Revising A3 refrigerants charge limits to promote wider application
A recent IJR paper examines the validity and form of charge limits for “highly flammable” A3 refrigerants, which constitute low-GWP alternatives to HFCs. Authors propose to eliminate upper charge limits for indoor systems whilst retaining allowable charge limits.
In an effort to reduce greenhouse gas emissions, the refrigeration, air conditioning and heat pump (RACHP) industry has been focusing more on the use of hydrocarbons (HCs). HCs are low-GWP alternatives to hydrofluorocarbons (HFCs), with the disadvantage of being more flammable (class “A3” according to ISO 817). The use of flammable refrigerants is carefully restricted by safety standards and the quantities of charge allowed within occupied spaces are constrained. 
Charge limits are intended to prevent the formation of large volumes of a flammable mixture in the event of a leak. To determine a charge limit, several assumptions are required, relating to system/equipment characteristics, the nature of the leaks, the conditions in the room, etc..
For small systems* with a cooling or heating capacity of up to 25 kW, there are two forms of charge limits: upper charge limits and allowable charge limits. “Upper charge” refers to the highest absolute mass of refrigerant permitted in a given refrigerant circuit. This value is irrespective of the size of the room and is generally a function of the access category such as general/public space, supervised space, or authorised space. The “allowable charge” is determined according to the dimensions of the space in which the system is installed. However, the allowable charge cannot exceed the upper charge.
In response to regional and international legislation on the phase-down of HFCs, the authors of the IJR article consider that a revision of A3 refrigerants charge limits is necessary to promote wider use of these low-GWP refrigerants. 
Whilst considering revised charge limits, it is important to ensure that they match the cooling or heating demand of common applications. There are two distinct situations in which the desired charge quantity may be determined:
- Systems whose cooling or heating capacity is dictated by the dimensions and characteristics of the room(s) they serve (e.g. air conditioners and cold rooms).
- Systems whose cooling or heating capacity is broadly independent of the room(s) in which theyr are located (e.g. display cabinets and hydronic heat pumps).
There are also instances that encompass both situations, such as “indoor” cold rooms with monobloc systems.
Rationale for eliminating upper charge limits for indoor systems
Upper charge limits are arguably used to minimise the intensity of a fire, either due to direct ignition of the refrigerant or due to an external fire (the latter being more likely). Yet, there are no such upper charge limits for non-flammable refrigerants.
Upper charge limits may thus be considered depending on the heat of combustion of the refrigerant and possibly in relation to the volume of the space in which the system is installed. For example, 1 kg of propane (R290) burning in a 10m3 space would be substantially more intense than in a 100m3 volume.
Accordingly, if the allowable charge limit is already a function of space volume, then the upper limits should also be accounted for automatically and should not be specified separately.
It can be concluded that for RACHP equipment installed inside and where the amount of refrigerant is proportional to the room size, there should be no upper charge limits, since it is self-limiting by means of the allowable charge limit. An exception would be for systems located in the open air, where excessive quantities of flammable refrigerant should be avoided.
Proposal for evaluating allowable charge limits
There is currently too much diversity in the way A3 refrigerant charge limits are dealt with in RACHP safety standards. It is suggested that charge limits rules be based on evaluating a given RACHP unit, including its prescribed protective measures. To evaluate allowable charge limits, the following are proposed:
- A clear and unambiguous calculation method, being a function of room size and RACHP equipment characteristics (such as housings, enclosures and amount of air flow or ventilation).
- A safety performance test where the refrigerant is released and the floor concentration is measured whilst allowing protective measures to function as intended.
- A flammability risk assessment method, based on anticipated charge, number and characteristics of sources of ignition, other characteristics of the RACHP unit and a certain level of tolerable risk against which the calculation result could be gauged.
Recommended safety measures
A risk assessment by the EU-funded Life Front project has concluded that the flammability risk associated with the use of hydrocarbon-based refrigerants is low compared to the fire risk associated with other comparable equipment and appliances . It highlights the example of an AHT Vento Hybrid multideck refrigerated display cabinet operating with 500g of propane (R290), which was found to present a flammability risk several times lower than the background situation or residual risks (such as electrical fires, etc).
The hazards identified often arise from “human errors or misbehaviour”. Detailed and easy-to-understand instruction manuals and well-trained technical personal for installation, commissioning and decommissioning are consequently seen as effective mitigation measures to reduce identified risks.
* Small systems include residential and light commercial air conditioning, commercial refrigeration such as display and food storage cabinets, small cold room units, and domestic and small commercial heat pumps.
 Colbourne, D., et al. "General framework for revising class A3 refrigerant charge limits – a discussion." International Journal of Refrigeration (2020).
Article available in FRIDOC (free of charge for IIR members), or on ScienceDirect (free of charge for IIR "standard members" after activation of their account).