Air source heat pump

Abstract

An air source heat pump (10, fig 1) comprises a controller and an air heat exchanger 300 receiving refrigerant to extract energy from ambient air that flows over the exchanger. The exchanger has a plurality of independently controllable flow paths A1-A6, B1-B6 communicating the refrigerant through the exchanger. A flow control arrangement 316A, 316B (expansion valves) coupled to the controller controls and selectively adjusts flow of the refrigerant through the flow paths. In a heating mode a first distribution of the refrigerant flows across the flow paths to extract energy from the air, and the controller controls the refrigerant flow in an incremental defrosting procedure whereby a distribution of the refrigerant flows through a subset of the flow paths and is adjusted to selectively defrost a first part of the heat exchanger while refrigerant continues to flow through at least some of the flow paths but enables the heat pump to continue to extract energy from the air during the defrosting procedure. The flow paths may comprise a plurality of interleaved conduits connected in parallel to a refrigerant distributor 302, and the plurality of conduits may be clustered into one or more groups of multiple conduits (fig 3B).

Description

Technical field The present invention relates to an air source heat pump and to an air heat exchanger matrix for an air source heat pump. Background 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 1 consequently configured to perform from time to time what is known as a defrost cycle 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. Generally the defrost cycle involves either the bypassing of hot gas from the compressor discharge or high pressure receiver to the air heat exchanger (known as hot gas defrost) or a reversal of the usual energy extraction cycle so that the heat pump operates in reverse, with energy being transferred from the home to the ambient air (which is essentially a short duration running of the heat pump in cooling mode). The frequency of defrost cycle events may be as often as every 35 minutes, for perhaps 10 minutes a time, but depends upon the ambient conditions, and the amount of heat load the system is trying to deliver, among other things. Understandably, the overall efficiency of an air source heat pump is reduced by this need to run the heat pump in reverse to defrost the air heat exchanger. 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 an aspect there is provided an air source heat pump comprising a controller and an air heat exchanger to receive liquid refrigerant to extract energy from ambient air that in use flows over the air heat exchanger, the air heat exchanger having a plurality of independently controllable flow paths to carry refrigerant through the air heat exchanger, the air heat exchanger being part of an assembly that includes a flow control arrangement coupled to the controller and operable under control of the controller to selectively adjust flow of refrigerant through one or more of the plurality of independently controllable flow paths; the controller being programmed to implement an incremental defrosting procedure involving controlling the flow control arrangement to perform a first change to the rate of refrigerant flow through one or more of the plurality of independently controllable flow paths to selectively defrost parts of the heat exchanger corresponding to only some of the independently controllable flow paths, and to perform a second change to the rate of refrigerant flow through one or more of the plurality of independently controllable flow paths to selectively defrost other parts of the heat exchanger corresponding to other of the independently controllable flow paths. According to a first aspect there is provided an air source heat pump comprising a controller and an air heat exchanger to receive liquid refrigerant to extract energy from ambient air that in use flows over the air heat exchanger, the air heat exchanger having a plurality of independently controllable flow paths to carry refrigerant through the air heat exchanger, the air heat exchanger being part of an assembly that includes a flow control arrangement coupled to the controller and operable under control of the controller to permit flow of refrigerant through one or more subsets of the plurality of independently controllable flow paths while inhibiting flow through other of the plurality of independently controllable flow paths; the controller being programmed to implement a defrosting procedure involving a first step in which the flow control arrangement is controlled to selectively inhibit flow of refrigerant through only some of the independently controllable flow paths while maintaining flow through one or more other of the independently controllable flow paths; and subsequently controlling the flow control arrangement to: resume flow of refrigerant through the independently controllable flow path(s) through which flow was selectively inhibited in the first step; and to selectively inhibit flow through at least some of the one or more independently controllable flow paths through which flow was maintained in the first step. According to a second aspect there is provided an air source heat pump comprising a controller and an air heat exchanger to receive liquid refrigerant to extract energy from ambient air that in use flows over the air heat exchanger, the air heat exchanger having a plurality of independently controllable flow paths to carry refrigerant through the air heat exchanger, the air heat exchanger being part of an assembly that includes a flow control arrangement coupled to the controller and operable under control of the controller to selectively adjust flow of refrigerant through the plurality of independently controllable flow paths; the heat pump having in a heating mode a first distribution of refrigerant flows across the plurality of independently controllable flow paths to extract energy from ambient air; and the controller being programmed to control the flow control arrangement to implement an incremental defrosting procedure in which the distribution of refrigerant flows through a subset of the plurality of independently controllable flow paths is adjusted to selectively defrost a first part of the air heat exchanger while refrigerant continues to flow through at least some of the independently controllable flow paths to enable the heat pump to continue to extract energy from ambient air during the defrosting procedure. With heat pump configurations according to any of the aspects of the invention the heat to melt ice built up on the air heat exchanger (as the result of the heat pump extracting energy from ambient air - which happens when the heat pump is run in heating mode) advantageously comes from ambient air that flows over the heat exchanger rather than from the refrigerant circuit as is the case when defrosting is performed either by reversing flow of liquid refrigerant or using a hot gas defrosting method (which typically also involves a reversal of flow). It can be seen that defrosting can be achieved without the need to use energy from (warm) refrigerant. Moreover, the heat pump of the first aspect is able to continue to extract ambient heat in a continuing heating phase while the heat exchanger is being defrosted. In heat pumps according to aspects of the invention the controller may be programmed to switch to a first defrost mode, in which the distribution of refrigerant flows through some of the plurality of independently controllable flow paths differs from that of the heating mode, to 3 selectively defrost a first part of the air heat exchanger before reverting to the first distribution of refrigerant flows. Optionally, the incremental defrosting procedure may comprise the controller switching from the first distribution of refrigerant flows to a second defrost mode, in which the distribution of refrigerant flows through some of the plurality of independently controllable flow paths differs from those of the heating mode and the first defrost mode, after reverting to the first distribution of refrigerant flows. Optionally, the first defrost mode may involve selectively inhibiting flow of refrigerant through only some of the independently controllable flow paths while maintaining flow through one or more other of the independently controllable flow paths. Optionally, the second defrost mode may involve resuming flow of refrigerant through the independently controllable flow path(s) through which flow was selectively inhibited in the first defrost mode, and selectively inhibiting flow through at least some of the one or more independently controllable flow paths through which flow was maintained in the first defrost mode. In an air source heat pump according to any aspect of the invention, the defrosting procedure may comprise a first step in which the flow control arrangement is controlled to selectively inhibit flow of refrigerant through only some of the independently controllable flow paths while maintaining flow through one or more other of the independently controllable flow paths; and the subsequently control of the flow control arrangement to resume flow of refrigerant through the independently controllable flow path(s) through which flow was selectively inhibited in the first step, and to selectively inhibit flow through at least some of the one or more independently controllable flow paths through which flow was maintained in the first step. The processor may be configured / programmed to control the flow control arrangement to maintain substantially the same overall flow rate while changing the flow rates of refrigerant through different flow paths. For example, if refrigerant flow through one or more flow paths is selectively reduced, the flow rate through one or more other flow paths may be increased to compensate so that the overall flow rate remains largely unchanged. Likewise, if refrigerant flow through one or more flow paths is selectively increased, the flow rate through one or more other flow paths may be reduced to compensate so that the overall flow rate remains largely unchanged. Maintaining an overall flow rate in this way may make it easier to maintain stable operation of the heat pump. There may be two, three or more independently controllable flow paths. Each of these independently controllable flow paths may have its own dedicated expansion valve, or two or more of the independently controllable flow paths may share an expansion valve. In an air source heat pump according to any aspect of the invention, the controller may be configured to resume flow of refrigerant through the independently controllable flow path(s) 4 through which flow was selectively inhibited in the first step before selectively inhibiting flow through the at least some of the one or more independently controllable flow paths through which flow was maintained in the first step. In an air source heat pump according to any aspect of the invention, the controller may be configured to resume flow of refrigerant through the independently controllable flow path(s) through which flow was selectively inhibited in the first step simultaneously with selectively inhibiting flow through the at least some of the one or more independently controllable flow paths through which flow was maintained in the first step. In an air source heat pump according to any aspect of the invention, the controller may be configured to initiate the resumption of flow of refrigerant through the independently controllable flow path(s) through which flow was selectively inhibited in the first step simultaneously with initiating the selective inhibition of flow through the at least some of the one or more independently controllable flow paths through which flow was maintained in the first step. In an air source heat pump according to any aspect of the invention, the air heat exchanger may have at least three independently controllable flow paths to carry refrigerant through the air heat exchanger, and the controller may be configured in the first step to control the flow control arrangement to selectively inhibit flow of refrigerant through only one of the independently controllable flow paths while maintaining flow through the other of the independently controllable flow paths. In an air source heat pump according to any aspect of the invention, the flow control arrangement may comprise, for each of the independently controllable flow paths, a dedicated expansion valve. In an air source heat pump according to any aspect of the invention, each of the independently controllable flow paths may include a plurality of conduits for the passage of refrigerant. Optionally for each independently controllable flow path the conduits of the plurality of conduits may be connected in parallel to a refrigerant distributor. Optionally, within the air heat exchanger, for each independently controllable flow path the conduits of the plurality of conduits may be clustered into one or more groups of multiple conduits. Optionally the groups of conduits for the different independently controllable flow paths may be interleaved. In an air source heat pump according to any aspect of the invention, the independently controllable flow paths may be interleaved so that within the air heat exchanger one or more conduits of a first independently controllable flow path are flanked on each side by one or more conduits of a different independently controllable flow path. In an air source heat pump according to any aspect of the invention, the assembly may include a refrigerant sub-cooler that is arranged to receive little or no airflow and through which refrigerant is arranged to flow before reaching the flow control arrangement. In an air source heat pump according to any aspect of the invention, the controller may be configured to determine a need to perform an air heat exchanger defrost event based, at least in part, on a change in power consumption characteristic of a fan motor of the heat pump. Optionally the power consumption characteristic is the amount of current consumed or a rate of change of current consumption. In an air source heat pump according to any aspect of the invention, the controller may be configured to control the flow control arrangement to selectively inhibit flow of refrigerant by ceasing the flow of refrigerant through the selected independently controllable flow paths while maintaining flow through the one or more other of the independently controllable flow paths. Alternatively, the controller may be configured to control the flow control arrangement to selectively inhibit flow of refrigerant by maintaining a reduced flow of refrigerant through the selected independently controllable flow paths while maintaining flow through the one or more other of the independently controllable flow paths. Air source heat pumps according to any aspect of the invention may be configured to perform a defrost procedure in response to a trigger to perform an air heat exchanger defrost event. According to a third aspect there is provided a method performed by the controller of an air source heat pump, the heat pump having an air heat exchanger including a plurality of independently controllable flow paths to carry refrigerant, the method comprising: operating the heat pump in a heating mode in which the air heat exchanger is used to extract energy from ambient air using a distribution of refrigerant flows through the plurality of independently controllable flow paths; implementing an incremental defrosting procedure by adjusting the distribution of refrigerant flows of the plurality of independently controllable flow paths to selectively defrost a portion of the air heat exchanger while continuing to flow refrigerant through at least some of the independently controllable flow paths to continue to extract energy from ambient air by operating the heat pump in heating mode. In a method according to the third aspect adjusting the distribution of refrigerant flows may involve selectively inhibiting flow of refrigerant through only some of the refrigerantcarrying conduits of the air heat exchanger while maintaining flow through others of the refrigerant-carrying conduits of the air heat exchanger. Optionally, the flow of refrigerant through the refrigerant-carrying conduits of the air heat exchanger through which flow was selectively inhibited may be resumed before selectively inhibiting flow through at least some of the refrigerant-carrying conduits of the air heat exchanger through which flow was maintained. In a method according to the third aspect resuming the flow of refrigerant through the refrigerant-carrying conduits of the air heat exchanger through which flow was selectively inhibited may be initiated simultaneously with selectively inhibiting flow through at least some of the refrigerant-carrying conduits of the air heat exchanger through which flow was maintained. In a method according to the third aspect the heat pump may be connected to supply heat to a heating system, and the method may further comprise performing the defrost event without interrupting the supply of heat to the heating system. According to a fourth aspect there is provided a method performed by the controller of an air source heat pump, the heat pump having an air heat exchanger including a plurality of refrigerant-carrying conduits, the method comprising, in response to a trigger to perform an air heat exchanger defrost event, defrosting the air heat exchanger by: in a first step selectively inhibiting flow of refrigerant through only some of the refrigerantcarrying conduits of the air heat exchanger while maintaining flow through others of the refrigerant-carrying conduits of the air heat exchanger; and subsequently resuming flow of refrigerant through the refrigerant-carrying conduits of the air heat exchanger through which flow was selectively inhibited; and selectively inhibiting flow through at least some of the refrigerant-carrying conduits of the air heat exchanger through which flow was maintained in the first step. In the heat pumps of any aspect of the invention, the heat pump may include a mechanism to detect the build-up of ice, or to detect climatic conditions when ice build-up is to be expected, or may have some other mechanism to ensure timely defrosting, based on one or more of these triggers, and have a processor configured to cause the heat pump to enter a defrost mode as appropriate. One possible trigger is a detected change in one or more characteristics of the power demanded by the motor of the fan of the heat pump. For example the processor may respond to a detected change in the amount of current drawn (e.g. an increase in current) or a change in the rate of change in drawn current (e.g. an or an increase or decrease in a rate of change in the current drawn). The processor may be configured to respond to a detected change in a vector characteristic rather than simply to an increase in an overall level, but may alternatively be configured to respond to a change in an overall level, or to a rate of change of a characteristic such as motor current. For example the processor may be configured to respond to an increase assessed against a known (stored) value of current drawn when the evaporator is free from ice build-up - for example determined during commissioning of the heat pump. Additionally or alternatively the heat pump may receive an externally generated trigger. Brief description of the 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; Figure 2 shows schematically the air source heat pump of Figure 1 operating in defrost mode; Figure 3 A shows schematically an air heat exchanger assembly according to an aspect of the invention; Figure 3B shows schematically details of an air heat exchanger matrix according to an aspect of the invention; and Figure 4 shows schematically an air source heat pump according to an aspect of the invention. Specific description Figure 1 shows schematically an air source heat pump 10 operating in heating mode. The main mechanical components of the heat pump are shown, but for the sake of clarity the electrical, sensing, and control elements and connections, including the controller or processor of the heat pump have been omitted. Starting in the bottom right hand corner of the Figure, cold liquid refrigerant is forced from a liquid receiver 12 along a conduit 14, which passes through the air heat exchanger 20, to a heating 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 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 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 8 heat pump’s processor to achieve a target value of refrigerant superheat as appropriate to the current 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, a plurality of output conduits (indicated with reference numeral 19) and an internal bypass conduit (not shown). 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 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 20 includes (or is coupled to) a fan 24 which moves (sucks or blows) ambient air over the matrix 22 of 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 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 water heat exchanger in which energy from the hot refrigerant is transferred to water (or some other suitable operating liquid in case of space heating), e.g. for space heating via 9 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 water 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 water 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 cooled 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 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 10 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, which is also used as the defrost mode, rather than in a heating 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 heating system heat exchanger 34, it passes to the 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 back to the liquid receiver 12. From the liquid receiver 12 the refrigerant flows along conduit 52. The conduit 52 has an open end 53 which in use is submerged within liquid refrigerant - so that 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 11 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 vale 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 refrigerant absorbs heat from the hot water in the heat exchanger (and that is possibly flowing 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 valve 30. The refrigerant leaves the 4-way valve via port 1. Because the input side of the compressor 32 is always at a lower pressure when the compressor is running refrigerant continues to flow through the heat exchanger 46 of the inverter towards the compressor 32, under the control of the processor of the heat pump. Turning now to Figure 3 A we can see, schematically, features of a new air heat exchanger assembly 300 according to an aspect of the invention. The air heat exchanger assembly 300 comprises an air heat exchanger matrix 322 which is coupled between a distribution arrangement 302, from which it receives refrigerant, and a collector or manifold arrangement 304 to which refrigerant flows after having passed through the matrix 322. Optionally, but preferably, the air heat exchanger assembly 300 also includes a liquid sub cooler 306 through which refrigerant flows on its way to the distribution arrangement 302 from the refrigerant conduit 14 which carries refrigerant from the liquid refrigerant reservoir 12.The liquid sub cooler 306 if present is preferably arranged to receive little or no air flow from the fan 24 such that the lower portion of the air heat exchanger is maintained at a temperature above ambient thus preventing ice build-up on the lower portion of the air heat exchanger, while serving to ensure that the liquid refrigerant received by the distribution arrangement 302 is cooled to a temperature below the refrigerant's boiling point. In the example shown, the air heat exchanger matrix 322 consists of two independently controllable flow paths / circuits A and B. Each of the independently controllable flow paths / circuits is provided with, and fed from, a dedicated distributor, and each dedicated distributor receives refrigerant from a dedicated expansion valve. So in this example we see that flow path A is fed by a dedicated distributor 318A which receives refrigerant from a dedicated 12 expansion valve 316A, while flow path B is fed by a dedicated distributor 318B which receives refrigerant from another dedicated expansion valve, 316B. More than two independently controllable flow paths / circuits may be provided (e.g. 3, 4, 5, 6, 7, 8, 9, or 10 or more). The number of independently controllable flow paths / circuits may be varied for different applications and situations. Although we show here each independently controllable flow path / circuit as having its own dedicated expansion valve 316, it will be appreciated that an expansion valve could be shared across plural independently controllable flow paths / circuits with the control of each of the independently controllable flow paths / circuits being provided by a respective additional (non-expansion, e.g. solenoid) valve downstream of the shared expansion valve. In addition, it may be convenient to use a multi-outlet expansion valve - for example one in which refrigerant is provided to each of multiple different outlets cyclically, thus serving both as an expansion valve and a distributor, allowing a reduction in the number of individual expansion valves used. In the example shown, conduits of the two independently controllable flow paths / circuits A and B are interleaved so that pairs of B conduits are separated by an A conduit, and pairs of A conduits are separated by a B conduit. It will be appreciated however that each pictorial element A1-A6, B1-B6) may actually represent one conduit or a plurality of conduits (2, 3, 4, 5, 6, 7, 8, 9, 10, or more). An example of such an arrangement is shown schematically in Figure 3B, where the air heat exchanger matrix again consists of two independently controllable flow paths / circuits A and B (although clearly more independently controllable flow paths / circuits could be provided), and these are provided as interleaved banks, A1-A8 and B1-B8, of groups of conduits. In this example each group consists of 5 individual conduits, although as previously mentioned the groups of conduits could each comprise from 2 to more than 10 individual conduits. Also, for ease of presentation, the schematic figure does not show the presence of gaps (air gaps) between adjacent elements of the air heat exchanger matrix, but it is to be understood that adjacent elements of the matrix are separated one from another so that air can flow over and between the conduits (into or out of the plane of the paper, assuming the figure is printed on paper). Each conduit may be a finned tube, although other forms of construction are contemplated. The air heat exchanger matrix may comprise just a single layer of conduits, or multiple layers (2,3,4,5,6,7,8,9, or 10 or more) of conduits - although in general it is preferred to have between 1 and 5 layers, e.g. 2 or 3. Although Figures 3A and 3B represent the air heat exchanger matrix as having a rectangular outline, it will be understood that this is merely for ease of illustration: in reality the air heat exchanger may have an external periphery of any suitable shape to match the housing within which it is to be mounted. For example, it may be found convenient to provide an air heat 13 exchanger with an external periphery that is generally circular, for mounting in a suitable housing, because this may facilitate the provision of more uniform air movement through the air heat exchanger - for example based on the use of a fan having a blade whose external periphery describes a circle whose diameter corresponds generally with that of the air heat exchanger matrix. Typically, for domestic sized heat pumps, tubing with 5mm-7mm OD may be used. Good thermal performance can be achieved using copper tubing provided with aluminium fins, for example the fins having run through holes to accept the tubing. With tubing having an OD of 5mm-7mm a convenient centre-to-centre spacing may be in the range 15mm-25mm, for example 18mm-22mm, e g. around 20mm. Conveniently, the tubes of adjacent layers may offset so that a gap between adjacent tubes in row 1 sits in front of a tube in layer 2, etc.. As an alternative to interlacing independently controllable flow paths / circuits it would also be possible to arrange the flow paths in banks - for example the upper half and lower half of the heat exchanger could each be provided as a bank of flow paths, provided that any consequent asymmetry in the air flow was acceptable. Also, if defrosting the upper half, water from melted ice could flow down onto the lower half which could still be frosty and continuing to be cooled and hence possibly refreeze. Such refreezing would tend to be much less of an issue with an interlaced arrangement, which is one reason why it is preferred. The reason for forming the air heat exchanger with multiple independently controllable flow paths / circuits (to make what is in effect a sectional air heat exchanger) will now be explained. As we saw from the discussion of Figure 2, in heating mode air source heat pumps suffer from ice build-up on their air heat exchangers, reducing efficiency and requiring the periodic running of defrost cycles that themselves reduce the overall heat pump efficiency. The need to reverse the direction of refrigerant flow (from the direction used in heating mode) to achieve defrosting is particularly harmful to overall efficiency. The present inventors have appreciated that by providing an air heat exchanger having multiple independently controllable flow paths / circuits it is possible to defrost some of the independently controllable flow paths / circuits, by changing the rate of flow of refrigerant (i.e. the amount of refrigerant being passed through a selected subset of the flow paths) rather than reversing the flow of refrigerant. Typically, the change in the rate of flow will be a reduction in the rate of flow - although it may also be possible to perform selective defrosting by increasing the rate of flow of refrigerant -although this may carry with it a risk that the refrigerant may not be fully vaporised on its transit through the heat exchanger - so that further control measures may be required to prevent the compressor from receiving liquid refrigerant. Changing the rate of flow may involve selectively inhibiting (reducing) the refrigerant flow to selected flow paths / circuits , e.g. optionally turning those flow paths / circuits off or at least severely restricting the flow of refrigerant through them, while keeping the heat pump running in heating mode (as in Figure 1) using the remaining independently controllable flow paths / circuits - at least when the ambient temperature is above freezing (frost build-up on air heat exchangers is primarily caused by the presence of refrigerant at temperatures of OC or below, rather than by sub-zero ambient temperatures). By altering the flow of refrigerant into the selected subset of controllable flow paths (e.g. under the control of the heat pump’s controller), the relevant conduits will no longer be being cooled to the same degree and will hence tend towards the ambient temperature resulting in thawing of the built-up ice on the areas of the air heat exchanger associated with these conduits. Once the relevant conduits have been defrosted, the heat pump’s controller can be used to control another subset of the controllable flow paths so that they in turn can be defrosted. This process may be repeated until all the conduits of the air heat exchanger matrix have been defrosted, and the cycle then repeated when it next becomes necessary or as determined according to a predetermined schedule. As previously noted, the processor may be configured / programmed to control the flow control arrangement in such a way as to maintain an overall refrigerant flow rate (the actual refrigerant flow rate will of course typically depend upon the operating conditions set - e g. the amount of power being extracted, and hence may not be a single constant value whenever the heat pump is in heating mode). Figure 4 illustrates schematically a heat pump 410 according to an aspect of the invention, and in particular an arrangement of elements to support the heat pump’s processor 400 in deciding that a defrost cycle is needed, and also in determining what approach to defrost cycling is appropriate, for example whether to selectively shut down parts of the air heat exchanger to allow the ice to melt as a result of the ambient temperature, or whether to supply heated vapour to the air heat exchanger. The heat pump has a housing 411 within which are located the air heat exchanger matrix 422, one or more fans 424 with an associated drive motor 404, the processor 400, and (optionally) the heating system heat exchanger 434 - although this may be mounted remotely, for example in place of a gas boiler that has been replaced by the heat pump. The processor 400, which has an associated memory 402 (storing software instructions to control operation of the processor, etc.), is operatively connected to dedicated expansion valves 416A, 416B, and 416C, one for each of three independently controllable flow paths / circuits A, B, and C. The processor 400 is also operatively connected to the motor 404 that drives the fan 424 that rotates to drive ambient air over the air heat exchanger matrix 422, and may be 15 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. Although Figure 4 shows the air heat exchanger matrix 422 located downstream of the fan 424 and its motor 404, in practice the fan (and preferably also its motor) may be located downstream of the air heat exchanger matrix 422 with the fan preferably arranged to draw air through the air heat exchanger matrix 422 rather than being arranged to push air through the air heat exchanger matrix. The processor 400 may also be operatively connected to a temperature and humidity sensing arrangement 406 that is configured to sense the ambient temperature and humidity at the site of the air heat exchanger 422. The processor 400 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 heated by the heat pump, and the air heat exchanger 422 in a separate housing outside the premises. In the latter case, the housing for the air heat exchanger 422 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. 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, Zigbee, Wi-Fi, 5G. etc.). The processor 400 may be operatively connected to a control arrangement 440 that controls the heating system of the premises 435. The control arrangement may include one or more thermostats 442 and a controller 444 that is operable to control the heating system, for example setting start and end times for periods of heating. The controller 444 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 400 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 400 may decide not to perform a defrost event if the heating controller 444 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 400 may be operatively connected to a camera (e.g. a video camera) and / or another optical sensor 408 to detect the build-up of ice on the air heat exchanger matrix 422, 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 400 may also be operatively connected to the Internet 450 and hence to a source of (local or hyper local) weather forecasts 460 and also to a system back-end 412 managed by the manufacturer / supplier or installer of the heat pump. Using data from the source of weather forecasts 460, the processor 400 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 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 412 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 400 may be configured to detect a build-up of frost on the air heat exchanger based on a characteristic change in an electrical behaviour or power consumption characteristic of a fan motor, for example the power consumption characteristic may be the amount of current consumed or 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 example detected based on an increased current consumption an increased supply 17 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. Thus the processor may usefully be configured to apply machine learning based on these factors in order to determine whether a detected increase 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 412) of the need to clean the air inlet - the alert optionally being sent by the system back-end 412, for example as a push notification to a user device or even as an automated phone call or email or letter. The processor 400 may be configured to detect and record the power consumption of the fan motor 404 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 performed during development of the heat pump. Continued monitoring (on a continuous or periodic basis) of fan power consumption may also reveal the need for maintenance (e.g. the clearing of air inlet(s), the removal of pest infestation - such as a wasp or bee nest, or the need to replace a damaged fan or fan motor. The motor 404 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 (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 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, power, voltage, pulse amplitude and / or duration and / or frequency, can be taken from the fan controller and used by the heat pump controller. The processor may also be able to detect successful defrosting of the air heat exchanger based on a characteristic change in a power consumption characteristic of a fan motor - e.g. a reduction in current consumption. It should be appreciated however that the effective unblocking of the heat exchanger may actually lead to an increase in current 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. Additionally or alternatively, the processor 400 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 470, and a pressure measured outside the housing e.g. by a second pressure sensor 472. In general the fan associated with an air heat exchanger sucks 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. It is possible instead to suck air into the housing through the air heat exchanger, but this is a less preferred arrangement, and in this case the pressure differential technique may not always be a reliable means of detecting the build-up of ice - depending very much on the precise configuration chosen. In any event, the processor 400 may also take advantage of information provided by a heating system controller 440 and / or a home automation system 444 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 400 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 400 with information on fan motor power consumption and / or other performance characteristics. The fan motor controller may optionally report to the processor 400 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 19 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. 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 power consumption characteristics of the fan motor is generally more reliable and avoids the need to provide sensitive air pressure sensors (whose reliability is not always good).

Claims

1. An air source heat pump comprising a controller and an air heat exchanger to receive liquid refrigerant to extract energy from ambient air that in use flows over the air heat exchanger, the air heat exchanger having a plurality of independently controllable flow paths to carry refrigerant through the air heat exchanger, the air heat exchanger being part of an assembly that includes a flow control arrangement coupled to the controller and operable under control of the controller to selectively adjust flow of refrigerant through the plurality of independently controllable flow paths; the heat pump having in a heating mode a first distribution of refrigerant flows across the plurality of independently controllable flow paths to extract energy from ambient air; andthe controller being programmed to control the flow control arrangement to implement an incremental defrosting procedure in which the distribution of refrigerant flows through a subset of the plurality of independently controllable flow paths is adjusted to selectively defrost a first part of the air heat exchanger while refrigerant continues to flow through at least some of the independently controllable flow paths to enable the heat pump to continue to extract energy from ambient air during the defrosting procedure.

2. An air source heat pump as claimed in claim 1, wherein the controller is programmed to switch to a first defrost mode, in which the distribution of refrigerant flows through some of the plurality of independently controllable flow paths differs from that of the heating mode, to selectively defrost the first part of the air heat exchanger before reverting to the first distribution of refrigerant flows.

3. An air source heat pump as claimed in claim 2, wherein the incremental defrosting procedure comprises the controller switching from the first distribution of refrigerant flows to a second defrost mode, in which the distribution of refrigerant flows through some of the plurality of independently controllable flow paths differs from those of the heating mode and the first defrost mode, after reverting to the first distribution of refrigerant flows.

4. An air source heat pump as claimed in claim 2 or claim 3, wherein the first defrost mode involves selectively inhibiting flow of refrigerant through only some, but not all, of the independently controllable flow paths while maintaining a flow through one or more other of the independently controllable flow paths.

5. An air source heat pump as claimed in claim 4, wherein the second defrost mode involves resuming flow of refrigerant through the independently controllable flow path(s) through which flow was selectively inhibited in the first defrost mode, and selectively inhibiting flow through at least some of the one or more independently controllable flow paths through which flow was maintained in the first defrost mode.

6. An air source heat pump as claimed in claim 1, wherein the defrosting procedure comprises a first step in which the flow control arrangement is controlled to selectively inhibit flow of refrigerant through only some, but not all, of the independently controllable flow paths while maintaining flow through one or more other of the independently controllable flow paths; and subsequently controlling the flow control arrangement to:resume flow of refrigerant through the independently controllable flow path(s) through which flow was selectively inhibited in the first step; and toselectively inhibit flow through at least some of the one or more independently controllable flow paths through which flow was maintained in the first step.

7. The air source heat pump of claim 6, wherein the controller is configured to resume flow of refrigerant through the independently controllable flow path(s) through which flow was selectively inhibited in the first step before selectively inhibiting flow through the at least some of the one or more independently controllable flow paths through which flow was maintained in the first step.

8. The air source heat pump of claim 6, wherein the controller is configured to resume flow of refrigerant through the independently controllable flow path(s) through which flow was selectively inhibited in the first step simultaneously with selectively inhibiting flow through the at least some of the one or more independently controllable flow paths through which flow was maintained in the first step.

9. The air source heat pump of claim 6, wherein the controller is configured to initiate the resumption of flow of refrigerant through the independently controllable flow path(s) through which flow was selectively inhibited in the first step simultaneously with initiating the selective inhibition of flow through the at least some of the one or more independently controllable flow paths through which flow was maintained in the first step.

10. The air source heat pump of any one of the preceding claims, wherein the air heat exchanger has at least three independently controllable flow paths to carry refrigerant through the air heat exchanger.

11. The air source heat pump of any one of the preceding claims, wherein the flow control arrangement comprises, for each of the independently controllable flow paths, a dedicated expansion valve.

12. The air source heat pump of any one of the preceding claims, wherein each of the independently controllable flow paths includes a plurality of conduits for the passage of refrigerant.

13. The air source heat pump of claim 12, wherein for each independently controllable flow path the conduits of the plurality of conduits are connected in parallel to a refrigerant distributor.

14. The air source heat pump of claim 12 or claim 13, wherein, within the air heat exchanger, for each independently controllable flow path the conduits of the plurality of conduits are clustered into one or more groups of multiple conduits.

15. The air source heat pump of claim 14, wherein the groups of conduits for the different independently controllable flow paths are interleaved.

16. The air source heat pump of any one of claims 1 to 12, wherein the independently controllable flow paths are interleaved so that within the air heat exchanger one or more conduits of a first independently controllable flow path are flanked on each side by one or more conduits of a different independently controllable flow path.

17. The air source heat pump of any one of the preceding claims, wherein the assembly includes a refrigerant sub-cooler (306) that is arranged to receive little or no airflow and through which refrigerant is arranged to flow before reaching the flow control arrangement (302).

18. The air source heat pump of any one of the preceding claims, wherein the controller (400) is configured to determine a need to perform an air heat exchanger defrost event based, at least in part, on a change in a power consumption characteristic of a fan motor (404) of the heat pump.

19. The air source heat pump of claim 18, wherein the power consumption characteristic is the amount of current consumed or a rate of change of current consumption.

20. The air source heat pump of any one of the preceding claims, wherein the controller (400) is configured to control the flow control arrangement to selectively inhibit flow of refrigerant by ceasing the flow of refrigerant through the selected independently controllable flow paths while maintaining flow through the one or more other of the independently controllable flow paths.

21. The air source heat pump of any one of claims 1 to 19, wherein the controller (400) is configured to control the flow control arrangement to selectively inhibit flow of refrigerant by maintaining a reduced flow of refrigerant through the selected independently controllable flow paths while maintaining flow through the one or more other of the independently controllable flow paths.

22. A method performed by the controller of an air source heat pump, the heat pump having an air heat exchanger including a plurality of independently controllable flow paths to carry refrigerant, the method comprising:operating the heat pump in a heating mode in which the air heat exchanger is used to extract energy from ambient air using a distribution of refrigerant flows through the plurality of independently controllable flow paths;implementing an incremental defrosting procedure by adjusting the distribution of refrigerant flows of the plurality of independently controllable flow paths to selectively defrost a portion of the air heat exchanger while continuing to flow refrigerant through at least some of the independently controllable flow paths to continue to extract energy from ambient air by operating the heat pump in heating mode.

23. The method of claim 22 wherein adjusting the distribution of refrigerant flows involves selectively inhibiting flow of refrigerant through only some of the refrigerant-carrying conduits of the air heat exchanger while maintaining flow through others of the refrigerant-carrying conduits of the air heat exchanger.

24. The method of claim 23, wherein the flow of refrigerant through the refrigerant-carrying conduits of the air heat exchanger through which flow was selectively inhibited is resumed24before selectively inhibiting flow through at least some of the refrigerant-carrying conduits of the air heat exchanger through which flow was maintained.

25. The method of claim 23, wherein resuming the flow of refrigerant through the5 refrigerant-carrying conduits of the air heat exchanger through which flow was selectively inhibited is initiated simultaneously with selectively inhibiting flow through at least some of the refrigerant-carrying conduits of the air heat exchanger through which flow was maintained.

26. The method of any one of claims 22 to 25, wherein the heat pump is connected to supply10 heat to a heating system, the method further comprising performing the defrost event without interrupting the supply of heat to the heating system.AMENDMENTS TO THE CLAIMS HAVE BEEN FILED AS FOLLOWS:Claims1. An air source heat pump comprising a controller and an air heat exchanger to receive liquid refrigerant to extract energy from ambient air that in use flows over the air heat exchanger, the air heat exchanger having a plurality of independently controllable flow paths to carry the refrigerant through the air heat exchanger, the air heat exchanger being part of an assembly that includes a flow control arrangement coupled to the controller and operable under control of the controller to selectively adjust flow of the refrigerant through the plurality of independently controllable flow paths, wherein the flow control arrangement comprises, for each of the independently controllable flow paths, a dedicated expansion valve;the heat pump having in a heating mode a first distribution of the refrigerant flows across the plurality of independently controllable flow paths to extract energy from the ambient air; and the controller being programmed to control the flow control arrangement to implement an incremental defrosting procedure in which the distribution of the refrigerant flows through a subset of the plurality of independently controllable flow paths is adjusted to selectively defrost a first part of the air heat exchanger while the refrigerant continues to flow through at least some of the independently controllable flow paths to enable the heat pump to continue to extract energy from the ambient air during the defrosting procedure.

2. An air source heat pump as claimed in claim 1, wherein the controller is programmed to switch to a first defrost mode, in which the distribution of the refrigerant flows through some of the plurality of independently controllable flow paths differs from that of the heating mode, to selectively defrost the first part of the air heat exchanger before reverting to the first distribution of the refrigerant flows.

3. An air source heat pump as claimed in claim 2, wherein the incremental defrosting procedure comprises the controller switching from the first distribution of the refrigerant flows to a second defrost mode, in which the distribution of the refrigerant flows through some of the plurality of independently controllable flow paths differs from those of the heating mode and the first defrost mode, after reverting to the first distribution of the refrigerant flows.

4. An air source heat pump as claimed in claim 2 or claim 3, wherein the first defrost mode involves selectively inhibiting flow of the refrigerant through only some, but not all, of theindependently controllable flow paths while maintaining a flow through one or more other of the independently controllable flow paths.

5. An air source heat pump as claimed in claim 4, wherein the second defrost mode involves resuming flow of the refrigerant through the independently controllable flow path(s) through which flow was selectively inhibited in the first defrost mode, and selectively inhibiting flow through at least some of the one or more independently controllable flow paths through which flow was maintained in the first defrost mode.

6. An air source heat pump as claimed in claim 1, wherein the defrosting procedure comprises a first step in which the flow control arrangement is controlled to selectively inhibit flow of the refrigerant through only some, but not all, of the independently controllable flow paths while maintaining flow through one or more other of the independently controllable flow paths; andsubsequently controlling the flow control arrangement to:resume flow of the refrigerant through the independently controllable flow path(s) through which flow was selectively inhibited in the first step; and toselectively inhibit flow through at least some of the one or more independently controllable flow paths through which flow was maintained in the first step.

7. The air source heat pump of claim 6, wherein the controller is configured to resume flow of the refrigerant through the independently controllable flow path(s) through which flow was selectively inhibited in the first step before selectively inhibiting flow through the at least some of the one or more independently controllable flow paths through which flow was maintained in the first step.

8. The air source heat pump of claim 6, wherein the controller is configured to resume flow of the refrigerant through the independently controllable flow path(s) through which flow was selectively inhibited in the first step simultaneously with selectively inhibiting flow through the at least some of the one or more independently controllable flow paths through which flow was maintained in the first step.

9. The air source heat pump of claim 6, wherein the controller is configured to initiate the resumption of flow of the refrigerant through the independently controllable flow path(s) through which flow was selectively inhibited in the first step simultaneously with initiating the selective27inhibition of flow through the at least some of the one or more independently controllable flow paths through which flow was maintained in the first step.

10. The air source heat pump of any one of the preceding claims, wherein the air heat exchanger has at least three independently controllable flow paths to carry the refrigerant through the air heat exchanger.

11. The air source heat pump of any one of the preceding claims, wherein each of the independently controllable flow paths includes a plurality of conduits for the passage of the refrigerant.

12. The air source heat pump of claim 11, wherein for each independently controllable flow path the conduits of the plurality of conduits are connected in parallel to a refrigerant distributor.

13. The air source heat pump of claim 11 or claim 12, wherein, within the air heat exchanger, for each independently controllable flow path the conduits of the plurality of conduits are clustered into one or more groups of multiple conduits.

14. The air source heat pump of claim 13, wherein the groups of conduits for the different independently controllable flow paths are interleaved.

15. The air source heat pump of any one of claims 1 to 11, wherein the independently controllable flow paths are interleaved so that within the air heat exchanger one or more conduits of a first independently controllable flow path are flanked on each side by one or more conduits of a different independently controllable flow path.

16. The air source heat pump of any one of the preceding claims, wherein the assembly includes a refrigerant sub-cooler that is arranged to receive little or no airflow and through which the refrigerant is arranged to flow before reaching the flow control arrangement.

17. The air source heat pump of any one of the preceding claims, wherein the controller is configured to determine a need to perform an air heat exchanger defrost event based, at least in part, on a change in a power consumption characteristic of a fan motor of the heat pump.

18. The air source heat pump of claim 17, wherein the power consumption characteristic is the amount of current consumed or a rate of change of current consumption.

19. The air source heat pump of any one of the preceding claims, wherein the controller is5 configured to control the flow control arrangement to selectively inhibit flow of the refrigerant by ceasing the flow of the refrigerant through the selected independently controllable flow paths while maintaining flow through the one or more other of the independently controllable flow paths.10 20. The air source heat pump of any one of claims 1 to 18, wherein the controller isconfigured to control the flow control arrangement to selectively inhibit flow of the refrigerant by maintaining a reduced flow of the refrigerant through the selected independently controllable flow paths while maintaining flow through the one or more other of the independently controllable flow paths.1521. A method performed by the controller of an air source heat pump, the heat pump having C\j an air heat exchanger including a plurality of independently controllable flow paths to carry refrigerant, the air heat exchanger being part of an assembly that includes a flow control arrangement coupled to the controller, wherein the flow control arrangement comprises, for each of the independently controllable flow paths, a dedicated expansion valve, the method1 comprising:operating the heat pump in a heating mode in which the air heat exchanger is used to extract energy from ambient air using a distribution of refrigerant flows through the plurality of independently controllable flow paths; and25 implementing an incremental defrosting procedure by adjusting the distribution ofrefrigerant flows of the plurality of independently controllable flow paths to selectively defrost a portion of the air heat exchanger while continuing to flow the refrigerant through at least some of the independently controllable flow paths to continue to extract energy from the ambient air by operating the heat pump in heating mode.3022. The method of claim 21, wherein adjusting the distribution of refrigerant flows involves selectively inhibiting flow of the refrigerant through only some of the refrigerant-carrying conduits of the air heat exchanger while maintaining flow through others of the refrigerantcarrying conduits of the air heat exchanger.

23. The method of claim 22, wherein the flow of the refrigerant through the refrigerant-carrying conduits of the air heat exchanger through which flow was selectively inhibited is resumed before selectively inhibiting flow through at least some of the refrigerant-carrying conduits of the air heat exchanger through which flow was maintained.

524. The method of claim 22, wherein resuming the flow of the refrigerant through the refrigerant-carrying conduits of the air heat exchanger through which flow was selectively inhibited is initiated simultaneously with selectively inhibiting flow through at least some of the refrigerant-carrying conduits of the air heat exchanger through which flow was maintained.1026. The method of any one of claims 21 to 24, wherein the heat pump is connected to supply heat to a heating system, the method further comprising performing the defrost event without interrupting the supply of heat to the heating system.19 04 2431

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