As the policy-driven refrigerant transition takes hold in the HVAC industry, mechanical engineers are encountering a new challenge to ventilation system design. Refrigerants widely used in central chiller plant systems that will be prohibited in new equipment, though high in their measure of global warming potential (GWP), are classified by ASHRAE as having “no flame propagation” (or in the common vernacular, “not flammable”). Low-GWP refrigerants entering the market to replace them include a new collection of refrigerants classified as having low flammability. As a result, chiller plant ventilation design guidelines have undergone rapid change in pursuit of chiller plant safety.

Through the past two decades, after the phase-out of earlier common refrigerants such as R-22 had begun in service of environmental stewardship, refrigerants R-134a and R-410A have dominated usage in central chiller plants containing centrifugal and positive displacement chillers. These plants include refrigerant leak detection and ventilation systems governed by ASHRAE Standard 15 Safety Standards for Refrigeration Systems and its companion, ASHRAE Standard 34 Designation and Classification of Refrigerants.

The language of Standard 15 includes use of the term “machinery room” to define a space that contains compressors and pressure vessels or whose volume of refrigerant per circuit exceeds the refrigerant concentration limit (RCL) defined in Standard 34. Italicized text in the previous sentence indicates code-specific language. For the purposes of this discussion focused on central plant design, the term “chiller room” is used interchangeably with “machinery room.” Note that Standard 15-2022 includes extensive HVAC system design requirements for refrigerant-containing spaces that are not code-defined as machinery rooms. This article excludes discussion of those space types.

Standard 34 classifies the safety of every refrigerant along the measures of toxicity and flammability. All refrigerants include an alphanumeric designation to signify their location in the matrix illustrated in Standard 34, Figure 6-1. The alphanumeric designation is the “Safety Group Classification,” and Figure 1 shows the relative location of refrigerants with the safety group classification of R-134a and R-410A as well as the new refrigerants replacing them.

R-134a and R-410A have a safety group classification of A1. Use of both refrigerants is governed under a phase-out timeline established by the U.S. Environmental Protection Agency (EPA) which prohibits their use in new chillers beginning January 1, 2024. As a result, chiller manufacturers have adapted to the varying state-by-state adoption of the requirement by ending the sale of equipment charged with the two phased-out refrigerants in 2022. Chiller rooms installed with new chillers beginning in 2024 will require ventilation systems designed for replacement refrigerants.

One set of low-GWP refrigerants emerging in industry to replace R-134a and R-410A is R-32, R-454B, R-1234yf, and R-1234ze. All four refrigerants in this group have a safety classification of A2L, which indicates “low flammability” per Standard 34. For the first time, flammable refrigerants are entering widespread usage in HVAC chiller plants, which carries significant implications for the work of mechanical engineers.

Prior to 2019, ASHRAE Standard 15 did not include guidelines for ventilation of equipment rooms containing chillers with A2L refrigerants. Ventilation for A1 refrigerants included the requirements listed below, and chiller room HVAC systems were designed to meet these requirements. (The list aims to highlight the differences in design requirements between machinery rooms containing A1 and A2L refrigerants and is not intended to be comprehensive.)

Chiller rooms containing R-134a or R-410A were designed for the following:

• Refrigerant leak exhaust system capacity governed by equation Q = 100 x G^0.5 where Q = minimum airflow (cfm) and G is the total weight of refrigerant per individual circuit (pounds).
• Refrigerant detection system that alarms at occupational exposure limit (OEL) levels exceeding the limits defined in Standard 34.

ASHRAE Standard 15 published in 2019 introduced requirements for chiller rooms containing A2L refrigerants. Because these refrigerants are a flammable substance, and because they exist in central plants in large volumes, a new set of design requirements govern their usage. ASHRAE 15-2019 and 2022 have provided guidance on A2L chiller room design that will continue to actively advance with the issuance of addenda to ASHRAE 15-2022.

The goal of design guidelines for A2L refrigerant-containing chiller rooms, whether enhancements to previously defined guidelines for A1 chiller rooms or new requirements altogether, is to mitigate fire hazard in the event of a refrigerant leak. Standard 15 accomplishes this goal with a multipronged approach.

The simplified option is to classify the chiller room as a National Electrical Code (NEC) / National Fire Protection Agency (NFPA) 70 Class 1, Division 2 hazardous location. Due to the nature of central plants, which often contain large quantities of electrically-powered equipment of a variety of types serving multiple HVAC systems in one contiguous space, classifying a chiller room may as an NEC Class 1, Division 2 hazardous may be difficult or impossible. For these cases, Standard 15 offers a prescriptive set of criteria to achieve A2L safety compliance.

Exhaust System Capacity

Guidelines for A2L refrigerants include a dramatically increased minimum capacity for refrigerant leak exhaust systems. Rather than the simple equation used for A1 refrigerants, a calculation dependent on refrigerant pressure and the refrigerant charge of the largest independent circuit is provided to calculate the minimum exhaust capacity. The calculation for A2L refrigerants may be performed either using the equations in ASHRAE 15 Table 8-3 or the graphical method in Figures 8-1 and 8-2. The resulting airflow rapidly removes contaminated air at a rate far exceeding the minimum required for A1 refrigerants. Example calculations using the graphical method are shown in Figure 2 and summarized in Figure 3.

In the case of a 250-ton centrifugal chiller, a chiller room with equipment using R-134a requires a refrigerant leak exhaust system with a minimum capacity of 3,162 CFM. A room with equipment using R-1234ze requires a 19,000 cfm exhaust system.

In an example using higher-pressure refrigerants, a chiller room containing a bank of multiple 70-ton heat pump modules with R-410A requires a minimum refrigerant leak exhaust system capacity of 775 cfm. The same system in a chiller room with modules using R-454B, an A2L refrigerant, requires a minimum capacity of 9,000 cfm.

The elevated minimum exhaust rate for A2L refrigerants has significant implications for air intake and discharge coordination and the sizing and location of ventilation system equipment.

Refrigerant Detection System

The refrigerant detection system for chiller rooms containing A2L refrigerants is enhanced due to the elevated hazard associated with a low-flammability substance. The refrigerant detection system must monitor refrigerant concentration and activate emergency shut-down of refrigerant equipment, including compressors, pumps, and valves, upon detection of refrigerant concentration levels (RCL) exceeding 25% of the lower flammability limit (LFL) of the refrigerant. Additionally, a manual emergency shut-down controller must be located outside the door of the chiller room.

Standard 15 also defines refrigerant detector system minimum control requirements including criteria such as setpoints, response times, and alarms. Detectors must include automatic self-testing with alarm on failure. Interlocks with building automation system (BAS) for emergency shut-down of all electrically powered equipment upon alarm demands an additional layer of controls design not previously required for A1 refrigerant chiller rooms.

The described features of the refrigerant detection system are listed in addition to the baseline system requirements for A1 refrigerants including, but not limited to, OEL limit alarms. Controls systems for A2L refrigerant monitoring are a considerably more complex design than the types of systems previously required for chiller rooms containing R-134a and R-410A chillers.

Standard 15 includes many additional safety requirements for A2L refrigerants not reviewed comprehensively in this article. For example, exhaust fans serving the refrigerant leak exhaust systems may not include motors in the air stream, and fan components must be nonferrous or non-sparking. Exhaust inlets must be located within one foot of the expected concentrated elevation of refrigerant. The requirements of Standard 15 are numerous and warrant a systematic review by any engineer accustomed to designing systems around R-134a and R-410A.

While designing a chiller room, it is critical for a mechanical engineer to select a target refrigerant early in chilled water system design in order to establish chiller room ventilation requirements; coordinate space constraints such as intake and discharge dimensions with architects; write ventilation equipment specifications; and provide direction on specifications for every electrically-powered device located in the central plant room with all other MEP engineers. The variation between A1 refrigerant and A2L refrigerant requirements are numerous, and late-stage design changes at the scale required for converting from A1 to low-GWP A2L refrigerants in a chiller plant may be costly.

A smooth transition to low-GWP chiller systems demands a close reading of ASHRAE Standard 15-2022 by HVAC central plant designers to accommodate the introduction of flammable substances into our chiller rooms.