Refrigeration glossary: 44 terms from A2L to subcooling

Glossary term

Short explanations of the most common terms in refrigeration, with links to the articles and calculators.

Last checked: October 7, 2026

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A

A2L (lower flammability refrigerant)

Safety class for refrigerants with lower toxicity that are mildly flammable: they burn, but the flame spreads at most 10 cm/s and a lot of energy is needed to ignite them. Examples: R32, R1234yf, R1234ze(E) and R454B. The charge limits in EN 378 are 1.5 times higher than for class 2.

Access category

Classification in EN 378 and ISO 5149 by who is present where the system is located: a (general, for example shops and homes), b (supervised, for example offices) and c (authorised, trained staff only, for example cold stores and production areas). The category affects how much refrigerant is allowed.

B

Brazing with nitrogen

Brazing copper pipes while dry nitrogen flows through the pipe, so that no oxide forms on the inside. Otherwise the oxide later comes loose, travels with the refrigerant and oil and can block filters and valves. A small flow is enough. The nitrogen must be oxygen-free.

Bubble point

The temperature at a given pressure at which a zeotropic refrigerant blend starts to boil, that is where the first vapour bubble forms. The bubble point is lower than the dew point. Subcooling is calculated from the bubble point.

C

CO2 equivalent (CO2e)

Measure of the climate impact of a quantity of gas: the mass in kg multiplied by the GWP and divided by 1,000 gives tonnes of CO2e. 10 kg of R404A with a GWP of 3,922 is 39.2 tonnes CO2e. The leak check thresholds of the F-gas Regulation are expressed in tonnes CO2e.

Source: Regulation (EU) 2024/573 – EUR-Lex

Compressor burnout

When the compressor motor winding burns out, acids and soot form and spread through the system. The oil smells sharp and an acid test shows a raised acid level. After a burnout the filter drier is replaced, often with one intended for acid removal, and the oil is changed. The system is cleaned according to the compressor manufacturer's instructions before it is restarted.

D

Dew point

The temperature at a given pressure at which a zeotropic refrigerant blend has finished boiling, that is where the last drop of liquid evaporates. The dew point is higher than the bubble point. Superheat is calculated from the dew point.

E

Evacuation

Removing air and moisture from a refrigerating system with a vacuum pump before it is charged with refrigerant. The vacuum is measured with an electronic vacuum gauge on the system. A standing test with the pump isolated shows whether the system is dry and tight.

Expansion valve

The throttling valve that lowers the pressure from condensing to evaporating pressure and controls how much refrigerant is fed to the evaporator. A thermostatic expansion valve (TXV) is controlled by the superheat via a bulb on the suction line. An electronic expansion valve (EEV) is controlled by a controller with pressure and temperature sensors.

F

F-gas certificate

Personal certificate showing that a technician has the knowledge required to install, repair, leak-check and recover refrigerant in equipment containing F-gases. Companies that carry out the work for others need a company certificate. In the EU the minimum requirements are in Implementing Regulation (EU) 2024/2215.

Source: Commission Implementing Regulation (EU) 2024/2215 – EUR-Lex

F-gases (fluorinated greenhouse gases)

Collective name for fluorinated greenhouse gases such as HFCs, PFCs and SF6 and, under EU Regulation 2024/573, also unsaturated HFCs and HCFCs (HFOs). They are regulated to reduce greenhouse gas emissions. Ozone-depleting substances such as R22 are regulated separately.

Source: Regulation (EU) 2024/573 – EUR-Lex

Filter drier

Filter in the liquid line that traps moisture, acids and particles. It must be replaced when the system has been open and after a compressor burnout. If the temperature drops noticeably across the filter drier during operation, it is blocked.

G

Gas cooler

The heat exchanger that cools the high-pressure gas in a transcritical CO2 system. Above the critical point, 31 °C and 73.8 bar for CO2, the gas does not condense. Instead it is cooled without a phase change, and the high pressure is controlled by the gas cooler outlet temperature.

Glide (temperature glide)

The difference between the dew point and the bubble point at the same pressure for a zeotropic blend. Glide means that the temperature changes during evaporation and condensation. R407C has a glide of about 6 K, and R448A and R449A a few kelvin. Blends with glide must be charged as liquid.

GWP (global warming potential)

How much a gas contributes to the greenhouse effect compared with the same mass of carbon dioxide, usually over 100 years. CO2 has a GWP of 1, R134a 1,430 and R404A 3,922 according to the values the F-gas Regulation uses for HFCs. Used to convert kilograms to tonnes CO2e and in the limits for bans.

H

Hermetically sealed equipment

Equipment in which all parts containing refrigerant are joined by welding, brazing or a similar permanent connection. Factory-made refrigerators are a typical example. If the equipment is labelled as hermetically sealed and contains less than 10 tonnes CO2e, it does not need leak checks under the F-gas Regulation.

HFC (hydrofluorocarbons)

Refrigerants made of hydrogen, fluorine and carbon, for example R134a, R32 and the blends R404A, R410A and R407C. They do not deplete the ozone layer but often have a high GWP. HFCs are being phased down under the Kigali Amendment to the Montreal Protocol and the F-gas Regulation.

HFC phase-down

Step-by-step reduction of how much HFC may be produced, imported or placed on the market, counted in tonnes CO2e. It is governed by the Kigali Amendment to the Montreal Protocol. In the EU the quota reaches zero in 2050, and in Australia imports reach 15 % of the baseline in 2036.

HFO (hydrofluoroolefins)

Synthetic refrigerants with a double bond that makes them break down quickly in the atmosphere, so they have a very low GWP, for example R1234yf and R1234ze(E). Most of them are mildly flammable (A2L). Blends of HFOs and HFCs, for example R448A and R454B, are often called HFO blends.

High-pressure switch

Pressure switch that stops the compressor if the condensing pressure becomes too high. When it has a safety function it must be type-approved, have a manual reset and be set so that it trips before the safety valve opens. It must not be possible to isolate it from the compressor with a valve.

L

Leak check

Check that a system containing refrigerant is tight, carried out by a certified person. Under the F-gas Regulation it must be carried out at least every 12, 6 or 3 months depending on the quantity in tonnes CO2e. The check covers the records, a visual inspection, direct leak detection and a comparison of operating data.

Leakage detection system

Calibrated mechanical, electrical or electronic device that detects refrigerant leaks and alerts the operator. Under the F-gas Regulation it is required for equipment with 500 tonnes CO2e or more, and it extends the leak check interval.

LFL (lower flammability limit)

The lowest concentration of a refrigerant in air that can ignite, in kg/m³. Propane (R290) has an LFL of 0.038 kg/m³ and R32 0.307 kg/m³. The charge limits for flammable refrigerants in EN 378 and IEC 60335 are calculated from the LFL.

Liquid receiver

Pressure vessel after the condenser that collects liquid refrigerant and evens out variations in the charge between operating conditions. It keeps the condenser from filling with liquid and can hold the refrigerant during servicing. Liquid receivers must have a safety valve, and it should be possible to check the liquid level.

Liquid slugging

Damage that occurs when liquid refrigerant or oil enters the compressor cylinders, usually at start-up after a standstill or when the superheat is too low. Liquid cannot be compressed and can destroy valves and connecting rods. Prevented with the right superheat, crankcase heating and pump down.

Location class

Classification in EN 378 by where the parts of the system containing refrigerant are located: I (in an occupied space), II (compressor and pressure vessels in a machinery room or outdoors), III (everything in a machinery room or outdoors) and IV (in a ventilated enclosure). The location class affects the maximum allowed charge.

Low-pressure switch

Pressure switch that stops the compressor when the suction pressure becomes too low. It is used as protection against freezing and operation at too low a pressure, and as control in systems with pump down, where the compressor stops when the solenoid valve has closed and the low-pressure side has been emptied.

N

Natural refrigerants

Refrigerants that occur in nature: carbon dioxide (R744), ammonia (R717), hydrocarbons such as propane (R290), isobutane (R600a) and propene (R1270), and water and air. They have a GWP close to zero and are not covered by the quotas of the F-gas Regulation. Hydrocarbons are flammable and ammonia is toxic, which brings its own requirements.

O

ODP (ozone depletion potential)

How much a substance depletes the ozone layer compared with CFC-11, which has an ODP of 1. R22 has an ODP of 0.055. HFCs, HFOs and natural refrigerants have an ODP of 0. In the EU, ozone-depleting substances are regulated by Regulation (EU) 2024/590.

Source: Regulation (EU) 2024/590 – EUR-Lex

Operator

The natural or legal person who exercises actual power over the technical functioning of equipment containing refrigerant. The operator is responsible for leak checks, records, repairing leaks and, in Sweden, for notification and the report to the supervisory authority. It is usually the owner.

P

PED category

Risk category I–IV for pressure equipment under the Pressure Equipment Directive 2014/68/EU. It is determined by the maximum allowable pressure, the volume or pipe size and the fluid group of the refrigerant. The category decides which assessment is required before CE marking. Below category I, sound engineering practice applies (Article 4.3).

Source: Pressure Equipment Directive 2014/68/EU – EUR-Lex

Practical limit

The highest refrigerant concentration in kg/m³ in an occupied room, under EN 378-1, that does not create a risk of poisoning, asphyxiation or fire if the whole charge leaks out. The maximum charge is essentially the limit multiplied by the volume of the room. ISO 5149 and ASHRAE 15 use similar concepts, for example RCL.

PS (maximum allowable pressure)

The highest pressure for which a part of the system is designed, as specified by the manufacturer. It is shown on the nameplate. PS sets the test pressure for the strength test and the safety valve that is needed, and must be at least as high as the highest pressure the part can be exposed to in operation and at standstill.

R

Reclamation

Reprocessing recovered refrigerant so that it meets the specification of new refrigerant. It is carried out at a reclamation facility. Reclaimed gas may be used for servicing even where virgin gas is banned, for example with a GWP of 2,500 or more until the end of 2029 in the EU.

Refrigerant recovery

Emptying refrigerant from a system into a recovery cylinder with a recovery unit, without releasing it. Done before repairs and disposal. The refrigerant can then be reused, reclaimed or destroyed. The cylinder must not be overfilled and must be labelled with the refrigerant.

S

Safety class

The class of a refrigerant under ISO 817 and EN 378: a letter for toxicity (A lower, B higher) and a number for flammability (1 no flame propagation, 2L lower flammability, 2 flammable, 3 higher flammability). Examples: R744 is A1, R32 A2L, R290 A3 and R717 B2L.

Safety valve

Valve that opens and releases refrigerant when the pressure reaches the set pressure, to protect the system against excessive pressure, for example in a fire. It must be set at no more than PS, have sufficient capacity and have a discharge line to a safe place. Sized according to EN ISO 24664:2024, which replaces EN 13136 (harmonised under the PED until 1 April 2028).

Saturation temperature

The temperature at which a refrigerant boils or condenses at a given pressure. Every pressure has a saturation temperature, and that is what you read in a pressure-temperature chart. Superheat and subcooling are calculated from the saturation temperature.

Sight glass

Small window in the liquid line that shows whether the liquid is free of vapour bubbles. There is often a moisture indicator that changes colour when the moisture content is too high. Bubbles at full load can be due to too little refrigerant or a pressure drop before the sight glass, but temporary bubbles can be normal at start-up and load changes.

Strength test

Pressure test of pipes and components at a pressure above PS, usually with nitrogen, to show that they withstand the pressure. Under EN 378-2 pipework is normally tested at at least 1.1 × PS or 1.43 × PS depending on the PED category. The test is carried out before the system is tightness tested and evacuated.

Subcooling

How much colder the liquid is than the saturation temperature at the condensing pressure. For blends with glide it is calculated from the bubble point. Subcooling shows that there is solid liquid before the expansion valve and is used to judge the charge in systems without a liquid receiver.

Superheat

How much warmer the gas in the suction line is than the saturation temperature at the suction pressure. For blends with glide it is calculated from the dew point. Superheat shows whether the evaporator is fed correctly. Too little risks liquid slugging, too much gives lower capacity and a high discharge temperature.

T

Tightness test

Test that shows a system does not leak. It is carried out after the strength test at a pressure of no more than PS, usually with nitrogen, and all joints are checked with leak detection fluid or a leak detector. A standing vacuum test does not replace the tightness test.

Transcritical system

Refrigerating system, usually with CO2 (R744), where the high-pressure side operates above the critical pressure of the refrigerant so that the gas does not condense. Common in warm climates and in supermarkets with booster systems. The high pressure is controlled with a high-pressure valve, and the system needs pipes and components for high pressures.