AN ARRANGEMENT AND METHOD FOR RETROFITTING AN AIR CONDITIONING SYSTEM TECHNICAL FIELD 5 The disclosure relates to air conditioning systems and in particular arrangements and methods for retrofit of air conditioning systems. The arrangement, method and kit may be employed in applications such as the split air conditioning systems in the domestic air conditioning industry and the disclosure is herein described in that context. However, it is to 10 be appreciated that the disclosure is not limited to that use and may be applied to other air conditioning systems such as industrial systems, commercial systems and the like. BACKGROUNDART 15 Air conditioning systems are often fitted to buildings to maintain a controlled environment within an occupied space. Domestic and commercial air conditioning systems typically include an indoor evaporator unit and an outdoor condenser unit. Air conditioning systems use the refrigeration cycle to deliver conditioned air. Refrigerants commonly used in air conditioning systems include hydrochlorofluorocarbons (HCFC), chlorofluorocarbons 20 (CFC) and hydrofluorocarbons (HFC), the most common of which include R-22 and R-410A. Fluorocarbon refrigerants are often referred to as 'ozone depleting substances', due to their relatively high global warming potential, and have the potential to damage the ozone layer when leaked into the atmosphere. Natural refrigerants, including hydrocarbons, are 25 refrigerants that may be used in lieu of fluorocarbon refrigerants. Hydrocarbon refrigerants are considered an environmentally sustainable alternative to fluorocarbon refrigerants due to their wide availability, low cost and relatively low global warming potential. However, hydrocarbon refrigerants are highly flammable. Due to their high flammability, the use of hydrocarbon refrigerants is restricted in Australia. 30 The above references to the background art do not constitute an admission that the art forms a part of the common general knowledge of a person of ordinary skill in the art. The above references are also not intended to limit the application of the apparatus as disclosed herein. 1 SUMMARY OF THE DISCLOSURE Disclosed herein is an air conditioning arrangement. The air conditioning arrangement 5 comprises an evaporator arranged to deliver an air stream. The evaporator comprises a first heat exchanger configured to receive a natural refrigerant. In some forms, the air conditioning arrangement also comprises a sensor arranged to detect the presence of the natural refrigerant in the airstream. 10 Disclosed herein is a method of retrofitting an air conditioning system. The method of retrofitting an air conditioning includes the steps of installing a sensor arranged to detect the presence of a natural refrigerant in an airstream and charging the air conditioning system with a natural refrigerant. 15 Further disclosed herein is a kit to retrofit an air conditioning system. The kit may comprise a natural refrigerant, a sensor arranged to detect the presence of the natural refrigerant in an airstream, in inlet and an outlet valve arranged in a closed condition to isolate a first heat exchanger from the natural refrigerant, a condenser comprising an inverter compressor and second heat exchanger, and a timer. 20 Further disclosed herein is an air conditioning arrangement comprising a body defining an interior. A first heat exchanger is located in the interior, configured to receive a natural refrigerant. A sensor is also located in the interior, arranged to detect the presence of the natural refrigerant. 25 Also disclosed is a method of arranging an air conditioning system. The method includes the steps of installing a sensor arranged to detect the presence of a natural refrigerant, securing an inlet valve and an outlet valve, the valves being arranged in a closed condition to isolate a first heat exchanger from the natural refrigerant and charging the air 30 conditioning system with a natural refrigerant. 2 BRIEF DESCRIPTION OF THE DRAWINGS Further embodiments of the present apparatus will become apparent from the following description which is given by way of example only and with reference to the 5 accompanying drawings in which: Fig. 1 illustrates a schematic of the air conditioning system; Fig. 2 illustrates a flow diagram of a method for retrofitting an existing domestic split air conditioning system; 10 Fig. 3 illustrates a schematic of a retrofitted indoor unit forming part of an existing domestic split air conditioning system. DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 15 In the following detailed description, reference is made to accompanying drawings which form a part of the detailed description. The illustrative embodiments described in the detailed description, depicted in the drawings and defined in the claims, are not intended to be limiting. Other embodiments may be utilised and other changes may be made without departing from the spirit or scope of the subject matter presented. It will be readily 20 understood that the aspects of the present disclosure, as generally described herein and illustrated in the drawings can be arranged, substituted, combined, separated and designed in a wide variety of different configurations, all of which are contemplated in this disclosure. The disclosure is directed generally towards the air conditioning industry, an energy 25 intensive industry that contributes substantially to the emissions of ozone depleting substances. Sustainability practices, including the reduction on the reliance of ozone depleting substances such as fluorocarbon refrigerants, are of critical importance in the industry. Commonly used 'traditional' refrigerants, including R-22, are being phased out worldwide to protect the environment. 30 Substitute refrigerants are required to replace the large volume of traditional refrigerants used in the air conditioning industry, preferably without adding significantly to the costs associated with the manufacture and servicing of air conditioning equipment. A 3 number of substitute materials have been tested in the air conditioning industry with varied levels of success. Of the substitute materials tested, ammonia has been moderately successful and is used commonly in industrial applications. While proving to be an adequate substitute in the air conditioning industry, the use of ammonia has major drawbacks in domestic 5 applications due to the high cost of equipment relative to the cost of equipment manufactured for fluorocarbon refrigerants, its high toxicity and flammability. Other substitutes include hyrdrocarbon refrigerants, such as propane (R-290), butane (R-600) and propylene (R-1270). Hydrocarbon refrigerants have a low ozone depletion 10 potential and are relatively low in cost and toxicity. However, hydrocarbons are highly flammable and are therefore a safety risk when used in commercial, and in particular, domestic air conditioning systems. Disclosed is an arrangement for installing and retrofitting existing air conditioning 15 systems addressing both the environmental pressures in the air conditioning industry and the problems associated with the safety of natural refrigerants in air conditioning systems. In some forms, disclosed is an air conditioning arrangement comprising an evaporator arranged to deliver an air stream. The evaporator comprises a first heat exchanger configured to receive a natural refrigerant. The air conditioning arrangement also includes a sensor 20 arranged to detect the presence of the natural refrigerant in the airstream. The air conditioning arrangement allows for the use of a natural refrigerant in an air conditioning system, and allows for an existing air conditioning system to be charged with natural refrigerant. In some forms, the air conditioning further comprising an inlet valve and an outlet 25 valve arranged in a closed condition to isolate the first heat exchanger from the natural refrigerant. In at least one embodiment, the sensor effects closure of the inlet and outlet valves upon detection of the natural refrigerant in the air stream. In some forms, the air conditioning arrangement furthers comprises a timer 30 configured to close the inlet valve before closing the outlet valve. In accordance with the air conditioning arrangement disclosed herein, the inlet and outlet valves may be solenoid valves. In at least one embodiment, the first discharging arrangement further comprises at least one fan. In some forms, the first discharging arrangement further comprises a supply air plenum. 4 In at least one embodiment the sensor is located in the evaporator. In another embodiment, the sensor is located between the first heat exchanger and the fan. In another embodiment, the sensor is located between the first heat exchanger and the supply air 5 plenum. In some forms, the air conditioning arrangement further comprises a condenser. In at least one embodiment, the condenser comprises a second heat exchanger and an inverter compressor. 10 In at least one embodiment, the second heat exchanger is fluidly connected to the first heat exchanger. In some forms, the air stream is warmed or cooled air. In some forms, the natural refrigerant includes a hydrocarbon refrigerant such as 15 propane, ethane or propylene or a combination thereof. In some forms the natural refrigerant is a blend of a hydrocarbon refrigerant and a hydrofluorocarbon refrigerant. Also disclosed herein is method of retrofitting an air conditioning system. The method may include the steps of installing a sensor arranged to detect the presence of a natural 20 refrigerant in an airstream and charging the air conditioning system with a natural refrigerant. In some forms, the method of retrofitting an air conditioning system further includes the step of securing an inlet valve and an outlet valve arranged in a closed condition to isolate a first heat exchanger from the natural refrigerant. 25 In some forms, the method of retrofitting an air conditioning system further includes the step of removing a first condenser and replacing it with a second condenser comprising an inverter compressor. In some forms, the sensor effects closure of the inlet and outlet valves upon detection of the natural refrigerant in the air stream. 30 In some forms, the method of retrofitting an air conditioning system further includes the step of installing a timer. In at least one embodiment, the timer is configured to close the inlet valve before closing the outlet valve. In some forms, the inlet and outlet valves are solenoid valves. 5 In some forms, the method of retrofitting an air conditioning system further includes the step of installing the sensor in an evaporator. In at least one embodiment, the natural refrigerant includes a hydrocarbon refrigerant such as propane, ethane or propylene or a 5 combination thereof. In some forms, the natural refrigerant is a blend of a hydrocarbon refrigerant and a hydrofluorocarbon refrigerant. Also disclosed herein is a kit to retrofit to an air conditioning system. The kit comprises a natural refrigerant, a sensor arranged to detect the presence of the natural 10 refrigerant in an airstream, in inlet and an outlet valve arranged in a closed condition to isolate a first heat exchanger from the natural refrigerant, a condenser comprising an inverter compressor and second heat exchanger. In some forms, the kit to retrofit to an air conditioning system further comprises a 15 timer configured to close the inlet valve before closing the outlet valve. Further disclosed is an air conditioning arrangement comprising a body defining an interior. In some forms, a first heat exchanger is located in the interior, configured to receive a natural refrigerant. In some forms, a sensor is located in the interior, arranged to detect the 20 presence of the natural refrigerant. Also disclosed herein is a method of arranging an air conditioning system. The method includes the steps of installing a sensor arranged to detect the presence of a natural refrigerant, securing an inlet valve and an outlet valve, the valves being arranged in a closed 25 condition to isolate a first heat exchanger from the natural refrigerant. The air conditioning system may be charged with a natural refrigerant. Referring to Fig. 1, an air conditioning arrangement is shown in the form of a domestic split system 1. An evaporator, in the form of an indoor unit 3, is arranged to deliver 30 an air stream 5 to a space 7, such as an occupied indoor space of a domestic building. The indoor unit may be fixed to the interior wall, ceiling or floor of the indoor space 7. The indoor unit 3 comprises a first heat exchanger, in the form of an indoor coil 9, configured to receive a natural refrigerant. 6 The domestic split system 1 also comprises a sensor 11 arranged to detect the presence of a natural refrigerant in the airstream 5. An inlet valve 13 and an outlet valve 15, in the form of solenoid valves, are moveable to adopt an open condition and a closed condition. In the closed condition the valves are arranged to isolate the indoor coil 9 from the natural 5 refrigerant. The inlet solenoid valve 13 and outlet solenoid valve 15 are open when the air conditioning system is in running mode. Upon detection of the natural refrigerant in the air stream 5, the sensor effects closure of the inlet solenoid valve 13 and outlet solenoid valve 15. The closure of the solenoid valves 13, 15 isolates the indoor unit 3, and consequentially the indoor space 7, from the natural refrigerant. 10 In the illustrated form, a timer is also installed within the air conditioning arrangement. The timer is configured to close the inlet valve 13 before closing the outlet valve 15. This operation takes place in both heating and cooling mode in a reverse cycle air conditioning system. The staggering of the closure of solenoid valves 13, 15 ensures that upon detection of 15 natural refrigerant in the airstream, the supply of natural refrigerant to the indoor coil is immediately ceased, while adequate time is allowed to discharge the natural refrigerant from the indoor coil and any extension of piping 17 that is within the indoor space 7 before the solenoid outlet valve 15 is closed. 20 The indoor unit 3 comprises at least one fan 19 arranged to deliver the air stream 5 to the indoor space 7. In an alternate arrangement to that shown in Fig. 1, the indoor unit also comprises a supply air plenum. It will also be apparent to those skilled in the art that other alternate arrangements for the indoor unit 3 may be implemented. 25 The sensor 11 is located in the indoor unit 3, between the indoor coil 9 and the fans 19. In an alternate arrangement not shown, the sensor 11 may be located between the indoor coil 11 and the supply air plenum, or in the indoor space 7. In a further alternate arrangement, a second sensor may be installed to detect the presence of natural refrigerant. The first sensor is positioned in the indoor unit 3, while the second sensor is positioned outside the indoor 30 unit 3. The split air conditioning system 1 further comprises a condenser in the form of an outdoor unit 21. The outdoor unit 21 is typically located external to the indoor space 7, in a well ventilated area such as a plant room or an outdoor space. The outdoor unit 21 comprises 7 a second heat exchanger, in the form of an outdoor coil, an inverter compressor and an expansion valve. The inverter compressor is configured to control the speed of the compressor motor in order to regulate the temperature of the air stream 5. The compressor, in combination with the expansion valve, assists to control the phase change of the natural 5 refrigerant from liquid to gas and vice versa. The outdoor coil is fluidly connected to the indoor coil 9. Piping 23 connects the outlet of the outdoor coil to the inlet of the indoor coil 9, while piping 17 connects the inlet of the outdoor coil to the outlet of the indoor coil 9. Piping 17, 23 is typically manufactured from 10 copper and may be insulated if required. Piping 17,23 allows for the closed loop flow of natural refrigerant between the indoor and outdoor coils. The natural refrigerant includes a hydrocarbon refrigerant such as propane, ethane or propylene or a combination thereof. For example, a natural refrigerant that has 85 - 95% 15 propane and 5-15% ethane or propylene, such as Minus 60TM from Australian refrigerant manufacturer HyChill, is used to increase the discharge pressure of the refrigerant from the outdoor unit. This is required for heating mode in cold climates. Natural refrigerant Minus 60TM has similar properties to commonly used refrigerant R410A. Minus 60 natural refrigerant is therefore advantageous as it can be used in a conventional R410A system, 20 whereby the compressor and associated controls do not need to be altered. Pure propane natural refrigerant products behave in a similar manner to refrigerant R22. Referring now to Fig. 2, a method 25 of retrofitting an air conditioning system in the form of a domestic split system is disclosed. A sensor, in the form of a propane sender unit, is 25 installed 27 in the existing indoor unit. The propane sender unit is arranged to detect the presence of a natural refrigerant in an air stream delivered by the indoor unit. The location of the propane sender unit within the indoor unit is dependent on the arrangement of the existing indoor unit. In an alternate arrangement, the propane sender unit may be installed exterior to the indoor unit. 30 Secondly, an inlet valve and an outlet valve, in the form of solenoid valves, are secured 29 to the existing refrigerant pipe work. The valves are arranged in a closed condition to isolate a first heat exchanger, in the form of an indoor coil, from the natural refrigerant. The solenoid valves are located such that when they are in a closed condition they isolate the 8 indoor unit, and consequentially the indoor space that the existing air conditioning system delivers conditioned air to, from natural refrigerant in the air conditioning system. The propane sender unit effects closure of the solenoid valves upon detection of the natural refrigerant in the air stream. A timer is installed within the air conditioning to offset the 5 closure of the solenoid valves. The timer is configured to close the inlet solenoid valve before closing the solenoid outlet valve. The closure of the solenoid valves therefore isolates the indoor space from the natural refrigerant when a leak of natural refrigerant is detected in the airstream. 10 The indoor electronics, including the indoor thermostat and wall controller, are removed and replaced 31. The existing first condenser, in the form of an outdoor unit, is replaced 33 with a second condenser, in the form of a replacement outdoor unit. The replacement outdoor unit comprises an inverter compressor, a heat exchanger in the form of an outdoor coil, an expansion valve and controls. When the retrofitted air conditioning system is charged with a 15 natural refrigerant such as Minus 60Tm, the replacement outdoor unit is an outdoor unit manufactured for R-410A refrigerant. The air conditioning arrangement may be supplied as a kit, used to retrofit an existing air conditioning system. The kit comprises a sensor, in the form of a propane sender unit, an inlet 20 and an outlet valve in the form of solenoid valves, and a condenser in the form of a replacement outdoor unit. The kit is supplied to retrofit an existing air conditioning system, such as a domestic split system. The kit allows an existing air conditioning system to safely use a natural refrigerant, 25 such as Minus 6 0 TM. The replacement outdoor unit comprises an inverter compressor. The inverter compressor maximises the efficiency of the retrofitted air conditioning system when using a natural refrigerant. The retrofit system is designed to not exceed burst pressures of interconnecting 30 copper pipe (with a factor of safety) that was originally designed and intended for used with R22. The system is configured to ensure that during operation, the pipe that connects the evaporator to the condenser installed as part of the retrofit, does not exceed a pressure of 450 psi. During cooling mode, the interconnecting liquid line of the retrofit system is designed to not exceed 450psi. During heating mode, the Superheated Gas line is designed to not exceed 9 450psi. Control during cooling mode is achieved by the EEV (Electronic Expansion Valve). Control during heating mode is achieved by the EEV and by controlling the frequency of the compressor. 5 Referring now to Fig. 3, an air conditioning arrangement in the form of an indoor evaporator unit 25 is disclosed. The indoor unit 35 comprises a body 37 defining an interior 39. The body 37 is rectangular in shape and the interior 39 is hollow. A first heat exchanger, in the form of an indoor coil 41, is located in the interior 39. The indoor coil 39 is configured 10 to receive a natural refrigerant. A sensor, in the form of a propane sender unit 43, is located in the interior 39. The propane sender unit 43 is arranged to detect the presence of a natural refrigerant in an air stream delivered by the indoor unit 25. The indoor unit 25, forming part of a domestic split air conditioning system, includes fans 41 arranged to deliver the air stream to an indoor space. 15 In the claims which follow and in the preceding description, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e., to specify the presence of the stated features but not to preclude the presence or addition of further 20 features in various embodiments of the apparatus. 10