R-744
Also known as: R744, CO2, Carbon Dioxide, Transcritical CO2
Natural refrigerant with GWP of 1. Transcritical CO2 systems are the leading direction for new supermarket refrigeration. A1 safety class. High operating pressures require specialized equipment.
Regulatory Timeline
Alexander Twining and others demonstrate CO2 as a natural refrigerant in early compression systems
CO2 largely displaced by CFCs in most applications due to lower operating pressure requirements
CO2 transcritical systems developed by Gustav Lorentzen, beginning the modern CO2 revival
First commercial CO2 transcritical supermarket systems installed in North America
CO2 transcritical becomes the standard specification for new supermarket refrigeration in some regions
What R-744 (CO2) Is and How It Works
CO2 (R-744) is a natural refrigerant with GWP of 1 — effectively zero climate impact relative to the HFCs it replaces. It has zero ODP and is non-toxic (though it can displace oxygen in confined spaces at high concentrations). In HVAC and refrigeration applications, CO2 operates in either subcritical mode (below its critical point of 88°F/7.4 MPa) or transcritical mode (where the high-pressure side operates above the critical point).
The transcritical cycle is the practical reality for CO2 refrigeration in climates where ambient temperatures exceed 88°F — which includes much of North America during peak summer conditions. In transcritical operation, the high-pressure side of the system operates at 1,200–1,500 psig, significantly higher than any HFC system. This requires specialized compressors, pressure vessels, controls, and piping — all of which must be rated for CO2 transcritical service.
Despite the engineering complexity, CO2 transcritical systems have achieved high adoption in European supermarkets where energy efficiency mandates and F-Gas regulations made them economically attractive. North American adoption is accelerating, driven by AIM Act HFC phasedown making high-GWP alternatives increasingly expensive.
Applications and Market Adoption
The primary growth area for CO2 in North America is supermarket refrigeration. CO2 transcritical rack systems for grocery stores combine the refrigerant benefits (zero GWP, low cost) with a system architecture that can handle the complex multi-temperature loads of a modern supermarket. Some large grocery chains have adopted CO2 as a corporate standard for all new store construction.
Heat pump water heaters represent another significant CO2 application. CO2 heat pump water heaters exploit the transcritical cycle's high-temperature heat rejection to efficiently produce 140-160°F hot water. Japanese and European manufacturers have commercialized CO2 heat pump water heaters successfully. US market adoption is growing, driven by efficiency standards and electrification incentives.
For HVAC contractors without commercial refrigeration expertise, CO2 systems are typically outside standard service scope. They require specialized training, high-pressure rated service equipment, and familiarity with transcritical cycle operation that is distinct from HFC service work.