Importance of the Refrigerant Concentration Limit in HVAC Design

Shubham More
Author : Shubham More
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    Modern HVAC systems are designed with low-toxicity refrigerants such as R-410a, but even these substances can be dangerous if their concentration becomes too high. HVAC engineers must consider the potential amount of refrigerant if there is a leak, ensuring it stays below the refrigerant concentration limit (RCL).

    The RCL value indicates an allowable refrigerant weight per cubic foot of room volume (lb/ft3). As long as the concentration stays below the RCL, dangerous effects are avoided: oxygen deprivation, flammability, cardiac sensitization, etc. Since these effects are prevented, occupants can easily escape from the affected area.

    To get an idea of how a nontoxic or low-toxicity substance can be dangerous, consider water. Although water is vital, it can cause drowning because it displaces air and is unbreathable - the same principle applies for any other liquid or gas that displaces air.

    Make sure your HVAC system is designed for safe operation.

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    Safety Standards for Refrigerants in HVAC Systems

    The American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) has developed standards that document best practices in the HVAC industry. These cover topics such as the safe use of refrigerants, proper ventilation of commercial buildings, temperature and humidity control, as well as measures that minimize energy and water consumption. In particular, the following standards address safety topics:

    • ASHRAE 15-2004: Safety Standard for Refrigeration Systems

    • ASHRAE 34-2007: Designation and Safety Classification of Refrigerants

    The R-410a refrigerant used in VRF systems has the safest rating according to ASHRAE standards, along with R-22, R-134a and R-407c. However, even a safe refrigerant can be dangerous when a large leak displaces the air in a room, creating a risk of asphyxiation. The refrigerant concentration limit (RCL) for R-410a is 0.025 lb per cubic foot.


    To maximize safety, ASHRAE standards assume the worst case scenario in their design requirements - having the full refrigerant charge of an HVAC system released into a small room, over a short period of time. The refrigerant level after a leak is determined by two main factors:

    • The amount of refrigerant used by HVAC equipment.

    • The air volume available for dispersion and dilution. Rooms having the smallest air volume are the most critical.

    The calculated refrigerant concentration for each room cannot exceed the RCL. There are also applications where the allowable limit is reduced to 50% of the RCL - some examples are asylums, nursing homes and hospitals.

    Brief Overview of the RCL Verification Process

    Once the dimensions of all building zones are known, HVAC engineers calculate the air volume of the smallest rooms. Then they divide the refrigerant charge of the system by this volume.

    If the calculated value exceeds the RCL, the installation does not meet ASHRAE standards and must be redesigned. As previously mentioned, the RCL for R-410a is 0.25 lb/ft3. There are many ways to reduce the refrigeration concentration during a leak:

    • Increasing the dimensions of rooms where concentration exceeds the RCL, or using the area above the ceiling to expand the room.

    • Modifying the piping system to reduce the refrigerant charge.

    • Increasing the air volume with measures such as transfer ducts between rooms, undercut doors, grilles on doors or walls, etc.

    Professional HVAC engineers can design mechanical systems according to all the requirements in ASHRAE standards and local building codes. They can also optimize energy efficiency, achieving a permanent reduction of your gas and electricity bills. In a few words, professional services ensure that your installation is efficient and compliant.


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    Tags : HVAC HVAC System Design building safety

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