R-744 vs R-290: Two Natural Refrigerants, Different Trade-offs
Two natural refrigerants with very low GWP (R-744 = 1, R-290 = 3). R-744 (CO₂) requires very high-pressure equipment (838 PSIG at 70°F) with transcritical operation in warm climates — non-flammable. R-290 (propane) operates at conventional pressures (110 PSIG at 70°F) but is A3 highly flammable with strict charge limits.
PT curves, overlaid
Both refrigerants are pure or near-azeotropic — single curve per series.
Pressure comparison at service temperatures
Side-by-side pressure values at common service temperatures, computed from CoolProp 7.2.0. Useful for retrofit feasibility — pressure deltas within ±20% typically allow drop-in compatible service equipment; larger deltas require component pressure-rating review.
| Temperature | R-744 | R-290 | Δ vs R-744 |
|---|---|---|---|
| -20°F | 200 PSIG | 11 PSIG | -94.6% |
| 0°F | 291 PSIG | 24 PSIG | -91.9% |
| 40°F | 553 PSIG | 64 PSIG | -88.4% |
| 70°F | 838 PSIG | 110 PSIG | -86.9% |
Pressure delta visualization: positive = R-290 runs higher than R-744; negative = lower. Service equipment pressure rating matters when delta exceeds ±20% on the discharge side. For R-744 (zeotropic blend) bubble pressure is shown; for R-290 same rule applies.
Property differences side by side
- Safety class change: R-744 (A1) → R-290 (A3). Same toxicity class, different flammability characteristics.
- GWP impact: R-744 = 1, R-290 = 3 (+200% vs R-744). Switching increases direct climate impact.
- Lubricant: R-744: POE/PAG; R-290: MO/AB/PAO. Lubricant systems differ; check compatibility per manufacturer.
Properties side by side
| Property | R-744 | R-290 |
|---|---|---|
| Type | natural | hc |
| ASHRAE class | A1 | A3 |
| Composition | Pure | Pure |
| GWP (AR5) | 1 | 3 |
| ODP | 0 | 0 |
| Lubricant | POE, PAG | MO, AB, PAO |
| Boiling point @ 1 atm | — | -42.1°C |
| Critical point | 31.0°C / 1055 PSIG | 96.7°C / 602 PSIG |
| Temp glide | 0.00°F | 0.00°F |
| AIM Act affected | No | No |
Choose R-744 if…
Commercial refrigeration at scale where the equipment cost of high-pressure CO₂ infrastructure is justified — supermarket booster systems, district refrigeration, large industrial cold storage. R-744 eliminates flammability concerns and clears regulatory phase-down hurdles permanently (GWP 1, no plausible future restriction).
Choose R-290 if…
Small heat pumps and commercial refrigeration where charge can be kept within A3 hydrocarbon limits (typically <150g for residential, larger for commercial with engineered controls). R-290 uses conventional pressure-range equipment and standard HVAC service practices adapted for hydrocarbon handling. Dominant in EU residential heat pumps and many small commercial refrigeration units.
When neither is ideal
For applications too small for R-744 economics but too large for R-290 charge limits, A2L HFC/HFO blends remain the practical choice (R-32, R-454B for residential AC; R-454C, R-455A for commercial refrigeration). For ultra-low temperature industrial applications, R-717 (ammonia) covers the largest installed base. Naturals are growing but the right application fit matters significantly.
Retrofit and transition
Neither R-744 nor R-290 is typically a retrofit refrigerant. Both require purpose-designed equipment due to their unique handling requirements. The decision between them is usually made for new installations.
**R-744 (CO₂) — design considerations:**
- **Very high pressures:** Standard high-side rating of 1450-2000 PSIG for transcritical systems; subcritical systems typically rated 600-900 PSIG. Standard HVAC manifold gauges (500-800 PSI) will fail catastrophically with R-744. - **Transcritical operation:** Above 87.8°F (CO₂'s critical temperature), no condensation occurs. The "condenser" becomes a "gas cooler" and high-side pressure is controlled by a valve to optimize efficiency rather than being set by ambient. - **Cost:** Equipment is 30-100% more expensive than HFC alternatives for the same capacity; refrigerant itself is essentially free. - **Application fit:** Supermarket commercial refrigeration (booster systems handling both medium- and low-temperature cases on one refrigerant), CO₂ heat pumps for water heating, large industrial refrigeration with high-pressure infrastructure already in place.
**R-290 (propane) — design considerations:**
- **Conventional pressures:** Standard HVAC manifold gauges (500-800 PSI) handle R-290 without modification. - **A3 highly flammable:** Hydrocarbon-rated equipment design required — sealed compressor compartments, intrinsically safe controls, ventilation interlocks. Brazing requires nitrogen purge AND combustible refrigerant displacement protocols. - **Charge limits:** Per IEC 60335-2-40, residential applications limited to small charges (typically <150g, expanding to <500g under recent revisions for some configurations). Commercial refrigeration per IEC 60335-2-89 allows larger charges in sealed equipment. Industrial applications can use much larger charges with engineered controls. - **Application fit:** Residential heat pumps (especially EU market with established R-290 equipment ecosystem), small commercial refrigeration (vending coolers, ice machines, beverage merchandisers), and some industrial process refrigeration where flammability can be managed through engineering controls.
**Choice framework for new equipment:**
- **Small residential heat pump (<5 kW):** R-290 is the dominant natural choice. Established equipment ecosystem in EU, growing in US. - **Commercial refrigeration display case (single):** R-290 is typical for self-contained refrigerated cases with sealed refrigerant circuit. - **Supermarket-scale refrigeration:** R-744 booster systems are the modern standard for new installations. R-290 distributed systems are an alternative. - **Large industrial refrigeration:** R-717 (ammonia) often outcompetes both R-744 and R-290 for capacity and efficiency. Naturals competition rather than HFC competition.
Regulatory and transition context
Both refrigerants sit in an active regulatory transition driven by climate-impact rules. The transitions affect availability, pricing, and new-equipment specification.
- EPA AIM Act (40 CFR Part 84): US HFC production / import phase-down. Cap declines from 90% allocation (2022) to 15% by 2036. Neither refrigerant is directly affected.
- EU F-Gas Regulation (517/2014, updated 2024/573): European stationary refrigeration GWP cap typically 150 (much tighter than AIM Act). Drives earlier adoption of very-low-GWP options in European markets.
- Kigali Amendment to Montreal Protocol (2016): international HFC phase-down framework (198 countries). The AIM Act and EU F-Gas are regional implementations. Schedules differ by country group.
- ASHRAE 34-2022: safety classification (A1, A2L, A3, B1, B2L). For A2L refrigerants like R-32, R-454B, R-454C, R-455A: equipment must be A2L-certified, charge limits per IEC 60335-2-40 apply.
Standard transition procedure — R-744 → R-290
Step-by-step service procedure for transitioning an existing R-744 system to R-290, derived from the property differences above. Always cross-check equipment OEM service literature for the specific equipment being serviced. The steps below codify EPA Section 608 requirements (recovery, evacuation, documentation) plus refrigerant-specific accommodations for lubricant, safety class, pressure envelope, and glide differences. Skipping any of the regulatory steps (leak check, recovery, evacuation, documentation) creates compliance liability; skipping refrigerant-specific accommodations creates equipment-failure risk.
- EPA Section 608 leak-check first.Verify the existing system isn't leaking before any work. If it's leaking, find and repair the leak — adding refrigerant (existing or new) to a leaking system violates 40 CFR Part 82.
- Recover R-744. Use a recovery machine rated for A1refrigerants. Recover into properly-labeled cylinders; don't mix recovered R-744 with virgin or recovered R-290 (cross-contamination invalidates reclaim).
- Drain POE lubricant and flush. R-744 runs on POE/PAG; R-290 requires MO/AB/PAO. Drain the compressor crankcase, accumulator, and any oil traps. Flush the system with a compatible flush solvent or run MO lubricant through the system and re-drain to clear residual POE. Mixing mineral oil with POE in an HFC system produces oil-return failures within hours of operation.
- Replace filter-drier. Install a new drier rated for R-290 (MOlubricant). Filter-driers are single-use after exposure to a refrigerant; the old drier may have absorbed contaminants you don't want carrying into the new charge.
- Pressure-test and evacuate to ≤500 microns. Pressure-test with dry nitrogen to verify no leaks. Pull deep vacuum and hold ≥30 minutes with vacuum pump isolated to confirm no leak-back. This step is non-negotiable — non-condensables (air, moisture) trapped in the system raise discharge pressure and damage the compressor.
- Charge R-290 by weight to nameplate. Use a calibrated recovery / charging scale. Charging by gauge feel produces frequent overcharge errors.
- Verify with SH and SC at steady state. R-290 has minimal glide (pure or near-azeotrope), so the bubble = dew curve and standard PT chart math applies. Target SC = 8-12°F for TXV systems; target SH per OEM nameplate.
- Document and label. Update the equipment data plate to reflect R-290. EPA Section 608 requires records of refrigerant added / recovered; OEM warranty may require documentation of approved-refrigerant substitution.
Lifecycle and operational context
Beyond the per-service-call decision, the R-744 ↔ R-290 choice sits inside a broader regulatory and lifecycle context. The transition direction (which is the predecessor, which is the successor) is driven by climate policy and the AIM Act phase-down, not technical preference alone.
- GWP profile: R-744 = 1 GWP (AR5); R-290 = 3 GWP. Switching from R-744 to R-290 increases direct refrigerant climate impact by 200%.
- AIM Act exposure: Neither refrigerant is directly affected by the AIM Act phase-down. Other regional regulations (EU F-Gas, Kigali signatory implementations) may still apply.
- EU F-Gas Regulation: Both refrigerants are below the EU F-Gas 150 GWP cap — compliant for European stationary refrigeration.
- Service supply outlook: Neither refrigerant faces near-term supply constraints from US AIM Act phase-down. Pricing follows normal commodity dynamics.
- TEWI / LCCP framing: Total Equivalent Warming Impact accounts for both direct refrigerant emissions (leakage, end-of-life) and indirect emissions from equipment energy consumption. For HVAC equipment with ≤5% annual leak rate, indirect emissions typically dominate TEWI by 80-90% — meaning equipment efficiency matters more than refrigerant GWP for total climate impact. For commercial refrigeration with higher leak rates, the balance can tip toward favoring low-GWP refrigerants.
Regulatory sources: EPA AIM Act (40 CFR Part 84), EU F-Gas Regulation 517/2014 and update 2024/573, Kigali Amendment to the Montreal Protocol (2016), Japan Fluorocarbon Emissions Control Law. GWP values per IPCC AR5 (2013) WG-I Table 8.A.1.
Service implications — R-744 → R-290
What a service technician needs to know when transitioning from R-744to R-290 (or comparing them for new equipment specification). Two real-world scenarios show how the difference plays out in practice.
Pressure envelope check for R-744 → R-290
Scenario · Field tech needs to know: do R-744 service tools handle R-290, or does the pressure delta require new equipment? PT chart comparison at service temperatures gives the answer.
| Temp | R-744 | R-290 | Δ |
|---|---|---|---|
| 40°F | 553 PSIG | 64 PSIG | -88.4% |
| 70°F | 838 PSIG | 110 PSIG | -86.9% |
Service-side implications: lubricant and safety
Scenario · Beyond pressure envelope, the switch from R-744 to R-290 affects lubricant, safety class, and operating procedure.
| Concern | R-744 | R-290 | Action |
|---|---|---|---|
| Lubricant | POE/PAG | MO/AB/PAO | Oil change required |
| Safety class | A1 | A3 | Same toxicity, different flammability |
| Glide | 0.0°F | 0.0°F | Minor |
When to use which tool for this comparison
- R-744 full reference — properties, PT chart, lubricant, retrofit options for R-744.
- R-290 full reference — properties, PT chart, lubricant, retrofit options for R-290.
- PT Comparison Tool — overlay any 2-4 refrigerants' PT curves interactively.
- Retrofit Compatibility Calculator — five-criterion compatibility analysis with verdict.
- Refrigerant Comparison Guide — long-form sourced reference for all common HVAC refrigerant comparisons.
Frequently asked
›Why are R-744 and R-290 both called 'natural refrigerants'?
Both are substances that occur in nature in their refrigerant form, distinguishing them from manufactured fluorocarbons (HFCs, HFOs). R-744 is CO₂ — directly recovered from industrial sources or atmospheric capture. R-290 is propane — extracted from natural gas or petroleum refining. Both have very low GWP (R-744 = 1 by definition, R-290 = 3) and zero ODP. The 'natural' designation positions them as long-term-viable alternatives to manufactured HFCs facing climate phase-down.
›Why does R-744 require such high pressures?
Physics of the molecule. CO₂'s small molecular size and high vapor pressure mean saturation pressures are very high at typical refrigeration temperatures: 838 PSIG at 70°F, 1280 PSIG at the critical temperature (87.8°F). There's no way to reduce R-744 operating pressures without changing the application range — equipment design accommodates this through thicker-walled components rated for the high pressures.
›What does 'transcritical' mean for R-744?
Above CO₂'s critical temperature (87.8°F), there is no phase distinction between liquid and vapor — CO₂ becomes a single supercritical fluid. The 'condenser' in a normal refrigeration cycle is replaced by a 'gas cooler' that removes heat from the supercritical fluid without phase change. The high-side pressure is no longer set by ambient (as it would be in subcritical condensation) but is actively controlled by a valve to optimize cycle efficiency. Most US commercial CO₂ systems operate transcritically during summer and subcritically during winter.
›Why does R-290 have a charge limit?
A3 flammability. Propane in air at concentrations of approximately 2-10% by volume is flammable; outside that range it's too lean (under-fueled) or too rich (over-fueled) to burn. Charge limits in IEC 60335-2-40 ensure that even total refrigerant release into the smallest enclosed space served by the equipment can't reach the lower flammability limit. The recent revision (2024) allows somewhat larger charges (up to ~500g) for residential equipment with appropriate engineering controls; commercial refrigeration limits per IEC 60335-2-89 allow larger charges in sealed equipment.
›Which is more efficient, R-744 or R-290?
Application-dependent. R-290 is more efficient than R-744 across most typical operating ranges (warm ambient, medium-temperature operation) — its thermodynamic properties (high COP, conventional pressure envelope) outperform R-744's transcritical operation in warm climates. R-744 is competitive or superior in cold ambient (Northern climates), for heat pump water heating (where the transcritical cycle's high heat-rejection temperature glide matches water heating well), and for low-temperature commercial refrigeration where the higher discharge temperatures of R-744 are tolerable. The efficiency comparison varies dramatically with application; published COP studies for specific applications are the authoritative source.
›Can R-744 or R-290 be retrofitted into existing HFC equipment?
Generally no. R-744 requires pressure ratings that legacy HFC equipment doesn't have; retrofit would risk catastrophic component failure. R-290 requires intrinsically safe electrical components in the refrigerant space and ventilation interlocks that legacy A1 equipment doesn't have; retrofit would create a flammable-atmosphere safety violation. Both naturals require purpose-designed equipment from the start.
›Are R-744 and R-290 affected by AIM Act or EU F-Gas?
Not directly. AIM Act targets HFCs; EU F-Gas targets HFCs and PFCs. Neither legislation phases down R-744 (a basic chemical) or R-290 (a hydrocarbon). The regulatory situation: zero phase-down risk for either natural refrigerant. The practical limits on adoption are equipment capital cost (R-744) and charge/safety limitations (R-290), not regulatory pressure on the refrigerant itself.
R-744 full reference
PT chart, properties, retrofit guidance.
R-290 full reference
PT chart, properties, retrofit guidance.