Mitigation Mode: How A2L Refrigerants Impact Telecom Systems

As HVAC equipment manufacturers continue transitioning toward lower-GWP refrigerants such as R-32 and R-454B, many telecom facilities, including telecom huts and shelters, already have, or soon will have, systems using A2L refrigerants.

What is an A2L refrigerant? A2L refrigerants are lower-toxicity, lower-flammability refrigerants used in newer HVAC equipment. Because they are mildly flammable, equipment using A2L refrigerants may include added safety features such as refrigerant leak detection and Mitigation Mode (UNEP/ASHRAE).

So far, most of the focus has been on procurement and avoiding equipment delays. But there’s an important operational detail that deserves attention: a built-in safety feature called Mitigation Mode.

This function is already programmed into equipment and can significantly impact how systems operate during a refrigerant leak – highlighting important system behaviors operators should understand.

WHAT IS MITIGATION MODE?

Mitigation Mode activates automatically when the unit’s refrigerant detection system identifies a leak inside the unit, typically near the indoor coil. Leaks that occur outside the unit or in piping are not detected and won’t trigger this mode.

Mitigation Mode activates automatically when a refrigerant leak is detected inside the unit, at the indoor coil. Leaks that occur outside the unit or in piping are not detected and won’t trigger this mode.

When activated, the system shifts into a safety response designed to do two things: reduce the number of refrigerants in the air and eliminate any risk of ignition (UL Solutions). The system remains in this mode until the leak condition clears and, in some cases, continues for a short delay before returning to normal operation.

Want a step-by-step breakdown? View our infographic on how Mitigation Mode works.

While this sequence is designed for safety, it can also affect how your system performs during a leak.

HOW MITIGATION MODE IMPACTS TELECOM FACILITIES

Mitigation Mode introduces new operational considerations, especially when it comes to maintaining cooling and responding to system events during a refrigerant leak.

Depending on how your HVAC system is configured, the impact can vary:

1. Facilities without redundant HVAC capacity: In facilities without redundancy, a refrigerant leak is effectively the same as a unit failure. Cooling is lost, and Mitigation Mode does little to maintain space conditions.
2. Facilities with lead-standby configurations: In lead-standby systems, Mitigation Mode can delay or complicate the transition to the standby unit. The affected unit may continue running its fan without providing cooling, which can make it appear operational. If the refrigerant leak alarm is not connected to the Building Management System (BMS), operators may not be aware of the issue until temperatures begin to rise or the unit shuts down. To avoid this, the control sequence should ensure the BMS:

• Recognizes the failure
• Stops attempting to run the affected unit
• Automatically starts the standby system

3. Facilities with shared ductwork or plenums: If lead and standby systems share ductwork or a plenum, both units may operate simultaneously during Mitigation Mode. The system should be designed to handle this condition, including the use of motorized or backdraft dampers to prevent airflow conflicts between units.

WHY THIS MATTERS: SAFETY & OPERATIONS

All refrigerant leaks create a potential hazard and should be addressed according to applicable standards and codes. A2L refrigerants are subject to stricter safety requirements due to their mild flammability compared to traditional A1 refrigerants.

The level of risk depends on the environment. Smaller occupied spaces with larger refrigerant charges present a greater concern. For example, a pre-fabricated outdoor telecom hut with a 15-ton system may carry higher risk due to the concentration of refrigerant in a confined area.

At the same time, leak detection sensors provide a valuable operational advantage. Instead of waiting for a compressor failure alarm, operations teams can receive early notification of a leak. This allows personnel to identify issues more quickly and reduce troubleshooting time.

WHAT SHOULD TELECOM FACILITIES DO NEXT?

Understanding how Mitigation Mode affects both system performance and safety is essential, but the next step is making sure your facility is prepared to respond.

Watch our video: What telecom facilities should do about Mitigation Mode

This resource outlines practical steps your team can take to maintain cooling reliability and respond safely to refrigerant leaks. If you have questions, please reach out to me at ebjorndal@wbengineering.com or 301.279.6312.