5.5 A2L and Low-GWP Refrigerants: Codes, Charge Limits, and Service Practice

5.5 A2L and Low-GWP Refrigerants: Codes, Charge Limits, and Service Practice

The Low GWP Refrigerants sheet is one of the fifteen competency areas on the HVAC Excellence task list, and it backs a dedicated Specialty Certification. Its closing block is explicit: candidates must "have a general understanding of the industry codes and standards that affect the safe use and handling of A2L refrigerants, including ASHRAE Standards 15 and 15.2, UL 60335-2-40 and UL 60335-2-89, EPA SNAP rulings, and the American Innovation and Manufacturing (AIM) Act." This is the newest material on the exam and the most likely to be updated.

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1. The Regulatory Driver: the AIM Act

The American Innovation and Manufacturing Act of 2020 gave EPA three distinct authorities:

Under the Technology Transitions rule, new residential and light commercial air conditioners and heat pumps manufactured from January 1, 2025 must use a refrigerant with a GWP at or below the applicable sector limit. The market answer was R-454B (GWP ≈ 466) and R-32 (GWP ≈ 675), both A2L, replacing R-410A (GWP ≈ 2,088). Existing R-410A equipment remains legal to service indefinitely.

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2. What A2L Actually Means

ASHRAE Standard 34 assigns two characters: toxicity (A = lower, OEL ≥ 400 ppm) and flammability. The 2L subclass was created specifically to separate mildly flammable refrigerants from genuinely flammable ones.

A refrigerant is Class 2L when it exhibits flame propagation, has a heat of combustion below 19,000 kJ/kg, and has a maximum burning velocity of 10 cm/s or less at 60°C and 101.3 kPa.

The practical meaning: an A2L will burn, but only within a narrow concentration band, only with a sustained high-energy ignition source, and with a flame front that travels slowly enough that a running blower disperses the vapor faster than it can accumulate. It is not equivalent to a propane leak. It is also not inert, and the old habit of ignoring a small leak is no longer acceptable.

Also note R-454B and R-32 are mildly flammable but still HFC-based; they are not "natural" refrigerants, and their glide differs: R-32 is a single component with essentially no glide, while R-454B is a zeotropic blend with a small glide (about 1.5°F) that still requires liquid charging.

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3. The Codes and Standards

• ANSI/ASHRAE Standard 15 — the safety standard for refrigeration systems: machinery rooms, refrigerant concentration limits, relief device sizing and discharge. Standard 15.2 was created specifically for residential and light commercial systems and carries the A2L installation requirements for that equipment class.
• ANSI/ASHRAE Standard 34 — assigns the safety group and the Refrigerant Concentration Limit (RCL), the maximum concentration allowed in an occupied space.
• UL/CSA 60335-2-40 — the product safety standard for heat pumps, air conditioners, and dehumidifiers. It replaced UL 1995 and contains the A2L mitigation requirements.
• UL/CSA 60335-2-89 — the parallel standard for commercial refrigerating appliances, which raised the allowable A3 (hydrocarbon) charge from 150 g to as much as 500 g in self-contained equipment.
• EPA SNAP (Significant New Alternatives Policy) — lists substitutes as acceptable, acceptable with use conditions, or unacceptable, by end use. A2Ls are listed acceptable subject to use conditions, which include the UL listings and specific installation requirements.
• Model codes — the IMC, IFGC, IRC, and UMC have been amended to adopt A2L provisions. Local adoption lags, so the authority having jurisdiction determines what applies on any given job.

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4. Charge Limits and Mitigation

The controlling idea in UL 60335-2-40 is simple: keep the concentration in the occupied space below 25% of the lower flammability limit (LFL) under any credible leak scenario.

• Maximum charge for a given room is a function of the refrigerant's LFL, the conditioned floor area, the installation height of the indoor unit, and whether mitigation is present. Larger rooms and mitigation permit larger charges. Manufacturers publish minimum-room-area tables; use them rather than a rule of thumb.
• Mitigation on ducted equipment normally means:
  1. A factory-installed refrigerant detection sensor (RDS) in the indoor unit or supply plenum.
  2. On detection, the control shuts off the compressor, opens dampers if fitted, and energizes the indoor blower to a specified minimum airflow, diluting the leak below 25% LFL.
  3. A fault/lockout condition that requires service attention rather than an automatic reset.
• The RDS is a service item. Sensors have a stated life (commonly 10–15 years) and must be replaced, not defeated. Jumpering out a refrigerant sensor to "get the customer running" defeats the entire listed safety system and voids the listing.
• No open ignition sources may be located in the airstream where A2L could accumulate. This is why an A2L air handler cannot share a cabinet with an open-flame appliance.

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5. Service Practice: What Changes

Most A2L service is identical to A1 service. The differences are specific and testable.

Practical field notes

• R-454B and R-410A pressures are similar but not identical, and their P-T behavior diverges at the extremes. Always use the correct chart.
• A2L systems often use POE oil, so the hygroscopic handling rules from Section 5.4 apply — keep containers sealed, evacuate deeply, replace the drier.
• Equipment labeling identifies the refrigerant, its safety group, and the required mitigation. Read it before touching anything; retrofitted or mislabeled equipment is a real hazard.
• Technician certification: EPA Section 608 certification remains the legal requirement for handling regulated refrigerants and their substitutes, including A2Ls. The HVAC Excellence Low GWP Refrigerant Safety Specialty Certification is a separate 50-question credential that documents A2L-specific competence.