Hybrid heating controller

Abstract

A central heating system 10 comprises an emitter circuit 50 circulating water that is heated either by a heat pump 30 (e.g. air source heat pump) or a boiler 40 (e.g. combination boiler). A hybrid controller determines whether the heat pump or boiler heats water based upon at least one of a fluid temperature and a delay timing. Ideally a valve 110 is actuated by the controller to interconnect a boiler circuit 42,44 or a heat pump circuit 32,34 with the emitter circuit. When selecting based on fluid temperature, a return water temperature sensed by a return water temperature sensor 112 may be used to determine whether the boiler or heat pump heats water. When selecting based on a delay timing it may be that the heat pump is initially activated to heat water and the heat pump remains selected as the initial heater for a predetermined time period, e.g. at least ten minutes.

Description

TECHNOLOGICAL FIELD Examples of the disclosure relate to a hybrid heating controller. Some relate to a system comprising the hybrid heating controller, a combination boiler, and an air source heat pump. BACKGROUND A central heating system for a building generally comprises a heater such as a boiler or a heat pump, one or more room emitters such as room radiators or ‘in floor’ heating pipes (underfloor heating), and a room thermostat (or multiple thermostats for zoned heating control) with a setpoint control and an ambient temperature sensor. Heating demand is controlled by the room thermostat. The room thermostat is usually in the form of a setpoint controller that compares a sensed room air temperature with a setpoint input by the user. One type of heater is an air source heat pump (ASHP), and another type of heater is a combination (‘combi’) boiler. A boiler that combines central heating with domestic hot water is referred to as a combination boiler. A combination boiler is operable in either central heating mode or in hot water mode. While one mode is active, the other mode is inactive. Hot water mode is always priority. ASHPs consume electricity whereas boilers consume natural gas. At peak efficiency, an ASHP is 300-400% efficient whereas a boiler is 90-95% efficient. However, the maximum coefficient of performance (COP) of an ASHP worsens with reducing ambient air temperatures outside the building. Therefore, small low-powered ASHPs cannot efficiently serve all-year heating demand such as peak evening demand during cold winter days. Most ASHPs are oversized to meet worst case scenarios. Financial and space constraints mean that it may not be practical to implement a large high-powered ASHP with an optional thermal store (hot water tank) and larger surface area radiators. Therefore, hybrid systems have been devised where an ASHP and a boiler are selectively coupled to the same emitters (radiators) by a hybrid heating controller and which may be hydraulically separated by a plate heat exchanger to maintain a barrier to prevent mixing of the temperature outputs of the heat sources. The hybrid heating controller controls heater selection based on various inputs. A ‘switch’ mode of a general hybrid heating controller relies on an external temperature setpoint, such as the ambient external air temperature outside the building. The boiler takes over when the sensed ambient external air temperature is too cold for the heating demand to be efficiently met by the ASHP. At such cold temperatures, the ASHP would have a low COP resulting in high electricity consumption for limited heating, and may not be capable of meeting the expected comfort levels of room heating. Use of the boiler minimises annoyance to the occupants, and the boiler may even operate more efficiently than the ASHP if the ambient external air temperature is very low. Some hybrid heating controllers rely on real-time weather forecasts of external temperature and solar gain, to influence source selection. Some hybrid heating controllers can perform fuel cost optimisation by using real-time electricity and gas price tariffs to influence source selection. BRIEF SUMMARY According to various, but not necessarily all examples of the disclosure there is provided a hybrid heating controller comprising means for controlling heater selection between a heat pump and a boiler for heating fluid for an emitter circuit, in dependence on temperature sensing of the fluid and / or in dependence on delay timing. An advantage is enabling heater selection for greater thermal efficiency. An advantage of delay timing is compensating for system inconsistencies which may incur short cycling or interfacing of the two heaters. An ASHP for example takes 10-15 minutes to reach its peak COP. Therefore, introducing delay timing enables the ASHP to reach a higher COP before a next switchover. The delay timing also enables temporary shortterm fluctuations in performance to be ignored, such as the combination boiler switching from central heating mode to water heating mode. An advantage of sensing the temperature of the fluid is that this is provides closed loop feedback directly indicating the actual performance of the heater. Therefore, if the heat pump is failing to satisfy heating demand, the hybrid heating controller can accurately detect this and switch over to the boiler. The hybrid system and hybrid heating controller together enable at least some of the following advantages: Suitability for older type homes with higher heating requirements; Maintaining expected comfort in very cold weather; In normal heating season conditions, using the energy efficient ASHP; Reducing installation costs of the ASHP; Less intrusive installation with less inconvenience to the home occupier; and Accelerating the market transition away from fossil fuels. The means may consist of or comprise an electronic controller. The electronic controller may comprise at least one processor; and at least one memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause an apparatus to perform any one or more of the functions described herein. The apparatus may comprise: a heat pump inlet to receive heated fluid from the heat pump; a boiler inlet to receive heated fluid from the boiler; an optional recirculation inlet to receive return fluid from the emitter circuit; an emitter outlet to provide heated fluid to the emitter circuit, wherein the recirculation inlet is fluidly coupled to the emitter outlet; a valve apparatus having at least a heat pump-connected state in which the heat pump inlet is fluidly coupled to the emitter outlet, and a boiler-connected state in which the boiler inlet is fluidly coupled to the emitter outlet; and the electronic controller to control the valve apparatus in dependence on the temperature sensing and / or the delay timing; The apparatus may comprise a temperature sensor, wherein the temperature sensing is based on information from the temperature sensor. The apparatus may comprise an isolation valve to control fluid flow through the recirculation inlet. The isolation valve may comprise an isolation check valve to control a fluid flow direction through the recirculation inlet. The isolation check valve may permit flow towards the emitter outlet. In some examples, the hybrid heating controller may comprise the apparatus. Alternatively, the hybrid heating controller may consist of an electronic controller and the apparatus may be remote. The temperature sensing referred to above may comprise return temperature sensing of the fluid returning from the emitter circuit. The return temperature sensing may indicate a temperature of return fluid after the fluid has passed through the emitter or emitters along the emitter circuit. The delay timing may control when the hybrid heating controller is permitted to switch heaters. Controlling the heater selection may be dependent on both the temperature sensing and on the delay timing. For example, the delay timing may control when the hybrid heating controller is permitted to switch heaters in dependence on the temperature sensing, such as the return temperature sensing. An advantage is that heating efficiency is improved. The heat pump is allowed to reach or approach its peak COP before any inferences are made about whether it is satisfying heating demand. In addition, the return temperature of the fluid in the emitter circuit is given time to reach or approach a steady state value before it is used as an evaluator of the effectiveness of the heat pump to satisfy heating demand. A condition for controlling heater selection may be dependent on an in-service controllable parameter. The parameter may be a water temperature setpoint or allowable range. An advantage is that conditions can be tailored for individual installations. The parameter may be a return water temperature setpoint value. The means may be configured to select the heat pump as an initial heater in dependence on initialisation of fluid heating. The delay timing may comprise an initialisation delay timer to control how long the heat pump remains selected as the initial heater. Wherein the initialisation delay timer may have a duration of at least 10 minutes. An advantage is that heating efficiency is improved by prioritising the heat pump. The initialisation delay timer enforces a reduction in short cycle switching between the boiler and the heat pump. The delay allows the heat pump to reach its steady state COP in the hope that it can continue to be used, which is advantageous because heat pumps are most efficient when used continuously for long periods. The delay also allows for conditions to stabilise such as the sensed temperature. Regarding human factors, the delay recognises the fact that members of a household may increase and decrease the room temperature setpoint several times in a short space of time, before a compromise is reached. This initialisation delay ensures that the HCU is not constantly switching in response to such changes. Further, the delay also allows time for hot tap water to be demanded from a combination boiler during this initial period, assuming that occupants often use hot water shortly after arriving home and requesting central heating. The means may be configured to receive a signal from the room thermostat, and trigger the initialisation delay timer in dependence on the signal indicating that room heating is demanded. The means may be configured to execute a first selection operation in dependence on timer expiry, comprising selecting in dependence on the temperature sensing which one of the heat pump or the boiler is connected to the emitter circuit to satisfy heating demand. If there is first an initialisation delay, the timer expiry may refer to expiry of the initialisation delay timer. An advantage is that the sensed temperature is given time to stabilise before it is measured and / or acted upon. In examples, the first selection operation can comprise selecting which one of the heat pump-connected state and the boiler-connected state the valve apparatus is to be actuated to. In dependence on the first selection operation, the means may be configured to actuate the valve apparatus into the selected state. The first selection operation may be configured to select the boiler in dependence on the temperature sensing indicating a below-threshold fluid temperature. Selecting the boiler may comprise selecting the boiler-connected state of the valve apparatus. The selection of the boiler may be dependent on the temperature sensing indicating a fluid temperature below a lower threshold. If the lower threshold is based on return temperature sensing, the lower threshold may have a value selected from the range 30 to 40 Celsius, such as <35 Celsius. The means may be configured to inhibit switching away from the selected boiler until a minimum boiler heating duration condition is satisfied, wherein the minimum boiler heating duration condition specifies a set period of time of multiple hours and / or specifies a time of day (e.g., midnight). An advantage is reducing short cycle switching, because the selection of the boiler implies that the heat pump has not been providing sufficient heating and is unlikely to be able to do so for at least several hours. The first selection operation may be configured to select the heat pump in dependence on the temperature sensing indicating an above-threshold fluid temperature. Selecting the heat pump may comprise selecting the heat pump-connected state of the valve apparatus. The selection of the heat pump may be dependent on the temperature sensing indicating a fluid temperature greater than the lower threshold. The first selection operation may be configured to select at least the boiler in dependence on the above-threshold fluid temperature being greater than an upper limit. The upper limit may be an upper threshold greater than the lower threshold. The heat pump may therefore be selectable within a middle band between the lower threshold and the upper threshold, the boiler being selected in a low band below the lower threshold and in a high band above the upper threshold. If the upper limit is based on return temperature sensing, the upper limit may have a value selected from the range 50 to 65 Celsius, such as >55 Celsius. This relates to the less common situation where the thermostat is still requesting heating even though the fluid temperature (e.g., return fluid temperature) is very high. Since heat pumps are not efficient in dealing with very high temperatures, the boiler is advantageously selected because the boiler is better suited to dealing with such high temperature operating conditions. Therefore, the boiler and heat pump are advantageously used within the temperature bands where they are efficient. The delay timing may comprise a second delay timer to control how long a selected one of the boiler or heat pump from the first selection operation remains selected. The second delay timer may be triggered in dependence on execution of the first selection operation. An advantage is that heating efficiency is improved by enforcing a reduction in short cycle switching between the boiler and heat pump. For example, if the hybrid heating controller switches to a combination boiler due to the heat pump failing to satisfy heating demand, the combination boiler will remain selected for at least the duration of the second delay even if the combination boiler switches from central heating mode to water heating mode. Therefore, advantageously, the second delay may have a duration selected from the range 15 minutes to 30 minutes. This corresponds to the typical duration of water heating demand, such as taking a shower, drawing a bath, or washing dishes. The means may be configured to execute a second selection operation in dependence on expiry of the second delay timer, comprising selecting in dependence on the temperature sensing which one of the heat pump or the boiler is connected to the emitter circuit to satisfy heating demand. In examples, this can comprise selecting which one of the heat pump-connected state and the boiler-connected state the valve apparatus is to be actuated to. In dependence on the second selection operation, the means may be configured to actuate the valve apparatus into the selected state. The means may be configured to trigger a further delay timer to control how long a selected one of the boiler or heat pump from the second selection operation remains selected. Triggering the further delay timer may comprise re-triggering the second delay timer, or a different third delay timer. The means may be configured to: a) determine whether a fluid heating condition is satisfied, associated with a presence of heating demand; b) control actuation of the valve apparatus to one of the heat pump-connected state or the boiler-connected state in dependence on the fluid heating condition being satisfied indicating a presence of heating demand, wherein in the heat pump-connected state a heat pump inlet is fluidly coupled an emitter outlet, and in the boiler-connected state a boiler inlet is fluidly coupled to the emitter outlet; and c) control actuation of the valve apparatus to the interconnected state in which the boiler inlet is fluidly interconnected with the heat pump inlet, in dependence on the fluid heating condition not being satisfied. Step b) may be in the form of the first selection operation. Steps a) to c) may also be repeated for the second selection operation. An advantage is that when the central heating system is not in operation, fluid circuits for the boiler and the heat pump are fluidly interconnected to aid with commissioning, filling, and maintenance. Further, where the emitter circuit may comprise a circulation pump, the interconnected state will not inhibit any circulation pump over-run operations employed by the boiler or heat pump. Determining whether the fluid heating condition is satisfied may be dependent on a room thermostat signal. The room thermostat signal may indicate whether a heating demand is present, for example whether the room thermostat is in an open state or a closed state. The fluid heating condition may be unsatisfied in dependence on the room thermostat signal indicating an absence of heating demand (e.g., open state), and satisfied in dependence on the room thermostat indicating a presence of heating demand (e.g., closed state). In examples, the means may be configured to select the heat pump as an initial heater in dependence on initialisation of fluid heating, by controlling actuation of the valve apparatus to the heat pump-connected state in dependence on the fluid heating condition being satisfied indicating a presence of heating demand. According to various, but not necessarily all examples of the disclosure there is provided a central heating system comprising the hybrid heating controller, the heat pump, and the boiler. The heat pump may be an air source heat pump. The boiler may be a combination boiler. According to various, but not necessarily all examples of the disclosure there is provided a method of controlling heater selection between a heat pump and a boiler for heating fluid for an emitter circuit, in dependence on temperature sensing of the fluid and / or in dependence on delay timing. According to various, but not necessarily all examples of the disclosure there is provided computer software that, when executed, controls heater selection between a heat pump and a boiler for heating fluid for an emitter circuit, in dependence on temperature sensing of the fluid and / or in dependence on delay timing. According to various, but not necessarily all examples of the disclosure there is provided a hybrid heating controller comprising means for controlling heater selection between a heat pump and a boiler for heating fluid for an emitter circuit, in dependence on temperature sensing and in dependence on delay timing. In this example, the sensed temperature could be the temperature of the fluid, or ambient external air temperature, for example. While the above examples of the disclosure and optional features are described separately, it is to be understood that their provision in all possible combinations and permutations is contained within the disclosure. It is to be understood that various examples of the disclosure can comprise any or all of the features described in respect of other examples of the disclosure, and vice versa. Also, it is to be appreciated that any one or more or all of the features, in any combination, may be implemented 10 by / comprised in / performable by an apparatus, a method, and / or computer program instructions as desired, and as appropriate. BRIEF DESCRIPTION Some examples will now be described with reference to the accompanying drawings in which: FIG. 1 illustrates an example of a central heating system with a valve apparatus in an interconnected state; FIG. 2 illustrates the valve apparatus in a heat pump-connected state; FIG. 3 illustrates the valve apparatus in a boiler-connected state; FIG. 4 illustrates an example method; FIG. 5 illustrates an example of an electronic controller; and FIG. 6 illustrates an example of a non-transitory computer-readable storage medium. DETAILED DESCRIPTION The Figures illustrate a central heating system 10, and a hybrid heating controller thereof, referred to herein as a Hybrid Connect Unit (HCU 100). The HCU 100 comprises means for controlling heater selection between heaters including a heat pump 30 and a boiler 40 for heating fluid for an emitter circuit 50, in dependence on temperature sensing and / or in dependence on delay timing. FIGS. 1-3 illustrate the central heating system 10 in various states. The illustrated central heating system 10 is for a building such as a residential or commercial building. The central heating system 10 can be a hot water central heating system 10, for heating and circulating hot water to one or more emitters 20 such as convector radiators and / or underfloor heating systems. One such emitter 20 is shown. The fluid may be other than water in some examples. The central heating system 10 comprises an air source heat pump (ASHP 30) and a combination boiler 40 (‘boiler’ herein). In other examples, the heat pump 30 can be a ground source heat pump, water source heat pump, or an exhaust air heat pump, for example. The heat pump comprises a vapor-compression refrigeration system. In examples, the heat pump is electrically powered. The hot water connections of the boiler 40 are not shown. In other examples, the boiler 40 can be a different type of boiler than a combination boiler, not configured for hot water heating. In examples, the boiler 40 is powered by hydrocarbon fuel such as natural gas or by hydrogen blend or 100% hydrogen. The emitter 20 is connected to an emitter circuit 50. The emitter circuit 50 has an inlet for receiving heated water, and an outlet for returning water to the currently selected heater. The ASHP 30 is connected to a hot water outlet line 32 and a water return line 34. The boiler 40 is connected to a hot water outlet line 42 and a water return line 44. The inlet of the emitter circuit 50 is selectively connected to a hot water manifold 116, the hot water manifold 116 being selectively fluidly connected to the hot water outlet lines 32, 42 by an automatically controlled valve apparatus 110. The valve apparatus 110 has an electrically-controlled actuator to move the valve apparatus 110 between respective states. The outlet of the emitter circuit 50 is connected to a return water manifold 118, the return water manifold 118 being fluidly connected to the water return lines 34, 44. A recirculation line 52 of the emitter circuit 50 interconnects the return water manifold 118 with the hot water manifold 116 via an isolation valve 114, to allow water to recirculate around the emitter circuit 50 while bypassing the heaters. The isolation valve 114 may be a check valve, or another type of isolation valve, or may be omitted. Each manifold is implemented as a four-way manifold. The various lines and manifolds may or may not form part of the central heating system 10. A room thermostat 70 of the central heating system 10 is also illustrated in FIGS. 1-3. The room thermostat 70 can comprise a dial, button, or touch screen interface for modifying a setpoint temperature. The setpoint temperature may be controllable manually and / or automatically by the room thermostat 70. The room thermostat 70 can also comprise an ambient temperature sensor, determine a setpoint error, and transmit a signal to the currently active heater to control said heater to minimise the setpoint error. For example, the room thermostat 70 can be configured as a bang-bang controller which has binary active and inactive states. It would be appreciated that control scheme used by the room thermostat 70 may vary with implementation. FIGS. 1-3 schematically illustrate an example of an HCU 100 fitted or retrofitted to the central heating system 10. The HCU 100 comprises the hot water manifold 116 and the valve apparatus 110, as well as an electronic controller 500 (FIG. 5) to control the valve apparatus 110. The electronic controller 500 may be local to the hot water manifold 116 and valve apparatus 110, or remote within the building or elsewhere. The HCU 100 can also comprise a return water temperature sensor 112 which is fitted between the emitter(s) 20 and the outlet of the emitter circuit 50 to sense the water temperature therein. The sensed water temperature will be cooler than that between the inlet of the emitter circuit 50 and the emitter 20, due to heat loss as the water passes through the emitter 20. Alternatively, the HCU 100 may be sold / supplied as an electronic controller 500 for use with separately-supplied apparatus 101 including the valve apparatus 110 and abovedescribed components. In an example implementation, an apparatus 101 that is supplied as part of the HCU 100 comprises some or all of the following components: part of the emitter circuit 50; recirculation line 52; secondary circulation pump 60; heat pump inlet 102; boiler inlet 104; recirculation inlet 106; emitter outlet 108; valve apparatus 110; return water temperature sensor 112; isolation valve 114; hot water manifold 116; and return water manifold 118. The HCU 100 comprises various inlets and outlets (e.g., inlet and outlet ports) for connecting the hot water manifold 116 to the relevant parts of the central heating system 10. These include: - a heat pump inlet 102 fluidly connected to the hot water outlet line 32 for the ASHP 30, to receive heated water from the ASHP 30; - a boiler inlet 104 fluidly connected to the hot water outlet line 42 for the boiler 40, to receive heated water from the boiler 40; - a recirculation inlet 106 to receive recirculating return water from the recirculation line 52; and - an emitter outlet 108 to provide heated water to the inlet of the emitter circuit 50. The valve apparatus 110 can selectively connect the heat pump inlet 102, the boiler inlet 104, and the recirculation inlet 106, to the emitter outlet 108. The valve apparatus 110 can comprise a four-way valve body with a rotating gate, for example. Alternatively, multiple separate valves can be provided to define the valve apparatus 110. Where underfloor heating systems are used, an additional thermostatic mixing valve (not shown) may be present or included to reduce or limit the temperature into the floor to meet system design temperature requirements. In FIG. 1, the valve apparatus 110 is in an optional interconnected state in which the boiler inlet 104 is fluidly interconnected with the heat pump inlet 102. This state is used when heating is not demanded, allowing water to flow between the boiler inlet 104 and the heat pump inlet 102. This state is useful when the central heating system 10 is not in operation, because the fluid circuits are fluidly interconnected to aid with commissioning, filling, and maintenance. Where the valve apparatus 110 comprises a rotating gate, the rotating gate is shown at an intermediate angle between the boiler inlet 104 and heat pump inlet 102 to keep both ports (inlets) at least partially open. While the valve apparatus 110 is in the interconnected state of FIG. 1, the recirculation inlet 106 is fluidly connected to the emitter outlet 108, allowing water to recirculate within the emitter circuit 50. This allows water to recirculate within the emitter circuit 50. In this state, the emitter circuit 50 may be isolated from the heat pump inlet 102 and boiler inlet 104. While the valve apparatus 110 is in the interconnected state of FIG. 1, the recirculation inlet 106 is fluidly connected to the emitter outlet 108 only when the preset spring tension of the isolation valve 114 is exceeded. Conditions when the isolation valve 114 opens to allow fluid access to recirculation inlet 106 are 1) when the rotating gate position is as shown in FIG. 1, or 2) when the isolation valve 114 is manually closed. The isolation valve 114 may also be isolation only for commissioning, filling and maintenance if so desired. Further, where the emitter circuit 50 may comprise a circulation pump 60, the interconnected state will not inhibit any circulation pump 60 over-run operations employed by the boiler 40 or ASHP 30. In FIG. 2, the valve apparatus 110 is in a heat pump-connected state in which the heat pump inlet 102 is fluidly interconnected with the emitter outlet 108. This state is used when heating by the ASHP 30 is demanded. The boiler inlet 104 is blocked. The rotating gate is shown at a second angle blocking the boiler inlet 104. The arrows in FIG. 2 show the flow of water in a cycle between the ASHP 30 and the emitter 20. Hot water flows from the ASHP 30 to the hot water outlet line 32, then to the hot water manifold 116, then through the valve apparatus 110, then to the inlet of the emitter circuit 50, then through an optional secondary circulation pump 60 (which may be active or not), then through the emitter 20, then past the return water temperature sensor 112, then to the return water manifold 118, then to the return water line 34, and then back to the ASHP 30. In FIG. 3, the valve apparatus 110 is in a boiler-connected state in which the boiler inlet 104 is fluidly interconnected with the emitter outlet 108. This state is used when heating by the boiler 40 is demanded. The heat pump inlet 102 is blocked. The rotating gate is shown at a third angle blocking the heat pump inlet 102. The arrows in FIG. 3 show the flow of water in a cycle between the boiler 40 and the emitter 20. Hot water flows from the boiler 40 to the hot water outlet line 42, then to the hot water manifold 116, then through the valve apparatus 110, then to the inlet of the emitter circuit 50, then through the secondary circulation pump 60 (which may be active or not), then through the emitter 20, then past the return water temperature sensor 112, then to the return water manifold 118, then to the return water line 44, and then back to the boiler 40. Due to the higher power of the boiler 40 than the ASHP 30, it is likely that the return water temperature sensor 112 will detect higher return temperatures when the boiler 40 is active. The logic and routines employed by the electronic controller 500 for controlling the valve apparatus 110 are now described, with reference to the flowchart of FIG. 4. FIG. 4 is illustrative of an example method 400, which may be a computer-implemented method or other automated or semi-automated method. Block 401 denotes a ‘START’ of the method 400. Block 402 denotes a ‘SETUP’ operation. For example, a configuration comprising the various conditions for controlling heater selection (temperature thresholds and timer durations) described herein may be updated automatically and / or by a human such as an installer. The configuration may be accessible via a wired or wireless interface. The configuration may allow modification of the thresholds and timer durations within allowed predetermined ranges, for example. At block 404, the method 400 comprises actuating the valve apparatus 110 to the interconnected state shown in FIG. 1. This initial state is useful for filling, as described earlier. Decision block 406 relates to detecting initialisation of heating. At block 406, the method 400 comprises determining whether a fluid heating condition is satisfied, associated with a presence of heating demand. Specifically, determining whether the fluid heating condition is satisfied may be dependent on a signal from the room thermostat 70, indicating whether a state of the room thermostat 70 is open (no heating demand) or closed (heating demanded). The closed / heating demanded state typically occurs when the setpoint error is below the room temperature setpoint set in the room thermostat 70. The HCU 100 can comprise any appropriate wired or wireless interface operable to receive signals from the room thermostat 70. In other examples, the HCU 100 can determine whether heating demand is present via other techniques, such as sensors or communication with the heaters. If there is no heating demand, the condition is not satisfied so the method 400 loops back to block 404, such that the valve apparatus 110 remains in the interconnected state. If there is heating demand, the condition is satisfied so the method 400 proceeds to block 408. At block 408, the method 400 comprises selecting the ASHP 30, in other words actuating the valve apparatus 110 to the heat pump-connected state shown in FIG. 2. This means that the ASHP 30 is selected as the initial heater in response to the initialisation of fluid heating of decision block 406. The ASHP 30 is the priority for room heating as this heater is the most efficient. The ASHP 30 will immediately respond to being signalled by the HCU 100 to provide heating, but to reach the set design return temperature this may take a few minutes, therefore an initialisation delay timer at block 410 delays the employment of the return water temperature sensor 112 (time set by the installer when commissioning.) This step may also comprise the HCU’s electronic controller 500 activating the circulation pump 60. Alternatively, or additionally, if the ASHP 30 and / or boiler 40 has its own integral pump then the integral pump may activate independently. At block 410, which may be part of the same operation as block 408, the method 400 comprises triggering an initialisation delay timer (delay 1). Upon expiry of the initialisation delay timer, the return water temperature sensor (112) monitors the return temperature and reports to the HCU electronic control 500 as to which temperature band the ASHP 30 is achieving that controls how long the ASHP 30 remains selected as the initial heater. An ASHP 30 for a typical building may take from 10-15 minutes to reach its peak COP. Therefore, the initialisation delay timer has a duration of at least 10 minutes, or another value selected from the range 10-60 minutes. While the initialisation delay timer is running, a flag or other configuration prevents the electronic controller 500 from selecting another heater (the boiler 40) even if the information from the return water temperature sensor 112 suggests that the boiler 40 should be used. This is because, during this initialisation process, the sensed temperature will not yet have stabilised and the COP of the ASHP 30 may still be increasing. Once conditions have stabilised, the temperature sensed by the return water temperature sensor 112 may in fact indicate that the ASHP 30 is the appropriate heater. Regarding human factors, it is also possible that members of a household will increase and decrease the room temperature setpoint several times in a short space of time, before a compromise is reached. This initialisation delay ensures that the HCU 100 is not constantly switching in response to such changes. The delay timing also allows for the boiler 40 to serve typical hot water demands that may be expected when occupants arrive home, without loss of central heating because the ASHP 30 can continue to operate while the boiler 40 is serving hot tap water demand. The initialisation delay timer may be triggered by the satisfaction block 406, and / or triggered by the initiation or completion of block 408. Regardless of the specific trigger, the triggering of the initialisation delay timer is dependent on the satisfaction of block 406. In response to the initialisation delay timer expiring, the method 400 proceeds to decision block 412. At decision block 412, the method 400 comprises executing a first selection operation. According to the first selection operation, the electronic controller 500 selects which one of the ASHP 30 or the boiler 40 is connected to the emitter circuit to satisfy the heating demand, in dependence on the sensed temperature from the return water temperature sensor 112. If the ASHP 30 is selected, the method 400 proceeds to block 414 in which the valve apparatus 110 is controlled to the heat pump-connected state of FIG. 2. If the boiler 40 is selected, the method 400 proceeds instead to block 416 or 418 in which the valve apparatus 110 is controlled to the boiler-connected state of FIG. 3. In block 414, the HCU’s electronic controller 500 will run circulation pump 60. An advantage of basing the decision on the temperature of the return water is that the return water temperature directly indicates the actual performance of the heater. The return water temperature takes into account the effect of ambient external air temperature, solar gain, and all other relevant factors, on the performance of the ASHP 30. If the initially-selected ASHP 30 is failing to satisfy heating demand, the return water temperature will be too low relative to the water temperature output by the ASHP 30. If the sensed temperature (T1) is within a middle band Tmin <T1 <Tmax, the ASHP 30 can remain connected so the method 400 proceeds to block 414. Example values for the middle band are 30 Celsius (Tmin) to 40 Celsius (Tmax). However, the values of these lower and upper thresholds / limits Tmin and Tmax may be different and / or reconfigurable. For example, Tmin may be a value less than 35 Celsius. Tmax may be a value higher than Tmin and selected from the range 50 to 65 Celsius. The values are in any case less than the hot water temperatures upstream of the emitter 20. In some examples, the value or permitted maximum value of the lower threshold (Tmin) is dependent on an in-service controllable parameter that is controllable in-service (during commissioning, operation, or maintenance) relative to a factory default value. In an example, the in-service controllable parameter is a setpoint Tset. The setpoint Tset may be a water temperature setpoint. The setpoint Tset controls the limit(s) of one or more of the bands. For example, in dependence on Tset being increased inservice automatically or by a user input at an input device, the electronic controller 500 may be configured to increase the value of the lower threshold Tmin. Additionally, or alternatively, the upper threshold Tmax may be similarly dependent on another variable such as Tset. Tset may be a target return water temperature setpoint for ASHP-heating. Therefore, the value of Tset may be selected or user-selectable from the range 35 to 50 Celsius, suitable for ASHP installations. The factory default value of Tset may be selected from the range 37 to 43 Celsius, because this is optimal for many ASHP installations. When Tset is at the factory default value, Tmin could be 35 Celsius for example. These are merely example values and could vary for different implementations. In examples, the electronic controller 500 may select a Tmin value that is less than 35 Celsius, in dependence on Tset being a value that is less than 40 Celsius. In examples, the electronic controller 500 may select a Tmin value that is less than 38 Celsius, in dependence on Tset being a value that is less than 45 Celsius. In examples, the electronic controller 500 may select a Tmin value that is less than or equal to 40 Celsius, in dependence on Tset being a value that is less or equal to 50 Celsius. In the above example, Tset is an individual value (setpoint) which controls the limits of the bands (conditions for controlling heater selection). Additionally, or alternatively, Tset is an allowable water temperature range rather than a value. Or, instead of being dependent on Tset, Tmin may be individually controllable. An advantage of providing a controllable Tset value is that it is intuitive. It is intuitive because its value is based on building standards such as BS EN 14511, which advises 40 Celsius for the return water in normal installations. Tset may be displayed on a display (not shown) and user-controllable via any appropriate input device (not shown), such as a dial, buttons, or a slider. If the sensed temperature is in a low band T1 <Tmin, this indicates that the ASHP 30 is failing to provide sufficient heating performance. Therefore, the method 400 proceeds to block 416 where the boiler 40 is selected. In other words, the HCU 100 will only revert to the boiler 40 for room heating when the monitored return temperature calculates that the ASHP 30 is not coping with the current heating demand, e.g., due to extremely low ambient external air temperatures or being in defrost mode. For the greater part of the heating season, the ASHP 30 will remain as the selected heat source. When the boiler 40 is selected (‘boiler mode’), the boiler 40 will start room heating. There may be a second delay at block 422 before returning to block 412 to monitor the return water temperature sensor 112. Once in boiler mode, the electronic controller 500 may not permit returning to ASHP heating for a set period of time of multiple hours or longer than six hours, such as 24 hours. Alternatively, or additionally, the electronic controller 500 may not permit returning to ASHP heating until a time of day such as 12.00am. This is provided for in decision block 415 (minimum boiler heating duration condition). The condition of block 415 is useful because once boiler mode has been activated, it is clear that the ASHP 30 is struggling. When the thermostat setpoint temperature is reached, the boiler 40 will shut off, but cold weather dictates that room temperature will fall due to building fabric heat losses and that the room 70 will again request heating. In this cold weather, it is counter-productive to go back to the ASHP 30 too quickly, and run through the cycle of switching back to the boiler 40 to achieve the required room thermostat setpoint temperature. Therefore, even when the return water temperature climbs above Tmin, the boiler will remain selected even if the second delay timer (described below) has expired, if the minimum boiler heating duration condition has not yet been satisfied. Advantages are 1) emitters 20 do not need upgrading, 2) the ASHP 30 does not need to be oversized to meet worst case heating loads, 3) a new hot water storage cylinder and necessary controls and pipework do not need fitting and purchasing, so additional space for those components is not required and 4) substantially reduced installation costs. If the sensed temperature is in a high band T1 >Tmax (upper limit / threshold), the method 400 proceeds to block 418 in which the valve apparatus 110 is actuated to the interconnected state (or boiler-connected state). Either way, the boiler 40 can supply energy to the emitter circuit 50. This relates to the less common situation where the room thermostat 70 is still requesting heating despite the sensed water return temperature being greater than 42 Celsius (or whatever the value of Tmax is). For example, heating demand may be especially high. In such a situation, the return temperature is too high for the ASHP 30 to operate efficiently, so the boiler 40 can provide heat. When the boiler 40 is selected in block 416 or 418, the HCU’s electronic controller 500 may continue running the circulation pump 60. At decision block 420, the method 400 checks decision block 406 again to check heating is still in demand (e.g., room thermostat open state). If there is no heating demand, this suggests that the room temperature is at or near the required room temperature setpoint. The method 400 can therefore loop back to block 404 by selecting the interconnected state of the valve apparatus 110. If the fluid heating condition is still satisfied, this means that heating demand is still ongoing. Therefore, the method 400 progresses to block 422. Although the separate blocks 406 and 420 implies that the fluid heating condition is checked more than once at specific times, the condition may be continuously monitored throughout the method 400. Before the heater can be switched again, block 422 comprises triggering a second delay timer (delay 2) to control how long the currently-selected heater (ASHP 30 if block 414, boiler 40 if block 416 / 418) can remain selected. The second delay timer may be triggered specifically in response to completion of any appropriate part of blocks 414-420. If the ASHP 30 is currently selected, this means that the fluid temperature T1 is in the middle band (Tset band) at the time of selection of the ASHP 30. the second delay timer usefully allows the ASHP 30 to stay at its peak COP for a further reasonable length of time. If the boiler 40 is currently selected, this may be because the fluid temperature T1 is in the lower band or upper band at the time of selection of the boiler 40. The second delay timer usefully allows the boiler 40 to build up the temperature from the lower band to significantly greater than Tmin. Or, if the temperature is in the upper band, the second delay timer allows the boiler 40 to continue satisfying the high demand. The second delay timer may be in the order of 15 to 30 minutes, for example to allow various conditions to stabilise, such as completion of hot tap water usage after arriving at home. The specific duration of the second delay timer may be predetermined or reconfigurable within this range. The second delay timer may however have a longer duration than that of the minimum boiler heating duration condition, where said condition specifies a set period of time such as multiple hours. In addition to or instead of the delays described above, other approaches could be employed to achieve a similar effect, such as hysteresis techniques. However, delay timers can advantageously provide a suitably long period of delay, and can be readily adjusted without significant calibration effort. Further timers can be employed, for example by repeating the second delay timer or implementing a third delay timer. Other possible features not shown in the FIGs: A flow limiter can be connected at the ASHP connection point (either flow or return) to monitor and adjust the flow as dictated by the ASHP manufacturer. A connection point on the HCU 100 can be provided should an additional expansion vessel be required. FIG. 5 illustrates an example of a controller (electronic controller) 500 suitable for use in an apparatus 100. Implementation of a controller 500 may be as controller circuitry. The controller 500 may be implemented in hardware alone, have certain aspects in software including firmware alone or can be a combination of hardware and software (including firmware). As illustrated in FIG. 5 the controller 500 may be implemented using instructions that enable hardware functionality, for example, by using executable instructions of a computer program 506 in a general-purpose or special-purpose processor 502 that may be stored on a computer readable storage medium (disk, memory etc) to be executed by such a processor 502. The processor 502 is configured to read from and write to the memory 504. The processor 502 may also comprise an output interface via which data and / or commands are output by the processor 502 and an input interface via which data and / or commands are input to the processor 502. The memory 504 stores a computer program 506 comprising computer program instructions (computer program code) that controls the operation of the apparatus 100 when loaded into the processor 502. The computer program instructions, of the computer program 506, provide the logic and routines that enables the apparatus to perform the methods illustrated in the accompanying Figs. The processor 502 by reading the memory 504 is able to load and execute the computer program 506. The apparatus 100 comprises: at least one processor 502; and at least one memory 504 including computer program code the at least one memory 504 and the computer program code configured to, with the at least one processor 502, cause the apparatus 100 at least to perform: controlling heater selection between a heat pump and a boiler for heating fluid for an emitter circuit 50, in dependence on temperature sensing of the fluid and / or in dependence on delay timing. The apparatus 100 comprises: at least one processor 502; and at least one memory 504 storing instructions that, when executed by the at least one processor 502, cause the apparatus at least to: control heater selection between a heat pump and a boiler for heating fluid for an emitter circuit 50, in dependence on temperature sensing of the fluid and / or in dependence on delay timing. As illustrated in FIG. 6, the computer program 506 may arrive at the apparatus 100 via any suitable delivery mechanism 508. The delivery mechanism 508 may be, for example, a machine readable medium, a computer-readable medium, a non-transitory computer-readable storage medium, a computer program product, a memory device, a record medium such as a Compact Disc Read-Only Memory (CD-ROM) or a Digital Versatile Disc (DVD) or a solid-state memory, an article of manufacture that comprises or tangibly embodies the computer program 506. The delivery mechanism may be a signal configured to reliably transfer the computer program 506. The apparatus 100 may propagate or transmit the computer program 506 as a computer data signal. Computer program instructions for causing an apparatus to perform at least the following or for performing at least the following: controlling heater selection between a heat pump and a boiler for heating fluid for an emitter circuit 50, in dependence on temperature sensing of the fluid and / or in dependence on delay timing. The computer program instructions may be comprised in a computer program, a non-transitory computer readable medium, a computer program product, a machine readable medium. In some but not necessarily all examples, the computer program instructions may be distributed over more than one computer program. Although the memory 504 is illustrated as a single component / circuitry it may be implemented as one or more separate components / circuitry some or all of which may be integrated / removable and / or may provide permanent / semi-permanent / dynamic / cached storage. Although the processor 502 is illustrated as a single component / circuitry it may be implemented as one or more separate components / circuitry some or all of which may be integrated / removable. The processor 502 may be a single core or multi-core processor. References to ‘computer-readable storage medium’, ‘computer program product’, ‘tangibly embodied computer program’ etc. or a ‘controller’, ‘computer’, ‘processor’ etc. should be understood to encompass not only computers having different architectures such as single / multi- processor architectures and sequential (Von Neumann) / parallel architectures but also specialized circuits such as field-programmable gate arrays (FPGA), application specific circuits (ASIC), signal processing devices and other processing circuitry. References to computer program, instructions, code etc. should be understood to encompass software for a programmable processor or firmware such as, for example, the programmable content of a hardware device whether instructions for a processor, or configuration settings for a fixed-function device, gate array or programmable logic device etc. The blocks illustrated in the accompanying Figs may represent steps in a method and / or sections of code in the computer program 506. The illustration of a particular order to the blocks does not necessarily imply that there is a required or preferred order for the blocks and the order and arrangement of the block may be varied. Furthermore, it may be possible for some blocks to be omitted. Where a structural feature has been described, it may be replaced by means for performing one or more of the functions of the structural feature whether that function or those functions are explicitly or implicitly described. The term ‘comprise’ is used in this document with an inclusive not an exclusive meaning. That is any reference to X comprising Y indicates that X may comprise only one Y or may comprise more than one Y. If it is intended to use ‘comprise’ with an exclusive meaning then it will be made clear in the context by referring to “comprising only one...” or by using “consisting”. In this description, the wording 'connect', 'couple' and 'communication' and their derivatives mean operationally connected / coupled / in communication. It should be appreciated that any number or combination of intervening components can exist (including no intervening components), i.e., so as to provide direct or indirect connection / coupling / communication. Any such intervening components can include hardware and / or software components. As used herein, the term "determine / determining" (and grammatical variants thereof) can include, not least: calculating, computing, processing, deriving, measuring, investigating, identifying, looking up (for example, looking up in a table, a database or another data structure), ascertaining and the like. Also, "determining" can include receiving (for example, receiving information), accessing (for example, accessing data in a memory), obtaining and the like. Also, "determine / determining" can include resolving, selecting, choosing, establishing, and the like. In this description, reference has been made to various examples. The description of features or functions in relation to an example indicates that those features or functions are present in that example. The use of the term ‘example’ or ‘for example’ or ‘can’ or ‘may’ in the text denotes, whether explicitly stated or not, that such features or functions are present in at least the described example, whether described as an example or not, and that they can be, but are not necessarily, present in some of or all other examples. Thus ‘example’, ‘for example’, ‘can’ or ‘may’ refers to a particular instance in a class of examples. A property of the instance can be a property of only that instance or a property of the class or a property of a sub-class of the class that includes some but not all of the instances in the class. It is therefore implicitly disclosed that a feature described with reference to one example but not with reference to another example, can where possible be used in that other example as part of a working combination but does not necessarily have to be used in that other example. Although examples have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the claims. Features described in the preceding description may be used in combinations other than the combinations explicitly described above. Although functions have been described with reference to certain features, those functions may be performable by other features whether described or not. Although features have been described with reference to certain examples, those features may also be present in other examples whether described or not. The term ‘a’, ‘an’ or ‘the’ is used in this document with an inclusive not an exclusive meaning. That is any reference to X comprising a / an / the Y indicates that X may comprise only one Y or may comprise more than one Y unless the context clearly indicates the contrary. If it is intended to use ‘a’, ‘an’ or ‘the’ with an exclusive meaning then it will be made clear in the context. In some circumstances the use of ‘at least one’ or ‘one or more’ may be used to emphasis an inclusive meaning but the absence of these terms should not be taken to infer any exclusive meaning. The presence of a feature (or combination of features) in a claim is a reference to that feature or (combination of features) itself and also to features that achieve substantially the same technical effect (equivalent features). The equivalent features include, for example, features that are variants and achieve substantially the same result in substantially the same way. The equivalent features include, for example, features that perform substantially the same function, in substantially the same way to achieve substantially the same result. In this description, reference has been made to various examples using adjectives or adjectival phrases to describe characteristics of the examples. Such a description of a characteristic in relation to an example indicates that the characteristic is present in some examples exactly as described and is present in other examples substantially as described. The above description describes some examples of the present disclosure however those of ordinary skill in the art will be aware of possible alternative structures and method features which offer equivalent functionality to the specific examples of such structures and features described herein above and which for the sake of brevity and clarity have been omitted from the above description. Nonetheless, the above description should be read as implicitly including reference to such alternative structures and method features which provide equivalent functionality unless such alternative structures or method features are explicitly excluded in the above description of the examples of the present disclosure. Whilst endeavouring in the foregoing specification to draw attention to those features believed to be of importance it should be understood that the Applicant may seek protection via the claims in respect of any patentable feature or combination of features hereinbefore referred to and / or shown in the drawings whether or not emphasis has been placed thereon. l / we claim:

Claims

1. A hybrid heating controller comprising means for controlling heater selection between a heat pump and a boiler for heating fluid for an emitter circuit, in dependence on temperature sensing of the fluid and / or in dependence on delay timing.

2. The hybrid heating controller of claim 1, wherein the temperature sensing comprises return temperature sensing of the fluid returning from the emitter circuit.

3. The hybrid heating controller of claim 1 or 2, wherein controlling the heater selection is dependent on both the temperature sensing and on the delay timing.

4. The hybrid heating controller of claim 1, 2, or 3, wherein the delay timing controls when the hybrid heating controller is permitted to switch heaters.

5. The hybrid heating controller of claim 4, wherein the delay timing controls when the hybrid heating controller is permitted to switch heaters in dependence on the temperature sensing.

6. The hybrid heating controller of any preceding claim, wherein a condition for controlling heater selection is dependent on an in-service controllable parameter.

7. The hybrid heating controller of any preceding claim, the means configured to select the heat pump as an initial heater in dependence on initialisation of fluid heating.

8. The hybrid heating controller of claim 7, wherein the delay timing comprises an initialisation delay timer to control how long the heat pump remains selected as the initial heater.

9. The hybrid heating controller of claim 8, wherein the initialisation delay timer has a duration of at least 10 minutes.

10. The hybrid heating controller of any preceding claim, the means configured to execute a first selection operation in dependence on timer expiry, comprising selectingin dependence on the temperature sensing which one of the heat pump or the boiler is connected to the emitter circuit to satisfy heating demand.

11. The hybrid heating controller of claim 10, wherein the first selection operation is configured to select the boiler in dependence on the temperature sensing indicating a below-threshold fluid temperature.

12. The hybrid heating controller of claim 11, the means configured to inhibit switching away from the selected boiler until a minimum boiler heating duration condition is satisfied, wherein the minimum boiler heating duration condition specifies a set period of time of multiple hours and / or specifies a time of day.

13. The hybrid heating controller of claim 10, 11 or 12, wherein the first selection operation is configured to select the heat pump in dependence on the temperature sensing indicating an above-threshold fluid temperature.

14. The hybrid heating controller of claim 13, wherein the first selection operation is configured to select at least the boiler in dependence on the above-threshold fluid temperature being greater than an upper limit.

15. The hybrid heating controller of any one of claims 10 to 14, wherein the delay timing comprises a second delay timer to control how long a selected one of the boiler or heat pump from the first selection operation remains selected.

16. The hybrid heating controller of claim 15, wherein the second delay timer has a duration of at least 15 minutes.

17. The hybrid heating controller of claim 15 or 16, the means configured to execute a second selection operation in dependence on expiry of the second delay timer, comprising selecting in dependence on the temperature sensing which one of the heat pump or the boiler is connected to the emitter circuit to satisfy heating demand.

18. The hybrid heating controller of claim 17, the means configured to trigger a further delay timer to control how long a selected one of the boiler or heat pump from the second selection operation remains selected.

19. The hybrid heating controller of any preceding claim, the means configured to: determine whether a fluid heating condition is satisfied, associated with a presence of heating demand;control actuation of a valve apparatus to one of a heat pump-connected state or a boiler-connected state in dependence on the fluid heating condition being satisfied, wherein in the heat pump-connected state a heat pump inlet is fluidly coupled an emitter outlet, and in the boiler-connected state a boiler inlet is fluidly coupled to the emitter outlet; andcontrol actuation of the valve apparatus to an interconnected state in which the boiler inlet is fluidly interconnected with the heat pump inlet, in dependence on the fluid heating condition not being satisfied.

20. The hybrid heating controller of any one of claims 1 to 18, the means comprising:a heat pump inlet to receive heated fluid from the heat pump;a boiler inlet to receive heated fluid from the boiler;an emitter outlet to provide heated fluid to the emitter circuit;a valve apparatus having at least a heat pump-connected state in which the heat pump inlet is fluidly coupled to the emitter outlet, and a boiler-connected state in which the boiler inlet is fluidly coupled to the emitter outlet; anda controller to control the valve apparatus in dependence on the temperature sensing and / or the delay timing.

21. The hybrid heating controller of claim 20, the means comprising a recirculation inlet to receive return fluid from the emitter circuit, wherein the recirculation inlet is fluidly coupled to the emitter outlet.

22. The hybrid heating controller of claim 20 or 21, the means comprising an isolation valve to control fluid flow through the recirculation inlet.

23. The hybrid heating controller of any preceding claim, the means comprising a temperature sensor, wherein the temperature sensing is based on information from the temperature sensor.5 24. A central heating system comprising the hybrid heating controller, the heatpump, and the boiler, of any one of the preceding claims.

25. The central heating system of claim 24, wherein the heat pump is an air source heat pump and the boiler is a combination boiler.1026. A method of controlling heater selection between a heat pump and a boiler for heating fluid for an emitter circuit, in dependence on temperature sensing of the fluid and / or in dependence on delay timing.15 27. Computer software that, when executed, controls heater selection between aheat pump and a boiler for heating fluid for an emitter circuit, in dependence on temperature sensing of the fluid and / or in dependence on delay timing.

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