R-744
Carbon dioxide — the natural refrigerant defining the GWP=1 reference. A1 non-flammable, transcritical above 87.8°F, very high pressures (sub-critical saturation 838 PSIG at 70°F). The long-term low-GWP destination for supermarket commercial refrigeration.
Lower toxicity (Occupational Exposure Limit ≥ 400 ppm). No flame propagation in air at standard atmospheric pressure and 60°C. R-134a, R-22, R-410A, R-404A, R-744 (CO2) are A1.
- Flammability
- None (no flame propagation)
- Toxicity
- Lower (OEL ≥ 400 ppm)
Classification per ANSI/ASHRAE Standard 34-2022. See full reference.
Saturation pressure-temperature curve
Saturation values from CoolProp 7.2.0 CarbonDioxide. Operating pressure on a running system differs — see what R-744 operating pressures should be.
R-744 PT chart PDF — printable saturation table
Looking for the R-744 PT chart PDF for shop reference? The complete pressure-temperature saturation table is below — every 1° increment from −40°F to 150°F (or to the refrigerant's critical temperature). Use the Print / Save as PDF button in the table header to download a clean, table-only PDF (the rest of the page is hidden from the print output). Important service temperatures (normal boiling point, freezing point of water, residential AC evap and condenser targets) are tinted and tagged in the table for at-a-glance shop reference.
R-744 PT Chart — Pressure-Temperature Saturation Table
1° increments · Source: CoolProp 7.2.0 / manufacturer datasheet · hvacptcharts.com
| Temp (°F) | Pressure (PSIG) |
|---|---|
| -40°FIndustrial LT evap | 131.0 |
| -39°F | 134.0 |
| -38°F | 137.1 |
| -37°F | 140.1 |
| -36°F | 143.3 |
| -35°F | 146.5 |
| -34°F | 149.7 |
| -33°F | 153.0 |
| -32°F | 156.3 |
| -31°F | 159.7 |
| -30°F | 163.1 |
| -29°F | 166.6 |
| -28°F | 170.1 |
| -27°F | 173.7 |
| -26°F | 177.3 |
| -25°F | 181.0 |
| -24°F | 184.8 |
| -23°F | 188.6 |
| -22°F | 192.4 |
| -21°F | 196.3 |
| -20°F | 200.2 |
| -19°F | 204.2 |
| -18°F | 208.3 |
| -17°F | 212.4 |
| -16°F | 216.5 |
| -15°F | 220.8 |
| -14°F | 225.0 |
| -13°F | 229.4 |
| -12°F | 233.8 |
| -11°F | 238.2 |
| -10°F | 242.7 |
| -9°F | 247.3 |
| -8°F | 251.9 |
| -7°F | 256.6 |
| -6°F | 261.3 |
| -5°F | 266.1 |
| -4°F | 271.0 |
| -3°F | 275.9 |
| -2°F | 280.9 |
| -1°F | 285.9 |
| 0°FIndustrial MT evap | 291.0 |
| 1°F | 296.2 |
| 2°F | 301.4 |
| 3°F | 306.8 |
| 4°F | 312.1 |
| 5°F | 317.6 |
| 6°F | 323.1 |
| 7°F | 328.6 |
| 8°F | 334.3 |
| 9°F | 339.9 |
| 10°F | 345.7 |
| 11°F | 351.6 |
| 12°F | 357.4 |
| 13°F | 363.4 |
| 14°F | 369.5 |
| 15°F | 375.6 |
| 16°F | 381.8 |
| 17°F | 388.0 |
| 18°F | 394.3 |
| 19°F | 400.7 |
| 20°F | 407.2 |
| 21°F | 413.8 |
| 22°F | 420.4 |
| 23°F | 427.1 |
| 24°F | 433.8 |
| 25°F | 440.7 |
| 26°F | 447.6 |
| 27°F | 454.6 |
| 28°F | 461.7 |
| 29°F | 468.8 |
| 30°F | 476.1 |
| 31°F | 483.4 |
| 32°FH₂O freeze | 490.8 |
| 33°F | 498.3 |
| 34°F | 505.8 |
| 35°F | 513.5 |
| 36°F | 521.2 |
| 37°F | 529.0 |
| 38°F | 536.9 |
| 39°F | 544.8 |
| 40°F | 552.9 |
| 41°F | 561.0 |
| 42°F | 569.3 |
| 43°F | 577.6 |
| 44°F | 586.0 |
| 45°F | 594.5 |
| 46°F | 603.0 |
| 47°F | 611.7 |
| 48°F | 620.5 |
| 49°F | 629.3 |
| 50°F | 638.3 |
| 51°F | 647.3 |
| 52°F | 656.5 |
| 53°F | 665.7 |
| 54°F | 675.0 |
| 55°F | 684.5 |
| 56°F | 694.0 |
| 57°F | 703.6 |
| 58°F | 713.3 |
| 59°F | 723.1 |
| 60°F | 733.0 |
| 61°F | 743.1 |
| 62°F | 753.2 |
| 63°F | 763.4 |
| 64°F | 773.8 |
| 65°F | 784.2 |
| 66°F | 794.8 |
| 67°F | 805.5 |
| 68°F | 816.2 |
| 69°F | 827.1 |
| 70°F | 838.1 |
| 71°F | 849.3 |
| 72°F | 860.5 |
| 73°F | 871.9 |
| 74°F | 883.3 |
| 75°F | 894.9 |
| 76°F | 906.7 |
| 77°F | 918.5 |
| 78°F | 930.5 |
| 79°F | 942.6 |
| 80°F | 954.9 |
| 81°F | 967.3 |
| 82°F | 979.8 |
| 83°F | 992.5 |
| 84°F | 1005.4 |
| 85°F | 1018.4 |
| 86°F | 1031.6 |
| 87°F | 1044.9 |
| Temp (°C) | Pressure (kPa) |
|---|---|
| -40°CIndustrial LT evap | 903 |
| -39°C | 941 |
| -38°C | 979 |
| -37°C | 1,019 |
| -36°C | 1,059 |
| -35°C | 1,101 |
| -34°C | 1,144 |
| -33°C | 1,188 |
| -32°C | 1,233 |
| -31°C | 1,279 |
| -30°C | 1,326 |
| -29°C | 1,375 |
| -28°C | 1,425 |
| -27°C | 1,476 |
| -26°C | 1,528 |
| -25°C | 1,581 |
| -24°C | 1,636 |
| -23°C | 1,692 |
| -22°C | 1,750 |
| -21°C | 1,808 |
| -20°C | 1,868 |
| -19°C | 1,930 |
| -18°CIndustrial MT evap | 1,993 |
| -17°C | 2,057 |
| -16°C | 2,122 |
| -15°C | 2,190 |
| -14°C | 2,258 |
| -13°C | 2,328 |
| -12°C | 2,400 |
| -11°C | 2,473 |
| -10°C | 2,547 |
| -9°C | 2,624 |
| -8°C | 2,701 |
| -7°C | 2,781 |
| -6°C | 2,862 |
| -5°C | 2,945 |
| -4°C | 3,029 |
| -3°C | 3,115 |
| -2°C | 3,203 |
| -1°C | 3,292 |
| 0°CH₂O freeze | 3,384 |
| 1°C | 3,477 |
| 2°C | 3,572 |
| 3°C | 3,669 |
| 4°C | 3,768 |
| 5°C | 3,868 |
| 6°C | 3,971 |
| 7°C | 4,075 |
| 8°C | 4,182 |
| 9°C | 4,290 |
| 10°C | 4,401 |
| 11°C | 4,514 |
| 12°C | 4,628 |
| 13°C | 4,745 |
| 14°C | 4,865 |
| 15°C | 4,986 |
| 16°C | 5,110 |
| 17°C | 5,236 |
| 18°C | 5,364 |
| 19°C | 5,495 |
| 20°C | 5,628 |
| 21°C | 5,764 |
| 22°C | 5,902 |
| 23°C | 6,043 |
| 24°C | 6,186 |
| 25°C | 6,333 |
| 26°C | 6,482 |
| 27°C | 6,635 |
| 28°C | 6,791 |
| 29°C | 6,950 |
| 30°C | 7,112 |
Full saturation values at 1° increments — toggle between °F / PSIG and °C / kPa. Use Print / Save as PDF for laminated shop reference, or download the CSV / JSON below for use in other tools. R-744 PT chart data: CoolProp 7.2.0 (REFPROP-compatible Helmholtz EOS) or manufacturer datasheet, validated against AHRI Standard 700-2019.
At a glance
Chemistry
Lubricant compatibility
Critical point at 31.0°C (87.8°F) — most commercial systems operate transcritically. Very high operating pressures (typically 60-130 bar). Specialty equipment, POE oil (specific CO2-rated formulations), and high-pressure components required.
Common applications
- Commercial refrigeration (supermarket booster and transcritical CO2 systems, expanding rapidly)
- Cascade refrigeration (CO2 low-stage)
- Heat pumps (water heating, with high COP at moderate ambient)
- Mobile AC (heat pumps, EU/Japan adoption)
Properties
- Critical point87.8°F at 1055 PSIG
- Molar mass44.01 g/mol
- Temperature glideNegligible (0.00°F)
- ODP0
- GWP (AR5, 100-yr)1
- GWP (AR6, 100-yr)1
What is R-744?
R-744 is carbon dioxide (CO₂) — a pure-substance natural refrigerant with the lowest possible global warming potential (GWP 1, defined as the reference value for the GWP scale) and zero ozone-depletion potential [ipccar5][ashrae34]. It is industrial, food-grade, and atmospheric in origin; refrigerant-grade R-744 typically comes from industrial CO₂ recovery streams rather than dedicated production.
R-744's defining property is its low critical temperature of 87.8°F (31°C). Above this temperature, no liquid/vapor phase distinction exists — the refrigerant behaves as a single supercritical fluid. In warm-ambient operation (the normal case for US summer cooling), R-744 systems run transcritically: the high-pressure side cannot condense and must be controlled as a gas cooler rather than a condenser [ashraehandbook].
Where R-744 is used
- Supermarket commercial refrigeration — CO₂ booster systems for both medium and low temperature loads
- Heat pump water heaters — R-744 transcritical cycle uniquely well-suited
- Cascade refrigeration — R-744 as low-stage in cascade with R-717 or HFC high stage
- Industrial CO₂ refrigeration — food processing, cold storage
- Mobile heat pumps (some EV applications) and selected automotive applications
Regulatory & phase-down status
R-744 faces no phase-down pressure — GWP 1, ODP 0, A1 safety class, natural origin. The Kigali Amendment and EPA AIM Act target HFCs; R-744 is not an HFC and is not on any phase-down schedule [epasnap].
Market adoption is growing rapidly in supermarket commercial refrigeration as a long-term low-GWP destination replacing R-404A and R-22 systems. Equipment cost is higher than HFC alternatives due to the high-pressure components, but operating costs are favorable and there is no GWP-driven future replacement risk.
Service notes
R-744 service equipment is fundamentally different from HFC service equipment. Manifold gauges, hoses, and recovery cylinders must be rated for the very high working pressures — typical R-744 commercial refrigeration systems operate at 1000-1500 PSIG on the high side, with safety relief valves set above 2000 PSIG.
Lubricant depends on application: POE for HFC-cascade high-stage, POE or PAG for transcritical, PAO (polyalphaolefin) for some industrial applications. R-744 is generally non-miscible with mineral oil at typical operating temperatures.
The triple-point pressure of 60 PSIA (above atmospheric) means R-744 cannot exist as liquid at atmospheric pressure — a leak to atmosphere causes immediate flash to gas plus solid (dry ice). Service procedures must account for the high pressure and the sublimation behavior on depressurization [iir].
Operating cycle
R-744 is used in: Commercial refrigeration (supermarket booster and transcritical CO2 systems, expanding rapidly), Cascade refrigeration (CO2 low-stage), Heat pumps (water heating, with high COP at moderate ambient) — applications with much lower condensing temperatures than residential AC.
Phase-down timeline
R-744 is not subject to AIM Act or EU F-Gas phase-down regulation. With a 100-year GWP of 1 (hydrocarbon / natural refrigerant) and zero ozone-depletion potential, it sits below both the EU F-Gas 150 GWP cap and the EPA AIM Act 700 GWP cap. No phase-down schedule applies — it is one of the refrigerants chosen for the transition away from high-GWP HFCs.
Global warming potential, in context
Industrial refrigeration & cascade systems
Retrofit and replacement paths
R-744 replaces
Reading the R-744 pressure-temperature chart
R-744's PT chart shows a single saturation curve from approximately −56°F (the sub-critical lower bound) up to the critical point at 87.8°F (CoolProp 7.2.0). Above 87.8°F there is no saturation — the chart truncates because no liquid/vapor distinction exists in supercritical territory.
This is fundamentally different from HFC PT charts, which extend well above any normal operating temperature. R-744's truncated chart is correct physics — saturation doesn't exist above critical, and any chart purporting to show R-744 saturation values above 88°F is fabricating data that does not correspond to any physical state.
On a 95°F summer day, R-744 high-side pressure is no longer set by ambient-driven condensation (the standard refrigeration mode for HFC systems). Instead, the gas cooler outlet temperature is set by ambient + approach, and the gas cooler outlet pressure is independently controlled by a high-pressure throttle valve to optimize cycle efficiency. This is the defining characteristic of transcritical R-744 operation [ashraehandbook].
Why R-744 operates 4-5× higher than any HFC
R-744's small molecule (CO₂, 44 g/mol — the lightest commercial refrigerant) gives it extremely high vapor pressure at any given temperature. At 70°F R-744 saturation is 838 PSIG — compare to R-410A at 202 PSIG or R-22 at 121 PSIG. The pressure delta is so large that R-744 service equipment shares almost nothing with HFC service equipment.
For sub-critical operation (ambient below 87.8°F), R-744 saturation at typical commercial refrigeration evaporator temperatures: −20°F (frozen) gives 200 PSIG; 0°F (freezer) gives 290 PSIG; 30°F (medium temp) gives 460 PSIG. The high suction pressures are an operational advantage — easier to maintain leak-tightness against atmospheric infiltration than the near-vacuum suction pressures of low-temp HFC systems.
For transcritical operation (ambient above 87.8°F), the high-side pressure is actively controlled rather than passively set by condensation. Typical gas cooler discharge pressures: 1100 PSIG at 88°F ambient, 1350 PSIG at 95°F, 1500-1700 PSIG at higher ambients. Equipment is designed for sustained operation at these pressures with safety relief valves typically set at 1740 or 2030 PSIG.
R-744 chemistry — the simplest possible refrigerant
R-744 is carbon dioxide: one carbon atom double-bonded to two oxygen atoms (CO₂). It is the smallest refrigerant molecule in commercial use, with molar mass 44.01 g/mol — smaller than R-32 (52 g/mol), R-22 (86.5 g/mol), or any HFC. The small molecule and the high vapor pressure are directly linked.
R-744 is a naturally-occurring atmospheric constituent (around 420 ppm in 2026, rising from 280 ppm pre-industrial). Refrigerant-grade R-744 typically comes from industrial CO₂ recovery streams — byproducts of ammonia synthesis, ethanol fermentation, hydrogen production. The refrigerant is purified to AHRI 700 spec but is not a dedicated production stream like HFCs are [ahri700].
The global warming potential scale measures climate impact relative to CO₂ over a defined time horizon (100 years for standard GWP). CO₂ is the reference, so its GWP is exactly 1.0. R-744's "natural refrigerant" status is partly chemistry (the molecule occurs naturally) and partly definitional (the GWP scale is anchored to CO₂). No other refrigerant can have a lower GWP than R-744 by the scale's construction.
Critical point 87.8°F — the defining R-744 characteristic
R-744's critical temperature is 87.8°F (31°C); critical pressure is 1071 PSIA (73.8 bar / 7.38 MPa) [coolprop]. The critical temperature is uniquely low among commercial refrigerants — comparable to ambient summer temperatures, which is why R-744 systems must accommodate both sub-critical and transcritical operating modes within a single refrigeration plant.
Below 87.8°F (cool weather, cold-climate operation): R-744 operates sub-critically. The high side condenses at saturation pressure corresponding to condenser outlet temperature; the cycle resembles a conventional vapor-compression refrigeration cycle. Sub-critical R-744 efficiency is excellent and competitive with HFC systems.
Above 87.8°F (warm weather, hot-climate summer operation): R-744 operates transcritically. The high side does not condense; instead, the refrigerant flows through a gas cooler where heat is rejected to ambient air without phase change. A high-pressure throttle valve downstream of the gas cooler controls the high-side pressure to optimize cycle efficiency for the current ambient — typically targeting 8-10°F approach to ambient at the gas cooler exit [ashraehandbook].
Reading R-744 service temperatures across operating modes
R-744 service temperatures span a wider range than HFC refrigerants because of the dual operating modes. Standard service temperatures for sub-critical operation:
- −40°F (deep freezer evaporator) — R-744 saturation approximately 145 PSIG.
- −20°F (frozen food evaporator) — R-744 saturation approximately 200 PSIG.
- 0°F (medium-low temp evaporator) — R-744 saturation approximately 290 PSIG.
- 30°F (medium-temp refrigerated case evaporator) — R-744 saturation approximately 460 PSIG.
- 70°F (mid-temperature reference) — R-744 saturation 838 PSIG.
- 87°F (just below critical) — R-744 saturation approximately 1050 PSIG; near the critical pressure of 1071 PSIA.
For transcritical operation, high-side pressures are independent of saturation: 95°F ambient gas cooler discharge typically 1200-1400 PSIG; 105°F approximately 1450-1650 PSIG; 115°F approximately 1650-1850 PSIG (near system safety relief setpoints). Manifold gauges for R-744 service typically have 2000-3000 PSI working pressure to provide safety margin above relief valve settings.
R-744 service equipment is fundamentally different from HFC equipment
The pressure delta between R-744 and HFC operation drives fundamentally different service equipment requirements. R-744 manifold gauges, hoses, recovery cylinders, recovery machines, leak detectors, and charging scales are purpose-built for the 1500-2000 PSI working envelope and the unique behavior of supercritical CO₂.
| Equipment / procedure | R-410A (typical HFC) | R-744 (CO₂) | | --- | --- | --- | | Manifold gauge rating | 800 PSI | 2000-3000 PSI | | Recovery cylinder | 600 PSI service | 1800 PSI service / 2030 PSI burst | | Charging hose | 800 PSI working | 2000 PSI working | | Service port | 5/16" SAE | Quick-disconnect designed for high pressure | | Lubricant | POE | POE / PAG / PAO (application-dependent) | | Recovery machine | R-410A rated | R-744 specific (different compression and storage) | | Leak detection | Electronic A1 | Electronic with infrared CO₂-specific sensors | | Safety relief setpoint | 450 PSI typical | 1740-2030 PSI typical |
Using HFC-rated equipment on R-744 is dangerous — rupture risk under pressure is real. The R-744 service ecosystem is its own equipment category with its own training and certification requirements.
R-744 lubricants — application-specific, not one-size-fits-all
R-744 lubricant selection depends on application. There is no single "R-744 oil" the way there is a clear "R-410A uses POE ISO 32" rule. Common patterns:
- POE oil — most common in HFC-cascade systems where R-744 is the low-stage. Compatible with the HFC high-stage lubricant chemistry.
- PAG (polyalkylene glycol) — used in some transcritical commercial refrigeration systems and in CO₂ mobile heat-pump applications.
- PAO (polyalphaolefin) — used in some industrial R-744 applications and as a synthetic alternative to mineral oil in cascade systems.
- Mineral oil — historically used in some industrial R-744 systems but generally non-miscible at typical operating temperatures; mostly displaced by synthetic oils.
The lubricant choice interacts with the operating envelope. Sub-critical R-744 has very different oil-return behavior than transcritical R-744; high-side pressures of 1500+ PSIG in transcritical operation create different challenges for oil management than the moderate condenser pressures of HFC systems. Verify lubricant grade against equipment OEM service literature; do not assume cross-application compatibility.
Safety considerations — high pressure, asphyxiation, dry ice on release
R-744 is ASHRAE class A1: non-toxic at refrigeration concentrations and non-flammable [ashrae34]. The primary safety concerns are mechanical and asphyxiation-related rather than toxicological:
High pressure. System pressures of 1500-2000 PSIG mean any rupture is energetic and dangerous. Pressure-relief valves are sized to limit maximum pressure but the energy release on relief activation is substantial. Service personnel must maintain safe distance during pressure tests and during initial system startup after major repairs.
Asphyxiation. R-744 is not toxic but is heavier than air (density approximately 1.5× air at the same temperature and pressure). A large refrigerant release in a confined space displaces breathable oxygen and can produce asphyxiation. ASHRAE 15 specifies maximum refrigerant concentration limits per occupied space; R-744 system installations must comply with these limits or include refrigerant leak detection with ventilation interlock [ashrae15].
Dry ice formation. R-744 cannot exist as liquid at atmospheric pressure (triple-point pressure is 60.4 PSIA, above atmospheric). A liquid R-744 leak depressurizing to atmosphere flashes immediately to vapor plus solid dry ice at −109°F. The dry ice is a thermal hazard (extreme cold) and a visibility hazard (a release produces a dense fog of cold gas and ice particles). Service personnel use cold-rated PPE during R-744 system service.
Supermarket R-744 transcritical — the dominant new-equipment trajectory
R-744 transcritical "booster" systems handle both medium-temperature (refrigerated cases) and low-temperature (frozen food cases) loads in a single supermarket refrigeration plant using one refrigerant. The architecture: a low-temperature compressor rack pumps R-744 vapor from the low-temp evaporators up to medium-pressure; medium-temp evaporators discharge directly to medium-pressure; medium-pressure compressors pump everything up to gas cooler pressure; the gas cooler rejects heat to ambient; refrigerant flashes through a high-pressure throttle valve and returns to the evaporators.
EU supermarket adoption is mature — most new EU supermarket installations from 2015 onward are R-744 transcritical. The Kigali Amendment and EU F-Gas Regulation pressure on R-404A made R-744 the natural long-term destination, and the equipment supply chain (compressors, valves, controls, gas coolers) is well-developed [shecco].
US adoption lagged EU by approximately 5 years but is accelerating through 2024-2027 as the EPA AIM Act Technology Transitions Rule restricts R-404A for new equipment in commercial refrigeration categories. Major US supermarket chains (Walmart, Whole Foods, Sobeys, Loblaws) have piloted and rolled out R-744 transcritical installations through the 2020s.
How to think about R-744 in 2026 and beyond
R-744 occupies a structurally permanent position in the refrigeration ecosystem. GWP 1 by definition; ODP 0; natural origin; no plausible future regulatory restriction. The barriers to wider adoption are equipment cost (high-pressure components, transcritical control systems) and equipment unfamiliarity (different service practice from HFC systems).
For supermarket commercial refrigeration: R-744 transcritical is increasingly the new-equipment standard, particularly in EU and accelerating in US. Capital cost is higher than R-454C / R-455A alternatives but operating cost and zero phase-down risk favor R-744 for installations expected to operate 15-25 years.
For other applications: R-744 is dominant in heat pump water heaters (Sanden, ECO² Systems, others) where the transcritical cycle's natural high-side temperature glide matches the water-heating temperature lift. Cascade low-stage CO₂ paired with R-717 ammonia high-stage is a mature industrial refrigeration architecture. Mobile heat pumps (EV thermal management) increasingly specify R-744 because the cycle works at extreme cold ambient where HFCs lose capacity.
Frequently asked
›What is the normal operating pressure of R-744?
Very high compared to any HFC. Sub-critical R-744 at 70°F saturation is 838 PSIG; at 0°F (typical low-temp commercial refrigeration evaporator), saturation is approximately 290 PSIG. Transcritical operation runs even higher on the gas-cooler side — 1100-1500 PSIG typical at 95°F outdoor ambient, set by the system's gas cooler control strategy.
R-744 cannot be serviced with standard HFC manifold gauges. R-744-rated equipment is purpose-built for the 1500-2000 PSI working pressure range.
›Why does R-744 operate transcritically?
Because the critical temperature is 87.8°F (CoolProp 7.2.0) — below most warm-climate condenser temperatures. When the heat-rejection medium (air or water at the condenser inlet) is warmer than 88°F, R-744 cannot condense on the high side. Instead, the system operates a transcritical cycle: the high side runs above the critical point as a supercritical fluid, cooled by a gas cooler, with a high-pressure throttle valve controlling the pressure rather than condensation [ashraehandbook].
›What's R-744's GWP?
1 by definition. The 100-year global warming potential scale is defined relative to CO₂, so CO₂'s GWP is exactly 1.0. All other refrigerants are measured against this reference. R-744 has the lowest possible GWP because it is the reference [ipccar5].
For practical comparison: R-744 at GWP 1 has 1/2088 the climate impact per kg of R-410A (GWP 2088), and 1/3922 the impact of R-404A (GWP 3922). Reducing refrigerant climate impact through R-744 adoption is the maximum possible reduction.
›What lubricant does R-744 use?
Application-dependent. POE is most common for HFC-cascade high-stage CO₂ systems. PAG (polyalkylene glycol) is used in some transcritical commercial refrigeration. PAO (polyalphaolefin) is used in some industrial applications. Mineral oil is not generally miscible with R-744 at typical operating temperatures.
Verify lubricant grade and viscosity against equipment OEM service literature — R-744 system designs vary significantly between manufacturers and applications.
›Is R-744 safe?
R-744 is ASHRAE class A1 — non-toxic at typical refrigeration concentrations and non-flammable [ashrae34]. The primary safety concern is not toxicity per se but asphyxiation: a large refrigerant release in a confined space can displace breathable air. ASHRAE 15 charge limits for R-744 in occupied spaces address this [ashrae15].
The high pressures present a mechanical safety concern: pressure-relief valve operation, hose ratings, and recovery cylinder ratings must all be appropriate for the 1500-2000 PSI working range. Standard HVAC pressure equipment is inadequate.
›Why is R-744 growing in supermarket refrigeration?
Three reasons converge. (1) GWP: R-404A's high GWP (3922) creates regulatory phase-down pressure under both EPA AIM Act and EU F-Gas; R-744 at GWP 1 has no future regulatory risk. (2) Single-refrigerant simplicity: R-744 transcritical booster systems handle both medium and low temperature loads with one refrigerant chain, simplifying equipment compared to traditional dual-refrigerant supermarket setups. (3) Operating cost: R-744 itself is essentially free (industrial CO₂ recovery byproduct); equipment depreciation and maintenance are higher but lifecycle costs favor R-744 for new supermarket installations.
EU adoption is ahead of US — most new European supermarkets install R-744 transcritical systems. US adoption is growing through 2024-2027 as AIM Act phase-down pressure on R-404A accelerates.
›Can R-744 be used in residential AC?
Technically possible but not commercially common. The high pressures require purpose-built equipment that is significantly more expensive than HFC residential AC. Heat-pump water heaters using R-744 (e.g., Sanden, ECO² Systems) are commercially available because the transcritical cycle's natural temperature glide on the high side is well-matched to water heating.
For residential AC specifically, the AIM Act 700-GWP threshold is met by HFCs like R-32 (675) and R-454B (466) at much lower equipment cost than R-744. R-744 residential AC is a niche option.
›What is dry ice?
Dry ice is solid CO₂ — R-744 below its triple-point pressure of 60.4 PSIA (4.16 bar) [nistwebbook]. At atmospheric pressure (14.7 PSIA), CO₂ cannot exist as liquid; it transitions directly from solid to gas (sublimes) at −109.3°F. This is why a CO₂ leak to atmospheric pressure produces a snow-like residue rather than a liquid puddle.
For refrigeration purposes, R-744 always operates above the triple-point pressure, so the solid phase is not encountered during normal operation.
Sources & citations
- [1]ASHRAE Standard 34-2022 — Designation and Safety Classification of Refrigerants
- [2]IPCC AR5 (2014) Working Group I, Chapter 8, Table 8.A.1 — GWP reference
- [3]CoolProp 7.2.0 (Bell, Wronski, Quoilin, Lemort 2014) — REFPROP-compatible Helmholtz EOS
- [4]International Institute of Refrigeration (IIR) — Natural refrigerants reference material
- [5]ASHRAE Standard 15-2022 — Safety Standard for Refrigeration Systems
- [6]ASHRAE Handbook of Refrigeration 2022 — CO₂ transcritical system design
- [7]EPA Significant New Alternatives Policy (SNAP) — R-744 acceptable for commercial refrigeration
- [8]NIST Chemistry WebBook — Carbon dioxide thermophysical properties (CAS 124-38-9)
- [9]AHRI Standard 700-2019 — Specifications for Refrigerants
- [10]Shecco / R744.com — Natural refrigerant market analysis