Resources / A2L Refrigerant Compatibility
A2L.
The refrigerant transition, the standards that govern it, what “A2L-compatible controls” means in practice, and where Merlin's Overrides land in the picture.
~10 minute read
What A2L Refrigerants Are
A2L refrigerants are a class of working fluids designated "mildly flammable" under ASHRAE Standard 34. They have a flame propagation velocity below 10 cm/s and a heat of combustion below 19,000 kJ/kg. They can burn, but slowly, and only under specific ignition conditions. The two most relevant for commercial HVAC right now are R‑32 and R‑454B.
A2Ls exist because the HVAC industry is converting away from the previous-generation high-GWP refrigerants (R‑410A in particular) under the EPA's AIM Act (American Innovation and Manufacturing Act, 2020). R‑410A has a global warming potential of 2,088. R‑32 sits at 675. R‑454B sits at 466. The math is straightforward: a system charged with the new refrigerants has roughly a quarter to a third of the climate impact per unit charged.
The trade-off is the mild flammability. Equipment, installation practices, and controls all have to account for it. That's what ASHRAE 15-2024, ASHRAE 34-2024, and UL 60335-2-40 codify.
The Transition Timeline
The phase-down operates on a schedule. The key dates for commercial HVAC controls:
January 1, 2025. Manufacturing transition for most new equipment. R‑410A production for new systems was effectively phased out under EPA technology transition rules. Equipment manufacturers had to design and manufacture against A2L-compatible refrigerants going forward. That date is now behind us; the manufacturing transition has happened.
Through 2026. Installation and field-replacement phase. Existing R‑410A equipment can still be serviced with recycled refrigerant where allowed, but new installations are A2L-class refrigerants. State-level variances exist; check local code.
2027 and beyond. Continued tightening. The AIM Act's phase-down ratchets down allowable production and import quotas of high-GWP refrigerants over multiple steps through 2036.
What this means for a project specified today: your equipment is going to use A2L refrigerant. Your mechanical room is going to need to meet ASHRAE 15-2024 ignition-source criteria. Your fan-array controls are going to be part of that conversation.
ASHRAE 15-2024 and 34-2024
ASHRAE Standard 15 governs safety in refrigeration systems. ASHRAE Standard 34 classifies refrigerants. The 2024 revisions of both standards reshape the mechanical room around A2L refrigerants.
The new ignition-source criteria. Equipment in a mechanical room that contains A2L equipment (even equipment that doesn't directly contain refrigerant, like a fan-array controls package) must not be an ignition source for an A2L leak. Sparking contactors, hot motor terminals, and inadequately sealed electrical components all become design problems.
Sealed-component construction. A2L-compatible controls equipment is built with sealed contactors and components rated for the environment. Fuses, terminal strips, and unsealed relays don't pass.
Detection and response. Mechanical rooms with A2L equipment require refrigerant leak detection that interacts with the ventilation and control systems. When a leak is detected, the response is to increase airflow, not stop it. Ramping the fans up dilutes the refrigerant concentration below its lower flammability limit and clears the space safely. The detection signal needs a wiring terminal to land at, and a documented behavior at the controls that executes that ramp.
That last point is where the Merlin controls architecture meets the standard.
UL 60335-2-40
UL 60335-2-40 is the safety certification standard for equipment using A2L-class refrigerants. Its full name is Particular Requirements for Electrical Heat Pumps, Air-Conditioners and Dehumidifiers, published under the broader IEC 60335 family. In North America it's the standard a UL listing for an A2L heat pump, AC unit, or dehumidifier references.
What it covers, in practical terms:
Refrigerant charge limits. Maximum allowable charge for a given room size and ventilation rate. Larger systems require more sophisticated installation conditions or active mitigation strategies.
Ignition source identification and control. Every electrical component in proximity to a refrigerant-containing zone gets evaluated. Switching devices (contactors, relays) must be sealed or rated for an A2L atmosphere. Hot surfaces must stay below the refrigerant's auto-ignition temperature, including under fault conditions. Exposed terminal strips, arc-vent fuses, and unsealed components fail.
Leak detection and ventilation response. When a refrigerant detection signal asserts, the standard requires the system to increase airflow, dilute the refrigerant concentration below its lower flammability limit, and hold the response until the detection clears. Specific concentration thresholds and timing windows are defined.
Construction and marking. Enclosure construction requirements, wiring practices, proximity rules around refrigerant-bearing components, and the required A2L equipment markings.
Testing protocols. Fault simulation, leak simulation, and abnormal-operation testing required for certification.
Where Merlin stands. All three Merlin tiers (Essentials, Pro, Elite) are designed against UL 60335-2-40 requirements. Both the Control Panel and the QCB are currently UL 508A listed (the industrial control panel standard). UL 60335-2-40 listing for all three tiers is in progress, expected mid-2026.
What "A2L-Compatible Controls" Actually Means
"A2L-compatible" on a controls package isn't a marketing claim. It's a set of construction and behavioral requirements.
Sealed contactors. Standard contactors are open-air devices, with arcing contacts that ionize the air briefly when they switch under load. In an A2L atmosphere that's an ignition source. A2L-compatible controls use sealed contactors or solid-state switching where ignition risk applies.
Component temperature ratings. Components that can reach surface temperatures above the refrigerant's auto-ignition threshold need to be enclosed, derated, or eliminated. Many off-the-shelf relay banks, exposed terminal strips, and traditional fuse holders fail here.
Sealed enclosure construction. The cabinet itself, particularly around the components most likely to spark, needs construction that prevents an A2L leak from contacting an ignition source.
A wiring terminal for the detection signal. An A2L-compatible controls package needs a documented input where a refrigerant detection signal lands, with documented behavior on assertion: ramp the fans up to maximum airflow (or a calculated dilution rate), alarm the BMS, and hold the response until the detection clears.
A standard off-the-shelf PLC controls package fails most of these tests. It has open contactors. It has fuses. Its enclosure was designed for a standard mechanical room, not an A2L one. And the detection-signal input is somewhere between "not present" and "undocumented."
Merlin's Approach: The Overrides as the A2L Architecture
Merlin's A2L architecture lives in the two configurable Override inputs on every Pro and Elite Control Panel.
Each override is a programmable digital input. Either one can accept a refrigerant detection signal. Either one can also accept a fireman's override, a smoke-detector chain, a tenant after-hours signal, or any other auxiliary input the application requires. The Service role configures each override's behavior matrix at commissioning:
- The fan speed or mode the override commands. For an A2L refrigerant detection signal that's typically a ramp to maximum airflow, in order to dilute the refrigerant below its lower flammability limit. For a fireman's override that's typically a full stop. For other applications, whatever the application requires.
- What alarming propagates to the BMS over BACnet.
- How long the override is asserted before clearing.
The override is a wiring terminal with documented behavior. The detection system raises a digital signal. The override terminal sees it. The behavior matrix executes. Measured, documented, repeatable.
Around that input, the rest of the controls package is constructed to A2L-compatible standards. Sealed components. Ignition-source-mitigated design. UL 508A listed today; UL 60335-2-40 listing for all three tiers in progress, expected mid-2026.
This is the difference between A2L-compatible by design and A2L-compatible by retrofit. Merlin built the overrides in. The QCB mounts on the AHU wall, outside the airstream, and uses sealed IP54 receptacles and resettable MMPs. Every competitor that mounts a junction box inside the airstream is shipping an enclosure that isn't sealed and that uses fuses, and neither meets code for A2L applications.
What to Specify in Your Sequence of Operations
For consulting engineers writing A2L-aware sequences of operations on a Merlin Fan Array, the following spec language is verifiable against the controls architecture:
Upon assertion of the A2L refrigerant detection signal at Override Input 1 or 2, the Merlin Controls shall: (a) ramp the fan array to MAXIMUM AIRFLOW (or the calculated dilution rate per room volume and refrigerant charge) within [TIME] seconds, in order to dilute the refrigerant concentration below its lower flammability limit; (b) raise alarm category "A2L Detection" to the building management system over BACnet MS/TP; (c) hold the system in the configured override state until the detection signal clears and a Service-level reset is acknowledged; (d) log the assertion event in the controls event log with timestamp.
Override behavior shall be configured at the cabinet HMI by personnel with the Service role. The Override behavior matrix shall be documented in the project's commissioning record as part of Book 02 of the Merlin five-book documentation set.
Adjust the bracketed values for the application. The control modes available are documented in the Pro/Elite controls architecture and selectable on the HMI: Manual, Constant Speed, Constant CFM, Constant Local Static, and Constant Duct Static, with ECO mode and Maximum CFM limiting available as overlays.
For project-specific spec language, talk to Merlin engineering at sales@merlinair.com. For a submittal package, find your local rep.