What Should R-744 (CO₂) Pressures Be?
R-744 (CO₂) operating pressures are unlike any other HVAC refrigerant — saturation at 70°F is 838 PSIG, and above critical temperature (88°F) there is no saturation pressure at all (transcritical operation). Subcritical pressures for cascade and low-temp; transcritical gas-cooler pressures for warm-ambient systems.
Saturation pressure ≠ operating pressure
The numbers below are operating pressures — what your manifold gauges read on a running system at a given outdoor ambient. Operating pressures depend on charge, ambient, indoor load, superheat, and subcooling. The R-744 saturation pressures are different — those are thermodynamic equilibrium values you can look up on the R-744 PT chart.
Operating pressure ranges
| Condition | Suction (low side) | Discharge (high side) | Superheat target | Subcooling target |
|---|---|---|---|---|
| Subcritical commercial refrigeration — medium-temp (cold ambient, 30°F evap) | 415–470 PSIG | 600–700 PSIG | 5–15°F | 3–10°F |
| Subcritical commercial refrigeration — low-temp (cold ambient, -20°F evap) | 215–260 PSIG | 600–700 PSIG | 5–15°F | 3–10°F |
| Subcritical operation (just below critical, 30°F evap) | 415–470 PSIG | 950–1100 PSIG | 5–15°F | 3–8°F |
| Transcritical operation — medium-temp (30°F evap, 95°F ambient) | 415–470 PSIG | 1300–1500 PSIG | 5–15°F | 0–0°F |
| Transcritical operation — medium-temp (30°F evap, 105°F ambient) | 415–470 PSIG | 1450–1650 PSIG | 5–15°F | 0–0°F |
| Cascade low-stage (CO₂ low side, -20°F evap, cascade condenser ~10°F) | 215–260 PSIG | 360–410 PSIG | 5–15°F | 3–10°F |
Source: ASHRAE Handbook of Refrigeration 2022 (CO₂ refrigeration); IIR Informatory Note on CO₂; Hillphoenix and other commercial CO₂ refrigeration OEM design literature; Carel and Danfoss control documentation for transcritical CO₂ booster systems
R-744 (CO₂) operates fundamentally differently from any other HVAC refrigerant. Three things matter for understanding R-744 pressures:
**1. Pressures are extremely high.** R-744 saturation pressure at 70°F is 838 PSIG — five to ten times higher than typical HFC refrigerants at the same temperature. Equipment must be rated for these pressures: typical R-744 system high-side pressure ratings are 1450-2000 PSIG (10-14 MPa). Standard HVAC manifold gauges (rated 500-800 PSI) are completely inadequate for R-744 service.
**2. Critical point at 87.8°F is the dividing line.** Below 87.8°F R-744 has a normal saturation curve and can condense to liquid (subcritical operation). Above 87.8°F there is no saturation — R-744 stays a supercritical fluid that cools through a "gas cooler" rather than condensing through a "condenser" (transcritical operation). In warm climates (most US summer ambient), R-744 systems operate transcritically.
**3. Transcritical operation requires high-side pressure control.** In subcritical operation, high-side pressure is determined by condenser exit temperature (just like any other refrigerant). In transcritical operation, there is no condensation — the high-side pressure is actively controlled by the system to optimize gas cooler outlet enthalpy for the ambient condition. Typical optimum is 1.2-1.5× the critical pressure (~1300-1700 PSIG). This is set by the system controller, not by ambient alone.
R-744 systems are common in commercial refrigeration (supermarket CO₂ booster systems for both medium- and low-temperature cases on a single refrigerant) and in heat pumps (especially water heating where the trans-critical cycle's high heat-rejection-side temperature glide matches the water-heating load well). Service requires R-744-specific training, equipment, and procedures.
R-744 saturation pressure quick reference
Saturation pressure at common service temperatures, from the verified PT dataset (CoolProp 7.2.0). Use this for quick mental cross-reference against your manifold readings — operating pressure on a running system varies around these saturation values based on charge, ambient, and load.
| Temperature | Saturation (PSIG) | PSIA | kPa gauge |
|---|---|---|---|
| -20°F | 200.2 | 214.9 | 1380 |
| 0°F | 291.0 | 305.7 | 2007 |
| 20°F | 407.2 | 421.9 | 2808 |
| 40°F | 552.9 | 567.6 | 3812 |
| 70°F | 838.1 | 852.8 | 5779 |
R-744 saturation curve over the service temperature range. Source: CoolProp 7.2.0 (REFPROP-compatible Helmholtz EOS), generated 2026-06-05.
Operating envelope across application conditions
Operating pressure ranges visualized — suction (blue) and discharge (red) bars at each application condition. Wider bars indicate larger variation expected; tighter bars indicate the operating point is more constrained.
R-744 property snapshot
| Safety class | A1 |
| Type | natural |
| GWP (IPCC AR5, 100-yr) | 1 |
| ODP | 0 |
| Normal boiling point | — |
| Critical temperature | 87.8°F |
| Critical pressure | 1055 PSIG |
| Temperature glide | 0.0°F |
| Lubricant compatibility | POE, PAG |
| AIM Act affected | No |
Real service scenarios for R-744
Three field scenarios showing common diagnostic patterns when reading R-744 system pressures. Each maps manifold readings to a verdict and specific service action.
Operating envelope and equipment context — R-744
R-744pressures sit inside an operating envelope bounded by the refrigerant's thermodynamic properties (saturation curve, critical point) and the equipment's pressure-rated components. Understanding both bounds tells you what pressure readings are normal versus what readings indicate a system fault.
- Saturation envelope: R-744 saturation pressure ranges from 200 PSIG at −20°F to — PSIG at 95°F. Critical temperature is 87.8°F — above this point no saturation state exists.
- Equipment pressure rating: R-744 critical pressure is 1055 PSIG. Per AHRI Standard 540-2020, the high-pressure cutout switch is typically set at approximately 85% of critical pressure to protect the compressor from running into the near-critical regime where small temperature swings produce large pressure excursions. For R-744, that's a practical cutout setpoint around 897 PSIG.
- Charging metric: R-744 is pure or near-azeotropic with minimal glide, so bubble ≡ dew on the saturation curve. Standard PT chart math applies without curve-selection concerns.
- Lubricant requirement: R-744 runs on POE / PAG lubricant. POE oil is hygroscopic — keep cylinder sealed, change filter-drier on every service visit, evacuate to ≤500 microns before recharging to remove residual moisture.
- Regulatory status: R-744 is not directly affected by the AIM Act. Service supply follows normal commodity dynamics.
Common R-744 measurement mistakes
- PSIG vs PSIA confusion. Service manifold gauges read PSIG; tables sometimes use PSIA. PSIA = PSIG + 14.696. Confusing the two shifts saturation lookups by ~5°F at low-side pressures.
- R-744 has minimal glide(pure refrigerant or near-azeotrope), so bubble ≡ dew on the saturation curve. Curve selection on the PT chart doesn't matter for R-744.
- Probing temperature without insulating. Ambient air pulls the reading toward room temperature, inflating apparent superheat or depressing apparent subcooling.
- Reading before steady state. Allow 10-20 minutes after compressor start for pressures and temperatures to stabilize.
- Treating saturation as operating. Saturation is the thermodynamic reference; operating pressure on a running system depends on charge, ambient, load, superheat, and subcooling.
- R-744 has a low critical temperature(87.8°F). Above this temperature there is no saturation state — for warm-ambient applications, transcritical operation or system shutdown applies. Look up the R-744 reference page for transcritical guidance.
When pressures fall outside R-744 normal range
Use the calculators on this site to convert your readings into superheat, subcooling, and diagnostic patterns:
- Superheat Calculator — suction PSIG + line °F → superheat for R-744.
- Subcooling Calculator — liquid PSIG + line °F → subcooling.
- Combined SH/SC/PT — both sides + pattern-matching diagnostic banner.
- System Pressure Diagnostic — multi-input diagnostic with approach temperatures.
- High head pressure causes — decision tree for high-side problems.
Diagnostic procedure
Step-by-step procedure to interpret R-744 pressure readings on a service call. Emitted as HowTo structured data for search-engine rich results.
1Confirm R-744-rated equipment before service
Standard HVAC manifold gauges, hoses, and recovery cylinders are not rated for R-744 pressures and will fail catastrophically if used. Verify equipment is rated for the working pressure (typically 1450-2000 PSIG for transcritical, 600-900 PSIG for subcritical). R-744-specific manifolds, hoses, and recovery equipment are required.
Tools: R-744-rated manifold gauge set, R-744-rated hoses, R-744 recovery cylinder
2Identify operating mode: subcritical or transcritical
Check the ambient temperature and system controller. Above ~85°F ambient most systems will be in transcritical operation; below ~75°F most systems will be in subcritical. Transcritical operation has no high-side saturation — the high-side temperature reflects gas cooler outlet, not condensing. Subcritical operation has saturation on both sides — measurement proceeds like other refrigerants.
Tools: Outdoor ambient measurement, Controller status display
3Read pressures with R-744-rated manifold
Service ports on R-744 systems are different from standard HVAC (typically high-pressure quick-couplings or threaded fittings). The system controller is usually the primary diagnostic interface — manifold readings supplement controller data. Verify steady-state operation; CO₂ systems can have rapid pressure swings during compressor cycling that don't reflect steady-state values.
Tools: R-744-rated manifold, System controller interface
4Subcritical mode: measure SH and SC normally
In subcritical operation R-744 has a normal saturation curve. Superheat = suction-line T − saturation T at suction pressure. Subcooling = saturation T at discharge pressure − liquid-line T. Standard procedures apply with R-744-specific PT chart values (the [PT calculator](/pt-calculator/) handles R-744 in its subcritical range, -40 to +87°F).
Tools: Contact temperature probe, R-744 PT chart or PT calculator
5Transcritical mode: superheat applies but no subcooling
In transcritical operation, the suction side still has saturation (evaporator operates at sub-critical temperature and pressure), so superheat measurement applies normally. The high side has no saturation — gas cooler outlet temperature and gas cooler outlet pressure are independent variables, not coupled through saturation. 'Subcooling' is not defined in transcritical operation. Optimum gas cooler outlet pressure for the ambient is what the controller manages — verify per OEM service literature.
Tools: Controller diagnostic display, OEM transcritical operation reference
Frequently asked
›Why does R-744 operate at such high pressures?
CO₂'s low critical temperature (87.8°F) and small molecular size produce very high vapor pressures at typical refrigeration temperatures. At 70°F R-744 saturation is 838 PSIG; at 95°F R-744 is in transcritical state with gas cooler pressures of 1300-1700 PSIG. This is the physics of the molecule — there's no way to reduce R-744 operating pressures without changing the application range. Equipment design accommodates the high pressures through thicker-walled components and higher-rated controls.
›What's the difference between subcritical and transcritical operation?
Below the critical temperature (87.8°F), R-744 can condense to liquid — subcritical operation, with a normal condenser and a saturation pressure-temperature relationship on the high side. Above the critical temperature, there is no condensation possible at any pressure — transcritical operation, with a 'gas cooler' instead of a condenser and high-side pressure controlled by a valve to optimize cycle efficiency. Most US commercial CO₂ systems operate transcritically during summer ambient and subcritically during winter.
›Can I use my regular HVAC service manifold on R-744?
No. Standard HVAC manifolds rated 500-800 PSI will fail catastrophically at R-744 pressures (838 PSIG saturation at 70°F, 1300-1700 PSIG transcritical operation). R-744-rated manifolds (typically 2000+ PSI working pressure) are mandatory. Same for hoses and recovery cylinders — R-744 has dedicated equipment that is not interchangeable with HFC service equipment.
›What is gas cooler pressure and why is it controlled?
In transcritical operation, the 'condenser' is replaced by a 'gas cooler' — heat is rejected from the supercritical refrigerant without phase change. The outlet temperature is set by the air-side conditions, but the outlet pressure is independent and can be optimized for cycle efficiency. The optimum pressure varies with ambient and gas cooler exit temperature; typical values are 72-90 bar (1050-1300 PSIG) for moderate ambient and 95-115 bar (1380-1670 PSIG) for warm ambient. System controllers manage this via a high-pressure expansion valve.
›Is R-744 used in residential AC?
Rarely in cooling-only residential AC in the US (the equipment cost is high and the transcritical operation in hot ambients reduces efficiency vs HFC alternatives). Common in residential heat pumps for water heating (heat pump water heaters with CO₂) and in some EU residential heat pumps, where the transcritical cycle's high temperature glide is well-matched to water heating. The dominant US application is commercial refrigeration (supermarket CO₂ booster systems).
›What's the GWP of R-744?
1 (defined as 1 — CO₂ is the reference for GWP). R-744 has by definition the lowest GWP of any commercial refrigerant. It's the long-term destination for many applications facing aggressive GWP phase-down — the trade-off is equipment cost (high-pressure components) and efficiency in warm climates (transcritical operation has lower efficiency than subcritical at the same temperatures).
›Why does subcooling appear as 0 in the transcritical operating ranges?
Subcooling requires a saturation temperature at the discharge pressure to subtract from the liquid-line temperature. In transcritical operation there is no saturation on the high side — no condensation occurs, the refrigerant stays in supercritical state throughout the gas cooler. 'Subcooling' as a concept doesn't apply. Gas cooler outlet temperature is the relevant high-side measurement, evaluated against the manufacturer's optimal operating envelope.
R-744 full reference
Saturation chart, properties, retrofit guidance.
Superheat Calculator
Suction PSIG + line °F → superheat.
Subcooling Calculator
Liquid PSIG + line °F → subcooling.