What Is Refrigerant? States, History and the Refrigeration Cycle Explained

Ask any apprentice what a refrigerant is and you will often get a vague answer about “the gas in the system”. That is only half the story, and on the City & Guilds 2079 assessment the examiners want you to understand why a refrigerant behaves the way it does — not just name it. This short guide walks through what refrigerant is, how the industry got from dangerous early chemicals to today’s HFC blends, the properties that make a fluid fit for the job, and exactly what state the refrigerant is in at each point of the cycle.

“The important job of refrigerant is to absorb the heat in the area where we want to cool.”

That single sentence is the heart of it. A refrigerant is simply a working fluid chosen because it can pick up heat in one place and dump it in another, changing between liquid and vapour to do so.

What Refrigerant Actually Is

A refrigerant is a substance used in cooling devices — air conditioners, refrigerators, heat pumps and chillers — to move heat from where it is not wanted to where it can be safely rejected. Crucially, it does not stay in one physical state. Depending on the temperature and pressure it is subjected to, the same refrigerant exists as a liquid at one point in the circuit and a vapour at another.

This ability to boil and condense at convenient temperatures is what makes cooling possible. When a liquid boils, it absorbs a large amount of heat (its latent heat of vaporisation) without getting hotter itself. That is the trick every refrigeration system exploits.

A Short History: From Ammonia to HFCs

The refrigerants we use today are the result of a century of trial, error and hard lessons.

• Early days — dangerous chemicals. The first refrigerants included ammonia, sulphur dioxide and methyl chloride. They worked, but a leak was genuinely hazardous to anyone nearby and could even prove fatal.
• 1928 — the first “safe” refrigerant. R12, a chlorofluorocarbon (CFC), was introduced as a non-flammable, low-toxicity alternative. For decades it seemed like the perfect solution.
• 1980s — the ozone problem. It became clear that the chlorine in CFCs was severely damaging the ozone layer. As the video puts it:

“When 1 kg of R12 refrigerant leaks it causes the same damage as 10 tons of carbon dioxide.”

• The phase-out. The manufacture, buying and selling of R12 was banned. It is no longer produced anywhere.
• HFCs today. Refrigerants such as R134a and R410A, based on hydrofluorocarbon (HFC) chemistry, were introduced in its place. They do not deplete the ozone layer and are now used in most air conditioners and fridges.

This history is exactly why EU F-Gas Regulation 517/2014 exists. Although HFCs solved the ozone problem, they still have a significant global warming potential (GWP). F-Gas therefore targets the climate impact of these fluorinated gases through the phase-down of HFC quotas, mandatory leak checks based on CO₂-equivalent tonnage, recovery obligations and the requirement that only certified technicians handle them. Understanding the ozone-layer-versus-climate distinction — Montreal Protocol tackled ozone depletion, F-Gas tackles global warming — is a favourite exam theme.

The Properties of a Good Refrigerant

No single fluid is perfect, but the ideal refrigerant would tick every one of these boxes. Expect to be tested on them:

1. Low boiling point — so it can evaporate at the low temperatures needed for cooling.
2. High critical temperature — so it can still be condensed with ambient air or water.
3. High latent heat — more heat absorbed per kilogram circulated means a smaller, more efficient system.
4. High thermal conductivity — for efficient heat transfer through the coils.
5. Non-corrosive — it must not attack the materials, seals and oils in the system.
6. Non-flammable and non-explosive — for safe handling and installation.
7. Non-toxic — safe for occupants and technicians.
8. Cheap and readily available.
9. Low ozone and environmental impact — the modern, F-Gas-driven priority.

Notice how several of these properties pull against each other. Low-GWP alternatives such as R32 and the hydrocarbons are more flammable, which is why the A1 / A2L / A3 safety classifications matter so much on site. This trade-off between environmental performance and flammability is central to both modern system design and the safe-handling section of your 2079 training.

How Refrigerant Moves Round the Cycle

Here is where states and pressures come together. Take R410A, which has a boiling point of around −45 °C. That is so low that if you put a drop on your hand it would boil off using the heat of your skin. Inside a system, that eagerness to boil is put to work at four stages.

1. Evaporator (indoor unit). Low-pressure, very cold liquid refrigerant enters the indoor coil. A fan blows warm room air across the coil, and the heat in that air makes the refrigerant boil and turn to vapour. The room loses its heat to the refrigerant — that is the cooling you feel. The refrigerant leaves as a low-pressure, low-temperature vapour (picking up a little heat, so we say it is superheated).

2. Compressor (outdoor unit). The compressor draws in that low-pressure vapour and squeezes it. This raises the pressure dramatically, which is the only way the fluid can be pushed around the rest of the circuit and later condensed. It leaves as a high-pressure, high-temperature vapour.

3. Condenser (outdoor coil). The hot vapour flows through the outdoor coil while a fan blows ambient air across it. The refrigerant rejects its heat to the outside air, its temperature falls, and it condenses back into a liquid. It leaves as a high-pressure, liquid refrigerant.

4. Expansion valve. Finally the liquid passes through the expansion device, where its pressure drops sharply. Because pressure and temperature are linked, that pressure drop also causes a large temperature drop. The refrigerant leaves as a low-pressure, very low-temperature liquid — ready to enter the evaporator and start all over again.

“So, in this video, I hope you understand what refrigerant is, its history, characteristics and the states of refrigerant in an AC unit.”

If you can recite those four state changes — and, just as importantly, the pressure and temperature at each point — you have the backbone of the refrigeration-cycle questions that appear throughout the exam. A useful memory hook: the compressor and expansion valve change the pressure, while the evaporator and condenser change the state.

Why This Matters for Certified Technicians

Under F-Gas, understanding these states is not academic. Recovering refrigerant correctly, brazing without contaminating the charge, diagnosing a low charge from gauge readings and calculating a system’s CO₂-equivalent charge all rest on knowing where the fluid is liquid, where it is vapour, and at what pressure. That knowledge is what separates a competent, certified engineer from someone who simply tops up gas and hopes for the best — and it is exactly what the assessment is designed to confirm.

How F-Gas Exam Prep Fits Into This

The refrigeration cycle, refrigerant properties and the CFC-to-HFC history all sit squarely within the City & Guilds 2079 syllabus, and they are areas where candidates lose easy marks simply through fuzzy recall. The F-Gas Exam Prep app is built to fix exactly that:

• 460+ exam questions spanning every skill group, including refrigerant properties, environmental legislation and the practical cycle, so you drill the state-change and pressure questions until they are second nature.
• Mock exams that mirror the real City & Guilds 2079 format, so exam day feels familiar rather than daunting.
• AI voice challenges that let you talk through the cycle out loud — perfect for cementing the “low-pressure vapour into the compressor, high-pressure liquid out of the condenser” sequence.
• Detailed explanations for every answer, so when you get a question on why R12 was banned or which property matters most, you learn the reasoning and not just the correct option.

Master what refrigerant is and how it behaves around the circuit, and a whole cluster of exam questions becomes straightforward. Put in the practice now, and you will walk into your 2079 assessment ready to explain the cycle as confidently as you can feel the cold air coming off the evaporator.