Heat interface units

Abstract

A heat interface unit (HIU) 104 comprises a heat network flow inlet 200 and a controller (214, figure 2). The controller is operable to activate an immersion heater 304 (e.g. 7 kW cartridge heater) to

Description

Field The present disclosure relates to heat interface units (HIUs). Background In general terms, a heat network supplies heat from a central source to consumers, via a distribution system of insulated pipes. The insulated pipes take hot waterfrom one or more energy centres and deliverthe hot waterto one or more domestic and / or non-domestic buildings. An energy centre is a central source of energy in a heat network. A heat network can deliver heat to thousands of buildings or even to an entire city. The term “primary circuit” is generally used, in relation to heat networks, to mean the insulated pipes that take hot water from the energy centre(s) to one or more buildings. The term “secondary circuit” is generally used, in relation to heat networks, to mean pipework within communal areas of a building. The term “tertiary circuit” is generally used, in relation to heat networks, to mean pipework within an individual dwelling of a building. Heat networks have been used for decades to deliver heating and domestic hot water (DHW) to multi-dwelling buildings, such as apartment blocks. However, recent developments to heat networks have provided an opportunity to leverage and / or incorporate renewable heat sources. This can help reduce carbon emission in heat networks. An example of such a renewable heat source is a heat pump. Indeed, manufacturers, specifiers and heating engineers are currently embracingthe renewable technology of heat pumps on the primary circuit. Such heat networks are commonly referred to as “fourth generation heat networks” or “low temperature heat networks” (LTHNs). Such heat networks generally operate at a lower temperature than heat networks that only use boilers. This can contribute to higher system efficiency, lower carbon emissions, and reduced air pollution. In practice, one or more commercial heat pumps are generally installed on the outside of a building and are connected to a set of HIUs via the distribution system. Each apartment typically has its own HIU. An HIU may be designed specifically to work with the lower temperatures associated with heat pumps, and may optimise performance in a well-insulated, newly built apartment block. Such HIUs may therefore be considered “heat pump ready”. An example of a heat pump ready HIU is the MTA Plus HIU™ from Modutherm™. The MTA Plus HIU™ provides high DHW and heating performance, even at low temperatures. Summary According to first embodiments, there is provided a heat interface unit, HIU, comprising: a heat network flow inlet; and a controller, wherein the controller is configured to be operable to control operation of an immersion heater, and wherein the immersion heater is configured to heat a supply of water to the heat network flow inlet when activated by the controller. According to second embodiments, there is provided a heat interface unit, HIU, comprising: a heat network flow inlet; a processor; and memory, the memory comprising instructions which, when executed by the processor, cause the HIU to control operation of an immersion heater, the immersion heater being operable to apply a temperature boost to a fluid supply to the heat network flow inlet. According to third embodiments, there is provided a computer-implemented method of controlling a heat interface unit, HIU, the method comprising: controlling the HIU to activate or deactivate an immersion heater, wherein activating the immersion heater causes the immersion heater to apply a temperature boost to a supply of fluid to the HIU. According to fourth embodiments, there is provided a computer program configured, when executed, to perform a method accordingto the third embodiments. According to fifth embodiments, there is provided an immersion heater comprising: a tank; and a cartridge heater, wherein the tank comprises an inlet and an outlet, wherein the inlet of the tank is to receive a heat network flow, wherein the outlet of the tank is fluidly couplable to a heat network flow inlet of a heat interface unit, HIU, and wherein the immersion heater is configured to be controllable by the HIU. Brief Description of the Drawings Various embodiments will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows a schematic diagram of an example of a heat network; Figure 2 shows a schematic diagram of an example of an HIU; Figures shows a schematic diagram of an example of a system comprising an HIU and an immersion heater; Figure 4 shows a flowchart of an example of a method of controlling an HIU; Figure 5 shows a perspective view of an example of an immersion heater; Figure 6 shows a perspective view of example components of the example immersion heater shown in Figure 5; Figure 7 shows a schematic cross-sectional view of the example immersion heater shown in Figure 5; Figure 8 shows a graph of an example of temperatures as a function of time; and Figure 9 shows a graph of another example of temperatures as a function of time. Detailed Description Referring to Figure 1, there is shown an example of a heat network 100. The heat network 100 may take various forms. Example forms of heat network include, but are not limited to, communal heating and district heating. Communal heating may refer to heating a single building or a small number of buildings. District heating may refer to heating multiple buildings, on a larger scale than communal heating. However, district heating may also sometimes refer to heating a single building or a small number of buildings. Communal heating and district heating may be referred to as “communal heating systems” and “district heating systems” respectively. For ease of understanding, in this specific example, the heat network 100 delivers heating and DHW to apartments in an apartment block. However, the heat network 100 could be deployed in a different one-to-many configuration in other examples. In this example, the heat network 100 comprises an energy centre 102. As explained above, a heat network may comprise more than one energy centre 102 in other examples. The energy centre 102 comprises one or more types of heat source 102-1. Example types of heat source 102-1 include, but are not limited to, heat pumps, boilers, and waste heat recovery systems. The energy centre 102 may comprise a combination of different types of heat source 102-1. As explained above, a heat pump is a renewable heat source that can contribute to higher system efficiency, lower carbon emissions, and reduced air pollution compared to some other types of heat source. Heat pumps operate more effectively at higher outside temperatures than at lower outside temperatures. Heat pumps may operate less efficiently when heating to higher temperatures. For instance, a heat pump may operate efficiently when heating water to 45°C, but less efficiently at higher temperatures. A boiler, such as a gas boiler, may operate more cost-effectively than a heat pump at lower outside temperatures. For example, when the outside temperature is below 3°C, the energy centre 102 may switch from using one or more heat pumps to using one or more gas boilers for increased cost-effectiveness. An example of a waste heat recovery system is a system that recovers waste heat from an underground public transport network. In this example, the energy centre 102 comprises a thermal store 102-2. In this specific example, the thermal store 102-2 comprises a buffer tank. The heat source(s) 102-1 may provide heated water to the buffer tank 102-2, and the heated water may be drawn from the buffer tank 102-2 based on demand in the heat network 100. For example, a heat pump may supply heated water to the buffer tank 102-2 when outside temperatures are relatively high, and a gas boiler may supply heated water to the buffer tank 102-2 when outside temperatures are relatively low. Heated water drawn from the buffer tank 102-2 may, therefore, have been supplied by any heat source(s) 102-1 of the energy centre 102. In this example, the heat network 100 comprises a plurality of HIUs 104. In this example, each apartment in the apartment block comprises a respective HIU 104. In this example, each HIU 104 receives a heat network flow 106 from the energy centre 102 and returns a heat network return 108 to the energy centre 102. In this example, each HIU 104 supplies DHW 110. In this example, each HIU 104 supplies DHW 110 to one or more sanitary appliances 112 in the respective apartment. In this specific example, DHW 110 is supplied to a sanitary appliance 112 in the form of a sink. Other examples of sanitary appliance 112 to which DHW 110 may be supplied include, but are not limited to, baths, showers, and bidets. In this example, each HIU 104 supplies domestic cold water (DOW) 114. In this example, the DOW 114 is also supplied to the one or more sanitary appliances 112. DOW may, alternatively or additionally, be supplied to other types of appliances, such as home appliances. Examples of home appliances include, but are not limited to, washing machines, refrigerators and dishwashers. In this example, each HIU 104 supplies a heating flow 116. In this example, each HIU 104 supplies a heating flow 116 to a heating system 118 in the respective apartment. In this specific example, the heating system 118 comprises one or more radiators. Another example of a heating system 118 to which the heating flow 116 may be supplied is an underfloor heating system. In this example, each HIU 104 receives a heating return 120 from its respective heating system 118. In this example, boosted cold water (BCW) 122 is supplied to each HIU 104. The BCW 122 supply provides pressurised, fresh water that the HIU 104 heats up to supply the DHW 110. Referring to Figure 2, there is shown an example of an HIU 104. In this example, the HIU 104 is a twin HIU in that the HIU 104 can supply both the DHW 110 and the heating system 118. Example inlets and outlets of the HIU 104 will now be described. In this example, the HIU 104 comprises a heat network flow inlet 200. The heat network flow inlet 200 may receive the heat networkflow 106 from the energy centre 102. The heat network flow inlet 200 may be referred to as a “district inlet” 200. In this example, the HIU 104 comprises a heat network return outlet 202. The heat network return outlet 202 may return the heat network return 108 to the energy centre 102. The heat network return outlet 202 may be referred to as a “district return” 202, or a “district return outlet” 202. In this example, the HIU 104comprisesa heatingflow outlet 204. The heatingflow outlet 204 may supply the heating flow 116, for example to an apartment. In this example, the HIU 104 comprises a heating return inlet 206. The heating return inlet 206 may receive the heating return 120, for example from an apartment. In this example, the HIU 104 comprises a DHW outlet 208. The DHW outlet 208 may supply the DHW 110, for example to an apartment. In this example, the HIU 104 comprises a mains cold water inlet 210. The mains cold water inlet 210 may receive mains cold water in the form of the BCW122. The mains cold water may be heated to provide the DHW 110 and may be supplied at the received temperature for the DCW 114. In this example, the HIU 104 comprises a DCW outlet 212. The DCW outlet 212 may supply the DCW 114, for example to an apartment. In this example, the HIU 104 comprises a pressure relief valve drain. If the heating system 118 becomes over-pressurised, excess water may be discharged by a pressure relief value to reduce the pressure. The excess water may be outlet via the pressure relief valve drain. Other example components of the HIU 104 will now be described. In this example, the HIU 104 comprises a controller 214. The controller 214 may be referred to as a “control unit”, a “control box”, or the like. The controller 214 may comprise one or more processors. The controller 214 may comprise one or more memories. The controller 214 may be configured to be operable to perform methods such as those described herein. The expression “configured to be operable” is used herein, in relation to the controller 214, to mean that the controller 214 has been configured to have the functionality to perform such methods and that such functionality may or may not (currently) be enabled. Such functionality may be enabled or disabled at manufacture and / or installation of the HIU 104. For example, such functionality may be built into the controller 214 at installation, use of such functionality may be disabled at installation, and use of such functionality may subsequently be enabled. The controller 214 may be configured to be operable to perform methods such as those described herein via one or more software updates. In this example, the HIU 104 comprises an automatic airvent 216. The automatic air vent 216 may automatically discharge air from the heating system 118. In this example, the HIU 104 comprises a first heat exchanger 218. In this example, the first heat exchanger 218 is for heating and may, therefore, be referred to as a “heating heat exchanger”. As can be seen from Figure 2, the heating heat exchanger 218 receives the heat network flow 106 via the heat network flow inlet 200 and transfers heat from the heat network flow 106 to the heating flow 116, which is output via the heating flow outlet 204. The heat network return 108 is output via the heat network return outlet 202. Thus, the heat networkflow 106 does not itself enter the tertiary circuit via the heating heat exchanger 218; it is used to heat the water in the tertiary circuit. A temperature drop of 5°C across the heating heat exchanger 218 may be assumed. A 5°C-drop may be assumed even though an actual temperature drop may vary depending, for example, on temperature and flow rate. In this example, the HIU 104 comprises a modulating valve 220. In examples, the modulating valve 220 is used to control or modulate the flow of hot water from the heat network flow 116. The modulating valve 220 may be referred to as a “district modulating valve”, a “primary modulating valve”, or a “primary control valve”. In some examples, the modulating valve 220 can be opened by a set number of steps from fully closed to fully open. In examples described herein, the modulatingvalve 220 can be operated from step 0 (fully closed) to step 250 (fully open). A different number of steps may be used in other examples. In this example, the HIU 104 comprises a differential pressure sensor 222. The differential pressure sensor 222 may read the pressure of the heat network flow 106. The heat network flow 106 pressure may determine howthe modulatingvalve 220 operates. In this example, the HIU 104 comprises a further pressure sensor 224. The further pressure sensor 224 may read the pressure of the tertiary circuit. If that pressure becomes too low, for example owing to a tertiary circuit leak, or too high, a heating pump may be stopped. In this example, the HIU 104 comprises a second heat exchanger 226. In this example, the second heat exchanger 226 is for hot water and may, therefore, be referred to as a “hot water heat exchanger” 226, or a “DHW heat exchanger” 226. As can be seen from Figure 2, the DHW heat exchanger 226 receives the heat network flow 106 via the heat network flow inlet 200 and transfers heat from the heat network flow 106 to the mains cold water received via the mains cold water inlet 210. This produces DHW 110, which is output via the DHW outlet 208. Thus, again, the heat network flow 106 does not itself enter the tertiary circuit via the DHW heat exchanger 226; it is used to heat the water being provided to the tertiary circuit. A temperature drop of 5°C may be assumed across the DHW heat exchanger 226. In this example, the HIU 104 comprises a heat meter sensor 228. In this example, the heat meter sensor 228 is inserted into the heat network flow 106 to sense usage of the heat network flow 106 for the dwelling. In this example, the HIU 104 comprises a heat meter230. In this example, the heat meter 230 measures usage of the heat network flow 106 for the dwelling. Usage data may be collected by a billing company, and the occupier may be billed accordingly. In this example, the HIU 104 comprises a heat network flow inlet sensor 232 to measure a temperature of the heat network flow 106. The heat network flow inlet sensor 232 may be referred to as a “district inlet sensor” 232. In this example, the HIU 104 comprises a filter 234 to filter the heat network flow 106. The filter 234 may be referred to as a “district filter” 234. In this example, the HIU 104 comprises a heat network return sensor 236 to measure a temperature of the heat network return 108. The heat network return sensor 236 may be referred to as a “district return sensor” 236. In this example, the HIU 104 comprises a heating flow sensor 238 to measure a temperature of the heating flow 116. In this example, the HIU 104 comprises a heating pressure gauge 240 to measure a pressure of the heating flow 116. In this example, the HIU 104 comprises a filling device 242. The filling device 242 may be used to top up the pressure of the heating system 118. In this example, the HIU 104 comprises a three-bar pressure relief valve 244. The three-bar pressure relief valve 244 may discharge excess water from the heating system 118 if the pressure of water in the heating system 118 exceeds 3 bar (300,000 Pascals (Pa)). In this example, the HIU 104 comprises a cold water sensor 246 to measure a temperature of the mains cold water supplied to the HIU 104. In this example, the HIU 104 comprises a hot water sensor 248 to measure a temperature of the DHW 110. The hot water sensor 248 may be referred to as a “DHW sensor” 248. In this example, the HIU 104 comprises a heating return sensor 250 to measure a temperature of the heating return 120. In this example, the HIU 104 comprises a hot water flow switch 252. The hot water flow switch 252 may be referred to as a “DHW flow switch” 252. The DHW flow switch 252 may be in either an activated state or a deactivated state. The activated state and the deactivated state may be referred to as the “active state” and “non-active” state respectively. The DHW flow switch 252 may be in the activated state when DHW 110 is being drawn through the HIU 104. The DHW flow switch 252 may be in the deactivated state when DHW 110 is not being drawn through the HIU 104. In this example, the HIU 104 comprises a pump 254. In this example, the pump 254 circulates the heating flow 116 around the heating system 118. The pump 254 may be referred to as a “heating pump”. In this example, the HIU 104 comprises a heating regulation valve 256. In this example, the heating regulation valve 256 is fully open when there is heating demand and is fully closed when there is no heating demand. In this example, the HIU 104 comprises a hot water regulation valve 258. The hot water regulation valve 258 may be referred to as a “DHW regulation valve” 258. In this example, the hot water regulation valve 258 is fully open when there is DHW 110 demand and is fully closed when there is no DHW 110 demand. In this example, the HIU 104 comprises an expansion tank 260. The expansion tank260 may be used to help maintain the correct level of pressure in the heatingsystem 118. Although various inlets, outlets and components of the HIU 104 have been described above, this is byway of example only and not byway of limitation. An HIU may comprise different inlets, outlets and / or components in other examples. The HIU 104 may serve as an energy meter. However, the HIU 104 may provide water separation. For example, the heating and DHW exchangers 218,226 may be in the form of heating plates. As explained above, such heat exchangers 218,226 may separate the secondary circuit from the tertiary circuit. A leak in the tertiary circuit in an apartment would therefore not drain the whole heat network 100. Additionally, the separation maintains treated water in the secondary circuit, which could become contaminated in the tertiary circuit. In this example, the HIU 104 is configured such thatthe DHW 110 and the heating flow 116 cannot both be supplied at the same time as each other. This may be controlled by the heating and hot water regulation valves 256,258. In examples, the heating and hot water regulation valves 256, 258 cannot both be open at the same time as each other. The heating system 118 may only need the heating flow 116 to be at 40°C to operate effectively. This may be the case for an underfloor heating system, for example. Thus, in principle, the heat network 100 could be run at 45°C for heating purposes, assuming a temperature drop of 5°C across the heating heat exchanger 218. However, current Chartered Institution of Building Services Engineers (CIBSE) guidance (for example, the CIBSE “Guidance Note: Domestic Hot Water Temperatures From Instantaneous HIUs”) specifies that DHW 110 must be at 45°C within 45 seconds of being demanded and must be at 50°C in a reasonable time thereafter. Assuming, again, a temperature drop of 5°C across the DHW heat exchanger 226, an expectation may be that the heat network 100 (for example, a fourth generation heat network) should be run at a temperature of at least 55°C to deliver the DHW 100 at 50°C. In practice, however, only a small proportion of heated water supplied by the HIU 104 to an apartment is used for DHW 110. For example, DHW 110 may account for around 20% of the heated water supplied by the HIU 104, with heating flow 116 accounting for around 80% of the heated water supplied by the HIU 104. Consistently running the heat network 100 at, for example, 55°C to satisfy the CIBSE guidance for DHW 110 is, therefore, inefficient. Examples that will now be described in more detail may generally improve energy efficiency, cost-effectiveness, and / or longevity of heat networks. Without loss of generality, such examples may enable the heat network 100 to run at a relatively low temperature, for example 45°C, by providing a temperature boost when needed for DHW 110. In examples, the HIU 104 detects when DHW 110 is being demanded and, if the temperature of water entering the HIU 104 is not sufficiently high, the temperature is boosted. Such on-demand and as-needed temperature boosting is more efficient than constantly running the heat network 100 at a higher temperature, such as 55°C. Examples also include safety mechanisms that may be triggered when water is heated to one or more threshold temperatures. This can help prevent, or at least reduce the risk of, scalding from excessively hot water. Referringto Figure 3, there is shown another example of a system 300. The system 300 may be referred to as a “heating system”. In this example, the system 300 comprises an HIU 104. In this example, the HIU 104 comprises a district inlet 200, a district return outlet 202, a heating flow outlet 204, a heating return inlet 206, a DHW outlet 208, a mains cold water inlet 210, a DCW outlet 212, and a pressure relief valve drain 302. In this example, the system 300 comprises an immersion heater 304. In general terms, the immersion heater 304 uses an electric heating element to heat water in which the heating element of the immersion heater 304 is immersed. In this example, the immersion heater 304 is operable to heat a supply of water to the district inlet 200 of the HIU 104. In this example, the system 300 comprises a sensor 306. In this example, the sensor 306 is referred to as an “external sensor” on the basis that the sensor 306 is external to the HIU 104. The external sensor 306 may be referred to as a “primary flow sensor”, a “primary flow temperature sensor”, or the like. In this example, the external sensor 306 senses the temperature of heat network flow 106 supplied to the immersion heater 304. In this example, the external sensor 306 is connected to the HIU 104. More specifically, in this example the external sensor 306 is communicatively coupled to the controller 214 of the HIU 104. The controller 214 may therefore use the external sensor 306 to obtain temperature readings of the water supplied to the immersion heater 304. In this example, the HIU 104 comprises an electromechanical switch 308 in the form of a relay. In this example, the relay 308 is referred to as an “internal relay” on the basis that the relay 308 is internal to the HIU 104. The relay 308 may also be referred to as an “internal HIU relay”, an “HIU relay”, or the like. In this example, the immersion heater 304 is controllable by an electromechanical switch 310 in the form of a relay. In this example, the relay 310 is referred to as an “external relay” on the basis that the relay 310 is external to the HIU 104. The relay 310 may also be referred to as an “external immersion heater relay”, an “immersion heater relay”, or the like, it being understood that the relay 310 may be separate from the immersion heater 304 itself. A different type of electromechanical switch may be used in other examples. Another type of electromechanical switch is a contactor. A contactor may handle a significantly higher current than a relay. In this example, the HIU 104 is configured to be operable to control the immersion heater 304 using the internal and external relays 308, 310. In this specific example, the controller 214 of the HIU 104 controls the internal relay 308, and the internal relay 308 controls the external relay 310. In this example, the internal relay 308 controls the external relay 310 using a control signal 312. The control signal 312 may be a zero-Volt (V) control signal. Additionally, in this example, the controller 214 being configured to be operable to control operation of the immersion heater 304 comprises the controller 214 being configured to be operable to activate and deactivate the electromechanical switch 310 (in this example, a relay) using the relay 308 of the HIU 104. Thus, in this example, the controller 214 of the HIU 104 controls the external relay 310 via the internal relay 308. The HIU 104 therefore digitally controls the immersion heater 304. In this example, the immersion heater 304 also has its own power supply 314, referred to herein as the “immersion heater power supply”. In this specific example, the immersion heater power supply 314 comprises a 230V alternating current (AC) power supply. Thus, in this example, the immersion heater 304 is connected to 230V AC. The immersion heater power supply 314 may have a different voltage, such as 240V, in other examples. In this example, the external relay 310 is, in effect, an intermediary between the immersion heater power supply 314 and the immersion heater 304, and is controllable by the control signal 312. Referringto Figure 4, there is shown an example of a method 400 of controlling an HIU 104. In this example, the method is performed by the controller 214 of the HIU 104. At item 402, the controller 214 detects a DHW flow. The controller 214 may detect the DHW flow based on a control signal from the DHW flow switch 252, or otherwise. At item 404, the controller 214 determines whether TPF >tsphw + TSPhwoffset- In this example, TPP is the temperature of the heat network flow 106 prior to the immersion heater 304, which may be referred to as the “primary flow temperature”. The primary flow temperature may be measured by the external sensor 306. The primary flow temperature may not be the same as the temperature of the water entering the HIU 106, for example when the immersion heater 304 is active. In this example, TSPHW is a hot water temperature setpoint. The hot water temperature setpoint is a target hot water temperature. The hot water temperature setpoint may be referred to as a “DHW temperature setpoint”, denoted TSPDHW. The DHW temperature setpoint may be set on the HIU 104. For example, the HIU 104 may comprise a control panel, and the control panel may be useable to set the DHW temperature setpoint. The DHW temperature setpoint may be set by a manufacturer of the HIU 104, by a heating engineer, by a user of the HIU 104, or otherwise. In this specific example, the DHW temperature setpoint is 50°C. This corresponds to the 50°C temperature that the DHW 110 should reach in accordance with current CIBSE guidelines. In this example, TSPHW 0PPSET is a hot water temperature setpoint offset. The hot water temperature setpoint offset is arranged to account for a temperature drop across the DHW heat exchanger 226. The hot water temperature setpoint offset may be referred to as a “DHW temperature setpoint offset”, denoted TSPDHW 0PPSET. In this specific example, the DHW temperature setpoint offset is 5°C. In this example, the DHW temperature setpoint offset accounts for an assumed 5°C temperature drop across the DHW heat exchanger 226. If, at item 404, the controller 214 determines that TPP >tspdhw + TSPDHW offset’ then, at item 406, the HIU 104 produces DHW 110, and the method 400 returns to item 402. If, at item 404, the controller 214 determines that TPP <tspdhw + TSPdhw offset’ then, at item 408, the controller 214 activates the internal relay 308, such that the state of the internal relay 308 is “ON”. Activating the internal relay 308 activates the external relay 310 which, in turn, activates the immersion heater 304. The immersion heater 304 then applies a temperature boost to the heat network flow 106, with temperature-boosted water being supplied to the district inlet 200 of the HIU 104. The HIU 104 may apply the temperature boost when the HIU 104 is operating in a heat network flow heating mode, referred to herein as a temperature boost mode. Following item 408, the method 400 proceeds to item 406, where DHW 110 is delivered. The value of TSPHW + TSPHW offset may therefore be a primary flow temperature threshold. If the primary flow temperature is at most the primary flow temperature threshold, the controller 214 activates the immersion heater 304; otherwise, if the primary flowtemperature is above the primary flow temperature threshold, the controller 214 does not activate the immersion heater 304. Thus, and by way of a summary, when there is DHW 110 demand (item 402), the HIU 104 checks (item 404) if the primary flow temperature measured by the external sensor 306 is higher than the DHW temperature setpoint plus 5°C. If the primary flow temperature is higher than the DHW temperature setpoint plus 5°C, then DHW 110 production starts (item 406). If the primary flow temperature is at most the DHW temperature setpoint plus 5°C, the HIU 104 connects (item 408) the internal relay 308, which starts the immersion heater 304. When the DHW demand stops, the HIU 104 disconnects the internal relay 308, which stops the immersion heater 304. In this example, it is necessary, but not sufficient, for the DHW flow switch 252 to be in an activated state (indicating a DHW flow) for the controller 214 to activate the immersion heater 304. Thus, activation of the immersion heater 304 is dependent “at least” on the DHW flow switch 252 being in an activated state in this example. In this specific example, a further immersion heater activation criterion is that TPP <TSPHW + TSPHW 0PPSET. In this example, the controller 214 deactivates, or at least does not activate, the immersion heater 304 if the DHW flow switch 252 is in a deactivated state (in which case there is no DHW flow). The controller 214 may therefore be configured to be operable to control operation of the immersion heater 304 dependent at least on whether the DHW flow switch 252 is in an activated or deactivated state. Referring to Figures 5 to 7, there is shown an example of an immersion heater 304. In this example, the immersion heater 304 comprises a tank 500 and a cartridge heater 502. In some examples, the cartridge heater 502 is a 7-kilowatt (kW) cartridge heater. In some examples, the tolerance of the power rating of the cartridge heater 502 is 7kW +5% and / or -10%. Thus, a cartridge heater 502 having a power from 6.3kW to 7.5kW may have a 7kW power rating. A 7kW cartridge heater may raise water temperature by around 10°C at a flow rate of around 10 l / m. At flow rates of around 5 l / m and around 2.5 l / m, a 7kW cartridge heater may raise water temperature by around 20°C and around 40°C respectively. In this example, the cartridge heater 502 is either on or off at any given point in time. In this example, the immersion heater 304 comprises an inlet 504 and an outlet 506. In this example, the inlet and outlet 504, 506 are in the tank 500 component of the immersion heater 304. The inlet 504 receives a supply of water to the immersion heater 304. The supply of water may correspond to the heat network flow 106. The outlet 506 supplies water from the immersion heater 304 to the district inlet 200 of the HIU 104. The outlet 506 may be hydraulically connected to the district inlet 200 of the HIU 104 in any manner. For example, the outlet 506 may be hydraulically connected to the district inlet 200 of the HIU 104 via one or more pipes. Depending on an activation state of the cartridge heater 502, the water supplied by the immersion heater 304 to the district inlet 200 of the HIU 104 may or may not have been heated by the cartridge heater 502. In this example, the tank 500 comprises a cartridge heater opening 508 through which the cartridge heater 502 can be inserted into, and removed from, the tank 500. In this example, the cartridge heater opening 508 comprises a screw thread 510. In this example, the cartridge heater 502 comprises an end piece 512 having a screw thread 514 that is complementary to the screw thread 510 of the cartridge heater opening 508. In use, the cartridge heater 502 can be inserted into the tank 500 via the cartridge heater opening 508 and screwed securely in place to provide a secure, watertight seal. The cartridge heater 502 may subsequently be removed from thetank500 by unscrewingand removing the cartridge heater 502 from the tank 500. The cartridge heater 502 may be removed for repair, replacement, upgrade, or otherwise. The end piece 512 of the cartridge heater 502 may comprise a resin sealing. In some examples, a blanking plug (not shown) having the same complementary screw thread as the screw thread 514 of the end piece 512 may be screwed into the cartridge heater opening 508 when the cartridge heater 502 is not present. Water can then still flow through the immersion heater 304, via the inlet and outlet 504, 506, to the HIU 104 in a watertight manner even if a temperature boost cannot be applied to such water. The HIU 104 may be configured to disable or bypass the temperature boost mode temporarily until the cartridge heater 502 is present again, at which point the temperature boost mode may be enabled or reinstated. Thus, and by way of a summary, the immersion heater 304 may comprise a tank 500 and a cartridge heater 502. The tank 500 may comprise an inlet 504 and an outlet 506. The outlet 506 of the tank 500 may be hydraulically couplable to the district inlet 200. The tank 500 may comprises a cartridge heater opening 508, and the cartridge heater 502 may be insertable into, and removable from, the tank 500 through the cartridge heater opening 508. The cartridge heater opening 508 may comprise a screw thread 510. The cartridge heater 502 may be securable to the tank 500 via a complementary screw thread 514. The cartridge heater opening 508 may be sealable by a blanking plug having the complementary screw thread 514. The cartridge heater 502 may comprise a 7kW cartridge heater. In this example, the immersion heater 304 comprises a thermostat 700. The thermostat 700 has a thermostat cutoff temperature threshold. The thermostat cutoff temperature threshold is a temperature threshold at which the thermostat 700 will turn the immersion heater 304 off. In this specific example, the thermostat 700 is an 85°C thermostat, having an 85°C thermostat cutoff temperature threshold. Thus, in this specific example, if the water temperature in the immersion heater 304 exceeds 85°C, the thermostat 700 will turn the immersion heater 304 off. The thermostat 700 may have a different thermostat cutoff temperature threshold in other examples. Another example thermostat cutoff temperature threshold is 65°C. In general, and for reasons that will be explained below, the thermostat cutoff temperature threshold is greater than a maximum district inlet temperature threshold (for example, 60°C). In this example, the immersion heater 304 comprises a thermofuse 702. The thermofuse 702 has a thermofuse cutoff temperature threshold. The thermofuse cutoff temperature threshold is a temperature threshold at which the thermofuse 702 will turn the immersion heater 304 off. In this specific example, the thermofuse 702 is a 130°C thermofuse, having a 130°C thermofuse cutoff temperature threshold. Thus, in this specific example, if the water temperature in the immersion heater 304 exceeds 130°C, the thermofuse 702 will turn the immersion heater 304 off. The thermofuse 702 may have a different thermofuse cutoff temperature in other examples. Another example thermofuse cutoff temperature is 110°C. In general, and for reasons that will be explained below, the thermofuse cutoff temperature threshold is greater than the maximum district inlet temperature threshold (for example, 60°C) and is greater than the thermostat cutoff temperature threshold (for example, 65°C or 85°C). Within the immersion heater 304, there are therefore two safety devices, namely the thermostat 700 and the thermofuse 702. If, for any reason, the HIU 104 fails to cut off the immersion heater 304 and the temperature inside the immersion heater 304 exceeds 85°C ± 5°C, the thermostat 700 will cut out the immersion heater 304. The ± 5°C value corresponds to the tolerance of the thermostat 700. The immersion heater 304 will not turn back on until the temperature inside the immersion heater 304 falls to 76°C ± - 5°C. In this example, there is therefore a 9°C differential relative to the thermostat cutoff temperature threshold. If there is a severe problem, and if the temperature inside the immersion heater 304 exceeds 130°C, the thermofuse 702 cuts out the immersion heater 304. If this happens, the immersion heater 304, or at least the cartridge heater 502, may need to be replaced. The immersion heater 304 may fail if the thermostat 700 exceeds its maximum number of cycles. The maximum number of thermostat cycles may be 100,000. The immersion heater 304 may also fail if the temperature inside the immersion heater 304 regularly reaches close to the thermofuse cutoff temperature, since this will weaken the thermofuse 702 and will cause the thermofuse 702 to fail over time. By using the HIU 104 to control the on / off temperature of the immersion heater 304 (in other words, the temperature at which the immersion heater 304 turns on or off), longevity of the immersion heater 304 may be significantly increased. This is because the thermostat 700 should very rarely be used. Additionally, the internal relay 308 of the HIU 104 may have a maximum number of 1,000,000 relay cycles. By using the relay 308 to activate and deactivate the immersion heater 304, longevity may be increased tenfold compared to using the thermostat 700 to activate and deactivate the immersion heater 304 during normal operation. Referring to Figure 8, there is shown an example of a graph 800 representing temperature boost in the case of high DHW flow. The graph 800 shows a thermocouple J (TCJ) inlet temperature 802. A thermocouple J is a type of sensor. The TCJ inlet temperature 802 may correspond to a temperature of the cartridge heater 502 of the immersion heater 304. The TCJ inlet temperature 802 may not be measured in practical HIU deployments. However, the TCJ inlet temperature 802 is shown in the graph 800 to facilitate understanding. The graph 800 shows a pipework inlet temperature 804. The pipework inlet temperature 804 may correspond to the primary flow temperature, for example as measured by the external sensor 306. The graph 800 shows a TCJ outlet temperature 806. The TCJ outlet temperature 806 may correspond to a temperature of water in the immersion heater 304. The TCJ outlet temperature 806 may not be measured in practical HIU deployments. However, the TCJ outlet temperature 806 is shown in the graph 800 to facilitate understanding. The graph 800 shows a pipework outlet temperature 808. The pipework outlet temperature 808 may correspond to the temperature of water in the district inlet 200, for example as measured by the district inlet sensor 232. The pipework outlet temperature 808 may be referred to as the “district inlet temperature” or, more generally, as the “heat network flow inlet temperature”. The graph 800 shows a return temperature 810. The return temperature 810 may correspond to the temperature of the heat network return 108, for example as determined by the district return sensor 236. The graph 800 shows a DHW temperature 812. The DHW temperature 812 may correspond to the temperature of DHW 110 supplied by the HIU 104. The DHW temperature may be measured at a DHW outlet, such as at a sink, and / or by the DHW sensor 248. The graph 800 shows cold water inlet temperature 814, for example as measured by the cold water sensor 246. The graph 800 shows a temperature boost 816 as the difference between the pipework outlet temperature 808 and the pipework inlet temperature 804. The temperature boost 816 is therefore the boost resulting from the immersion heater 304. Item 818 shows where DHW production starts. In this example, the DHW flow rate is 8.51 litres per minute (l / m) when DHW production starts at item 818. This is a relatively high DHW flow rate. For example, a hot water tap of a sink may be turned on. The DHWflow switch 252 may be activated as a result. The controller 214 of the HIU 104 may detect that the DHW flow switch 252 has been activated and may check the pipework inlet temperature 804. In this instance, the pipework inlet temperature 804 is around 43-45°C, i.e. below the 55°C value of the DHW temperature setpoint (50°C) plus the DHW temperature setpoint offset (5°C). In this example, the controller 214 activates the immersion heater 304 accordingly. As a result of activating the immersion heater 304, the TCJ inlet temperature 802 rapidly increases. The TCJ outlet temperature 806 rises and, consequently, the pipework outlet temperature 808 also rises to around 50°C. The temperature boost 816 of approximately 10°C can also be seen. The DHW temperature 812 rises, following the rise in the pipework outlet temperature 808. The DHW temperature 812 is lower than the pipework outlet temperature 808 because of the approximately 5°C temperature drop across the DHW heat exchanger 226. Additionally, it may not be desired for the DHW temperature 812 to be higher than 50°C, where 50°C is the DHW temperature setpoint. If the DHW temperature 812 started to exceed 50°C, the modulating valve 220 would start to close and the temperature of the DHW 110 produced would start to decrease. The pipework inlet temperature 804, the return temperature 810, and the cold water inlet temperature 814 are substantially unaffected by the temperature boost. The pipework inlet temperature 804 shows a small spike after item 818, caused by primary temperature stabilisation. Referring to Figure 9, there is shown an example of a graph 900 representing temperature boost in the case of high DHW flow followed by reduced DHW flow. The lines shown in the graph 900 correspond to those shown in the graph 800. The same reference numerals have been used, but incremented by 100. In particular, the graph 900 shows a TCJ inlet temperature 902, a pipework inlet temperature 904, a TCJ outlet temperature 906, a pipework outlet temperature 908, a return temperature 910, a DHW temperature 912, and a cold water inlet temperature 914. Item 918 shows where DHW production starts. In this example, the DHW flow rate at item 918 is 8 l / m. This corresponds to a relatively high DHW flow rate. In this example, the DHW flow rate decreases at item 920. In this example, the decreased DHW flow rate at item 920 is 4 l / m. This corresponds to a relatively low DHW flow rate. The high DHW flow rate period is represented by arrow 922 and the low DHW flow rate period is represented by arrow 924. In more detail, DHW production starts at item 918. The graph 900 corresponds generally to the graph 800 during the high DHW flow rate period 922. However, at item 920, the DHW flow rate is reduced. For example, the hot water tap of the sink may be partially closed such that the DHW demand and, hence, DHW flow rate decreases. The lower DHW flow rate causes the TCJ outlet temperature 906 and the pipework outlet temperature 908 to increase. This is because the water passing through the immersion heater 304 is heated by the cartridge heater 502 for a longer duration at lower flow rates than at higher flow rates. If the pipework outlet temperature 908 were allowed to continue to increase, it could become sufficiently hot to present a scalding risk. In accordance with examples, when the pipework outlet temperature 908 reaches a maximum pipework outlet temperature threshold, the controller 214 of the HIU 104 deactivates the immersion heater 304. This causes the TCJ inlet temperature 902 to drop rapidly. The TCJ outlet temperature 906 and the pipework outlet temperature 908 drop accordingly. In examples, the maximum pipework outlet temperature threshold is higher than the primary flow temperature threshold. In this specific example, the maximum pipework outlet temperature threshold is60°C and is, therefore, higher than the example primary flow temperature threshold of 55°C described above. The maximum pipework outlet temperature threshold may be referred to as a “maximum district inlet temperature threshold” or, more generally, as a “maximum heat network flow inlet temperature”. Thus, the controller 214 may be configured to be operable to deactivate the immersion heater 304 based at least on the pipework outlet temperature 908 being above a pipework outlet temperature threshold (for example, 60°C). In some examples, the controller 214 is configured to be operable to deactivate the immersion heater 304 in response to the pipework outlet temperature 908 being above the pipework outlet temperature threshold (for example, 60°C). In such examples, the pipework outlet temperature 908 being above the pipework outlet temperature threshold is sufficient for the controller 214 to deactivate the immersion heater 304. In accordance with examples, when the pipework outlet temperature 908 drops to a minimum pipework outlet temperature threshold, the controller 214 of the HIU 104 activates the immersion heater 304 again. This causes the TCJ outlet temperature 906 and the pipework outlet temperature 908 to increase again. When the pipework outlet temperature 908 reaches the maximum pipework outlet temperature threshold (in this example, 60°C) again, the controller 214 of the HIU 104 deactivates the immersion heater 304 again, which causes the TCJ inlet temperature 902 to drop rapidly again. The TCJ outlet temperature 906 and the pipework outlet temperature 908 drop accordingly again. In this specific example, the minimum pipework outlet temperature threshold is 55°C and is, therefore, the same as the example primary flow temperature threshold of 55°C described above and lower than the example maximum pipework outlet temperature threshold of 60°C described above. The minimum pipework outlet temperature threshold may be referred to as a “minimum district inlet temperature threshold” or, more generally, as a “minimum heat network flow inlet temperature”. This cycling continues while DHW 100 is being demanded. Despite the rapid temperature changes to the TCJ inlet temperature 902 and the pipework outlet temperature 908, the DHW temperature 912 remains relatively stable at around 50°C during the low DHW flow rate period 924. Thus, HIU control is facilitated. If the primary flow temperature is below 55°C, the immersion heater 304 turns on when DHW production starts. If the temperature entering the HIU 104, namely the district inlet temperature 908, exceeds 60°C, the HIU 104 turns the immersion heater off 304. The district inlet temperature exceeding 60°C will likely happen during low DHW flow. The HIU 104 turns the immersion heater 304 back on when the temperature entering the HIU 104, namely the district inlet temperature 908, falls below 55°C. This controls the on / off cycling during the low DHW flow rate period 924, where the DHW flow rate may be around 3-4 l / m. It may take around 90 seconds for the district inlet temperature 908 to drop by 5°C from 60°C to 55°C, even if the primary flow temperature is around 45°C. The immersion heater 304 may not be a particularly efficient heat source, for example compared to a heat pump. However, in accordance with examples, the immersion heater 304 is only activated when DHW is being used (accountingfor around 20% of heated water usage), and only then when the primary flow temperature is below the primary flow temperature threshold. Thus, the immersion heater 304 may not be used particularly regularly in practice and may only be used to boost water temperature, rather than heat the water fully. In cases of low (continuous) flow, the immersion heater 304 may only be activated for around 50% of the DHW 110 demand time. In some examples, the HIU 104 is configured to limit a maximum DHW flow rate, for example using the modulating valve 220. At higher flow rates, the cartridge heater 502 may not have sufficient time to apply a sufficient temperature boost, for example 10°C, to the water passing through the immersion heater 304. The maximum DHW flow rate may, for example, be set as 10 l / m. Thus, in examples, an electric HIU 104 comprises a flow rate limiter. In addition to enabling sufficient temperature boost, the flow rate limiter may restrict the amount of heated water drawn from the secondary circuit. This may be especially effective in smaller apartments, where less heated water may be needed than for larger apartments. Thus, the controller 214 may be configured to be operable to control the modulating valve 220, when DHW 110 is being produced, to limit a maximum flow rate of water flowing through the heat network flow inlet 200. This enables the primary flow temperature to be boosted to at least 55°C. If a flow rate of 10 l / m were exceeded, a sufficient temperature boost may not be achieved. This may affect the ability of the HIU 104 to deliver the DWH 110 at the target temperature. Examples described herein may therefore provide DHW stability and may ensure that DHW temperature never exceeds 60°C, or at least reduces the likelihood of it doing so, as this would pose a scalding risk. The HIU 104 may operate such that, if the DHW temperature reaches 55°C, the modulating valve 220 fully closes. The DHW temperature may reach 55°C if the DHW flow rate suddenly drops, for instance if a hot water tap changes from high flow to low flow very quickly. The modulating valve 220 may fully close on the basis that the DHW heat exchanger 226 is still sufficiently hot that the DHW heat exchanger 226 can heat the mains cold water, using residual heat, to produce DHW at 50°C. However, this may not always be effective. For example, if the DHW temperature is 55°C and the district inlet temperature has not yet exceeded 60°C, the modulating valve 220 will fully close, but the immersion heater 304 will remain on. Since the modulating valve 220 is fully closed, water stops flowing through the immersion heater 304 and into the HIU 104. The temperature of the water in the immersion heater 304 will therefore continue to increase. This is because water from the immersion heater 304 with a temperature of at least 60°C is not entering the HIU and, therefore, the HIU 104 does not turn the immersion heater 304 off. In accordance with examples, the controller 214 is configured to keep the modulating valve 220 open by at least a small number of steps during DHW production. For example, the controller 214 may be configured to keep the modulating valve 220 open by at least 10 steps during DHW production, even during very low DHW flow, such that the modulating valve 220 is always open to some degree during DHW production. Even if the modulating valve 220 is open by a small number of steps during DHW production, the immersion heater 304 will turn off. This is because the modulating valve 220 being shut by such a significant amount indicates that there is sufficient heat in the DHW heat exchanger 226 to produce DHW 110 at 50°C without new primary heated water. Thus, the controller 214 may be configured to be operable to control the modulating valve 220, when the immersion heater 304 is active, to impose a minimum flow rate of water through the district inlet 200. Another feature of the present disclosure concerns heating temperatures for apartments. In general, the primary flow temperature is stable, whether that be 55°C, 60°C, or otherwise. A 5°C loss across the heating heat exchanger 218 is assumed. Thus, the temperature of the heating flow 116 may be set to be 5°C less than the primary flow temperature. As explained above, in some examples, a heat pump may be selected for use as a heat source when the outside temperature is above 3°C. Heating may still be used in apartments in such situations, for example when the outside temperature is not significantly above 3°C. A heat pump may provide a primary flow temperature of 45°C. In such situations, the temperature of the heating flow 116 may be set to be 40°C. In general, a heating system 118 is designed (including, for example, the size of radiators) based on operating temperatures, for example of the heating flow 116. Underfloor heating, for example, may only need the heating flow 116 to be at 40°C. However, a heat network flow 106 may typically run at 60°C all year round such that DHW 110 may be produced at up to 55°C. Running the heating flow 116 at, for example, 55°C may be problematic for underfloor heating. Thus, the temperature of the heating flow 116 may be set to be, for example, 40°C. When the outside temperature is less than 3°C, one or more boilers may be used instead of the heat pump(s).This may raise the primary flow temperature to 60°C. In such situations, the temperature of the heating flow 116 for the apartment should be 55°C. In some examples, the temperature of the heating flow 116 may be set on the HIU 104 and may not be updated, for example based on the heat source being used. An end user should not be expected to adjust the temperature of the heating flow 116 manually when the primary flow temperature changes. In accordance with examples, the HIU 104 is configured such that, when the HIU 104 is in the temperature boost mode, the HIU 104 sets the setpoint temperature of the heating flow 116 to be 5°C less than the primary flow temperature. As the primary flow temperature changes (increases or decreases), the temperature of the heating flow 116 changes accordingly. Thus, the controller 214 may be configured to be operable to set a heating flow temperature setpoint, when the temperature boost mode is activated, based at least on: (i) the primary flow temperature; and (ii) a heating flow temperature setpoint offset. The heating flow temperature setpoint offset is arranged to account for a temperature drop across the heating heat exchanger 218 of the HIU 104. Thus, when the HIU 104 is in the temperature boost mode, the heating flow temperature setpoint may be 5°C less than the primary flow temperature. Additionally, the method 400 described above with reference to Figure 4 may be followed. Examples described herein relate generally to district heating systems. Such examples are generally appliable to any other type of heat network, such as communal heating systems. Examples described herein relate specifically to domestic hot and cold water. The term “domestic” is used in this context in relation to water indicate water that may contact humans. Domestic hot and cold water may therefore be used in domestic and / or commercial settings. Examples described herein generally relate to domestic applications. Such examples are generally appliable to any other application, such as commercial applications. In examples described above, the immersion heater 304 is not comprised in the HIU 104. In other examples, the immersion heater 304 is comprised in the HIU 104. For example, the immersion heater 304 may be a component of the HIU 104. For example, the immersion heater 304 may be incorporated within a hydraulic manifold of the HIU 104, with casing of the HIU 104 increased in size to accommodate the immersion heater 304. Thus, an HIU 104 comprising a heat network flow inlet 200 (which may also be referred to as a district inlet) and a controller 214 may be provided. The controller 214 may be configured to be operable to control operation of an immersion heater 304. The immersion heater 304 may be configured to heat a supply of water to the heat network flow inlet 200 when activated by the controller 214. The HIU 104 may comprise a hot water flow switch 252 (which may also be referred to as a DHW flow switch 252). The controller 214 may be configured to be operable to control operation of the immersion heater 304 dependent at least on whether the hot water flow switch 252 is in an activated or deactivated state. In some examples, the controller 214 is configured to be operable to activate the immersion heater 304 based at least on the domestic hot water flow switch 252 being in the activated state. As explained above and below, activation of the immersion heater 304 may be dependent on one or more additional immersion heater activation criteria. In some examples, the controller 214 is configured to be operable to deactivate the immersion heater 304 based at least on the domestic hot water flow switch 252 being in the deactivated state. Deactivation of the immersion heater 304 may be dependent on one or more additional immersion heater deactivation criteria. In some examples, the controller 214 is configured to be operable to deactivate the immersion heater 304 in response to the domestic hot water flow switch 252 being in the deactivated state. In such examples, it is sufficient for the domestic hot water flow switch 252 to be in the deactivated state for the controller 214 to deactivate the immersion heater 304. In some examples, the controller 214 is configured to be operable to control operation of the immersion heater 304 dependent at least on a primary flow temperature. The primary flow temperature is a temperature of a supply of water to the immersion heater 304. As explained above, the supply of water to the immersion heater 304 may be referred to as the “primary flow”. In such examples, control of the immersion heater 304 may be dependent on both: (i) whetherthe hot water flow switch 252 is in an activated or deactivated state; and (ii) the primary flow temperature. The primary flow temperature may be obtained from the external sensor 306 (which may also be referred to as a primary flow sensor 306). In some examples, the controller 214 is configured to be operable to activate the immersion heater 304 based at least on the primary flow temperature being at most a primary flow temperature threshold. In some examples, the HIU 104 comprises a hot water heat exchanger 226. The primary flow temperature threshold may be based at least on: (i) a hot water temperature setpoint; and (ii) a hot water temperature setpoint offset. The hot water temperature setpoint may be a target hot water temperature and may be denoted as TSPHW. The hot water temperature setpoint offset may be arranged to account for a temperature drop across the hot water heat exchanger 226 and may be denoted as TSPHW 0PPSET. In the case of DHW, the primary flow temperature threshold may be expressed as TSPdhw + TSPDHW 0PPSET, where TSPDHW is a DHW temperature setpoint and TSPdhw offset is a DHW temperature setpoint offset. In some examples, TSPDHW = 50°C. In some examples, TSPDHW 0PPSET = 5°C. Such values of TSPDHW and TSPdhw offset may enable DHW to be provided at a temperature that meets current CIBSE guidance, considering heat exchanger temperature drops. Thus, in some examples, the primary flow temperature threshold is 55°C. In some examples, the HIU 104 comprises a heating heat exchanger 218. The controller 214 may be configured to be operable to set a heating flow temperature setpoint, when a heat network flow heating mode is activated, based at least on: (i) the primary flow temperature; and (ii) a heatingflowtemperature setpoint offset. The heating flow temperature setpoint offset may be arranged to account for a temperature drop across the heating heat exchanger 218. For example, the heating flow temperature setpoint offset may be -5°C such that the heatingflow temperature setpoint is 5°C below the primary flow temperature. In some examples, the controller 214 is configured to be operable to deactivate the immersion heater 304 based at least on a heat network flow inlet temperature being above a maximum heat network flow inlet temperature threshold. The heat network flow inlet temperature may be a temperature of the water supplied to the heat network flow inlet 200. In some examples, the controller 214 is configured to be operable to activate the immersion heater 304 based at least on the heat network flow inlet temperature falling belowa minimum heat network flow inlet temperature threshold. In some examples, the immersion heater 304 comprises a thermostat 700. The thermostat 700 may have a thermostat cutoff temperature threshold. In some examples, the thermostat cutoff temperature threshold is 85°C. In some examples, the thermostat cutoff temperature threshold is 65°C. The thermostat cutoff temperature threshold may be greater than the maximum heat network flow inlet temperature threshold. In some examples, the immersion heater 304 comprises a thermofuse 702. The thermofuse 702 may have a thermofuse cutoff temperature threshold. In some examples, the thermofuse cutoff temperature threshold is 130°C. In some examples, the thermofuse cutoff temperature threshold is 110°C. The thermofuse cutoff temperature threshold may be greater than the thermostat cutoff temperature threshold. In some examples, the HIU 104 comprises a relay 308 and the immersion heater 304 is controllable by an electromechanical switch 310. The controller 214 being configured to be operable to control operation of the immersion heater 304 may comprise the controller 214 being configured to be operable to activate and deactivate the electromechanical switch 310 usingthe relay 308. In some examples, the HIU 104 comprises a modulating valve 220. The controller 214 may be configured to be operable to control the modulating valve 220, when DHW 110 is being produced, to: (i) limit a maximum flow rate of water flowing through the heat network flow inlet 200; and / or (ii) impose a minimum flow rate of water through the heat network flow inlet 200. In some examples, the immersion heater 304 comprises a tank 500 and a cartridge heater 502. The tank may comprise an inlet 504 and an outlet 506. The outlet 506 of the tank 500 may be hydraulically couplable to the heat network flow inlet 200. In some examples, the tank 500 comprises a cartridge heater opening 508. The cartridge heater 502 may be insertable into, and removable from, the tank 500 through the cartridge heater opening 508. In some examples, the cartridge heater opening 508 comprises a screw thread 510. The cartridge heater 502 may be securable to the tank 500 via a complementary screw thread 514. The cartridge heater opening 508 may be sealable by a blanking plug havingthe complementary screw thread 514. In some examples, the cartridge heater 502 comprises a 7kW cartridge heater. Additionally, an HIU 104 comprising a heat network flow inlet 200, a processor, and memory may be provided. The memory may comprise instructions which, when executed by the processor, cause the HIU 104 to control operation of an immersion heater 304. The immersion heater 304 may be operable to apply a temperature boost to a fluid supply to the heat network flow inlet 200. Examples described herein relate to an HIU 104 and an immersion heater 304. In some examples, the HIU 104 comprises the immersion heater 304. The HIU 104 may have a power supply, and that power supply may also power the immersion heater 304. In some examples, the HIU 104 and the immersion heater 304 are comprised in a system in which the HIU 104 does not comprise the immersion heater 304. The HIU 104 may have a power supply and the immersion heater 304 may have a separate power supply. A heat network 100 comprising a plurality of HIUs 104 and an energy centre 102 may be provided. The energy centre 102 may comprise at least one heat source and at least one buffer tank. A computer-implemented method of controlling an HIU 104 may also be provided. The method may comprise controlling the HIU 104 to activate or deactivate an immersion heater 304. Activating the immersion heater 304 may cause the immersion heater 304 to apply a temperature boost to a supply of fluid to the HIU 104. A computer program may be provided. The computer program may be configured, when executed, to perform one or methods as described herein. A heating system 300 may also be provided. The heating system 300 may comprise: (i) an HIU 104 comprising a district inlet 200 and a controller 214; and (ii) an immersion heater 304 operable to heat a supply of fluid to the district inlet 200. The controller 214 may be operable to activate and deactivate the immersion heater 304. An electronic HIU 104 may be provided. The electronic HIU 104 may comprise a controller 214 and a modulating valve 220. The controller 214 may be configured to be operable to control the modulating valve 220 to impose a maximum and / or minimum flow rate to fluid flowing through the HIU when an immersion heater 304 is heating a supply of fluid to the HIU 104. An immersion heater 304 may be provided. The immersion heater 304 may comprise a tank 500 and a cartridge heater 502. The tank 500 may comprises an inlet 504 and an outlet 506. The inlet 504 of the tank may be to receive a heat network flow 106. The outlet 506 of the tank 500 may be fluidly couplable to a heat network flow inlet 200 of an HIU 104. The immersion heater 304 may be configured to be controllable by the HIU 104.

Claims

1. A heat interface unit, HIU, comprising:a heat network flow inlet; anda controller,wherein the controller is configured to be operable to control operation of an immersion heater, andwherein the immersion heater is configured to heat a supply of water to the heat network flow inlet when activated by the controller.

2. An HIU accordingto claim 1, comprising:a hot water flow switch,wherein the controller is configured to be operable to control operation of the immersion heater dependent at least on whether the hot water flow switch is in an activated or deactivated state.

3. An HIU according to claim 2, wherein the controller is configured to be operable to activate the immersion heater based at least on the domestic hot water flow switch being in the activated state.

4. An HIU according to claim 2 or 3, wherein the controller is configured to be operable to deactivate the immersion heater based at least on the domestic hot water flow switch being in the deactivated state.

5. An HIU according to any of claims 1 to 4, wherein the controller is configured to be operable to control operation of the immersion heater dependent at least on a primary flow temperature, the primary flow temperature being a temperature of a supply of water to the immersion heater.

6. An HIU according to claim 5, wherein the controller is configured to be operable to activate the immersion heater based at least on the primary flow temperature being at most a primary flow temperature threshold.

7. An HIU accordingto claim 6, comprising:a hot water heat exchanger,wherein the primary flow temperature threshold is based at least on:a hot water temperature setpoint; and a hot water temperature setpoint offset, wherein the hot water temperature setpoint is a target hot water temperature, and wherein the hot water temperature setpoint offset is arranged to account for a temperature drop across the hot water heat exchanger.

8. An HIU accordingto any of claims 5 to 7, comprising:a heating heat exchanger,wherein the controller is configured to be operable to set a heating flow temperature setpoint, when a heat network flow heating mode is activated, based at least on:the primary flow temperature; anda heating flow temperature setpoint offset, and wherein the heating flow temperature setpoint offset is arranged to account for a temperature drop across the heating heat exchanger.

9. An HIU according to any of claims 1 to 8, wherein the controller is configured to be operable to deactivate the immersion heater based at least on a heat network flow inlet temperature being above a maximum heat network flow inlet temperature threshold, the heat network flow inlet temperature being a temperature of the water supplied to the heat network flow inlet.

10. An HIU according to claim 9, wherein the controller is configured to be operable to activate the immersion heater based at least on the heat network flow inlet temperature falling below a minimum heat network flow inlet temperature threshold.

11. An HIU according to claim 9 or 10, wherein the immersion heater comprises a thermostat, wherein the thermostat has a thermostat cutoff temperature threshold, andwherein the thermostat cutoff temperature threshold is greater than the maximum heat network flow inlet temperature threshold.

12. An HIU according to claim 11, wherein the immersion heater comprises a thermofuse, wherein the thermofuse has a thermofuse cutoff temperature threshold, and wherein the thermofuse cutoff temperature threshold is greater than the thermostat cutoff temperature threshold.

13. An HIU accordingto any of claims 1 to 12, comprising:a relay,wherein the immersion heater is controllable by an electromechanical switch, andwherein the controller being configured to be operable to control operation of the immersion heater comprises the controller being configured to be operable to activate and deactivate the electromechanical switch using the relay.

14. An HIU accordingto any of claims 1 to 13, comprising:a modulatingvalve,wherein the controller is configured to be operable to control the modulating valve, when hot water is being produced by the HIU, to:limit a maximum flow rate of water flowing through the heat network flow inlet; and / orimpose a minimum flow rate of water through the heat network flow inlet.

15. An HIU according to any of claims 1 to 14, wherein the immersion heater comprises:a tank; anda cartridge heater,wherein the tank comprises an inlet and an outlet, andwherein the outlet of the tank is fluidly couplable to the heat network flow inlet.

16. An HIU according to claim 15, wherein the tank comprises a cartridge heater opening, and wherein the cartridge heater is insertable into, and removable from, the tankthrough the cartridge heater opening.

17. An HIU according to claim 16, wherein the cartridge heater opening comprises a screw thread, wherein the cartridge heater is securable to the tank via a complementary screw thread, and wherein the cartridge heater opening is sealable by a blanking plug havingthe complementary screw thread.

18. An HIU according to any of claims 15 to 17, wherein the cartridge heater comprises a 7-kilowatt cartridge heater.

19. A heat interface unit, HIU, comprising:a heat network flow inlet;a processor; andmemory,the memory comprising instructions which, when executed by the processor, cause the HIU to control operation of an immersion heater, the immersion heater being operable to apply a temperature boost to a fluid supply to the heat network flow inlet.

21. An HIU according to any of claims 1 to 20, wherein the HIU comprises the immersion heater.

22. A system comprising:an HIU accordingto any of claims 1 to 20; and the immersion heater.

23. A heat network comprising:a plurality of HIUs accordingto any of claims 1 to 21; andan energy centre.

24. A computer-implemented method of controlling a heat interface unit, HIU, the method comprising:controlling the HIU to activate or deactivate an immersion heater,wherein activating the immersion heater causes the immersion heater to apply a5 temperature boost to a supply of fluid to the HIU.

25. An immersion heater comprising: a tank; anda cartridge heater,10 wherein the tank comprises an inlet and an outlet,wherein the inlet of the tank is to receive a heat network flow,wherein the outlet of the tank is fluidly couplable to a heat network flow inlet of a heat interface unit, HIU, andwherein the immersion heater is configured to be controllable by the HIU.31 01 25AMENDMENTS TO THE CLAIMS HAVE BEEN FILED AS FOLLOWS:Claims1. A heat interface unit, HIU, comprising: a heat network flow inlet; and5 a controller,wherein the controller is configured to be operable to control operation of an immersion heater, andwherein the immersion heater is configured to heat a supply of water to the heat network flow inlet when activated by the controller.

102. An HIU according to claim 1, comprising: a hot water flow switch, wherein the controller is configured to be operable to control operation of the immersion heater dependent at least on whether the hot water flow switch is in an15 activated or deactivated state.

3. An HIU according to claim 2, wherein the controller is configured to be operable to activate the immersion heater based at least on the domestic hot water flow switch being in the activated state.

204. An HIU according to claim 2 or 3, wherein the controller is configured to be operable to deactivate the immersion heater based at least on the domestic hot water flow switch being in the deactivated state.25 5. An HIU according to any of claims 1 to 4, wherein the controller is configured tobe operable to control operation of the immersion heater dependent at least on a primary flow temperature, the primary flow temperature being a temperature of a supply of water to the immersion heater.30 6. An HIU according to claim 5, wherein the controller is configured to be operableto activate the immersion heater based at least on the primary flow temperature being at most a primary flow temperature threshold.31 01 257. An HIU accordingto claim 6, comprising:a hot water heat exchanger,wherein the primary flow temperature threshold is based at least on:5 a hot water temperature setpoint; anda hot water temperature setpoint offset, wherein the hot water temperature setpoint is a target hotwatertemperature, and wherein the hot water temperature setpoint offset is arranged to account for a temperature drop across the hot water heat exchanger.

108. An HIU accordingto any of claims 5 to 7, comprising:a heating heat exchanger,wherein the controller is configured to be operable to set a heating flow temperature setpoint, when a heat network flow heating mode is activated, based at 15 least on:the primary flow temperature; anda heating flow temperature setpoint offset, and wherein the heating flow temperature setpoint offset is arranged to account for a temperature drop across the heating heat exchanger.

209. An HIU according to any of claims 1 to 8, wherein the controller is configured to be operable to deactivate the immersion heater based at least on a heat network flow inlet temperature being above a maximum heat network flow inlet temperature threshold, the heat network flow inlet temperature being a temperature of the water 25 supplied to the heat network flow inlet.

10. An HIU according to claim 9, wherein the controller is configured to be operable to activate the immersion heater based at least on the heat network flow inlet temperature falling below a minimum heat network flow inlet temperature threshold.3011. An HIU according to claim 9 or 10, wherein the immersion heater comprises a thermostat, wherein the thermostat has a thermostat cutoff temperature threshold, andwherein the thermostat cutoff temperature threshold is greater than the maximum heat network flow inlet temperature threshold.31 01 2512. An HIU according to claim 11, wherein the immersion heater comprises a 5 thermofuse, wherein the thermofuse has a thermofuse cutoff temperature threshold, and wherein the thermofuse cutoff temperature threshold is greater than the thermostat cutoff temperature threshold.

13. An HIU according to any of claims 1 to 12, comprising:10 a relay,wherein the immersion heater is controllable by an electromechanical switch, andwherein the controller being configured to be operable to control operation of the immersion heater comprises the controller being configured to be operable to activate 15 and deactivate the electromechanical switch using the relay.

14. An HIU according to any of claims 1 to 13, comprising: a modulating valve, wherein the controller is configured to be operable to control the modulating20 valve, when hot water is being produced by the HIU, to:limit a maximum flow rate of water flowing through the heat network flow inlet; and / orimpose a minimum flow rate of water through the heat network flow inlet.25 15. An HIU according to any of claims 1 to 14, wherein the immersion heatercomprises:a tank; anda cartridge heater,wherein the tank comprises an inlet and an outlet, and30 wherein the outlet of the tank is fluidly couplable to the heat network flow inlet.31 01 2516. An HIU according to claim 15, wherein the tank comprises a cartridge heater opening, and wherein the cartridge heater is insertable into, and removable from, the tankthrough the cartridge heater opening.5 17. An HIU accordingto claim 16, wherein the cartridge heater opening comprises ascrew thread, wherein the cartridge heater is securable to the tank via a complementary screw thread, and wherein the cartridge heater opening is sealable by a blanking plug havingthe complementary screw thread.10 18. An HIU according to any of claims 15 to 17, wherein the cartridge heatercomprises a 7-kilowatt cartridge heater.

19. A heat interface unit, HIU, comprising: a heat network flow inlet;15 a processor; andmemory,the memory comprising instructions which, when executed by the processor, cause the HIU to control operation of an immersion heater, the immersion heater being operable to apply a temperature boost to a fluid supply to the heat network flow inlet.2021. An HIU according to any of claims 1 to 20, wherein the HIU comprises the immersion heater.

22. A system comprising:25 an HIU accordingto any of claims 1 to 20; andthe immersion heater.

23. A heat network comprising:a plurality of HIUs according to any of claims 1 to 21; and30 an energy centre.

24. A computer-implemented method of controlling a heat interface unit, HIU, themethod comprising:controlling the HIU to activate or deactivate an immersion heater,wherein activating the immersion heater causes the immersion heater to apply a5 temperature boost to a supply of fluid to the HIU.31 01 25

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