Air Purification Apparatus

Abstract

Air purification apparatus 100 comprises an airflow conduit 106 extending between an inlet 102 and an outlet 104, the conduit containing an air purification device 108 and a heat source 138. An airflow driving device 110 drives air from the inlet to the outlet. The inlet receives air from an upper air zone 12 of an indoor space 10; the outlet is below, and laterally offset from, the inlet and discharges purified and heated air into a lower air zone 14 of the indoor space 10. The centre of the inlet is typically 1.2 to 1.4 metres above the outlet, with the outlet typically at floor level. The purification device may comprise a germicidal UV light source 126, an absorption filter 132, and an activated carbon adsorption filter 134 downstream of the UV light source. The UV light source may be elongate and arranged parallel to the conduit. The inlet may have a coarse pre-filter 131. A filter support frame 136 may define a filter aperture which has a cross-section area greater than that of the air conduit, with the aperture angled relative to a longitudinal axis of the conduit. A method of controlling the apparatus is also claimed.

Description

FIELD OF THE INVENTION Embodiments of the present invention relate to an air purification apparatus. In particular, but not exclusively, they relate to an air purification and convection apparatus for indoor spaces. BACKGROUND TO THE INVENTION Compact or portable air purifiers are typically desk-mounted or floor-standing, located wholly in the lower air zone / region of an indoor space. Contaminated air close to ground level is ingested, purified, and exhausted upwardly and / or laterally. Larger or permanent air purifiers may be affixed to air handler units or HVAC (heating ventilation and air conditioning) units. Such units are fluidly coupled to one or more indoor spaces via ventilation ducts that comprise vents, allowing airflow to be moved into or out of individual rooms. The vents are typically located in the upper air zone / region of each indoor space. The lower air zone of an indoor space is defined herein as the zone below head height of a standing adult human, regardless of the overall height of the indoor space. The upper air zone is the zone at or above head height, regardless of the overall height of the indoor space. BRIEF DESCRIPTION OF VARIOUS EMBODIMENTS OF THE INVENTION According to various, but not necessarily all, embodiments of the invention there is provided an air purification apparatus comprising: an air inlet; an air outlet; an airflow conduit between the air inlet and the air outlet; one or more air purification devices in the airflow conduit; a heat source configured to heat air in the airflow conduit; and an airflow driving device configured to drive air from the air inlet to the air outlet, wherein the air outlet is below the air inlet, wherein the air inlet is at an elevation configured to receive air from an upper air zone of an indoor space, wherein the air outlet is at an elevation configured to discharge purified air into a lower air zone of the indoor space, and wherein the air outlet is laterally offset from the air inlet. An advantage of circulating air downwards from the upper air zone to the lower air zone is improved air purification and thermal destratification. The air in the upper air zone tends to be naturally warmer due to thermal stratification, especially if the indoor space has poor convection. The air in the upper air zone also tends to be more contaminated because more pollutants are generated at head height, such as pathogens and mould spores. Therefore, the naturally warmer air from the upper air zone is purified and recycled / harvested to the lower air zone. Heat energy that is ‘wasted’ in the upper air zone becomes useful once it has been transported to the lower air zone where it will usefully contribute to occupant comfort. This heat recycling reduces the perceived ‘wind chill’ temperature drop caused by the airflow driving device, and may perceptibly increase the ambient temperature of the indoor space, especially when the heat source is active. As will be described later, the heat source may comprise either heat generated by an optional germicidal light source, and / or heat generated by an optional heater device A related advantage of the heat source is that the warming of the air by the heat source assists with convection due to the heated air being inserted into the indoor space at a low height. An advantage of the air outlet being laterally offset some distance away horizontally from the inlet is that the warm and purified air is exhausted further away from the air inlet. This helps to improve the air changes per hour within an indoor space, because the air purification apparatus aspirates less of its own purified air. Optionally, a bottom of the air inlet is at least 1 metre above a top of the air outlet. Optionally, a centre of the air inlet is at least 1.2 metres or at least 1.4 metres above a centre of the air outlet. An advantage is that the air inlet is at a suitably high elevation to receive air which is half a degree Celsius warmer or more, than the ambient temperature at the elevation of the air outlet. Furthermore, the air is captured close to or above head height, where many contaminants are generated. It would be appreciated that if the indoor space is largely seated (e.g., lounge, cinema, or office desk area), the air inlet may be at a lower elevation. Optionally, the air inlet is at and / or proximal to a top end of the air purification apparatus, and the air outlet is at and / or proximal to a bottom end of the air purification apparatus. Optionally, the air inlet is proximal to and / or comprised in a duct end face of the airflow conduit. Optionally, a vertical separation from a bottom of the air outlet to a top of the air inlet is selected from the range 1.4 metres to 2.2 metres. Optionally, the air purification apparatus is located in the indoor space, wherein the air inlet is located in the upper air zone of the indoor space at a height of at least 1.5 metres, and wherein the air outlet is located in the lower air zone of the indoor space closer to the floor of the indoor space than the ceiling of the indoor space. Optionally, the air inlet is below ceiling level of the indoor space and the air outlet is wall-supported and elevated above floor level of the indoor space. An 4 advantage is that the exhausted purified air is not too close to the floor so it is less likely to pick up dust. Alternatively, the air outlet is at or below floor level (the air outlet comprising a floor vent). An advantage is that the greatest temperature gradient occurs from floor to ceiling. Optionally, the airflow conduit comprises an upright elongate duct section (upright portion), and wherein at least one of the one or more air purification devices is in the upright elongate duct section. Optionally, the upright elongate duct section is vertically orientated. Optionally, the upright elongate duct section is a straight duct section. Optionally, the airflow conduit has an inner duct wall cross-sectional area selected from the range 200cmA2 to 500cmA2 or from 500cmA2 to 5000cmA2. Optionally, the air outlet is laterally offset from the air inlet and faces away from the air inlet. Optionally, the air conduit comprises a laterally extending duct section (lateral portion) between the upright elongate duct section and the air outlet, downstream of the upright elongate duct section, and leading to the air outlet. Optionally, the air outlet faces away from the air inlet. An advantage is that purified air is exhausted further away from the air inlet, and in a direction away from the air inlet. This helps to improve the air changes per hour within an indoor space, because the air purification apparatus aspirates less of its own purified air. Optionally, the air purification apparatus comprises feet, casters, or a pad to support the air purification apparatus as a free-standing unit. Additionally, or alternatively, the air purification apparatus comprises one or more wall fixing points, such as rear brackets, to secure the air purification apparatus to a wall. Optionally, the air purification apparatus is mounted to a wall skin and protrudes into a room, or is mounted to a wall structure behind a wall skin so as not to protrude into the room, or to protrude less into the room. Optionally, the airflow driving device comprises a fan. Optionally, the fan is a variable-speed fan. Optionally, the air purification apparatus comprises a control apparatus configured to control a fan speed of the variable-speed fan. Optionally, the control apparatus is configured to control the fan speed in dependence on a signal from a filter loading sensor and / or a signal from a pollutant sensor. Optionally, the airflow driving device is monodirectional. This means that the airflow direction cannot be reversed by the airflow driving device. Optionally, the one or more air purification devices comprise a germicidal light source. Optionally, the germicidal light source comprises one or more ultraviolet lamps such as UV-C lamps. An advantage is that pathogens, such as viruses, which are too small to be absorbed by filters are destroyed. A further advantage of including a germicidal light source is that the filter(s) of the air purification apparatus can be coarser without reducing efficacy, reducing pressure losses and therefore energy consumption. A further advantage is that ultraviolet lamps generate significant heat. A heatsink of the ultraviolet lamps may be located in the airflow conduit to both cool the ultraviolet lamp and warm the air passing between the air inlet and air outlet. Optionally, the germicidal light source is elongate and arranged in a parallel or mostly parallel orientation to the airflow conduit. An advantage is increased germicidal effectiveness because pathogens in the driven airflow are exposed to germicidal light for longer. Optionally, the air purification apparatus is located in a central section of the air purification device, between a bottom of the air inlet and a top of the air outlet. Optionally, the air purification apparatus is configured to block line of sight to the germicidal light source from the air inlet, and configured to block line of sight to the germicidal light source from the air outlet. For example, the air purification apparatus can comprise one or more opaque or mostly-opaque barriers to block all possible lines of sight to the germicidal light source from the respective air inlet or air outlet. An advantage is reduced skin exposure and eye exposure to germicidal light. Optionally, the one or more air purification devices comprise an absorption filter, such as a HEPA filter. Optionally, the absorption filter is in a line of sight of the germicidal light source. An advantage is improved purification effectiveness and filter life, because the germicidal light source prevents pathogen growth on the HEPA filter. The HEPA filter may be sized to absorb mould spores and bacteria, which are destroyed by the germicidal light source. Optionally, the one or more air purification devices comprise an adsorption filter, such as an activated carbon or zeolite filter. An advantage is improved air purification because volatile organic compounds (VOCs) and odours are removed from the driven air. Optionally, the adsorption filter is downstream of the germicidal light source. An advantage is extended filter life due to initial irradiation. Optionally, the adsorption filter is downstream of the absorption filter. An advantage is extended filter life of the adsorption filter due to pre-filtering by the absorption filter. Optionally, the air purification apparatus comprises a heater device to heat the driven air, so that air passing through the air outlet is at a higher temperature than the air at the air inlet. The heater device may comprise a resistive heater device or any other type of heater device configured to primarily output heat. This helps to improve the air changes per hour within an indoor space, because the air leaving the air purification apparatus at a low elevation has a higher temperature than the air entering the air purification apparatus at a high elevation. Therefore, the rate of convective air mixing and heat dispersion in the indoor space increases, and the air purification apparatus aspirates less of its own purified air. Since the air inlet is above the air outlet, the heater device will create an upwards buoyancy effect of the air within the airflow conduit, resisting the downwards driving force of the airflow driving device. The airflow driving device may therefore be configured to overcome the buoyancy effect of the heater device in addition to overcoming the pressure losses caused by the filters. Optionally, a control apparatus of the air purification apparatus is configured to control an operating state (activation / deactivation state) of the heater device or a power of the heater device, in dependence on a signal indicative of heating demand. Optionally, the signal indicative of heating demand is a thermostat control signal. Optionally, the signal is based at least in part on a setpoint error (actual vs measured temperatures). Optionally, the control apparatus is configured to control the fan speed of the variable-speed fan in dependence on an operating state of the heater device or a setpoint error associated with the heater device. Optionally, the control apparatus is configured to increase the fan speed in dependence on a signal indicating activation or an increasing setpoint error. The signal may comprise a thermostat control signal. An advantage is that when the heater device is in a deactivated operating state, the fan consumes less energy and is quieter. When the heater device is on, the fan speed is increased to overcome the buoyancy effect of the heater device. Optionally, the heater device is arranged downstream of a germicidal light zone of the air purification apparatus. The germicidal light zone refers to the portion in the airflow conduit within line of sight of the germicidal light source. An advantage is that the germicidal light source works better in colder temperatures. Optionally, the heater device is located downstream of the absorption filter and / or adsorption filter. An advantage is a reduced rate of pathogen growth on the filters because the filters are in a cooler location. Optionally, the heater device has a fixed power consumption or peak selectable power consumption selected from the range 40 watts to 200 watts, or from 40 watts to 100 watts, or from 100 watts to 200 watts. An advantage is that enough heat is added to improve convection and mixing of the exhausted purified air, without acting as a room heater device. If the heater device is too powerful then the fan would need to work harder to overcome the upwards buoyancy effects caused by the heater device within the airflow conduit. Alternatively, the heater device has a fixed power consumption or peak selectable power consumption selected from the range 200 watts to 2000 watts, or from 200 watts to 800 watts, or from 800 watts to 2000 watts. This is suitable for use cases where the heater device acts as a secondary space heater, or even a primary space heater if the indoor space is small. Optionally, the one or more air purification devices comprise a pre-filter proximal to the air inlet, and one or more main filters downstream of the prefilter, wherein the pre-filter is coarser than at least one of the main filters. Optionally, the one or more main filters comprises the absorption filter and / or the adsorption filter as described earlier. An advantage is that large particles are caught at the pre-filter, extending the filter life of the main filters. Optionally, at least one of the main filters comprises a filter support frame defining a filter aperture in which a filter medium is mounted, wherein the filter aperture has a cross-sectional area greater than a cross-sectional area of the airflow conduit, and wherein the filter aperture is at an oblique or parallel angle relative to a longitudinal axis of the airflow conduit. For example, the main filter can be a diagonally-mounted flat panel filter, or can comprise a radial flow filter. An advantage is that the filtration area is increased without needing an enlarged conduit cross-section. The main filters may be configured to remove any one or more of the following: car brake dust, car tyre dust, pollen, organic shedding, general dust, smoke particles, cooking emissions, volatile organic compounds emitted from furniture and flooring, etc. The main filter mesh size and germicidal light source frequency / intensity may be sized differently depending on targeted pathogens which will probably depend on application. Domestic applications may be more concerned with mould. Medical facilities may be more concerned with bacteria and viruses. According to various, but not necessarily all, embodiments of the invention there is provided a control apparatus to control the air purification apparatus, the control apparatus configured to: control the airflow driving device to drive air from the air inlet to the air outlet, so that air is received from the upper air zone of the indoor space, and purified air is discharged into the lower air zone of the indoor space. Optionally, where the airflow driving device may comprise a variable-speed fan, the control apparatus may be configured to control a fan speed of the variablespeed fan. Optionally, the control apparatus is configured to control the fan speed in dependence on a signal from a filter loading sensor and / or a signal from a pollutant sensor. Optionally, the control apparatus is configured to output an alarm signal in dependence on a predetermined limit being reached, wherein the predetermined limit is associated with the fan speed. The predetermined limit may further be associated with the filter loading sensor. Optionally, the control apparatus is configured to control an operating state (activation / deactivation state) or a power of the heater device, in dependence on a signal indicative of heating demand, such as a thermostat control signal. The thermostat control signal may depend on a setpoint error (actual vs measured temperature). Optionally, the control apparatus is configured to control the fan speed of the variable-speed fan in dependence on an operating state of the heater device or a setpoint error associated with the thermostat control signal. Optionally, the control apparatus is configured to control an intensity of the germicidal light source in dependence on a signal from a light source monitoring indicator. Optionally, the control apparatus is configured to output an alarm signal in dependence on the light source monitoring sensor indicating light failure. Optionally, the control apparatus is configured to control fan speed in dependence on a signal from a space occupancy indicator. According to various, but not necessarily all, embodiments of the invention there is provided a method of controlling the air purification apparatus, the method comprising: controlling the airflow driving device to drive air from the air inlet to the air outlet, so that air is received from the upper air zone of the indoor space, and purified air is discharged into the lower air zone of the indoor space. Optionally, where the airflow driving device may comprise a variable-speed fan, the method may comprise controlling a fan speed of the variable-speed fan in dependence on a signal from a filter loading sensor and / or a signal from a pollutant sensor. Optionally, the method comprises controlling an operating state (activation / deactivation state) of the heater device or a power of the heater device, in dependence on a signal indicative of heating demand, such as a thermostat control signal. Optionally, the method comprises controlling the fan speed of the variable-speed fan in dependence on an operating state of the heater device or a setpoint error associated with the heater device. According to various, but not necessarily all, embodiments of the invention there is provided an air purification apparatus comprising: an air inlet; an air outlet; an airflow conduit between the air inlet and the air outlet; one or more air purification devices in the airflow conduit; and an airflow driving device configured to drive air from the air inlet to the air outlet, wherein the air outlet is below the air inlet, wherein the air inlet is at an elevation configured to receive air from an upper air zone of an indoor space, and wherein the air outlet is at an elevation configured to discharge purified air into a lower air zone of the indoor space. According to various, but not necessarily all, embodiments of the invention there is provided an air purification apparatus comprising: an air inlet; an air outlet; an airflow conduit between the air inlet and the air outlet; one or more air purification devices in the airflow conduit; and an airflow driving device configured to drive air from the air inlet to the air outlet, wherein the air outlet is below the air inlet. BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of various examples of embodiments of the present invention reference will now be made by way of example only to the accompanying drawings in which: FIG. 1 illustrates a cutaway side view of an example air purification apparatus; FIGS. 2A-2B illustrate front perspective views of air purification apparatus; FIG. 3 illustrates a control apparatus; and FIG. 4 illustrates an example method. DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS OF THE INVENTION FIG. 1 illustrates an indoor space 10 such as a room or hall. The indoor space 10 comprises a floor 18, a ceiling 16, and walls 20. The indoor space 10 is divided into an upper air zone 12 and a lower air zone 14. The lower air zone 14 of an indoor space is defined herein as the zone below head height of a standing adult human, regardless of the overall height of the indoor space. The upper air zone 12 is the zone at or above head height, regardless of the overall height of the indoor space. Due to thermal stratification, ambient air gradually becomes warmer with height, so the average ambient temperature in the upper air zone 12 of the indoor space 10 is greater than that in the lower air zone 14 of the indoor space 10. The average temperature difference may be 0.5 Celsius or more. The difference can be even greater for rooms with poor convection. Pathogen concentration also tends to increase at or above head height, due to pathogen emissions from humans in the indoor space 10, and because mould and damp problems tend to occur at higher elevations. Heavier particles tend to settle quickly, whereas lighter particles may remain airborne for long periods of time. FIG. 1 further illustrates a non-limiting example of a tall air purification apparatus 100 within the indoor space 10, comprising the features and optional features defined in the above summary. The air purification apparatus 100 receives warmer ambient air from the upper air zone 12, purifies the air, optionally further heats the air, and then exhausts the purified air in the cooler lower air zone 14 of the indoor space 10. This simultaneously assists with thermal destratification as well as pathogen, pollution, and odour reduction. The air purification apparatus 100 is fan-driven by an airflow driving device to ensure that air is transported downwardly, counteracting the natural thermal buoyancy of the air within the air purification apparatus 100. The air purification apparatus 100 of FIG. 1 comprises an upright elongate duct section 112 (upright portion) and an optional laterally extending duct section 120 (lateral portion) below the upright elongate duct section 112, together defining an L-shaped airflow conduit 106. Optionally, the air purification apparatus 100 further comprises an upper laterally extending duct section 120 (not shown) for the air inlet 102. Airflow enters a top region of the upright elongate duct section 112 via an air inlet 102, is fan-propelled downwardly through or past a plurality of air purification devices 108 located inside the upright elongate duct section 112, and past a heater device 138, and is then exhausted back into the indoor space 10 through an air outlet 104 via the optional laterally extending duct section 120. The exhausted air exits the air purification apparatus 100 at a significantly lower elevation than the air inlet 102 because the air outlet 104 is lower than the air inlet 102. Depending on the fan speed of the air purification apparatus 100, the number of air changes per hour of the indoor space 10 can be significantly increased, resulting in cleaner air and less thermal stratification in the indoor space 10. FIG. 2A illustrates an apparatus 100 in which the laterally extending duct section 120 is omitted or supplied separately. The airflow conduit 106 is straight, and the air outlet 104 is comprised in the upright elongate duct section 112 rather than in a separate laterally extending duct section 120. First, the exterior shape and geometry of the air purification apparatus 100 is described in more detail, referring to FIGS. 1-2B collectively. The illustrated upright elongate duct section 112 is shown as a substantially straight and substantially vertically extending duct, with an internal cross-section area selected from the range 200cmA2 to 500cmA2 to minimise room intrusion. This is only one example of an upright elongate duct section 112. The laterally extending duct section 120 in FIG. 1 can have various shapes, including but not limited to a straight horizontal duct section extending substantially perpendicularly away from the upright elongate duct section 112. In some non-illustrated examples, several laterally extending duct sections 120 could be fluidly coupled to the upright elongate duct section 112, each leading to a separate air outlet vent. Just one is shown in FIG. 1. In some examples, an enlarged base of the air purification apparatus 100, e.g., circular or multispoked base, can comprise one or more laterally extending duct sections 120 for the air outlet 104. The function of the laterally extending duct section 120 in FIG. 1 is to ensure a lateral separation between the air inlet 102 and air outlet 104, in addition to the vertical separation, to reduce re-aspiration of purified and warm air. This same result could be achieved without a laterally extending duct section 120, by instead shaping the upright elongate duct section 112 so that the air outlet 104 is laterally offset from the air inlet 102. For instance, the upright elongate duct section 112 could be non-straight or could taper laterally. As shown in FIG. 1, the upright elongate duct section 112 may house the air purification devices 108 within, and may also house the heater device 138 within. Therefore, the upright elongate duct section 112 and its internal components is common to versions with and without laterally extending duct sections 120. If laterally extending duct sections 120 are omitted or supplied separately, then the upright elongate duct section 112 may be as shown in FIG. 2A. The upright elongate duct section 112 comprises both the air inlet 102 and the air outlet 104, located towards opposite respective upper and lower ends of the upright elongate duct section 112. If a laterally extending duct section 120 can be retrofitted, then the upright elongate duct section 112 may optionally comprise a fixing point (e.g., flanges / brackets) enabling a laterally extending duct section 120 to be fitted against the air outlet 104 and / or may comprise a fixing point enabling a laterally extending duct section 120 to be fitted against the air inlet 102, if required by the end user. The air purification apparatus 100 illustrated in FIG. 1 is wall-mounted. It is also shown as resting on the floor 18, to demonstrate that the air purification apparatus 100 could be either wall-mounted via wall fixing points 124 such as brackets, or floor-standing (free-standing) on feet 122, casters, and / or an enlarged pad. In practice, the air purification apparatus 100 may be elevated above floor level if it is wall-mounted. The air purification apparatus 100 may be capable of both wall-mounted and free-standing configurations. In some examples, a plurality of air purification apparatuses may be in the same indoor space 10, and are all wall-mounted, or all floor-standing, or comprise a combination of wall-mounted and floor-standing units. Where the air purification apparatus 100 may be wall-mounted, the laterally extending duct section 120 may extend in a direction away from the wall 20, as shown in FIG. 1, rather than parallel to the wall 20. If the air purification apparatus 100 is configured to be wall-mounted centrally on a wall 20, away from the corners, then it can be defined as having a rear side facing the wall 20, and front and lateral sides facing the indoor space 10. If the air purification apparatus 100 is configured to be corner-mounted against a corner between two walls, then it has two rear sides facing the respective walls, and two front sides facing the indoor space 10. The air purification apparatus 100 illustrated in FIG. 1 is mounted with its rear side against a wall skin of the wall 20 of the indoor space 10. Therefore, the other sides (e.g., front side and lateral sides) protrude into the indoor space 10. However, the air purification apparatus 100 could instead be recessed into the wall 20 so that the front side is flush with the wall 20, or even concealed so that the upright elongate duct section 112 is concealed behind the wall skin of the wall 20. The air inlet 102 and air outlet 104 would remain fluidly coupled to the indoor space 10, either directly or indirectly via a laterally extending duct section(s) 120. A floor-standing version of the air purification apparatus 100 may comprise an omnidirectional or multi-directional air inlet 102 and / or an omnidirectional or multi-directional air outlet 104. The floor-standing air purification apparatus 100 may or may not have a defined ‘front’ and Tear’. The air inlet 102 and air outlet 104 illustrated in FIGS. 1-2B are located between floor level and ceiling level. It would be appreciated that the air inlet 102 could instead be suspended from the ceiling 16. It would also be appreciated that the air outlet 104 could be located at floor level, flush with floor level and / or mounted to the floor 18. In such embodiments, the wall-mounted or floor-standing upright elongate duct section 112 may be fluidly coupled to the remote ceiling-suspended air inlet 102 and / or floor air outlet 104 via lateral ducting, at least part of which extends above the ceiling 16 and / or below the floor 18. However, an advantage of FIGS. 1,2A, and 2B, where the air inlet 102 and air outlet 104 are within the indoor space 10, is that the air purification apparatus 100 can be installed or retrofitted to an indoor space 10 without requiring destruction or modification of the ceiling 16 and floor 18 of the indoor space 10. As best shown in FIGS. 2A-2B, the upright elongate duct section 112 is optionally polygonal in cross-section. The polygonal duct section is defined by a plurality of elongate faces 118, which extend upwardly. The elongate faces 118 include one or more front elongate faces 118F at the front side of the air purification apparatus 100, side elongate faces 118S at the lateral sides of the air purification apparatus 100, and a rear elongate face 118R at the rear side of the air purification apparatus 100. The elongate faces 118 are fluidly capped by an upper end face 114 of the upright elongate duct section 112, defining an upstream end of the airflow conduit 106. Some or all of the elongate faces 118 may be fluidly capped at or proximal to their bottom ends by a lower end face 116 of the upright elongate duct section 112. The top and bottom ends of the elongate faces 118 of the upright elongate duct section 112 may define respective top and bottom ends of the air purification apparatus 100. Additionally, or alternatively, the upper and lower end faces 114, 116 of the upright elongate duct section 112 may define the respective top and bottom ends of the air purification apparatus 100. The upper end face 114, and the top ends of the elongate faces 118 of the upright elongate duct section 112 may be below ceiling level. The lower end face 116, and the lower ends of the elongate faces 118 of the upright elongate duct section 112 may be above floor level. Therefore, the upright elongate duct section 112 may not extend from full floor-to-ceiling height. In a wall-mounted implementation, the lower end face 116 may be elevated and suspended above floor level optionally by at least 0.1 metres or at least 0.5 metres. Each elongate face 118 of the upright elongate duct section 112 may be defined by a separate duct wall. Likewise, the upper and lower end faces 114, 116 may be defined by end walls secured to ends of the duct walls. Alternatively, a duct section 112, 120 may be non-polygonal / circular in crosssection, the duct section being defined by a single rounded elongate duct wall fluidly capped at or proximal to its top and bottom ends by respective upper and lower end faces 114, 116. FIG. 1 shows in cross-section front and rear surfaces of the upright elongate duct section 112. The front and rear surfaces are upright and optionally parallel as shown. The front surface may be defined by a front elongate face 118F or front duct wall, if the upright elongate duct section 112 is polygonal in crosssection. If the upright elongate duct section 112 is circular in cross-section, the front surface may be defined by a front side of a rounded elongate duct wall. In FIG. 1, the laterally extending duct section 120 is connected to the front surface of the upright elongate duct section 112, at an elevation proximal to a lower end / bottom edge of the front surface. Regarding the air inlet 102, FIG. 1 shows that the front side of the air purification apparatus 100, e.g., the front surface / front face of the upright elongate duct section 112 may comprise an aperture proximal to an upper end / top edge of the front surface / front face, the aperture defining the air inlet 102. The air inlet 102 is proximal to the top end of the air purification apparatus 100. The aperture may be covered by a grille. The air inlet 102 in FIG. 1 is therefore mono-directional to receive air in a horizontal or mostly horizontal direction. The air inlet 102 faces away from the wall 20, facing the indoor space 10. Regarding the elevation of the air inlet 102, FIG. 1 illustrates the air inlet 102 being at about head level for a standing adult human. Therefore, the elevation of the centre and / or bottom and / or top of the air inlet 102 may be selected from the range 1.5 metres to 2.2 metres above floor level, or from the range 1.5 metres to 1.9 metres. If the air inlet 102 has multiple vents (e.g., 102A-102C of FIG. 2A), the centre is taken to be the centre of the centres of the individual vents 102A-102C, the top is the top of the uppermost vent 102C, and the bottom is the bottom of the lowermost vent 102A / 102B. If the air purification apparatus 100 is wall-mounted, the centre of the air inlet 102 may be at least 1.3 metres above the lower end face 116 and / or bottom end of the air purification apparatus 100. FIG. 2A illustrates a variant in which the air inlet 102 may be defined by more than one side of the upright elongate duct section 112. For example, the air inlet 102 comprises vents 102A, 102B, 102C in multiple faces of the upright elongate duct section 112. FIG. 2A shows a pair of front faces which face in different horizontal directions than each other, wherein each front face comprises an aperture defining a separate vent 102A, 102B of the air inlet 102. The air inlet 102 in FIG. 2A is therefore multi-directional in a horizontal plane, to receive air from multiple horizontal directions. FIG. 2A shows that in addition to, or alternatively to the multiple front-facing vents 102A, 102B, an upper end face 114 of the upright elongate duct section 112 may comprise an aperture defining a further separate upwards-facing vent 102C of the air inlet 102, in addition to the vent(s) 102A, 102B in the front face(s). The air inlet 102 in FIG. 2A is therefore also multi-directional in a vertical 20 plane, to receive air from multiple vertical directions. Further, as shown in FIG. 2A, the vent 102C in the upper end face 114 may be tilted / sloped away from the wall 20. The air inlet 102 is proximal to the top end of the air purification apparatus 100. FIG. 2B is a perspective view of an apparatus 100 similar to FIG. 1 and illustrates the air inlet 102 being wholly comprised in the upper end face 114 of the upright elongate duct section 112. Therefore, the air inlet 102 is monodirectional to receive air in a vertical or mostly vertical downwards direction. The air inlet 102 is at the top end of the air purification apparatus 100. The precise location of the air inlet 102 may depend on the likely use of the indoor space 10. Thermal gradients and convection are generated not only by room heating devices such as radiators, but also by solar gain through windows, by people, and by heat sinks. Therefore, computational fluid dynamics simulations may be employed to optimise the position of the air inlet 102 based on a thermal model of the indoor space 10, and the likely use of the indoor space 10. Turning now to the air outlet 104, the air inlet 102 and air outlet 104 of FIG. 1 are vertically and laterally offset, the air inlet 102 being at or above head height and the air outlet 104 being proximal to or at the bottom end of the air purification apparatus 100. The laterally extending duct section 120 may comprise an aperture defining a vent of the air outlet 104. The aperture may be covered by a grille. The air outlet 104 may optionally be located in an end wall of the laterally extending duct section 120, defining the downstream end of the airflow conduit 106. From centre-to-centre, the air outlet 104 may be at least 1.2 metres or at least 1.4 metres vertically below the air inlet 102, so as to be close to floor level. If the air inlet 102 or air outlet 104 has multiple vents (e.g., FIG. 2A), the centre is taken to be the centre of the centres of the individual vents. In dimensionless terms, the centre-to-centre distance may be at least 65% or at least 75% of a total height of the air purification apparatus 100 or of the total height of the upright elongate duct section 112. Another dimension is the minimum vertical separation from the top of the air outlet 104 to the bottom of the air inlet 102, which may be at least 1 metre or at least 1.2 metres. In dimensionless terms, the minimum vertical separation may be at least 55% or at least 65% of the total height as defined above. If the air outlet 104 and / or air inlet 102 comprise multiple vents, the minimum vertical separation is measured between the bottom of the lowermost vent 102A, 102B of the air inlet 102 and the top of the uppermost vent 104A, 104B of the air outlet 104. Another dimension is the maximum vertical separation from the bottom of the air outlet 104 to the top of the air inlet 102, which may be selected from the range 1.3 metres to 2.2 metres, which is generally less than floor-to-ceiling height in most indoor space 10. In dimensionless terms, this is 85% to >99% of the total height as defined above. If the air outlet 104 and / or air inlet 102 comprise multiple vents (e.g., FIG. 2A), the maximum vertical separation is measured between the bottom of the lowermost vent 104A, 104B of the air outlet 104 and the top of the uppermost vent 102C of the air inlet 102. Another dimension is the top of the air outlet 104, which may be no more than 0.8 metres above floor level. Regarding the lateral offset, FIG. 1 illustrates the air outlet 104 being located at or towards a laterally (horizontally) distal end of the laterally extending duct section 120, distal from the upright elongate duct section 112 and from the wall 20 of the indoor space 10. The centre-to-centre lateral (horizontal) offset between the air inlet 102 and air outlet 104 is defined by at least the laterally extending duct section 120, and may be at least 0.2 metres or at least 0.4 metres horizontally. FIG. 2A illustrates an apparatus 100 without the laterally extending duct section 120, wherein the air outlet 104 is comprised in the upright elongate duct section 112. The upright elongate duct section 112 therefore comprises one or more apertures defining one or more vents 104, 104A, 104B fluidly coupled to the indoor space 10, each aperture covered by a grille. There may be either no lateral offset between the air inlet 102 and air outlet 104, or the centre-to-centre lateral offset may be less than a front-rear depth of the upright elongate duct section 112 or of the air purification apparatus 100 (e.g., less than 0.3 metres or less than 0.2 metres). FIG. 2A illustrates the air outlet 104 being defined by more than one side of the upright elongate duct section 112. For example, the air outlet 104 comprises vents 104A, 104B in multiple faces of the upright elongate duct section 112. FIG. 2A shows a pair of front faces which face in different horizontal directions than each other, wherein each front face comprises an aperture defining a separate vent 104A, 104B of the air outlet 104, and proximal to the bottom end of the air purification apparatus 100. The air outlet 104 in FIG. 2A is therefore multi-directional in a horizontal plane, to exhaust air in multiple horizontal directions. Specifically, FIG. 2A shows that each front face of the elongate faces 118 of the upright elongate duct section 112 can comprise an upper vent 102A / 102B and a lower vent 104A / 104B, wherein the upper vent 102A / 102B is for the air inlet 102, and the lower vent 104A / 104B is for the air outlet 104. The upper and lower vents may be co-planar with each other on the face, the air flowing in opposite lateral directions. Although FIG. 2A does not illustrate the lower end face 116 of the upright elongate duct section 112 comprising a vent for the air outlet 104, one could be provided. FIG. 2B illustrates an example in which the air outlet 104 is connected to the end of a laterally extending duct section 120 connected to the bottom of the upright elongate duct section 112. The air outlet 104 is mono-directional to exhaust air in a horizontal or mostly horizontal direction. Turning now to the internal components of the air purification apparatus 100, FIG. 1 is a cross-section view schematically illustrating the internal components inside the upright elongate duct section 112, within the airflow conduit 106. The order of the components is not limited to that shown. Starting from upstream, the air purification apparatus 100 comprises a coarse pre-filter 131 proximal to the air inlet 102. The coarse filter may be a dust filter, having a particle size efficiency less than 80% of its peak efficiency once particle size falls below 5 micrometers. Downstream of the pre-filter 131, the air purification apparatus 100 includes an airflow driving device which comprises a variable-speed fan 110. The variablespeed fan 110 drives air downwardly from the air inlet 102 to the air outlet 104. The illustrated variable-speed fan 110 is located closer to the air inlet 102 than the air outlet 104, but could alternatively be located closer to the air outlet 104 than the air inlet 102. The variable-speed fan 110 may have an automatically- and / or manually-selectable range of speeds to allow one or more average flow rates selected from the range 20 - 200 cubic feet per minute. A peak selectable speed of the variable-speed fan 110 may correspond to an average flow rate of less than 3000 cubic feet per minute. Optionally, the variable-speed fan 110 is monodirectional so that airflow only travels downwardly. This means it is not capable or not programmable by a user to drive airflow upwardly. After passing through the pre-filter 131, the airflow enters a germicidal light zone 130 inside the upright elongate duct section 112. The germicidal light zone 130 is a central section of the upright elongate duct section 112, between the bottom of the air inlet 102 and the top of the air outlet 104. In the germicidal light zone 130 is a germicidal light source 126 to disinfect the passing airflow in the zone 130. The germicidal light source 126 comprises ultraviolet (UV) lamps 128 configured to emit light with a peak output from the range 180-280 nanometres, in the ultraviolet-C (IIV-C) spectrum. A plurality of mercury excimer lamps may be used, or alternatively an array of UV-C light emitting diodes (LEDs). In a nonlimiting implementation, the UV-C LEDs may provide one or more of the following peak wavelengths within the band 200-280 nanometers (nm), such as a peak at 266nm, 270nm, 275nm, or 279nm. Or, if a mercury excimer lamp is provided, the peak wavelength may be 254nm. One or more of the peak wavelengths correspond with the peak absorption of various pathogens. A plurality of UV lamps 128 may be provided, wherein each one is elongate. The UV lamps 128 are arranged in a parallel or mostly parallel orientation to the airflow conduit 106, and to the upright elongate duct section 112. Therefore, the UV lamps 128 are upright. The UV lamps 128 may be parallel to each other. The length of the germicidal light source 126, and / or the length of each UV lamp, in said orientation, may be at least 5 or at least 10 or at least 15 centimetres, to ensure a good light exposure time for a given flow velocity. Each UV lamp 128 may be of this length. The UV lamps 128 may be mounted to the rear side of the air purification apparatus 100, closer to an electrical power source. The airflow may pass in front of the UV lamps 128. All possible direct lines of sight to the UV lamps 128 may be blocked from the air inlet 102 and the air outlet 104, to keep the high-energy UV radiation inside. If the internal components shown in FIG. 1 do not provide a sufficiently opaque barrier, then a dedicated barrier, e.g., baffle, may be incorporated. After passing through the UV lamps 128, the airflow reaches a pair of main filters in the form of an absorption filter 132 for capturing small particles, and an adsorption filter 134 for capturing smaller particles such as volatile organic compounds (VOCs) and odours. They are referred to herein as a high-efficiency particulate absorbing filter (HEPA) filter and an activated carbon filter, respectively. It would be appreciated that a HEPA filter 132 is just one example of a high-efficiency filter. The absorption filter may meet EPA (Efficiency Particulate Air), HEPA, or ULPA (Ultra-low particulate air) standards. The absorption filter may have an average EN 1822 / ISO 29463 particle size efficiency greater than 99% or greater than 99.9% at a 0.3 micrometer particle diameter. For a HEPA filter 132 satisfying H13 or H14 standards, the efficiency is generally greater than 99.9%. It would also be appreciated that an activated carbon filter 134 (e.g., activated charcoal) is just one example of an adsorption filter. The adsorption filter could comprise zeolites, silica gel, activated alumina, metal-organic frameworks, porous polymers, carbon nanotubes, etc. FIG. 1 shows the HEPA filter 132 in a line of sight of the UV lamps 128. For example, at least 70%, or at least 85%, or at least 90% of the frontal area of the upstream side (lamp-facing side) of a filter medium of the HEPA filter 132 may be in a direct line of sight of the UV lamps 128. This ensures that any mould spores, bacteria, or other pathogens caught in the HEPA filter 132 are destroyed and cannot propagate. The HEPA filter 132 is the one which 26 pathogens are most likely to settle on due to its high filtration efficiency for small particle sizes. The HEPA filter 132 comprises a filter support frame 136 surrounding filter medium, such as a pleated filter medium. The filter support frame 136 defines a filter aperture in which the filter medium is located. The filter support frame 136 may define a flat panel filter, or may define a non-planar shape such as a radial flow filter (e.g., cone). An advantage of a flat panel filter over radial flow filters is that more of it faces the UV lamps 128. To increase the area of the filter aperture and therefore allow a larger-perimeter filter medium, the HEPA filter 132 may be tilted diagonally within the upright elongate duct section 112, as shown in FIG. 1. As illustrated, the front side of the filter support frame 136 of the HEPA filter 132 is at a higher elevation than the rear side of the filter support frame 136, so that the HEPA filter 132 is tilted to an oblique angle relative to the longitudinal axis of the upright elongate duct section 112 / airflow conduit 106. This orientation reduces the angle of incidence from the UV lamps 128 because the UV lamps 128 are mounted at the rear side of the air purification apparatus 100. The oblique angle of the HEPA filter 132 may be selected from the range 15 to 75 degrees or 30 to 60 degrees. In other examples, the filter support frame 136 may be tilted rear-up, or laterally (from left to right rather than front to back). The optimum tilt depends on the locations of the UV lamps 128 or aerodynamic optimisation. In further alternatives, the HEPA filter 132 may be upright so that the filter aperture is parallel to the airflow. The HEPA filter 132 may be parallel to and alongside the UV lamps 128. However, this would require a 90-degree change of direction. As shown in FIG. 1, the activated carbon filter 134 may be located downstream of the UV lamps 128 and HEPA filter 132, so cleaner air reaches the activated carbon filter 134 which extends the life of the activated carbon filter 134. The activated carbon filter 134 may be tilted in addition to, or instead of, the HEPA filter 132, to allow a larger-perimeter filter medium to be used. The illustrated activated carbon filter 134 is optionally tilted in a different, e.g. opposite direction than the HEPA filter 132. However, it is not necessary for all implementations that the activated carbon filter 134 is tilted in this direction or even tilted at all. The activated carbon filter 134 does not need to be exposed to IIV-C light. The opacity of the HEPA filter 132 may prevent UV-C light from reaching the activated carbon filter 134, or at least reduce its intensity. In other examples, the activated carbon filter 134 may be in line of sight of the UV lamps 128. After passing through the main filters, the now-purified air reaches a heater device 138, e.g., electric heater 138, to increase a temperature of the air. The heater device 138 may not always be required. A manual or automatic thermostat 310 (FIG. 3) may control the heater device 138 depending on a user request or setpoint error. The heater device 138 is a low-power heater, only being powerful enough to improve convection and mixing of the exhausted purified air, without acting as a room heater device. The heater device 138 may therefore have a fixed power consumption or peak selectable power consumption selectable by the thermostat 310, which is selected from the range 40 watts to 200 watts, or from 40 watts to 100 watts, or from 100 watts to 200 watts. Alternatively, the heater device 138 has a fixed power consumption or peak selectable power consumption selected from the range 200 watts to 2000 watts, or from 200 watts to 800 watts, or from 800 watts to 2000 watts. This is suitable for use cases where the heater device 138 acts as a secondary space heater, or even a primary space heater if the indoor space 10 is small. The heater device 138 in FIG. 1 is elevated wholly or mostly above the air outlet 104. However, the heater device 138 could alternatively be at a lower elevation. The heater device 138 in FIG. 1 is downstream of the air purification devices 108, but could alternatively be upstream of one or more of them. For serviceability, the upright elongate duct section 112 may comprise an openable access panel (not shown) to facilitate access to the HEPA filter 132 and / or the activated carbon filter 134 and / or the UV lamps 128. The access panel may be located in one of the elongate faces 118. Alternatively, for a wall-mounted implementation, an openable access point for servicing may be located in the rear side to enable access from outside the indoor space 10, so that engineers do not need to arrange access to the indoor space 10 to allow servicing to be performed. FIG. 3 illustrates a control apparatus 300 for the air purification apparatus 100. The control apparatus 300 comprises at least one controller 301. The controller 301 comprises at least one processor 304, and at least one memory 306 including computer-program code 308 that, when executed by the at least one processor 304, controls one or more functions of the air purification apparatus 100. An interface 302 of the control apparatus 300 is configured to receive signals from sensors such as a thermostat 310, a filter loading sensor 312, and / or a pollutant sensor 314, and is configured to output a control signal to the variablespeed fan 110 and / or the heater device 138 which is based on the signals. The control apparatus 300 is configured to control the variable-speed fan 110 to drive air from the air inlet 102 to the air outlet 104, so that air is received from the upper air zone 12 of the indoor space 10, and purified air is discharged into the lower air zone 14 of the indoor space 10. Further, the control apparatus 300 is configured to control an operating state (activation / deactivation state) of the heater device 138, and / or a power output of the heater device 138, based on a setpoint error. The control of the variable-speed fan 110 may depend on the signal from the filter loading sensor 312, to compensate for the increased back-pressure as the filters age. The filter loading sensor 312 may comprise a timer, or pressure sensors for detecting the changing back-pressure, optical sensors to detect physical contamination of the filters, or any other sensing means for indicating filter loading. The control of the variable-speed fan 110 may depend on the signal from the pollutant sensor 314, to increase the fan speed when air quality is low. The pollutant sensor 314 may comprise an optical sensor or chemical sensor or other sensing means, for measuring the concentrations of various substances such as particulates, or specific chemicals. The control apparatus 300 may further control the activation state of the UV lamps 128 in dependence on the pollutant sensor 314. The control of the variable-speed fan 110 may also depend on a signal indicative of heating demand. The signal may originate from the thermostat 310 as a thermostat control signal, or may be an internal state signal indicating the activation state of the heater device 138. The fan speed may increase when the heater device 138 is started or increased in power, to cancel out the increased upwards buoyancy pressure in the upright elongate duct section 112. Other example sensors can include, for example, a light source monitoring indicator to detect output of the germicidal light source 126. The indicator can comprise a UV-C sensitive photo sensor to detect UV-C output, or an over-current / open circuit sensor to detect lamp failure, for example. The control apparatus 300 may output an alarm signal in dependence on lamp failure. Additionally, or alternatively, the control apparatus 300 may control UV-C intensity based on the detected UV-C output. Other example sensors can include, for example, a space occupancy indicator (not shown) such as an optical sensor or a day / night timer. The control apparatus 300 may limit fan speed (and therefore noise / energy consumption) when the indoor space 10 is empty, or at specific times when there is no occupation or no likely occupation, such as night. FIG. 4 illustrates a method 400 according to an embodiment of the invention. The method 400 is a method of controlling the air purification apparatus 100. The method 400 may be performed by the control apparatus 300 illustrated in FIG. 3. In particular, the memory 306 may comprise computer-readable instructions 308 which, when executed by the processor 304, perform the method 400. At block 402, the method 400 comprises controlling, e.g., activating, the variable-speed fan 110 to drive air from the air inlet 102 to the air outlet 104, so that air is received from the upper air zone 12 of the indoor space 10, and purified air is discharged into the lower air zone 14 of the indoor space 10. For example, block 402 can comprise activating the variable-speed fan 110. Block 402 may be performed in response to a manual or automatic control signal. If automatic, the activation may be performed in dependence on a signal from a sensor such as the pollutant sensor 314, and / or in dependence on the signal indicative of heating demand. If manual, the activation may be performed in response to a user input of an appropriate input device. At block 404, the method 400 comprises controlling a fan speed of the variable-speed fan 110 in dependence on a signal from a filter loading sensor 312 and / or a signal from a pollutant sensor 314. If filter loading increases or pollutant levels increase, the fan speed may be increased, and if they decrease then fan speed may fall. In use, the fan speed and the air purification apparatus 100 as a whole may be configured to provide a required number of air changes per hour for the indoor space 10. The average size of a living room in the UK varies between 16mA2 to 24mA2 depending on the year of build, which represents a room volume of 40 to 60 cubic metres (based on a ceiling height of 2.5m). In a domestic environment, 2x air changes per hour (120mA3 / hr) may be acceptable, whereas 6x air changes per hour (360mA3 / hr) might be preferred in a medical type clinic or surgery. The UV-C size / intensity and filter size specifications may be selected to target pathogens / particles of concern, and further optimised to provide a predetermined number of air changes per hour of between 2x and 6x, plus an optional margin of +x % (e.g., 5% to 25%). The fan may be set using open loop or closed loop control to the predetermined number of air changes per hour, along with the option to increase speed within the x% margin to overcome backpressure. Once the x% margin (limit) is reached, the control apparatus 300 / method 400 may be configured to output an alarm signal to an appropriate rendering device (e.g., speaker / display) and / or to a communication interface (e.g., transmitter). At block 406, the method 400 comprises controlling an operating state or a power level of the heater device 138, in dependence on a signal indicative of heating demand, such as a control signal from the thermostat 310. For example, when the thermostat 310 first indicates a requirement for heating such as actual temperature being below a setpoint (non-zero setpoint error), then block 406 may activate the heater device 138. If the heating requirement increases, e.g., increased setpoint error, then block 406 may increase the power of the heater device 138 if the power is a controllable variable. If the heating demand falls below a threshold or ceases, then the heater device 138 may be deactivated. At block 408, the method 400 comprises controlling the fan speed further in dependence on the signal indicative of heating demand. The signal may indicate the operating state of the heater device 138, or the power level of the heater device 138, or the setpoint error. The fan speed may therefore increase when the heater device 138 is activated, and / or increase when the power output of the heater device 138 increases. When the heater device 138 is deactivated and / or when the power output decreases, the fan speed may fall. The blocks illustrated in FIG. 4 may represent steps in a method 400 and / or sections of code in the computer program 308. The illustration of a particular order to the blocks does not necessarily imply that there is a required or preferred order for the blocks and the order and arrangement of the block may be varied. Furthermore, it may be possible for some steps to be omitted. Although embodiments of the present invention have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the invention as claimed. For example, the heater device 138 may be omitted. The order of the components may vary. Features described in the preceding description may be used in combinations other than the combinations explicitly described. Although functions have been described with reference to certain features, those functions may be performable by other features whether described or not. Although features have been described with reference to certain embodiments, those features may also be present in other embodiments whether described or not. 5 Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and / or shown in the drawings whether or not particular emphasis has been placed thereon. 10

Claims

1. An air purification apparatus comprising:an air inlet;an air outlet;an airflow conduit between the air inlet and the air outlet;one or more air purification devices in the airflow conduit;a heat source configured to heat air in the airflow conduit;an airflow driving device configured to drive air from the air inlet to the air outlet, wherein the air outlet is below the air inlet,wherein the air inlet is at an elevation configured to receive air from an upper air zone of an indoor space,wherein the air outlet is at an elevation configured to discharge purified air into a lower air zone of the indoor space, andwherein the air outlet is laterally offset from the air inlet.

2. The air purification apparatus of claim 1, wherein a bottom of the air inlet is at least 1 metre above a top of the air outlet.

3. The air purification apparatus of claim 1 or 2, wherein a centre of the air inlet is at least 1.2 metres or at least 1.4 metres above a centre of the air outlet.

4. The air purification apparatus of claim 1, 2, or 3, wherein a vertical separation from a bottom of the air outlet to a top of the air inlet is selected from the range 1.4 metres to 2.2 metres.

5. The air purification apparatus of any preceding claim, wherein the air purification apparatus is located in the indoor space, wherein the air inlet is located in the upper air zone of the indoor space at a height of at least 1.5 metres, and wherein the air outlet is located in the lower air zone of the indoor space closer to the floor of the indoor space than the ceiling of the indoor space.

6. The air purification apparatus of any preceding claim, wherein the one or more air purification devices comprise a germicidal light source.

7. The air purification apparatus of claim 6, wherein the germicidal light source comprises one or more ultraviolet lamps, wherein the one or more ultraviolet lamps define, at least in part, the heat source.

8. The air purification apparatus of claim 6 or 7, wherein the germicidal light source is elongate and arranged in a parallel or mostly parallel orientation to the airflow conduit.

9. The air purification apparatus of claim 6, 7, or 8, configured to block line of sight to the germicidal light source from the air inlet, and configured to block line of sight to the germicidal light source from the air outlet.

10. The air purification apparatus of any one of claims 6 to 9, wherein the one or more air purification devices comprise an absorption filter, and wherein the absorption filter is in a line of sight of the germicidal light source.

11. The air purification apparatus of claim 10, wherein the absorption filter is a HEPA filter.

12. The air purification apparatus of any one of claims 6 to 11, wherein the one or more air purification devices comprise an adsorption filter downstream of the germicidal light source.

13. The air purification apparatus of claim 12, wherein the adsorption filter comprises activated carbon.

14. The air purification apparatus of any preceding claim, wherein the heat source comprises a heater device to heat the driven air, wherein the heaterdevice defines, at least in part, the heat source, so that air passing through the air outlet is at a higher temperature than the air at the air inlet.

15. The air purification apparatus of claim 14, wherein the heater device is arranged downstream of a germicidal light zone of the air purification apparatus.

16. The air purification apparatus of claim 14 or 15, wherein the heater device has a fixed power consumption or peak selectable power consumption selected from the range 40 watts to 200 watts, or wherein the heater device has a fixed power consumption or peak selectable power consumption selected from the range 200 watts to 2000 watts.

17. The air purification apparatus of any preceding claim, wherein the one or more air purification devices comprise a pre-filter proximal to the air inlet, and one or more main filters downstream of the pre-filter, wherein the pre-filter is coarser than at least one of the main filters.

18. The air purification apparatus of claim 17, wherein the one or more main filters comprise an absorption filter and an adsorption filter.

19. The air purification apparatus of claim 18, wherein the adsorption filter of the main filters is downstream of the absorption filter of the main filters.

20. The air purification apparatus of claim 17,18, or 19, wherein at least one of the main filters comprises a filter support frame defining a filter aperture in which a filter medium is mounted, wherein the filter aperture has a cross-sectional area greater than a cross-sectional area of the airflow conduit, and wherein the filter aperture is at an oblique or parallel angle relative to a longitudinal axis of the airflow conduit.

21. The air purification apparatus of any preceding claim, wherein the airflow conduit comprises an upright elongate duct section, and wherein at least one of the one or more air purification devices is in the upright elongate duct section.

22. The air purification apparatus of any preceding claim, wherein the air outlet is laterally offset from the air inlet and faces away from the air inlet.

23. The air purification apparatus of any preceding claim, comprising feet, casters, and / or a pad to support the air purification apparatus as a floorstanding unit.

24. The air purification apparatus of any preceding claim, comprising one or more wall fixing points to secure the air purification apparatus to a wall.

25. A method of controlling the air purification apparatus as claimed in any one of the preceding claims, the method comprising:controlling the airflow driving device to drive air from the air inlet to the air outlet, so that air is received from the upper air zone of the indoor space, and purified air is discharged into the lower air zone of the indoor space.

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