What Pressure Should R-744 (CO₂) Be?
Updated October 1, 2026
R-744 A1 Lower toxicity, no flame propagation
You are on a CO₂ system — a booster rack, a cascade low stage, or a transcritical gas cooler — and the gauges read several times higher than any HFC you have worked on. Nothing about the numbers is familiar.
Bring HFC instincts to CO₂ and you will misjudge both the equipment rating and the high side, which above the critical point is not a saturation pressure at all. At these pressures, that is a safety issue, not just a service one.
This page gives CO₂ suction for coolers and freezers, the subcritical high-side pressure, the critical point from the dataset, and standstill pressure — and it stops where the physics does, above the critical temperature.
R-744 (CO₂) suction runs about 420–462 psig at a 25°F cooler coil and 189–212 psig at a −20°F freezer coil — several times higher than an HFC. Below its 87.8°F critical temperature the high side is a saturation pressure; above it the system is transcritical and the high-pressure control valve, not a chart, sets the gas-cooler pressure.
CO₂ suction pressure for coolers and freezers
Suction is set by the evaporator coil temperature, read on the dew curve at the AHRI 1250-2020 walk-in points — a 25°F cooler coil and a −20°F freezer coil. The pressures are high, but the logic is the same as any other refrigerant: the box sets suction, not the weather.
| Application | Suction (psig / kPa) |
|---|---|
| Walk-in cooler (25°F coil) | 420–462 psig 2,898–3,183 kPa |
| Walk-in freezer (−20°F coil) | 189–212 psig 1,300–1,464 kPa |
The high side below and above the critical point
Below the critical temperature CO₂ condenses, so the high side is a saturation (condensing) pressure — the table gives it from 40 to 85°F. As the condensing temperature approaches the critical point the pressure rises steeply.
| Condensing temperature | High side (psig / kPa) |
|---|---|
| 40°F 4.4°C | 553 psig 3,812 kPa |
| 45°F 7.2°C | 594 psig 4,099 kPa |
| 50°F 10.0°C | 638 psig 4,401 kPa |
| 55°F 12.8°C | 684 psig 4,719 kPa |
| 60°F 15.6°C | 733 psig 5,054 kPa |
| 65°F 18.3°C | 784 psig 5,407 kPa |
| 70°F 21.1°C | 838 psig 5,779 kPa |
| 75°F 23.9°C | 895 psig 6,170 kPa |
| 80°F 26.7°C | 955 psig 6,584 kPa |
| 85°F 29.4°C | 1,018 psig 7,021 kPa |
Critical point (from the dataset): 87.8°F / 1,055 psig. Above 87.8°F there is no saturation pressure — operation is transcritical and the high-pressure control valve, not a saturation temperature, sets the gas-cooler pressure. There is no single “normal” transcritical number.
Standstill pressure
Once the system has been off long enough for the pressures to equalize, CO₂ reads its saturation pressure at the temperature of the coolest part of the system — as long as that temperature is below the critical point. This is why CO₂ systems need a high standstill pressure rating, or a way to hold the pressure down during a long shutdown.
| Off temperature | Standstill (psig / kPa) |
|---|---|
| 65°F 18.3°C | 784 psig 5,407 kPa |
| 70°F 21.1°C | 838 psig 5,779 kPa |
| 75°F 23.9°C | 895 psig 6,170 kPa |
| 80°F 26.7°C | 955 psig 6,584 kPa |
| 85°F 29.4°C | 1,018 psig 7,021 kPa |
Above the critical temperature the fluid is supercritical, so there is no standstill saturation pressure to read.
Frequently asked questions
What is normal R-744 suction pressure?
About 420–462 psig at a 25°F cooler coil and 189–212 psig at a −20°F freezer coil, read on the dew curve. That is several times an HFC's suction, which is normal for CO₂ — pair the reading with the box temperature before judging it.
What is CO₂ standstill pressure at 70°F?
About 838 psig — with the system off at 70°F, CO₂ sits at its 70°F saturation pressure. Standstill climbs quickly with temperature and only exists below the 87.8°F critical temperature; above that the fluid is supercritical and there is no standstill saturation pressure.
Why can't a PT chart give the CO₂ high side above 87.8°F?
Because 87.8°F is CO₂'s critical temperature (1,055 psig). Above it there is no liquid–vapor boundary, so there is no saturation pressure to look up. The system runs transcritical and the high-pressure control valve sets the gas-cooler pressure for efficiency — there is no single "normal" number to read from a chart.
How these numbers are calculated
Every pressure here is R-744's saturation pressure at the stated temperature, calculated with CoolProp 7.2.0. Suction (low side) uses the saturated-vapor (dew) pressure; head (high side) uses the saturated-liquid (bubble) pressure.
CO₂ suction uses the AHRI 1250-2020 walk-in coil temperatures (25°F cooler, −20°F freezer). Below the critical temperature the high side is a saturation (condensing) pressure; above it the system is transcritical and the high-pressure control valve sets the gas-cooler pressure.
Sources: CoolProp 7.2.0; AHRI 1250-2020 (walk-in ratings). Your equipment maker's charging chart or the unit data plate overrides these reference values for that specific system.