuit in the first extended time period.

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