Blockage detection using an electrical characteristic of a fan motor

Abstract

An air source heat pump 310 having an air heat exchanger 322, a motor 304 driven fan 324 moving air between an inlet and an outlet and over the exchanger located therebetween and defining an air flow

Description

According to Directive 2012 / 27 / EU buildings represent 40 % of the final energy consumption and 36% of CO2 emissions. The EU Commission report of 2016 “Mapping and analyses of the current and future (2020 - 2030) heating / cooling fuel deployment (fossil / renewables)” concluded that in EU households, heating and hot water alone account for 79% of total final energy use (192.5Mtoe). The EU Commission also report that, “according to 2019 figures from Eurostat, approximately 75% of heating and cooling is still generated from fossil fuels while only 22% is generated from renewable energy. To fulfil the EU’s climate and energy goals, the heating and cooling sector must sharply reduce its energy consumption and cut its use of fossil fuels. Heat pumps, with energy drawn from the air, the ground or water, have been identified as potentially significant contributors in addressing this problem. Because only a small percentage of domestic households have access to a body of water to support the use of a heat pump that draws energy from water, and because the cost and space requirements for installing a ground source heat exchanger are very significant, it is generally cheaper and more convenient to install an air source heat pump. It is also generally recognised that an air source heat pump is the better replacement for an existing gas central heating boiler. Air source heat pumps do however have one disadvantage not shared by heat pumps that extract energy from a body of water or from the ground, and that is the need in temperate and cooler climates periodically to defrost the heat pump’s air heat exchanger when the heat pump has been run to extract energy from ambient air (“heating mode”). The air heat exchanger is a heat exchanger in which in heating mode heat from ambient air is transferred to liquid refrigerant. A matrix of, typically finned, tubes (more generally conduits) receive cold liquid refrigerant, and ambient air is moved over the tubes by the action of a fan or impeller. In heating mode, energy is extracted from the ambient air because the liquid refrigerant is colder than the ambient air, and the liquid refrigerant vaporises as a result of its increased temperature, and the temperature of the ambient air drops. Water vapour carried by the ambient air will consequently tend to condense on the surface of the air heat exchanger. If the ambient air temperature is sufficiently low, the water that condenses on the surface of the air heat exchanger will freeze to form ice. A build-up of ice reduces the heat transfer efficiency of the air heat exchanger, and a significant build-up of ice can block the gaps between adjacent conduits of the air heat exchanger leading to a loss of air flow and hence further reduction in efficiency. Air source heat pumps are consequently configured to perform from time to time what is known as a defrost cycle (which in effect involves briefly running the heat pump in cooling mode, in which heat is given up from the refrigerant to ambient via the air heat exchanger) during which warm refrigerant is fed into the air heat exchanger to melt the ice and hence restore the efficiency of the air heat exchanger. The defrost cycle involves a reversal of the usual energy extraction cycle used in the heating mode: the heat pump operates in reverse, with energy being transferred from the home (e.g. from the heated space within the home or from the hot water of the space heating system or of the domestic hot water supply) to the air heat exchanger and hence to ambient. The defrosting process obviously reduces the overall efficiency of an air source heat pump. The frequency of defrost cycle events may be as often as every 35 minutes, for perhaps 10 minutes a time. A heat pump’s processor may be programmed to run a defrost cycle periodically - either according to a fixed periodicity or according to a periodicity that depends upon the ambient conditions (such as ambient temperature and possibly humidity), and for example the amount of heat load the system is trying to deliver, among other things. Under some circumstances the processor of a heat pump may force the performance of defrost events when they are not actually necessary. Given the loss of efficiency inherent in performing a defrost event, it would clearly be beneficial to reduce any incidence of unnecessary defrost events, and given the loss of efficiency inherent in running an air source heat pump with an iced-up heat exchanger, it would be desirable also to ensure that defrost events are performed whenever they are actually needed. Air source heat pumps can typically be run either in a heating mode, in which ambient energy is extracted via the air heat exchanger and used to provide space heating and, often, hot water for premises, or in a cooling mode in which energy is extracted from the premises and lost to ambient via the air heat exchanger. For efficient cooling, the cooling mode is typically provided as part of an air-conditioning system. Simple heat pump installations, particularly those where a heat pump has replaced a conventional central heating boiler, may not be configured to support air-conditioning - instead the heat pump may simply be connected to supply hot water and a heated fluid (often also largely water) to a plurality of radiant heat transfer elements such as radiators or underfloor heating installations. In such installations, an air source heat pump may be unused from early spring to mid to late autumn - a period which may be as much as six months, or just run to provide hot water, typically for no more than about an hour or so each day. But it is not uncommon for hot water to be heated by some means other than the heat pump, so that in such installations the heat pump may only be used during colder times of the year, possibly remaining unused for many months at a time. Under such circumstances, a further problem that can arise with air source heat pumps is blockage of the air flow paths leading to or 10 15 CXI 20 25 30 from the air heat exchanger. Typically both the air inlet(s) and the air outlet(s) of an air source heat pump include some kind of grating or grill that permits the free flow of air but which, in particular, serves to keep the fingers, hands, arms, and feet of inquisitive children or other humans safe from rotating parts such as fan blades, from the potentially freezing surfaces of the air heat exchanger, and from the heat pump’s electrical system and electrical supply. Left for months, these air inlets -and corresponding internal air passageways may become home to nesting birds or rodents, insects such as bees, wasps, or termites, or may also become blocked by fallen leaves, or by plant growth -particularly from creeping plants such as ivy. Such blockages may develop quickly - over the space of days or weeks, rather than months, and others such as those caused by playful children, careless people, or malevolent actors may arise in a matter of minutes or hours. Installers of air source heat pumps generally recommend users to check the visual appearance of their air source heat pump before turning it on after it has been left for any extended period of time, so that blockages caused by, for example, fallen leaves may be detected and cleared before the heat pump is restarted after a long interval, and / or the installer may recommend that the heat pump is professionally serviced each year before its first use of the new heating season, but often such recommendations are ignored. The efficiency of an air source heat pump is significantly decreased if it is run with partially blocked air intakes / outlets, and in some cases, damage may occur to the fan or fan motor of a heat pump run with blocked or partially blocked airways or with rotation of the fan impeded (for example as the result of a nest, insect infestation, or a child’s stick pushed between the fan’s blades). The present invention seeks to provide an air source heat pump in which at least some of the drawbacks of existing air source heat pumps are mitigated in whole or in part. Summary According to a first aspect there is provided an air source heat pump having an air heat exchanger, a motor driven fan to move ambient air from an air inlet to an air outlet, the heat exchanger being positioned in an air flow path that extends between the air inlet and the air outlet, a compressor to compress refrigerant and to cause passage of refrigerant through the air heat exchanger, and a processor programmed to determine existence of an obstruction or blockage of the air inlet, air outlet, or air flow path based on a detected or determined behaviour of the motor of the fan, wherein the processor is programmed to perform the determination before attempting to start the compressor. According to a second aspect there is provided an air source heat pump having an air heat exchanger, a motor driven fan to move ambient air from an air inlet to an air outlet, the heat exchanger being positioned in an air flow path that extends between the air inlet and the air outlet, a compressor to compress refrigerant and to cause passage of refrigerant through the air heat 3 exchanger, and a processor programmed to determine existence of an obstruction or blockage of the air inlet, air outlet, or air flow path based on a detected or determined electrical characteristic of the motor of the fan, wherein the processor is programmed to perform the determination before attempting to start the compressor. 5 In variants of the first or second aspect: Optionally, the fan motor has an associated controller and the processor is configured to obtain from the controller data on the detected behaviour. 10 15 CXI 20 Optionally the processor has at least one operating mode in which it is programmed only to power up the compressor in the event that the determination indicates that there is either no obstruction or blockage, or that any obstruction or blockage is below a threshold level of significance. Optionally, the detected behaviour is an electrical characteristic of the motor. In a heat pump according to any variant of the first or second aspect the electrical characteristic may be motor power consumption and the detected change may be an increase in the power consumption or an increase in the rate of change of power consumption. In a heat pump according to any variant of the first or second aspect the electrical characteristic may comprise one or more of: motor drive current, effective duty cycle in a pulse width modulation drive arrangement, voltage supplied to the motor, the amplitude and / or phase of current pulses applied to the motor. In a heat pump according to any variant of the first or second aspect the electrical characteristic may be motor drive current and the detected change may be a decrease in the drive current or a decrease in the rate of change of the drive current. In a heat pump according to any variant of the first or second aspect the processor may have access to a database of electrical characteristics of the motor. 25 In a heat pump according to any variant of the first or second aspect the motor may be an electronically commutated motor. Optionally, the motor may be coupled to an electronic motor controller which is itself coupled to the processor. Optionally, the electronic motor controller and the processor may be coupled via 30 MODBUS. According to a third aspect there is provided a method performed by the processor of an air source heat pump having an air heat exchanger, and a motor-driven fan to move ambient air from an air inlet to an air outlet, the heat exchanger being positioned in an air flow path that extends between the air inlet and the air outlet, and a compressor to compress refrigerant and to cause 35 passage of refrigerant through the air heat exchanger, the method comprising determining 4 existence of an obstruction or blockage of the air inlet, air outlet, or air flow path based on a detected behaviour or electrical characteristic of the motor of the fan, wherein the determination is performed before attempting to start the compressor. Optionally, in the method according to the third aspect, the fan motor has an associated 5 controller, the method further comprising receiving from the controller data on the detected behaviour. Optionally the method according to the third aspect further comprises only starting to run the compressor of the heat pump in the event that the determination indicates that there is either no obstruction or blockage, or that the effect of any obstruction or blockage is below a threshold level 10 of significance. Brief description of Figures Embodiments of the invention will now be described, by way of example only, with reference to the accompanying Figures, in which: Figure 1 shows schematically an air source heat pump operating in heating mode; 15 Figure 2 shows schematically the air source heat pump of Figure 1 operating in defrost mode; and Figure 3 illustrates schematically a heat pump installation according to embodiments of the £\J invention. Specific description Figure 1 shows schematically an air source heat pump 10 operating in heating mode. The ^^.20 main mechanical components of the heat pump are shown, but for the sake of clarity the electrical, 1 sensing, and control elements and connections, including the controller or processor of the heat pump, have been omitted. Starting in the bottom right-hand comer of the Figure, cold liquid refrigerant is forced from a liquid receiver 12, under the action of the compressor 32, along a conduit 14, which passes through the lower portion of the air heat exchanger 20, to a heating 25 expansion valve 16. The conduit 14 has an open end 15 which in use is submerged within liquid refrigerant - so that refrigerant liquid rather than refrigerant vapour is supplied to the conduit 14 from the reservoir 12. In the heating expansion valve 16 the refrigerant passes through a small opening, resulting in a reduction in pressure which in turn reduces the boiling point of the refrigerant. Depending on 30 the temperature of the refrigerant at the expansion valve inlet and the pressure of the refrigerant at the expansion valve outlet, a portion of the refrigerant may spontaneously vaporise, resulting in a reduction in temperature of the refrigerant, such that the refrigerant leaves the expansion valve as a mixture of liquid and vapour. From the expansion valve 16 the refrigerant passes into a distributor 18 in which a single input conduit is coupled to a plurality of output conduits each of which 35 supplies a conduit of the air heat exchanger 20. A one-way valve 50 is provided in parallel with the heating expansion valve 16, but this is oriented to permit flow from the distributor 18 to the conduit 14 (the direction in which refrigerant flows during the cooling mode of operation) and to prevent flow of refrigerant from the conduit 14 to the distributor - so that in heating mode the refrigerant must pass through the heating expansion valve 16 to reach the distributor 18. As the skilled person will understand, the heating expansion valve 16 (like other expansion valves in the heat pump) is controlled by the heat pump’s processor to achieve a target value of refrigerant superheat as appropriate to the prevailing operating conditions. A filter-drier 51 may be provided in the flow path to the heating expansion valve 16, and if present this too is bypassed by flow through the one-way valve 50 during cooling and defrosting. In one variant, the distributor 18 may include 3 flow paths - a single input conduit 17 with a small internal fixed orifice (not shown), a plurality of output conduits (indicated with reference numeral 19) and an internal bypass conduit. In heating mode, the refrigerant flows through the heating expansion valve 16 to the single input conduit 17 of the distributor 18, through the small internal fixed orifice and out of the plurality of output conduits 19. In heating mode there is no flow via the internal bypass conduit. Expansion of the refrigerant is achieved through the heating expansion valve and fixed orifice. In cooling mode, the refrigerant is able to flow from the plurality of output conduits 19 and out of the internal bypass conduit and through the one-way valve 50, bypassing the internal fixed orifice and heating expansion valve 16 such that the heating expansion valve 16 and internal fixed orifice of the distributor 18 have negligible effect in cooling mode. In another variant, the distributor may be formed without the small internal fixed orifice and the internal bypass conduit. The air heat exchanger 20 is shown schematically as comprising just four conduits traversing the air heat exchanger, but in reality, many more such conduits are provided, and each is typically provided with thermally conductive fins to improve heat transfer from the air to the refrigerant inside the conduits. The air heat exchanger 12 includes (or is coupled to) a fan 24 which causes ambient air to move over the air heat exchanger 20. In heating mode, refrigerant inside the air heat exchanger 20 is warmed by energy absorbed from the ambient air and this causes it to change state into vapour, and the vapour may itself be further heated by absorbing energy from the air so that the vapour may be superheated. The vapourised refrigerant leaves the air heat exchanger 20 via a collector (or manifold) 26 which feeds a return conduit 28 which supplies a first port, port 1, of a 4-way valve 30. The vapourised refrigerant exits the 4-way valve 30 via port 2 from where it flows to a compressor 32. In the compressor the warmed vapourised refrigerant, which is received at low pressure, is 6 compressed to become high pressure vapour, and in the process of being compressed the refrigerant further increases in temperature. From the compressor 32 the refrigerant passes back through the 4-way valve 30, entering via port 3 and exiting via port 4 which feeds a heat exchanger matrix, not shown, in heating system heat exchanger 34. The heating system heat exchanger 34 may be, as shown here, a heating system heat exchanger in which energy from the hot refrigerant is transferred to water (or some other suitable operating fluid in the case of space heating), e.g., for space heating via a central heating system, or to provide hot water for domestic or industrial use. Hot refrigerant enters the heat exchanger 34 at port 36, condenses and then exits, as cooled refrigerant at port 38. Water for heating enters the heat exchanger 34 at port 40, on the cooler side of the heat exchanger 34, and exits at port 42 on the hotter side of the heat exchanger 34. Within the heating system heat exchanger 34 the refrigerant condenses from a vapour to a liquid, giving up heat to water in the water circuit of the heat exchanger 34. From port 38 the cooled liquid refrigerant passes to a one-way valve 54 that is oriented to permit flow from the heating system heat exchanger 34 to the liquid reservoir 12. This one way valve 54 therefore serves as a bypass for another expansion valve 44 which is provided to expand refrigerant as it flows from the liquid reservoir 12 towards the heating system heat exchanger 34 in the cooling mode of operation (and hence also during defrosting of the air heat exchanger). In heating mode, the refrigerant is able to flow through the one-way valve 54, bypassing the cooling expansion valve 44 with minimal restriction such that the cooling expansion valve 44 has negligible effect in heating mode. The liquid refrigerant then passes back into the liquid reservoir 12 at which we started the process. It will be appreciated that the heating system heat exchanger 34 may, instead of using the refrigerant to heat water, use the refrigerant to heat air - for example for warm air space heating, but the principal remains the same. Figure 1 also includes a further heat exchanger 46 which is fed from the refrigerant conduit 14 via a further expansion valve 48. This further heat exchanger 46 is optional but is preferably provided as a means of cooling (and controlling the temperature of) an electrical inverter (not shown) that powers the compressor and various other of the heat pump’s electrical components and systems. The further heat exchanger 46 may include a heat sink that is thermally coupled to the inverter and optionally other of the heat pump’s electronics and power components. The cooling provided by the further heat exchanger 46 helps to ensure correct operation of the inverter and other electronics, extending their lifetimes, and also captures what would otherwise be waste heat and feeds this into refrigerant which is then fed into the compressor 32 - thus further contributing to the efficiency of the heat pump. The function of, and flow direction through, the inverter cooling expansion valve and inverter cooler heat exchanger is independent of and regardless of operating mode (i.e., heating, cooling, defrost). The inverter cooling expansion valve is controlled by the processor of the heat pump to regulate the refrigerant conditions through the inverter cooler heat exchanger to achieve a desired cooling effect, e.g. a target value of inverter drive temperature. If the heat pump of Figure 1 is run in a temperate climate on a cool day (e.g. less than about 10 degrees Celsius) water vapour in the ambient air may condense on the air heat exchanger 20, and then turn to ice. The build-up of ice on the air heat exchanger 20 both restricts the flow of air through the air heat exchanger and also reduces the efficiency of energy transfer from the ambient air to the refrigerant. The heat pump will typically include either a mechanism to detect the build-up of ice, or to detect climatic conditions when ice build-up is to be expected, or have some other mechanism to ensure timely defrosting, and have a processor configured (e.g. programmed) to cause the heat pump to enter a defrost mode as appropriate. Throughout this specification, references to the processor are references to a processor or controller (e.g. a microcontroller or microprocessor) configured or programmed to perform the relevant functions and operations. The processor or controller will typically include, and or be coupled to, a memory arrangement that stores relevant code, instructions, programs, algorithms, and data to enable the processor to perform the relevant operations, functions, methods, and procedures. Although not shown in Figure 1, a drip tray is preferably provided beneath the air heat exchanger 20 to collect condensate when the heat pump is running in defrost or heating mode, for the reasons given above. At the inlet of the compressor 32 sensors 31 may be provided to measure the temperature and pressure of the refrigerant. Based on values obtained from these sensors 31 the processor or controller of the heat pump is able to determine the state of the refrigerant, and in particular the degree of superheating, at or just upstream of the compressor inlet. Figure 2 corresponds to Figure 1, but shows the heat pump operating in a cooling mode (e.g., for cooling premises served by the heat pump), rather than in a heating mode, which is also used as the defrost mode. In the cooling mode the flow of refrigerant is reversed compared to that of the heating mode, and this reversal is achieved by controlling the 4-way valve 30 so that the output from the compressor 32, which is received on port 3, flows to port 1 rather than port 4. So instead of refrigerant heated by the compressor 32 passing next to the heat exchanger 34, it passes to the collector (or manifold) 26 and thence to the air heat exchanger 20. By supplying hot refrigerant to the air heat exchanger, ice build-up can be thawed. Cooled refrigerant leaves the air heat exchanger 20 via the distributor 18 and then passes via one-way bypass valve 50 and the sub-cooler back to the liquid receiver 12. From the liquid receiver 12 the expanded refrigerant flows out along conduit 52. The conduit 52 has an open end 53 which in use is submerged within liquid refrigerant - so that 8 refrigerant liquid rather than refrigerant vapour is supplied to conduit 52 from the reservoir 12. Refrigerant from the reservoir 12 flows along conduit 52 through the cooling expansion valve 44, and thence to port 38 of the heating system heat exchanger 34. In cooling mode, there is no flow via the bypass conduit due to the position and orientation of the one-way valve 54. Expansion of the refrigerant is achieved through the cooling expansion valve 44. In the expansion valve the refrigerant passes through a small opening, resulting in a reduction in pressure which in turn reduces the boiling point of the refrigerant. Depending on the temperature of the refrigerant at the expansion valve inlet and the pressure of the refrigerant at the expansion valve outlet, a portion of the refrigerant may spontaneously vaporise, resulting in a reduction in temperature of the refrigerant, (such that the refrigerant leaves the expansion valve as a mixture of liquid and vapour). The expansion valve may be controlled by the heat pump’s processor based on data from the pressure and temperature sensors 31 at the input side of the compressor 32. Flowing through the heating system heat exchanger 34 the expanded refrigerant absorbs heat from the hot water in the heat exchanger (water flowing, in use, from port 40 to port 42) and is vaporised. Warmed vaporised refrigerant leaves the heat exchanger by port 36 and passes to port 4 of the 4-way valve 30 and exits via port 2. From port 2 of the 4-way valve 30 the refrigerant vapour passes to the compressor 32. It is important that the refrigerant arrives at the compressor in the form of a vapour because the compressor can be damaged if it receives liquid refrigerant. The compressor compresses the refrigerant, further increasing its temperature, and supplies the refrigerant to port 3 of the 4-way switch 30. The refrigerant leaves the 4-way switch via port 1. Because the input side of the compressor 32 is always at a lower pressure when the compressor is running, in cooling mode refrigerant continues to flow through the heat exchanger 46 of the inverter towards the compressor 32, as it did in heating mode, under the control of the processor of the heat pump. As the skilled person will be aware, defrosting techniques other than that just described may be used to defrost an air heat exchanger 20 - for example using electrically powered heaters associated with the heat exchanger 20. Such a technique, or others, may be used as alternatives to or in addition to the technique described with reference to Figure 2. As mentioned in the introduction, blockages or obstructions of the air flow path(s) through an air source heat pump may arise not only as the result of the air heat exchanger icing up when the heat pump is used in heating mode, they may also occur during periods when the heat pump is unused (e.g. from late spring to late autumn in mild temperate climates, if the heat pump is not used for air-conditioning or to provide hot water). There exists a need for an automated method to detect such blockages and obstructions. 9 According to aspects of the invention such a method is provided by monitoring the electrical behaviour or an electrical characteristic of the motor of the heat pump’s fan. Thus, in a first aspect there is provided an air source heat pump having an air heat exchanger, a motor driven fan to move ambient air from an air inlet to an air outlet, the heat exchanger being positioned in an air flow path that extends between the air inlet and the air outlet, a compressor to compress refrigerant and to cause passage of refrigerant through the air heat exchanger, and a processor programmed to determine existence of an obstruction or blockage of the air inlet, air outlet, or air flow path based on a detected behaviour of the motor of the fan. An obstruction or blockage of the air inlet, air outlet, or air flow path, however caused, may lead to the fan motor drawing more or less power, or the rate of change of power drawn as the result of a variation in drive conditions may increase or decrease. By characterising such changes in electrical behaviour it becomes possible to determine the existence of an obstruction or blockages, and its significance based on a detected change in electrical characteristic. Based on such a determination it may be possible to detect the presence of an obstruction on starting up a heat pump after it has been unused for some time, and also to determine the existence of an obstruction caused by a need to defrost the air heat exchanger during normal operation. At start up, whether or not after an extended period of non-use, the processor of the heat pump may be arranged to compare the electrical behaviour (e.g., power drawn and / or rate of change of power drawn) of the fan motor to stored values indicative of normal behaviour and indicative of obstructions in different locations and of differing severity in order to determine the existence of an obstruction and its characteristics. Conveniently such determination may be performed before the processor attempts to start the heat pump’s compressor, thereby avoiding high / low pressure trips (which may be damaging) and also avoiding wasting energy by running the compressor when the heat pump will be unable to run efficiently due, for example to an obstructed air flow path. The processor may be programmed to signal an error state, based on an obstruction determined based on fan motor behaviour, for example by messaging a user via a user device such as a smart phone, a message on a control panel, a message or warning light on the heat pump assembly, and / or an audible (e.g. voiced) warning. Figure 3 illustrates schematically a heat pump 310 according to embodiments of the invention, and in particular an arrangement of elements to support the heat pump’s processor 300 in deciding that a defrost cycle is needed, and also in determining for how long to perform a defrost cycle. The heat pump has a housing 311 within which are located the air heat exchanger matrix 322, one or more fans 324 with an associated drive motor 304, the processor 300, and (optionally) the heating system heat exchanger 334 - although this may be mounted remotely, for example within house (more generally premises) 335 in place of a gas boiler that has been 10 replaced by the heat pump. The processor 300, which has an associated memory 302 (storing software instructions to control operation of the processor, etc.), is operatively connected to a valve arrangement 330 for use in controlling the direction of flow of refrigerant within the heat pump, and in particular to enable the direction of flow through the air heat exchanger matrix to be reversed between a heating mode of operation and a defrosting mode of operation. As previously noted, the valve arrangement 330 may comprise a 4-way valve or some suitable combination of simpler valves 330A, 330B, and 330C. The processor 300 is also operatively connected to the motor 304 that drives the fan 324 that rotates to drive (pull or push) ambient air over the air heat exchanger matrix 322, and may be configured both to control the fan motor and to receive information from appropriate circuitry on the motor’s power consumption, for example based on the current drawn by the motor 304, or on the effective duty cycle in a pulse width modulation drive arrangement, voltage supplied, the amplitude and / or phase of current pulses applied to the motor, or the like, or some combination of these. The motor 304 is preferably an electronically commutated motor (EC motor) also known as brushless DC electric motor (BLDC motor) using a direct current (DC) electric power supply. It may use an electronic controller 305 (typically situated within the fan motor housing) to switch DC currents to the motor windings producing magnetic fields which effectively rotate in space and which the permanent magnet rotor follows. The controller may adjust the phase and amplitude of the DC current pulses, and / or use some form of pulse width modulation and / or voltage control, to control the speed and torque of the motor. This control system is an alternative to the mechanical commutator (brushes) used in many conventional electric motors. The fan motor controller is itself controlled by, and reports to, the heat pump controller which provides a fan speed demand signal based on the heat pump's operating condition. There are many means of communication between the fan controller and the heat pump controller (analogue, PWM, MODBUS, etc.) of which we prefer to use MODBUS as it allows 2-way communication such that information from the fan controller, such as fan motor current, voltage, power, pulse amplitude and / or duration and / or frequency, can be taken from (or provided by) the fan controller and used by the heat pump controller. Although Figure 3 shows the air heat exchanger matrix 322 located downstream the fan 324 and its motor 304, in practice the fan (and preferably also its motor) may be located downstream of the air heat exchanger matrix 322 with the fan preferably arranged to draw air through the air heat exchanger matrix 322 rather than being arranged to push air through the air heat exchanger matrix. The processor 300 may also be operatively connected to a temperature and humidity sensing arrangement 306 that is configured to sense the ambient temperature and humidity at the 11 site of the air heat exchanger 320. The processor 300 and the air heat exchanger matrix may be contained in a unitary structure, or they may be housed in separate structures / housings, for example with the processor positioned within a housing within the premises 335 heated by the heat pump, and the air heat exchanger 320 in a separate housing outside the premises 335. In the latter case, the housing for the air heat exchanger 320 may also contain a processor or controller (e.g. a microcontroller) that is controlled by or works in tandem with the processor within the premises 335. Also in the latter case, the processor within the premises may communicate with the components and systems of, or associated with, the external housing by wireline or wirelessly (for example using Matter, Bluetooth RTM, Zigbee RTM, Wi-Fi RTM, 5G etc.). The processor 300 may be operatively connected to a control arrangement 340 that controls the heating system of the premises 335. The control arrangement may include one or more thermostats 342 and a controller 344 that is operable to control the heating system, for example setting start and end times for periods of heating. The controller 344 may form part of a “smart home” or home automation system that can be used to schedule actions of the heating system (both space heating and domestic hot water, for example), a security monitoring system, lighting, and more. Such a home automation system may be programmed to control the operation of systems and subsystems - such as space heating and or hot water heating, based on occupancy data provided by the occupants of the premises, or derived from diary / schedule information about the occupants’ activities and planned presence. The home automation system may also use presence information provided by motion sensors or the opening and closing of doors (both internal and external) - using sensors that also support a security monitoring system, or derived from the activation of electrical switches, to track actual occupancy. Such actual and predicted occupancy data, together with data on scheduled periods of heating, may be used by the heat pump processor 300 in determining whether to perform a defrost event - for example, if it is determined that the air heat exchanger may benefit from being defrosted the processor 300 may decide not to perform a defrost event if the heating controller 344 is scheduled to turn off the heating system - and hence cease to call for energy from the heat pump within the near future (e.g. less than an hour or less than half an hour). The processor 300 may be operatively connected to a camera (e.g. a video camera) and / or another optical sensor 308 to detect the build-up of ice on the air heat exchanger matrix 322, optionally using image processing in the event that a camera is used, and also optionally using machine learning to detect the build-up of ice, and the need or otherwise for defrosting, based on analysis of signals / images. The processor 300 may be operatively connected to the Internet 350 and hence to a source 360 of (local or hyper local) weather forecasts and also to a system back-end 312 managed by the manufacturer / supplier or installer of the heat pump. Using data from the 12 source 360 of weather forecasts, the processor 500 may be configured to prepare for adverse weather situations, such as those in which the build-up of frost or ice on the air heat exchanger is likely to be quicker and or more severe than usual, for example by limiting (reducing) the maximum output of the heat pump (i.e. reducing the amount of heat extracted from the air, thereby reducing ice build-up - for example rather than extracting 16kW, the controller of the heat pump may be configured to reduce the amount of energy extracted to 10-12kW - e.g. a 25-50% reduction in energy extraction), or reducing the superheat setting so that refrigerant is supplied to the air heat exchanger at a higher temperature to reduce ice build-up. In this way the heat pump can prepare for forthcoming adverse weather conditions and possibly enable a reduction in the frequency of defrost events. Alternatively, particularly adverse weather may lead to the heat pump increasing the frequency of defrost cycles, or switching to supplying hot refrigerant vapour to the air heat exchanger rather than initially relying simply on shutting down some part(s) of the air heat exchanger. The system back end 312 may also be configured to supply advance notification of weather events, for example based on reports from other heat pumps installed in the same neighbourhood or installed in locations experiencing the arrival of a weather front that is on its way to the location of the heat pump in question. The system back end 312 may also of course provide any necessary firmware upgrades, respond to “incident alerts’ for example by detecting system or component failure, or responding to reports of system or component failure, by for example arranging for the despatch of an engineer or service agent together with any necessary replacement parts or service items. The processor 300 may be configured to distinguish between a blockage or obstruction caused by a build-up of frost on the air heat exchanger, and a blockage or obstruction due to some other cause (such as a build-up of leaves, or the partial or complete blockage of an air inlet or outlet) based on information from the temperature and humidity sensing arrangement 306 or from an external source of weather information (such as an online meteorology resource), and knowledge of the heat pump’s recent operational history. These information sources may be supplemented with information from system camera 308, and information from system back end (optionally following a request for relevant information) for example information on the likelihood of frost build-up and / or on the behaviour of other air heat pumps in the vicinity of the installation that are also supported by the system back-end 312. The processor 300 may be configured to detect a blockage, such as a build-up of frost on the air heat exchanger, based on a characteristic change in power consumption characteristic of a fan motor, for example the power consumption characteristic may be the amount of current consumed, a rate of change of current consumption, an effective duty cycle in a pulse width modulation drive arrangement. An increase in the power consumed by the fan motor, for 13 example detected based on an increased current consumption, an increased supply voltage, or an increased effective duty cycle in a PWM drive arrangement, or the like may indicate that the fan motor is having to work harder to drive air through the air heat exchanger possibly as a result of the air heat exchanger being partially or wholly blocked. A detected increase in power consumption may also be correlated with certain environmental factors - typically temperature and humidity (e.g. relative humidity), and the rate of increase may have significance. Conversely, blockage or obstruction may under certain circumstances lead to a reduction in the amount of current drawn. Thus, the processor may usefully be configured to apply machine learning based on these factors in order to determine whether a detected increase / decrease in power consumption is likely to be in consequence of a build-up of ice or snow on the air heat exchanger, or possibly some other cause - such as a build-up of leaves or other debris at the air inlet of the housing containing the air heat exchanger. In the latter case, the processor may be configured to generate an alert for the owner / occupier of the premises (and also sent to the system back-end 312) of the need to clean the air inlet - the alert optionally being sent by the system back-end 312, for example as a push notification to a user device or even as an automated phone call or email or letter. The processor 300 may be configured to detect and record the power consumption of the fan motor 304 at the time of initial installation, and use this as a datum against which later power consumption figures may be compared, or may be pre-programmed before installation - for example based on machine learning or empirical testing performed during development of the heat pump. Continued monitoring (on a continuous or periodic basis) of fan power consumption characteristic may also reveal the need for maintenance (e.g. the clearing of air inlet(s), the removal of pest infestation - such as an insect or rodent nest, or the need to replace a damaged fan or fan motor. It is possible that the heat pump’s air inlet(s) and / or air outlet(s) may be blocked by an external obstruction (such as a towel or clothing draped over the inlet, the placing of equipment such as a paddle board or surfboard, or building materials over the inlet(s) or outlet(s)), or an internal obstruction (such as an insect or rodent nest or other result of insect or rodent activity, or a build-up of leaves or vegetation) may obstruct the usual path of air flow between the air inlet(s) and air outlet(s). Any of these obstructions should, if significant enough, be detectable by the processor of a heat pump according to aspects of the invention. Similarly, the processor may be able to detect successful removal of a blockage, such as the defrosting of the air heat exchanger, based on a characteristic change in a power consumption characteristic of a fan motor - e.g. a reduction (or possibly an increase) in current consumption or a reduction (or possible increase) in the effective duty cycle in a pulse width modulation drive arrangement, voltage supplied, the amplitude and / or phase of current pulses applied to the motor, 14 or the like, or some combination of these. It should be appreciated however that the effective unblocking of the heat exchanger may actually lead to an increase in power consumption - for example if the fan has in effect been starved of a supply of air. For this reason it is important to observe the behaviour of the fan motor under varied test conditions so that the processor can be suitably programmed to best detect the condition of the air heat exchanger based on motor drive characteristics and hence optimise control. The location of any blockage or obstruction of the heat pump’s air flow path through the air heat exchanger may influence the behaviour of the fan and hence the electrical behaviour / characteristics of the fan’s motor. For example, partial or complete blockage of the air flow path upstream of the fan (either outside the housing of the heat pump or within the housing of the heat pump) may starve the fan of air, meaning that the fan motor has less work to do, possibly increasing the rotational speed of the fan, and possibly reducing the amount of power consumed. A comparable partial or complete blockage of the air flow path downstream of the fan (either within or outside the housing of the heat pump) may result in the fan motor having to work harder with a possible increase in the power consumed. The aerodynamic properties of the fan and of the air flow paths upstream and downstream may affect the behaviour of the fan, and the changes in fan motor characteristics, under different obstruction situations. Because of this it is desirable to determine fan motor behaviour and characteristics under normal conditions, and under the different foreseeable blockage situations, during pre-production (for example), so that the electrical characteristics signifying the different natures and locations of any obstruction can be determined (e.g. learnt) for use in programming the processor 300. The heat pump processor 300 may also be provided with some capacity for machine learning so that site specific adaptation can be performed, for example based on feedback from engineer’s site visits, user input, or downloads from a remote control centre. As previously mentioned, the processor 300 is preferably operatively connected to a controller 340 of the heating system (space heating and / or hot water supply system) of the premises 335 served by the heat pump. The processor 300 may therefore use status information, heating system schedules, in determining whether to act on a determination that defrosting would be appropriate. If the premises is served by a home automation system 344, the processor 300 is preferably operatively coupled to the system 344 so that occupancy data, occupant schedules, and other factors can be taken into account in the processor’s control of the heat pump. Additionally or alternatively, the processor 300 may be configured to detect the build-up of ice on the air heat exchanger based on a detected pressure difference across the air heat exchanger - that is between a pressure measured within the housing of the air heat exchanger (on the “upstream” side of the air heat exchanger as it were), e.g. by a first pressure sensor 370, and a 15 pressure measured outside the housing e.g. by a second pressure sensor 372. The fan associated with an air heat exchanger may be arranged to suck air into the housing through one or more air inlets 480 remote from the air heat exchanger, the air being expelled from the housing over / through the air heat exchanger and out an exit of the housing - that is, the fan is arranged upstream of the air heat exchanger, the fan effectively pushing air over the downstream air heat exchanger. Alternatively, the air heat exchanger may be located on the inlet side of the fan, with the fan downstream of the air heat exchanger, so that the fan sucks air into the housing through the air heat exchanger. Accuracy of the the pressure differential technique may be influenced by whether the fan is upstream or downstream of the air heat exchanger, so that under some circumstances this may not prove to be a reliable means of detecting a blockage such as a buildup of ice - the accuracy potentially depending on the precise configuration chosen. In any event, the processor 300 may also take advantage of information provided by a heating system controller 340 and / or a home automation system 344 in determining whether to perform a defrost event when one appears to be needed. If the fan motor has an associated fan motor controller, the processor 300 may receive information on fan motor behaviour from the fan motor controller, rather than directly detecting fan motor behaviour. The fan motor controller may be configured (e.g. programmed) to update, periodically or continuously, the processor 300 with information on fan motor power consumption and / or other performance characteristics. The fan motor controller may optionally report to the processor 300 based on consideration of the operating behaviour of the fan motor and all (or a sub-set of) the motor drive factors. The controller of the fan motor (which may or may not be an electronically commutated motor) may be programmed or configured to control the rotational speed of the fan motor, for example to hold a certain speed (X rpm) or to hold a speed within a certain range (Y to Z rpm). and may additionally or alternatively be programmed or configured to control the fan motor to achieve a certain rate of increase of fan speed or a rate of increase in fan speed that is within a certain range, for example when accelerating the fan from a stationary or zero rpm (or low rpm state) to a desired or target fan speed or range of fan speeds. The controller may be programmed or configured to achieve and hold certain fan speeds or ranges of speeds that differ according to different control states of the heat pump, for example as signalled by the processor of the heat pump, such as heat pump power setting and / or heat pump operation mode (heating, cooling, defrosting, for example). In order to control fan speed and / or rate of increase in fan speed, the controller may adjust one or more of multiple variables such as pulse width and / or effective duty cycle in a pulse width modulation drive arrangement, current and / or voltage supplied to the motor, the amplitude and / or phase of current pulses applied to the motor, among others. The fan motor controller may receive information on speed of rotation of the motor from one or more sensors, and / or may be able to determine speed of rotation, and hence rate of change of speed of rotation based on information derived from electrical characteristics / demands and / or behaviour of the motor. In the cases where there is no separate fan motor controller, a (the) processor of the heat pump may be programmed or configured to behave as just described with reference to the fan motor controller. Where a dedicated fan motor controller is provided, the fan motor controller may be programmed or configured to provide a (the) processor of the heat pump with information about speed of rotation, rate of increase in speed of rotation, and / or electrical characteristics / demands and / or behaviour of the motor. The supply of such information may be continuous or periodic, and may be automatic or may be upon request from the processor of the heat pump. It will be appreciated that the detection of obstructions / blockages at start up, and the detection of a need to perform a defrost cycle are separate aspects and a heat pump processor may be configured to perform just one of these methods or both. Moreover, any variant of either method may be combined with any variant of the other method, according to choice or need. Under some circumstances it may be preferred to provide more than one motor driven fan in an air source heat pump. The preceding discussion has generally, for the sake of clarity and simplicity, focussed on the control of a single motor driven fan, but it will be appreciated that the principles of the previously described methods and systems apply equally to multi-motor set ups. While the pressure differential method of detecting blockage of the air heat exchanger is useable, we have found that detecting air heat exchanger blockage based on changes in the electrical behaviour or an electrical characteristic of the fan motor is generally more reliable and avoids the need to provide sensitive air pressure sensors (whose reliability is not always good). Although we have discussed and described the detection of a blockage based on a change in an electrical characteristic or electrical behaviour of the motor of a fan it will be appreciated the detection of a blockage may instead be based on a torque measurement of the motor or based on a strain measurement of (one or more) fan blades. Currently, relevant strain measurement technology is relatively expensive, which might preclude the use of such a technique in heat pumps intended for domestic installations. But such an approach may be cost effective in more expensive installations, and as the cost of deploying such technology decreases it may well become cost effective even for heat pumps for use in domestic installations. Torque measurement technology is currently less unattractive financially than strain measurement technology, so it is more likely to be attractive in the shorter term, especially for more expensive installations - and again in the longer term expected cost reductions will make this approach increasingly attractive. However, in the near term, blockage detection based on an electrical characteristic or electrical behaviour of the motor of the fan is likely to remain the most cost-effective of the three approaches. It should be appreciated that the inventive concept nevertheless extends to the use of all three detection techniques.

Claims

1. An air source heat pump having an air heat exchanger, a motor driven fan to move ambient air from an air inlet to an air outlet, the heat exchanger being positioned in an air flow5 path that extends between the air inlet and the air outlet, a compressor to compress refrigerant and to cause passage of refrigerant through the air heat exchanger, and a processor programmed to determine existence of an obstruction or blockage of the air inlet, air outlet, or air flow path based on a detected behaviour or electrical characteristic of the motor of the fan, wherein the processor is programmed to perform the determination, before attempting to start the10 compressor.

2. A heat pump as claimed in claim 1, wherein the fan motor has an associated controller, and the processor is configured to obtain from the controller data on the detected behaviour or electrical characteristic.

153. A heat pump as claimed in claim 1 or claim 2, wherein the processor has at least one operating mode in which it is programmed only to power up the compressor in the event that the determination indicates that there is either no obstruction or blockage, or that any obstruction or1 blockage is below a threshold level of significance.

204. A heat pump as claimed in claim 1, wherein the electrical characteristic is motor power consumption, and the detected change is an increase in the power consumption or an increase in the rate of change of power consumption.25 5. A heat pump as claimed in claim 1, wherein the electrical characteristic is motor powerconsumption, and the detected change is a decrease in the power consumption or a decrease in the rate of change of power consumption.

6. A heat pump as claimed in claim 1, wherein the electrical characteristic comprises one or 30 more of: motor drive current, effective duty cycle in a pulse width modulation drivearrangement, voltage supplied to the motor, the amplitude and / or phase of current pulses applied to the motor.

7. A heat pump as claimed in any one of the preceding claims, the processor having access35 to a database of electrical characteristics of the motor.

8. A heat pump as claimed in any one of the preceding claims, wherein the motor is an electronically commutated motor.5 9. A heat pump as claimed in claim 8, wherein the motor is coupled to an electronic motorcontroller which is itself coupled to the processor.

10. A heat pump as claimed in claim 9, wherein the electronic motor controller and the processor are coupled via MODBUS.

11. A heat pump as claimed in any one of the preceding claims, wherein the processor is programmed to provide a notification or signal in the event that it is determined that there is an obstruction or blockage of the air flow path.15CXI2012. A heat pump as claimed in claim 11, provided with wired or wireless Internet access, the processor being programmed to transmit a notification to a user device in the event that it is determined that there is an obstruction or blockage of the air flow path.

13. A heat pump as claimed in any one of the preceding claims, wherein the processor is further programmed to determine a need to defrost the air heat exchanger based on a detected change in an electrical characteristic of the motor of the fan.

14. A heat pump as claimed in claim 13, wherein the electrical characteristic used to determine a need to defrost the air heat exchanger is motor power consumption and the detected 25 change is an increase in power consumption or an increase in a rate of change of power consumption.

15. A heat pump as claimed in claim 13, wherein the electrical characteristic used to determine a need to defrost the air heat exchanger is motor power consumption and the detected 30 change is a decrease in power consumption or a decrease in a rate of change of power consumption.

16. A heat pump as claimed in claim 13, wherein the electrical characteristic used to determine a need to defrost the air heat exchanger comprises one or more of: motor drive current,effective duty cycle in a pulse width modulation drive arrangement, voltage supplied to the motor, the amplitude and / or phase of current pulses applied to the motor.

17. A heat pump as claimed in any one of claims 13 to 16 wherein the processor is5 programmed also to take account of one or more other factors before initiating an air heat exchanger defrost event based on a determined need and the one or more other factors.

18. A heat pump as claimed in claim 17, wherein heat pump serves premises and the other factors include one or more of the following: predicted energy demand, premises occupancy, 10 predicted changes in occupancy, occupant schedules or appointments, historic patterns of occupancy and / or user behaviour, weather forecast data.

19. A heat pump as claimed in claim 18, wherein the processor of the heat pump is operatively connected to a home automation system.1520. A heat pump as claimed in any one of claims 13 to 19, the processor having access to a database of electrical characteristics of the motor.1 21. A method performed by the processor of an air source heat pump having an air heat20 exchanger, a motor-driven fan to move ambient air from an air inlet to an air outlet, the heat exchanger being positioned in an air flow path that extends between the air inlet and the air outlet, and a compressor to compress refrigerant and to cause passage of refrigerant through the air heat exchanger, the method comprising determining existence of an obstruction or blockage of the air inlet, air outlet, or air flow path based on a detected behaviour or electrical25 characteristic of the motor of the fan, wherein the determination is performed before attempting to start the compressor.

22. The method as claimed in claim 21, wherein the fan motor has an associated controller, the method further comprising receiving from the controller data on the detected behaviour.3023. The method as claimed in claim 21 or claim 22, further comprising comparing a value of sensed drive current with one or more stored reference values of drive current.

24. The method as claimed in any one of claims 21 to 23, further comprising only starting to run the compressor of the heat pump in the event that the determination indicates that there is either no obstruction or blockage, or that the effect of any obstruction or blockage is below a threshold level of significance.

525. The method as claimed in any one of claims 21 to 24, further comprising providing a notification or signal in the event that it is determined that there is an obstruction or blockage of the air flow path.10 26. The method as claimed in claim 25, wherein the notification is a notification sent to auser device such as a wireless transmit receive unit (WTRU).17 04 24

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