R744 RefrigerantCarbon dioxide · Natural refrigerant · Transcritical & subcritical systems
R744 is carbon dioxide used as a refrigerant. It has GWP 1, zero ODP and an A1 safety classification, but its system pressures and phase behavior are fundamentally different from conventional HFC/HFO refrigerants. Above its critical point near 31.0°C and 73.8 bar, systems operate transcritically rather than condensing in the usual way.
What is R744 refrigerant?
R744 is CO₂ used as a refrigerant in transcritical, subcritical and cascade systems. Its environmental profile is attractive, but engineering must address very high pressure, the low critical temperature, dry-ice formation risk during depressurization and direct CO₂ exposure hazards in occupied or machinery spaces.
R744 refrigerant properties
These values are screening and reference data. Refrigerant-property values can vary slightly by equation of state, assessment report, supplier dataset and regulatory basis. Use the source required by the applicable equipment standard or regulation.
| Property | Reference | Engineering / source note |
|---|---|---|
| Designation | R744 | Refrigerant designation for carbon dioxide |
| Chemical name | Carbon dioxide | Inorganic / natural refrigerant |
| Formula | CO₂ | Single component |
| CAS number | 124-38-9 | NIST Chemistry WebBook |
| Molecular weight | 44.0095 g/mol | NIST Chemistry WebBook |
| Triple point | 216.58 K (−56.57°C) at about 5.185 bar | NIST phase-change data |
| Critical temperature | 304.18 K (≈31.03°C) | NIST phase-change data |
| Critical pressure | Approx. 73.8 bar | NIST phase-change data |
| ODP | 0 | No ozone depletion |
| 100-year GWP | 1 | Reference value |
| Safety classification | A1 | Lower toxicity group / no flame propagation |
Scientific assessment reports periodically update climate metrics, while regulations may continue using a fixed reference value. This page identifies the regulatory or technical basis for the value shown instead of presenting one number as universal.
Why R744 systems can be transcritical
CO₂ has a critical temperature of only about 31°C. When the high side of a refrigeration cycle is above that temperature, there is no conventional liquid–vapor condensation process; heat rejection occurs above the critical point in a gas cooler. That transcritical behavior is one of the defining engineering differences between R744 and conventional HFC/HFO systems.
Pressure management is equally distinctive. Even when a system is off, trapped CO₂ can develop high standstill pressure as temperature rises. Relief devices, pressure-rated components, receiver strategy and service procedures are therefore integral to the architecture rather than secondary details.
Refrigerant choice affects pressure, compressor operation, lubricant, controls, charge, safety classification, component approval and regulatory compliance. Only use a refrigerant in equipment and conversion procedures explicitly approved for it.
Where is R744 used?
Applications depend on equipment design, refrigerant charge, climate, capacity, location and local rules. The examples below describe established or emerging use contexts, not blanket authorization for every system.
Supermarket refrigeration
Transcritical booster systems and cascade architectures in retail food.
Heat pumps & water heating
High-temperature water-heating applications can benefit from CO₂ temperature glide on the water side.
Transport refrigeration
Used in selected truck, trailer and container systems.
Industrial & cold-chain refrigeration
Cascade, low-temperature and process applications designed for R744 pressures.
For broader research routes, explore the Refrigerant Database, Refrigerant Replacements and Low-GWP Refrigerants guides.
R744 vs R290
Direct comparisons are useful only when the application, system architecture and regulatory basis are defined. The table highlights major differences rather than implying interchangeability.
| Characteristic | R744 | R290 |
|---|---|---|
| Refrigerant | R744: carbon dioxide | R290: propane |
| GWP | 1 | About 3 |
| Safety class | A1 | A3 |
| Flammability | No flame propagation classification | Higher flammability |
| Primary engineering challenge | Very high pressure / CO₂ exposure | Ignition control / charge limits |
| Critical temperature | ≈31°C | ≈96.7°C |
| System architecture | Often transcritical or cascade | Conventional subcritical vapor-compression architectures |
R744 safety: high pressure and CO₂ exposure are the central hazards
R744 is A1, but carbon dioxide is physiologically active at concentrations far below those required to displace all oxygen. Therefore, an oxygen sensor alone should not be treated as equivalent to direct CO₂ monitoring in spaces where a refrigerant release could occur. High-pressure release, cold jets and dry-ice formation also need dedicated engineering controls.
Direct CO₂ monitoring
Use concentration monitoring where the risk assessment or applicable standard requires it.
Pressure relief
Receivers, piping and isolated liquid volumes need correctly sized and routed pressure protection.
Dry ice / cold injury
Rapid depressurization can form solid CO₂ and create severe cold-burn hazards.
Actual risk depends on refrigerant charge, release rate, room volume, ventilation, ignition sources, pressure, occupancy, equipment construction, alarm/mitigation logic, service procedure and applicable codes.
See the site-wide Refrigerant Safety Guide for an overview of A1, A2L, A3 and B2L classifications.
R744 refrigerant leak detection
Define the monitoring function first
For occupied or machinery spaces, direct CO₂ measurement is usually the relevant concentration-monitoring method. NDIR is widely used because carbon dioxide has strong infrared absorption. Detector placement should be based on credible release paths, ventilation and room geometry rather than a simplistic ‘CO₂ always sinks’ rule.
Regulatory position of R744 in 2026
United States
EPA SNAP lists R744 as acceptable in multiple refrigeration end uses, including stand-alone equipment, supermarket systems, remote condensing units, cold storage and transport refrigeration. The exact listing and equipment requirements should be checked by end-use.
European Union
Because R744 is not a fluorinated greenhouse gas, it avoids many GWP-based F-gas placing-on-the-market restrictions. It still must comply with pressure-equipment, machinery, product safety and national refrigeration requirements.
Always verify the exact equipment category, manufacture/import date, installation date, charge size, safety standard and jurisdiction before making a compliance decision.
Is R744 being phased out?
R744 is not an HFC and is not subject to the HFC phasedown. Its constraints are primarily high-pressure system design, application economics, ambient-temperature performance strategy and safety standards.
Use the Refrigerant Regulations Hub for country- and region-level research.
What should be compared with R744?
There is no universal “best” refrigerant. Compare options by system architecture, safety class, GWP basis, expected efficiency, charge, climate, supply chain, serviceability and regulatory lifetime.
R290
Very-low-GWP A3 hydrocarbon with lower pressures but higher flammability.
R32
A2L HVAC refrigerant with conventional vapor-compression architecture.
R454B
A2L fluorinated blend for conventional R410A-class HVAC architectures.
Use the Refrigerant Tools hub for additional screening and calculation routes.
R744 refrigerant FAQ
What is R744 refrigerant?
What is the GWP of R744?
Is R744 flammable?
Why does R744 operate at such high pressure?
What is the critical point of R744?
What is a transcritical CO₂ system?
Can an oxygen sensor detect an R744 leak safely?
Where is R744 used?
R744 sources and further reading
The page was built from primary regulatory, standards, scientific and manufacturer technical references. Re-check the latest source before equipment design, service or compliance decisions.
Working with R744 or another refrigerant?
Use Refrigerants.net to compare refrigerants, understand safety classifications and research the current transition path for your application.
Technical Notice: This page is for refrigerant research and general technical reference. Refrigerant suitability, permitted charge, equipment approval, leak mitigation, servicing procedures and regulatory compliance depend on the specific equipment, installation and jurisdiction. Always verify current manufacturer documentation, applicable standards, SDS and local regulations before selecting, replacing, charging or servicing a refrigerant.