A2L refrigerants drive thermal drift concerns in HVAC systems

Much has been made about the shift of many HVAC systems to A2L refrigerants to comply with new environmental regulations, along with the continued expansion of A3-driven heat pumps in Europe and other regions.

The area that has received considerable attention is the development of refrigerant leak detection systems that HVAC/R manufacturers are implementing to comply with regulations such as UL60335-2-40.

However, the transition to new refrigerants comes with changing temperature ratings within these systems. These shifts are creating a demand for new product design with tighter accuracy to address the phenomenon of thermal drift.

A2L system development years in the making

The transition to A2L refrigerants is rooted in the global environmental initiatives that began with the Montreal Protocol, ratified in 1989 under the United Nations. This agreement aimed to protect the ozone layer by phasing out ozone-depleting substances such as chlorofluorocarbons (CFCs), which were widely used in HVAC systems, aerosols, and other industrial applications.

In response, the industry shifted toward hydrofluorocarbons (HFCs) – compounds that do not harm the ozone layer but carry a high global warming potential (GWP). This tradeoff eventually prompted further regulatory action, culminating in the Kigali Amendment, which the United States ratified in 2022. This amendment mandates a global phase-down of HFCs, targeting an 80-85% reduction in developed countries by the late 2040s.

A2L refrigerants offer a significantly lower GWP compared to the HFCs they replace. However, they introduce a new engineering consideration: mild flammability.

Under ISO 817, refrigerants are classified by both flammability and toxicity:

For a flammable refrigerant to pose a hazard, two conditions must be met:

• The refrigerant concentration must exceed a critical threshold.
• An ignition source must be present.

To mitigate this risk, the UL 60335-2-40 standard was developed. The standard mandates leak detection and mitigation to prevent refrigerant concentrations from reaching dangerous levels.

Each refrigerant is evaluated based on its Lower Flammability Limit (LFL)—the minimum concentration in air required for ignition. For R454A (as an example)—that LFL is 0.278 kg/m³. R290 is much more flammable, so its LFL is far more restrictive—A3 gases generally have an LFL of ≤ 0.10 kg/m³ or high heat at combustion ratings.

UL regulations require that systems prevent refrigerant concentrations from exceeding 25% of the LFL, ensuring a safety margin.

In meeting these requirements, HVAC OEMs also face a practical challenge: integrating them without compromising system cost, reliability, or lifecycle performance. These systems carry added components and compliance costs that end users may not fully appreciate. As a result, the burden falls on OEMs to deliver maintenance-free solutions to reduce the long-term cost of ownership.

Leak detection sensors employ acoustic resonance technology to continuously monitor for refrigerant leaks in critical zones—typically near the condenser coil, where leak probability is highest. These sensors are designed for extended operational life, exceeding 15 years, to help OEMs deliver systems that comply with evolving safety standards while being optimized for total cost of ownership and field reliability.

Downstream impacts — temperature and thermal drift

The shift to A2L designs has also introduced changes to HVAC manufacturers’ pressure and temperature ratings. The higher temperatures required can create additional drift in pressure output values, mainly on the high-pressure side of the refrigerant circuit.

These high-pressure cutoff switches are a critical component for maintaining system integrity and are mandated by system standards for most equipment. In situations where thermal drift exceeds normal levels, this can lead to a decrease in reliability for pressure cutoffs, as well as the potential for malfunctions.

Additionally, these complications arise at a time when energy efficiency is a critical selling point and a key competitive advantage in the market. Incorrect system responses can prevent systems from running the compressor, pump, fan and other components at peak efficiency—resulting in higher energy consumption and utility costs, along with the potential for increased maintenance costs.

Sensitive heating and cooling applications also need accurate data and integrated smart control systems. Having low thermal drift provides reliable data, improving overall system intelligence and responsiveness.

Global roadmap pushes toward continued change

As new systems driven by A2L and A3 refrigerants continue to enter the global market and expand into different verticals in the coming years, it is clear that the next period will not be a one-time market shift, but instead likely lead to a time of ongoing design optimization for these HVAC platforms.

Worldwide efforts to reduce energy consumption have put HVAC systems in the spotlight, as they typically account for about 40% to 60% of a commercial building’s total energy use, making them the largest opportunity for reducing energy costs and consumption.

As changes to required temperatures are incorporated into the newest designs, engineers will need to continually evaluate the other components built into their systems to ensure that the downstream impacts of these shifts still allow systems to run at their optimal levels.