ASHRAE Standard 15 & VRF

It’s a fact: Variable Refrigerant Flow (VRF) technology is taking off in the Southeast United States in a big, big way. As more owners, architects, engineers, and contractors become familiar with the technology and its proper application similar questions begin to come up fairly regularly. One of these questions has to do with health and safety concerns surrounding the R410a refrigerant which acts as the heat transfer medium in VRF systems.

Fortunately we have ASHRAE Standard 15. Originally developed in the 1930′s to address the dangers of chiller systems with thousands of pounds of ammonia or ether-based refrigerants dumping their total charge, ASHRAE 15 is now the go-to reference for HVAC systems that utilize refrigerant. Standard 15 has also filtered down and become part of some local and state codes.  Due to slight variations in the enforced code from municipality to municipality and state to state it is best to reference Standard 15 as well as local code when designing Variable Refrigerant Flow systems. ASHRAE Standard 34 is also key to VRF system design – Standard 15 relies on ASHRAE Standard 34 for refrigerant safety.

The purpose of Standard 15 is to ensure the safe design, construction, installation and operation of refrigeration systems by establishing safeguards for life, limb, health, and property and prescribing specific safety requirements. Standard 15 is very inclusive – it applies to the design, construction, test, installation, operation, and inspection of mechanical and absorption refrigeration systems. A key step in understanding how to apply ASHRAE Standard 15 to VRF systems is to determine which of the broad range of requirements to apply. This is accomplished by reviewing three basic sorting classifications: occupancy, system, and refrigerant.


Occupancy classification divides facilities according to the ability of individuals to respond to a refrigerant-related emergency and provides allowances for six unique occupancy classifications. The same general requirements apply to all of the occupancy groups with the exception of institutional and industrial occupancies, which have special considerations. For example: Patients in the intensive care wing of a hospital will respond much slower to a catastrophic refrigerant leak than those exercising in an athletic facility. Institutional occupancies as defined by Standard 15 include hospitals, nursing homes, asylums, and spaces containing locked cells, such as prisons.

System classification divides refrigeration system types according to the potential of the refrigeration equipment to expose the occupants to refrigerant. A low probability system is one “…in which the basis design, or location of the components, is such that leakage of refrigerant from a failed connection, seal or component can not enter the occupied space.” A high probability system is defined as one “…in which the basic design, or the location of components, is such that a leakage of refrigeration from a failed connection, seal, or component will enter the occupied space.” We consider VRF a high probability system because it has refrigerant containing components (air handlers, refrigerant piping, heat recovery boxes, etc) that reside in the occupied space, or in an airstream serving an occupied space.

Standard 15 uses the refrigerant safety classifications from ASHRAE Standard 34 as a model for classifying the refrigerant used.  Standard 34  classifies refrigerants according to their toxicity and flammability. R410a, the refrigerant used in most modern VRF systems, is classified as an A1 Safety Group refrigerant. This means that R410a has no flame propagation and low toxicity (i.e. it can’t catch on fire and can’t poison you). R410a is dangerous in that injury or death can occur by asphyxiation as this refrigerant displaces oxygen (because it is heavier than air).

ASHRAE Standard 34 also sets the refrigerant concentration limits used in ASHRAE Standard 15 calculations. The refrigerant concentration limit (RCL) is used to determine the allowable pounds of refrigerant per 1,000 cubic feet of occupied space. Standard 15 defines an occupied space as “…that portion of the premises accessible to or occupied by people, excluding machinery rooms.” Refrigerant concentration limits can not be exceeded in any occupied space –  if your calculations do exceed the RCL for your VRF system design you will need to start over and redesign the system. The RCL for R410a is 25 lbs / 1,000 cubic feet. This amount is reduced by 50% for all areas of institutional occupancies and some 24 hour a day occupancies.

In order to ensure ASHRAE Standard 15 compliance and provide safety for all occupants, we need to start with a value for the total R410a refrigerant charge in your VRF design. A typical design featuring LG’s Multi-V Series Variable Refrigerant Flow equipment is comprised of several air handlers served from a common set of refrigerant piping which has been connected to a condensing unit. Remember, while LG’s VRF condensing units come factory-charged with R410a they will also require an additional field charge to compensate for the additional piping and components (heat recovery boxes, air handlers, etc) in the system. Fortunately, LG’s LATS design software can perform all of these calculations for you and provide you with a total figure on which to base your calculations.

Once the total charge has been determined we need to determine the dilution volume. ASHRAE Standard 15, Section 7.3.2, provides the following: “Where a refrigerating system or a part thereof is located within an air handler, an air distribution duct system, or in an occupied space served by a mechanical ventilation system, the entire air distribution system shall be analyzed to determine the worst case distribution of leaked refrigerant.” Compared to a traditional packaged rooftop, the dilution volume of a VRF system is going to be smaller because if an air handler in a room leaks the entire refrigerant charge will leak into this room. This is an important distinction for VRF systems – the small occupied space served by the VRF system and the system’s entire refrigerant charge are used as the basis for the dilution volume. This is where Section 7.3.1 of Standard 15 comes into play: “Where a refrigerating system or a part thereof is located in one or more enclosed occupied spaces that do not connect through permanent openings or HVAC ducts, the volume of the smallest occupied space shall be used to determine the refrigerant quantity limit in the system.”


The plenum space above a suspended ceiling is not counted unless the plenum space is part of the air supply or return system. In this case both the supply and return air are ducted, meaning the plenum cannot be counted in the occupied space volume.

The facility’s smallest occupied space is now the driving factor for the overall size of your VRF system. Fortunately strategies exist for extending the size of this occupied space while staying within the confines of Standard 15. These approaches mainly rely on extending the boundary of the space or  modifying the VRF system design to change the total refrigerant charge. Engineering judgement will need to be used to determine which of these can properly be applied in any given situation:

  • Split larger refrigerant systems into smaller ones (i.e. instead of one 24 ton condensing unit use two separate 12 ton systems)
  • Undercut doors
  • Install doors with transfer grilles
  • Use the area above a drop ceiling as a return air plenum
  • Install wall-mounted transfer grilles
  • Use a ducted air handler in order to count the volume of the supply and return air ducts in the occupied space volume
  • Use a ducted air handler to serve multiple zones, expanding the occupied space volume
  • Remove the smallest space from the system and serve it with a ductless mini-split

The compliance calculations for Standard 15 are not difficult, you simply verify the total refrigerant charge of the VRF system, verify the volume of the smallest space, and finally verify the refrigerant system charge does not exceed the RCL. What you should keep in mind is that VRF technology is a relatively new system type in the United States, so be prepared with thoughtful answers about the strategies you decide to use above as there might be variations in how building inspectors interpret compliance requirements.

McNellage & Associates are absolutely passionate about energy efficient Variable Refrigerant Flow equipment. Every year we work with owners, architects, engineers, and contractors all over the Gulf Coast to design hundreds of tons of this equipment. We would love to work with you and your team! Click here to contact us and learn how to make your HVAC systems starting saving you thousands of dollars a year in energy costs.

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