Insights from Engineered Sales Manager Mario Skertchly

As the HVAC industry has fully adopted A2L refrigerants for VRF systems, specifying engineers are encountering new questions around safety, code compliance and system design. One of the most common misconceptions reloves around refrigerant charge limits, especially as design standards have evolved with the transition away from legacy refrigerants.

To help clarify how these limits work in practice, we sat down with Mario Skertchly, Engineered Sales Manager for Lennox Commercial, to discuss how engineers can design compliant VRF systems using A2L refrigerants while maintaining flexibility and performance.

According to Skertchly, understanding the relationship between three key design terms is critical: Lower Flammability Limit (LFL), Effective Dispersal Volume Charge (EDVC), and Refrigerant Concentration Limit (RCL).

What are the key A2L refrigerant limits engineers need to understand?

When designing VRF systems with A2L refrigerants, engineers must understand three important limits that appear throughout the codes and standards:

• Lower Flammability Limit (LFL)

• Effective Dispersal Volume Charge (EDVC)

• Refrigerant Concentration Limit (RCL)

Both LFL and EDVC have become central to system design, plan review and code compliance. However, all three values serve distinct purposes within the design process.

Understanding how they work together allows engineers to design compliant systems without unnecessarily restricting refrigerant charge.

What is the Lower Flammability Limit (LFL)?

The Lower Flammability Limit (LFL) is published in ASHRAE 34 and ISO 817 and represents a physical property of the refrigerant itself.

LFL defines the minimum concentration at which the refrigerant could become flammable if released into an enclosed space.

While LFL is used to determine flammability concentrations, it is not itself a regularoty compliance limit. Instead, it forms the foundation for other calculations used in A2L system design.

What is Effective Dispersal Volume Charge (EDVC)?

The Effective Dispersal Volume Charge (EDVC), sometimes referred to as Mmax, is published in ASHRAE 15 and is derived from two factors:

• The refrigerant's LFL

• The volume of the occupied space

EDVC represents the maximum refrigerant amount that could be released into an occupied space before flammability concentration is reach.

Because EDVC depends on the size of the room or space being served, it varies from one application to another.

This value is important because it is used to established the maximum allowable refrigerant charge in a space and to calibrate refrigerant detection sensors (RDS). These sensors trigger mitigation actions such as alarms, system shutdowns or ventilation sequences if a leak is detected.

While both LFL and EDVC relate to flammability limits, the key difference is the LFL is a fixed refrigerant property, while EDVC changes based on room volume.

What is the Refrigerant Concentration Limit (RCL)?

The Refrigerant Concentration Limit (RCL) is published in ASHRAE 34 and defines the maximum refrigerant concentration allowed in an occupied space for compliance purposes.

However, Skertchly notes that this value is often misunderstood.

Instead, RCL determines whether mitigation measures are required in the system design at all.

For A2L refrigerants, RCL values are significantly more conservative than those for legacy A1 refrigerants.

For example:

• A1 refrigerants such as R-410A allowed RCL values around 25-26 lb per 1,000 ft³

• A2L refrigerants sucha as R-32 or R-454B have RCL values around 4-5lb per 1,000 ft³

While this appears to represent a major reduction, it does not mean an A2L system must carry one-sixth of the refrigerant charge.

With proper design and mitigation strategies, engineers can still achieve comparable or even higher circuit charges while remaining compliant.

How do RCL, EDVC and LFL work together in VRF system design?

When engineers design a VRF system using A2L refrigerants, the three limits work together in a structured sequence.

First, the RCL value is determined using hte selected refrigerant and the volume of the occupied space. This establishes whether mitigation strategies will be required.

If the calculated refrigerant mass in the occupied space does not exceed the RCL, mitigation measures are not required.

If it does exceed the RCL, mitigation strategies must be incorporated. These strategies may include circuit subdivision, reduced refrigerant charge or architectural separation.

Once mitigation is required, the EDVC becomes the next critical value.

Derived from the refrigerant's LFL and the space volume, EDVC defines the maximum allowable refrigerant release before flammability concentration could occur.

View together:

• RCL determines whether mitigation strategies are required

• LFL defines the refrigerant's flammability limit

• EDVC converts that limit into a space-specific safety threshold

Another helpful way to think about the three limits is through the questions they answer during system design:

• RCL asks: Do mitigation strategies need to be incorporated?

• EDVC asks: At what refrigerant concentration should mitigation actions activate?

• LFL provides the physical flammabilty limit used to calculate those thresholds.

Why understanding these limits matters for A2L VRF design

For engineers designing modern VRF systems, understanding the distinct roles of RCL, EDVC and LFL is critical.

Now that the industry has transitioned to lower-GWP refrigerants, a clear understanding of these design principles will help engineers confidently specify compliant systems while maintaining performance and occupant safety.

Engineers interested in learning more about applying A2L refrigerants in VRF system design can fill out the form below to get in contact with their local Lennox Commercial Engineered Sales Manager.