R-744 (CO2)
R-744 is carbon dioxide used as a refrigerant. Its GWP of 1 is the reference value every other refrigerant is measured against, so it passes every federal and state GWP limit, including the 150 limits that arrive for large new systems on January 1, 2032 (91 FR 31284)[2].
CO2 equipment is its own world: operating pressures several times higher than HFC systems, dedicated components, and transcritical operation on warm days. The walk-in lines documented here rate CO2 unit coolers alongside their HFC and A2L siblings[1].
R-744 (CO2) quick facts
| Type | Natural refrigerant: carbon dioxide, single compound |
| ASHRAE safety class | A1 (no flame propagation) |
| GWP (AIM Act exchange values) | 1 (the reference gas) |
| Critical point | 87.8 °F at about 1,055 psig, above which systems run transcritical |
| Temperature glide | None (single compound) |
| Common use | Supermarket racks, cold storage, walk-in systems built for CO2 |
Rules
Federal
CO2 is under every GWP limit in the Technology Transitions rule, so each compliance date that closes a subsector to high-GWP blends opens it further to CO2 (91 FR 31284)[2]. The 2032 limits for supermarket systems, remote condensing units, and cold storage make CO2 one of the few field-piped options for large systems.
| Subsector | Limit | Date | R-744 (CO2) outcome |
|---|---|---|---|
| Supermarket systems, remote condensing units, cold storage | GWP 150/300 from Jan 1, 2032 | Jan 1, 2032[2] | CO2 qualifies everywhere |
State
No state HFC rule restricts CO2, since state programs target high-GWP substances. Pressure-vessel and mechanical codes govern CO2 systems. Verify with your local authority having jurisdiction before an install decision.
Buying R-744 (CO2)
Refrigerant-grade CO2 (R-744) is not restricted under 40 CFR 82.154. The Section 608 sales restriction covers ozone-depleting substances and non-exempt substitutes, and CO2 is an exempt substitute in its listed end-uses[3]. Buy refrigerant grade, not industrial or beverage grade. Moisture specs differ and CO2 systems are unforgiving about water.
Safety and handling
CO2 is nonflammable but runs at pressures far above HFC practice: standstill pressure in a warm room can exceed 1,000 psig. Components, hoses, and gauges must be rated for CO2 service. In confined spaces a large leak displaces oxygen, which is why machine rooms use CO2 monitors.
Replacements
CO2 is a destination refrigerant for new systems rather than a service alternative for existing ones. An HFC system cannot be converted to CO2. The transition happens at equipment replacement[4].
R-744 (CO2) PT chart
Saturation pressures for CO2 up to 85 °F, computed from reference equations of state[4]. The table stops below the critical point at 87.8 °F: above it, liquid and vapor stop being distinct phases and saturation pressure stops existing. Transcritical systems control gas-cooler pressure instead of condensing pressure on warm days.
| Temp (°F) | Liquid / bubble (psig) | Vapor / dew (psig) |
|---|---|---|
| -40 | 131.0 | 131.0 |
| -35 | 146.5 | 146.5 |
| -30 | 163.1 | 163.1 |
| -25 | 181.0 | 181.0 |
| -20 | 200.2 | 200.2 |
| -15 | 220.8 | 220.8 |
| -10 | 242.7 | 242.7 |
| -5 | 266.1 | 266.1 |
| 0 | 291.0 | 291.0 |
| 5 | 317.6 | 317.6 |
| 10 | 345.7 | 345.7 |
| 15 | 375.6 | 375.6 |
| 20 | 407.2 | 407.2 |
| 25 | 440.7 | 440.7 |
| 30 | 476.1 | 476.1 |
| 35 | 513.4 | 513.4 |
| 40 | 552.9 | 552.9 |
| 45 | 594.5 | 594.5 |
| 50 | 638.3 | 638.3 |
| 55 | 684.4 | 684.4 |
| 60 | 733.1 | 733.1 |
| 65 | 784.2 | 784.2 |
| 70 | 838.1 | 838.1 |
| 75 | 894.9 | 894.9 |
| 80 | 954.9 | 954.9 |
| 85 | 1018.4 | 1018.4 |
Use the vapor (dew) column to check superheat at the suction line. Use the liquid (bubble) column to check subcooling at the liquid line and for charging. CO2 is a single compound with no glide. Above 87.8 °F the system runs transcritical and this table no longer applies. R-744 (CO2) values are computed, not transcribed; see Sources.
Equipment rated for R-744 (CO2)
Walk-in unit cooler models whose maker publishes CO2 ratings. They are dedicated CO2 unit coolers, and their capacity tables behave differently from the HFC models, which is visible on each model page.
720 models in the database carry R-744 (CO2) ratings. A sample:
Questions
- Why does the CO2 PT chart stop at 85 F?
CO2's critical point is 87.8 F. Above it there is no boiling line, no distinct liquid and vapor, and no saturation pressure to tabulate. Systems rejecting heat above that temperature run transcritical and control gas-cooler pressure directly.
- Do I need a 608 certification to buy R-744?
No. CO2 is an exempt substitute under 40 CFR 82.154, so the federal sales restriction does not apply. Refrigerant-grade purity still matters. Industrial CO2 can carry too much moisture.
- Can I convert my R-404A rack to CO2?
No. CO2 operates at several times the pressure of an R-404A system, and vessels, piping, valves, and controls are all different. CO2 arrives with new equipment, not through a retrofit.
- Is CO2 refrigeration efficient?
In cool and moderate climates, competitive to better, especially with parallel compression and ejectors. In hot climates transcritical operation costs efficiency, which newer system designs work to claw back. The capacity tables on the CO2 model pages show the temperature dependence directly.
References
Updated