1) What is an A2L refrigerant?
“A2L” is a safety classification used for refrigerants:
- EN = lower toxicity (based on occupational exposure criteria)
- 2L = mild flammability with lower burning velocity; A2L refrigerants must have burning velocity ≤ 10 cm/s per common interpretations tied to ASHRAE 34 language and industry guidance
A2L was created to distinguish “mildly flammable” refrigerants from more easily ignitable flammables (A3), enabling different safety rules and mitigation strategies.
2) Common A2L refrigerants (what your sensor may need to detect)
A2L is not one gas—your “A2L sensor” typically targets specific refrigerants used in modern HVACR:
- R32 (widely adopted for room AC / heat pumps)
- R454B / R454A / R454C (common R410A-replacement family, region dependent)
- R1234YF (A2L used heavily in mobile A/C and some stationary applications)
Practical point: “A2L sensor” usually means a refrigerant detection system (RDS) designed and validated for A2L refrigerants, not a generic combustible-gas sensor.
3) The most important requirement: detect and mitigate below 25% of LFL
Across modern A2L appliance safety discussions, the most repeated requirement is:
- Leak detection must activate at a maximum concentration of < 25% of the LFL (Lower Flammability Limit)
- Some guidance highlights a performance expectation such as producing output within a defined time at 25 % LFL (e.g., 30 seconds in certain discussions/engineering summaries of IEC 60335-2-40-related requirements).
Why 25% LFL matters: it builds a safety buffer so the system can turn on ventilation / mitigation before a flammable mixture becomes possible.
4) Standards that drive A2L sensor design (what OEMs must align with)
UL/IEC 60335-2-40 (air conditioners, heat pumps, dehumidifiers)
UL and industry summaries describe updated requirements for refrigerant detection systems, including testing around 25 % LFL and lifetime reliability considerations.
ASHRAE 15 + ASHRAE 34 (system safety + classification)
ASHRAE 15 addenda and updates include requirements around refrigerant detectors, ventilation activation, and setpoint logic in machinery-room contexts.
I 378 / ISO 5149 family (EU/International safety framework)
EN 378 guidance commonly states detector placement principles and machinery-room requirements (e.g., at least one detector; mounting low for heavier-than-air refrigerants, high for lighter-than-air).
5) How to choose alarm thresholds for A2L sensors
Because different A2L refrigerants have different LFL values, use a fraction-of-LFL approach:
Formel
Alarm setpoint (vol%) = LFL (vol%) × target fraction
Common fractions:
- Tidlig advarsel: 10% LFL (site preference)
- Mitigation trigger: ≤ 25% LFL (commonly cited requirement for RDS activation)
Worked example: R32
Multiple sources cite R32 LFL ≈ 14.4% vol.
So:
- 25% LFL = 14.4% × 0.25 = 3.6% vol = 36,000 ppm
- 10% LFL = 1.44% vol = 14,400 ppm
Note: Some safety discussions also connect RCL logic to fractions of LFL (e.g., 20% of 14.4% = 2.88% ≈ 28,800 ppm in one engineering discussion).
Worked example: R1234yf
One reference table lists R1234yf LFL ≈ 6.5% vol.
- 25% LFL = 1.625% vol = 16,250 ppm
Takeaway: Don’t publish a single ppm number as “the A2L threshold.” Publish %LFL targets and show your conversion method.
6) A2L sensor technology options (what works best for mildly flammable refrigerants)
Option A — NDIR / Infrared refrigerant sensors
Many A2L-focused modules and datasheets explicitly use Er n for refrigerant detection (R32, R454 blends), emphasizing accuracy and long-term stability.
Fordeler
- Good long-term stability
- Better selectivity than many generic combustible sensors
- Easier to map to %LFL and compliance-style thresholds
Watch-outs
- Optical contamination risks (dust/oil) → use protective housing/filters
Option B — Catalytic bead (pellistor) combustible sensors
Catalytic sensors are widely used for %LEL combustible gas detection and can work for hydrocarbons; however, for A2L refrigerants, OEMs must consider poisoning, oxygen dependence, and correction-factor practices.
7) Placement and installation rules (why A2L detection fails in the field)
A2L refrigerants (and many HFC/HFO vapors) are commonly treated as denser than air, so they can accumulate low in poorly ventilated areas. EN 378 guidance explicitly states mounting logic: install detectors at the lowest underground room / low points for heavier-than-air, and at the highest point for lighter-than-air.
Practical placement checklist (OEM + contractor friendly)
- Place near likely leak sources: coil connections, valves, compressor compartment
- Avoid direct supply-air blasts that dilute the leak plume
- In large rooms, use multiple sensors and cover dead zones
- Protect from water, dust, oil mist (enclosure design matters)
8) Integration: what the detector should do when gas is detected
A2L sensor systems are not just “readouts.” They typically trigger mitigation:
- Ventilasjonsaktivering (machinery room ventilation triggered by detectors is part of ASHRAE 15 updates/addenda discussions)
- Release mitigation controls (standards presentations and compliance discussions reference mitigation controls activated by an RDS in certain applications)
- Fault behavior: if the sensor fails, the equipment should move to a safe state consistent with the applicable product standard approach
Recommended alarm logic (easy to communicate)
- Alarm 1: early warning (e.g., 10% LFL) → notify + log + pre-ventilation (design choice)
- Alarm 2: ≤ 25% LFL → mitigation required (fan/valve/shutdown strategy aligned to standard)
9) Calibration & long-term reliability (the hidden differentiator)
A2L sensors used in safety functions must be reliable over the lifetime of HVAC equipment. UL notes the importance of evaluating reliability over life and discusses drift/deviation allowances in updated requirements context.
What to publish on your product page
- Response time at a defined concentration (e.g., 25% LFL test condition)
- Drift specification and recommended test interval
- Self-test / fault output behavior
- Temperature/humidity compensation method (especially for embedded indoor units)
10) Vanlige spørsmål
What does A2L mean?
A2L indicates lower toxicity (A) og mild flammability with low burning velocity (2L); A2L refrigerants have burning velocity ≤ 10 cm/s in common classification guidance.
Which refrigerants are A2L?
Common examples include R32 og R454A/B/C for HVAC, and R1234YF in many mobile A/C contexts.
What is the key detector activation threshold?
A widely cited requirement is activation at < 25% of the refrigerant’s LFL to reduce ignition risk.
How fast does an A2L sensor need to respond?
Engineering summaries of IEC 60335-2-40-related requirements commonly reference producing output within a defined time when exposed to 25 % LFL (often quoted as 30 seconds in industry summaries).
Where should an A2L sensor be installed?
EN 378 guidance generally recommends installing at least one detector in each machinery room/occupied space considered, and placing detectors low for heavier-than-air refrigerants.
Which sensor technology is best for A2L refrigerants?
Many A2L refrigerant modules use NDIR (infrared) for stability and selectivity (R32/R454 blends). Catalytic sensors can work but require careful handling of poisoning/correction-factor issues.
Can one “A2L sensor” detect all A2L refrigerants?
Some modules support multiple A2L gases (e.g., R32 and R454 blends), but accuracy is best when the sensor is validated for the target refrigerant(s) and outputs in %LFL for those gases.
Winsen Solution
If you’re designing A2L HVAC equipment (R32, R454 blends, etc.), an A2L sensor is part of the safety system, not just a measurement component. A supplier should support gas selection, %LFL alarm strategy, placement guidance, og integration formats (UART/RS485/Modbus/analog) aligned with UL/IEC 60335-2-40 and the relevant system standards.
