The HVAC industry is mid-transition. R-410A, with a global warming potential (GWP) of 2,088, is being phased down under the EPA's AIM Act, and A2L refrigerants like R-32 (GWP 675) and R-454B (GWP 466) are the immediate replacements in residential equipment. But A2Ls are a bridge, not a destination. The real endgame is natural refrigerants — substances that exist in nature, have GWP at or near zero, and eliminate the synthetic fluorinated chemistry entirely.
The EPA's Significant New Alternatives Policy (SNAP) program, established under the Clean Air Act, evaluates refrigerant substitutes based on ozone depletion potential, GWP, toxicity, flammability, and exposure risk ([EPA SNAP](https://www.epa.gov/snap)). NIST's review of low-GWP options identifies three natural refrigerants as the leading candidates: carbon dioxide (R-744), propane (R-290), and ammonia (R-717), all with GWP values at or below 1 ([NIST](https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=932668)).
Each comes with a distinct set of trade-offs. Here's how they compare.
R-744 (CO2) — the safe, high-pressure option
Carbon dioxide is classified as A1 under ASHRAE Standard 34 — non-toxic and non-flammable — with a GWP of 1 ([Colmac Coil Refrigerant Guide, citing ASHRAE Std. 34](https://www.colmaccoil.com/media/394090/refrigerant-guide-final.pdf)). That safety profile is its biggest advantage: R-744 can be used in occupied spaces where flammable or toxic refrigerants are restricted.
The trade-off is pressure. CO2 has a critical temperature of only 31.1°C (88°F), meaning that above that ambient temperature it operates in a transcritical cycle where it cannot condense. This requires specialized components rated for pressures up to 7.38 MPa (1,070 psi) at the critical point — roughly five times the operating pressure of R-410A systems ([NIST](https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=932668)). That drives up equipment cost and service complexity.
Where R-744 shines is water heating and cold-climate performance. Peer-reviewed research shows transcritical CO2 heat pumps can achieve a COP of 3.1 at -20°C outdoor temperature and deliver hot water up to 90°C (194°F), with a combined COP of 8.0 when providing simultaneous heating and cooling ([MDPI, Recent Advances in Transcritical CO2 Heat Pump Systems](https://www.mdpi.com/1996-1073/12/3/457)). Japan's EcoCute program has deployed millions of CO2 heat pump water heaters nationally. The weakness is cooling mode efficiency in hot climates, where the transcritical cycle loses ground to conventional refrigerants.
R-290 (propane) — the efficient, flammable up-and-comer
Propane has a GWP of approximately 3 and excellent thermodynamic properties — high latent heat, low compression ratios, and compatibility with standard compressor oils. ASHRAE classifies it as A3: higher flammability, which is the main barrier to residential adoption in the U.S. ([Colmac Coil Refrigerant Guide](https://www.colmaccoil.com/media/394090/refrigerant-guide-final.pdf)).
That barrier is eroding in Europe. In March 2026, a major manufacturer announced a complete R-290 product range covering 17 to 2,000 kW across monobloc and modular heat pump configurations, delivering leaving water temperatures up to 75°C ([Daikin Europe](https://www.daikin.eu/en_us/press-releases/daikin-drives-decarbonisation-and-safety-forward.html)). In August 2026, Italian manufacturer Rhoss launched R-290 heat pumps for multi-family residential applications with heating capacities up to 64.7 kW and operating ranges from -20°C to 40°C ([PV Magazine](https://www.pv-magazine.com/2026/08/11/rhoss-launches-propane-heat-pumps-for-multi-family-residential-applications/)).
The key engineering challenge is charge size. Because propane is flammable, building and safety codes limit the allowable refrigerant charge in indoor spaces. Monobloc designs — where the refrigerant circuit is entirely outdoors and only water circulates inside — are the primary way manufacturers are solving this. R-290 systems are not yet common in U.S. residential HVAC, but the European product pipeline suggests they will arrive within the next few years.
Why charge limits matter.ASHRAE Standard 15 and international safety codes cap the refrigerant charge based on room volume and flammability class. For an A3 refrigerant like propane, the allowable charge in a typical residential room is small — often under 150 grams for direct-expansion indoor coils. Monobloc heat pumps sidestep this by keeping the entire refrigerant charge outdoors and delivering heated or cooled water via a hydronic loop.
R-717 (ammonia) — the industrial workhorse
Ammonia has a GWP of zero and superior thermodynamic efficiency, but it is classified B2 under ASHRAE Standard 34 — toxic and lower flammability ([Colmac Coil Refrigerant Guide](https://www.colmaccoil.com/media/394090/refrigerant-guide-final.pdf)). Even small leaks can be hazardous to occupants, which is why R-717 is effectively restricted to industrial and large commercial applications with dedicated refrigeration rooms and trained operators.
The EPA notes that the majority of cold storage warehouses in the United States already use ammonia as their primary refrigerant ([EPA SNAP Refrigeration and Air Conditioning](https://www.epa.gov/snap/refrigeration-and-air-conditioning)). For residential or light commercial HVAC, R-717 is not on the table — but it remains the benchmark for industrial refrigeration efficiency.
How they stack up
| Refrigerant | GWP | Safety class | Best fit | Key barrier |
|---|---|---|---|---|
| R-744 (CO2) | 1 | A1 (safe) | Hot water, cold-climate heat pumps | High pressure (7.38 MPa), cost |
| R-290 (propane) | ~3 | A3 (flammable) | Residential and commercial heat pumps | Charge limits, code adoption |
| R-717 (ammonia) | 0 | B2 (toxic) | Industrial refrigeration | Toxicity, trained operators only |
| R-410A (baseline) | 2,088 | A1 (safe) | Being phased down | High GWP — AIM Act target |
| R-32 (A2L bridge) | 675 | A2L (lower flam.) | Current residential transition | Still synthetic, GWP too high for long-term |
What this means for homeowners and contractors
If you're buying a residential heat pump or AC today in the U.S., you'll get an A2L system — most likely R-32 or R-454B. That's the right choice for now: the equipment is available, serviced by existing technicians, and meets current code. But the industry's trajectory is clear. The EPA's SNAP program and AIM Act are steadily tightening the screws on high-GWP refrigerants, and European manufacturers are already proving that R-290 and R-744 systems work at commercial scale ([EPA SNAP](https://www.epa.gov/snap)).
The practical timeline: R-290 monobloc heat pumps are likely to enter the U.S. residential market within the next 2-3 years as safety codes catch up. CO2 systems are already available for commercial hot water and cold-climate applications but are unlikely to displace A2Ls in standard residential split systems due to cost and pressure complexity. Ammonia will remain industrial-only.
Not a reason to wait.If your system needs replacement now, an A2L heat pump is a sound investment. Waiting for natural refrigerant equipment to arrive in the U.S. market means losing another season of efficiency savings and potential rebate eligibility. The best system is the one that's installed, running, and properly maintained.
The bottom line
Natural refrigerants are the endgame for HVAC's environmental footprint. R-744, R-290, and R-717 all have GWP at or near zero, and each has a clear lane where it outperforms synthetic alternatives. The barriers — pressure for CO2, flammability for propane, toxicity for ammonia — are engineering problems, not fundamental flaws. European manufacturers are already solving them at commercial scale, and U.S. code bodies are following.
Naming specific manufacturers or products in this article is for factual reporting only — not an endorsement. HVAC Zone Inc is a multi-brand dealer with no preference for any manufacturer's refrigerant strategy.
Wondering which refrigerant strategy makes sense for your building or project? Request a consultation — we'll evaluate your site, your existing equipment, and your efficiency goals to recommend the right system for your situation.