Flammability basics
A flammable gas can only burn when its concentration in air sits between a lower and an upper limit, and only if there is an ignition source. Propane's lower flammability limit is around 2% by volume in air; isobutane's is slightly lower. Below the lower limit the mixture is too lean to burn.
To put the numbers in context, 500 g of propane is roughly a quarter of a cubic metre of gas at room temperature. Spread evenly through a shop of about 50 m³, that would be around 0.5% by volume, a fraction of the lower limit. In a tiny store cupboard of a few cubic metres, the same leak could exceed it. And gas rarely spreads evenly: a leak that pools near the floor or in a closed compartment can locally pass the limit. That is why ventilation, room size and avoiding low pockets matter.
The whole safety strategy is to keep any leak from creating a flammable concentration where there might be a spark or flame. That is done by limiting how much refrigerant is in a circuit, how it can escape and what electrical parts are nearby.
Safety classes in brief
| Class | Meaning | Examples |
|---|---|---|
| A1 | Lower toxicity, no flame propagation | R134a, R404A, R744 (CO2) |
| A2L | Lower toxicity, lower flammability | Some HFO and HFO-blend refrigerants |
| A2 | Lower toxicity, flammable | R152a |
| A3 | Lower toxicity, higher flammability | R290, R600a |
Charge limits for commercial cabinets
The international safety standard for commercial refrigerating appliances, IEC 60335-2-89, sets how much flammable refrigerant a cabinet may contain. Its 2019 edition raised the limit for A3 refrigerants from 150 g to about 500 g per refrigerant circuit, and for A2 and A2L refrigerants to about 1.2 kg, subject to further design and installation conditions in the standard. The change allowed larger cabinets to use a single hydrocarbon circuit instead of several small ones.
A standard only becomes binding where a national regulation or code adopts it, and adoption varies by country. Ask suppliers which safety standard each model is certified to.
How safe hydrocarbon cabinets are designed
- Hermetically sealed circuits with brazed joints, minimising mechanical joints that can leak.
- Electrical components near the refrigerant, such as thermostats, relays and fans, chosen or sealed so they cannot ignite a leak.
- Charge kept as small as possible, with compact heat exchangers.
- Air movement inside the cabinet that disperses small leaks.
- Clear flammable-refrigerant warning labels on the unit.
Installation do's and don'ts
- Follow the manufacturer's minimum room size and ventilation requirements.
- Do not install units in small, sealed cupboards or below-ground pits where heavy gas can collect.
- Keep LPG cylinders, open flames and unprotected switches away from the compressor compartment.
- Do not stack several high-charge cabinets in a tiny unventilated room without checking the instructions.
- Use a properly earthed socket and avoid loose, sparking connections.
Service precautions
Most of the risk sits with servicing. A technician working on a hydrocarbon unit should check the area with a flammable-gas detector before starting, ventilate the space, use tools and recovery equipment rated for flammable refrigerants, remove the refrigerant and purge the circuit with nitrogen before any brazing, and charge by weight to the exact rating-plate amount. Never use an open flame on a circuit that may still contain refrigerant.
An owner's checklist
- Ask service providers to confirm training and equipment for hydrocarbon refrigerants.
- Keep the flammable-refrigerant labels visible and intact.
- Report any smell of gas near a cabinet; switch off at the wall if safe to do so and ventilate.
- Do not allow refrigerant 'top-ups' with a different gas.
- At end of life, use a recycler who recovers refrigerant safely.
Frequently asked questions
- Is a 500 g limit the same in every country?
- No. The figure comes from the 2019 edition of IEC 60335-2-89, and each country decides when and how to adopt it. Conditions in the standard also affect how much charge a particular design may hold.
- What about larger systems such as cold rooms on hydrocarbons?
- Cold rooms and other systems built on site fall outside the appliance standard for self-contained cabinets and are covered by separate system-safety rules, which are generally stricter about charge size and room volume. They need specialist design; do not scale up cabinet practice.
- Do hydrocarbon fridges smell of gas if they leak?
- Refrigerant-grade propane and isobutane are usually not odorised like LPG, so a leak may not be noticed by smell. That is why design limits and trained service matter so much.
