SLAAEQ2 September   2024 MSPM0G1506 , MSPM0G3507

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Example Mitigation Control Board Design
    1. 2.1 Overview of Mitigation Board Functions
    2. 2.2 Audible and Visible Detection Alarm
    3. 2.3 Freeze Detection (Water-Based Systems)
    4. 2.4 Isolation and or Safety Shut-off Valves (SSOVs) in the Refrigeration System to Limit Releasable Charge
    5. 2.5 Electronic Controls With A2L Logic to Perform Required Actions in The Event of a Leak — Such as Shutting off the Compressor and or Other Components
    6. 2.6 Ventilation and or Circulation Fans Within Cases or Walk-in Units
  6. 3Summary
  7. 4References

1 Introduction

A2L refrigerants get the respective name from the ASHRAE safety classification and are mildly flammable, low toxicity, and low global warming potential, or GWP. These refrigerants also have a zero-ozone depletion potential, or ODP. As far as the chemical makeup of these refrigerants, A2L refrigerants are hydrofluoroolefins (HFO) or a blend of HFOs. Compared to previous HFC and HCFC refrigerants used in HVAC systems, namely R-410A and R-22 among others, the A2L refrigerants are more energy efficient while also conforming to greener energy initiatives worldwide. Table 1-1 shows some common HVAC refrigerants along with the respective GWP/ODP values.

Table 1-1 Common Refrigerants and GWP/ODP Values
RefrigerantTypeGWPODP
R-22HCFC18200.055
R-410AHFC20880
R-134AHFC14300
R-1234YFHFO40
R-1234ZEHFO60
R-32HFC6750
R-454BHFC4660

The drawback to A2L refrigerants is the inherent mild flammability associated with the chemical makeup, which means there are equipment manufacturer regulations needed to mitigate any dangerous situations where the refrigerant can combust given an ignition source. Figure 1-1 shows a breakdown of different types of these refrigerants along with the respective ignition combustion data.

HVAC Refrigerant Combustion and Propagation DiagramFigure 1-1 HVAC Refrigerant Combustion and Propagation Diagram

A2L-based systems and the additional required electronics to comply with UL 60335-2-40. To make sure safe operation, air conditioning equipment can feature special components, such as refrigerant detection systems (RDS), as per safety standard UL 60335-2-40. RDS detects specific refrigerants and, upon exceeding predefined thresholds, triggers mitigation actions like activating fans to dilute refrigerant concentration via the mitigation board. Requirements for the RDS include:

  • The leaking gas shall not exceed 25% of the LFL,
  • In addition to not exceeding 15% of the LFL for a period of 5 min after the RDS has reset.
  • Refrigerant detection system shall make output within 30 seconds of direct exposure to 25% LFL

The deployment of RDS and adherence to safety standards are critical steps in the industry's response to regulatory changes. More information on the requirements of A2L sensor modules can be found in A2L Refrigerant Adoption: Considerations and Requirements.

The second crucial component of this standard is the mitigation board which handles all of the required tasks once the refrigerant concentration threshold has been reached as described above. This includes:

  • Switching off compressor unless the operation reduces the leak rate, or the total amount released into the indoor space
  • Turning on the indoor blower fan
  • Audible and visual indication that a refrigerant leak has been detected (or that the A2L sensor has malfunctioned and needs to be replaced)
  • Freeze detection circuitry for water-based systems
  • Fully opening zoning dampers (if applicable) to disperse the gas within the system
  • Engaging any safety shut-off valves (if applicable)