Four steps for designing a VRF system
Variable refrigerant flow offers an alternate HVAC solution
- Identify the codes and standards that govern refrigerant safety design of variable refrigerant flow systems.
- Assess the VRF design and implement refrigerant safety systems.
- Discuss refrigerant safety technology.
Variable refrigerant flow heating, ventilation and air conditioning technology systems are direct-expansion heat pump systems similar to other DX systems and have many of the same system components. There are two different terms used to refer to this type of technology:
- Variable refrigerant volume, a trademarked term.
VRF is the most widely used term for these types of systems. VRF was introduced to the HVAC commercial building market in the early 1980s in Japan. Since then, the VRF market has expanded its global presence to Europe in the early 1900s and to the United States in the early 2000s.
The U.S. VRF market is relatively small when compared to the Japanese market. In Japan, variable refrigerant flow systems are installed in nearly 50% of midsized commercial buildings and 33% of all large commercial buildings.
Many factors are contributing to VRF system growth in the U.S. market including but not limited to:
- Budgetary constraints.
- Ease of VRF system application in retrofit projects.
- U.S. energy efficiency codes constraints on commercial market energy consumption.
- Reasonable return on investments.
- Small mechanical footprints which can increase leasable space.
Variable refrigerant systems use a refrigerant as the heat transfer fluid like traditional ductless mini-split direct expansion systems. Multiple indoor fan coil units are piped to a central outdoor condensing unit. VRF systems can be set up to operate as a heat pump, cooling-only or heat-recovery arrangement.
Cooling-only VRF systems can only cool and heating is not an option. Heat pump VRF systems can provide heating or cooling to the indoor zones, but not both at the same time. All the indoor units connected to the heat pump system will operate in the same heating or cooling mode.
Heat recovery VRF systems can provide simultaneous heating or cooling to the indoor spaces. The indoor units connected to a heat-recovery system can operate independently in heating or cooling mode. Dependent on the manufacturer and selected system type, the VRF system will be a two- or three-pipe system with several refrigerant branch/circuit controllers to direct the refrigerant to the indoor fan coil units.
Refrigerant safety for VRF systems
A principle issue for many consulting engineers and owners is the presence of refrigerant piping in occupied zones. Occupant safety should not be a concern when applicable refrigerant safety standards and guidelines are applied. In the United States, VRF system safety requirements are addressed and incorporated into the design by using ASHRAE Standard 15 (packaged with Standard 34): Safety Standard for Refrigerant Systems and Designation and Classification of Refrigerants.
Standard 15 establishes guidelines and practices for the “design, construction, test, installation, operations and inspection of mechanical and absorption refrigeration systems, including heat pump systems.” Standard 34 creates a “uniform system for assigning refrigerant reference numbers, safety classifications and refrigerant concentration limits to refrigerants” dependent on refrigerant type and space occupancy classification.
Standard 15 and Standard 34 are National Voluntary Consensus Standards, which means its provisions are not mandatory until adopted as code by a state or local government. Because the adoption process usually includes amendments, it is recommended to review your local codes for any alterations or revisions to Standard 15 and/or Standard 34. If you are designing a project outside the United States, you may be required to design using the local refrigerant safety standards. A few examples of international standards equivalent to ASHRAE Standard 15 are the International Organization for Standards 5149 and Japanese Refrigeration and Air Conditioning Association GL-13:2012.
As with any HVAC system, the designer must follow a standard of care when designing VRF systems. The refrigerants used in these systems are often heavier than air and can be a health hazard to the public. In addition, your client (the owner) may have additional requirements that are more stringent than the minimum standard of care established by the above documents.
ASHRAE Standards 15 and 34 establish minimum refrigerant concentration limits, minimum allowable floor area in occupied spaces and require monitoring for refrigerant leaks in mechanical rooms under certain conditions. How does this standard apply to occupied spaces where refrigerant is the heat transfer fluid in a variable refrigerant flow system? How do you apply refrigerant safety standards and what safety measures apply to commercial refrigerant air conditioning systems in occupied buildings? The following sections will attempt to answer these questions and provide recommendations to comply with the Standards. Additionally, ASHRAE recently released a new guideline to provide information and guidance on VRF systems called Guideline 41-2020: Design, Installation and Commissioning of Variable Refrigerant Flow Systems.
Using ASHRAE 15 and 34 to design VRF
Based on the 2016 editions of ASHRAE Standards 15 and 34, variable refrigerant systems are categorized as follows.
- Standard 15 classifies variable refrigerant flow systems as “direct systems” and “high-probability systems.” This means the VRF indoor unit evaporator coils are in direct contact with the conditioned air stream and have a high potential to leak refrigerant into the occupied space.
- All VRF systems sold in the U.S. market use refrigerant 410A. ASHRAE Standard 34 Table 4-2 lists R-410A as a safety classification group A1. Group A1 refrigerants are nontoxic and nonflammable with zero ozone depletion potential.
- Refrigerant 410A is heavier than air and will displace oxygen. Therefore, Standard 34 has established the maximum refrigerant concentration limit of 26 pounds/1,000 cubic foot of room volume for occupied spaces as listed in Table 4-2.
ASHRAE Standard 15 and ASHRAE Standard 34 requirements can be applied to VRF system designs in the following four steps:
Step 1 — Develop a preliminary system layout: The first step in the VRF system design process is similar to traditional HVAC design procedures, and should be familiar. Develop a complete preliminary system layout, including all refrigerant piping, indoor fan coil units, branch/circuit controllers and outdoor condensing units that meet the HVAC load, control and performance requirements. Goals during this step are to locate the branch selectors in accessible locations, limit the number of indoor fan coil units and minimize piping lengths to control costs and reduce refrigerant charge.
Step 2 — Estimate the refrigerant charge: During this step, you will determine the total amount of refrigerant 410A in the VRF system. The preferred option to complete this step is to request initial equipment selections and refrigerant quantities from a local manufacturer’s representative based on the preliminary layout developed in step 1. In addition to efficiently and effectively using your time, the representative can provide you with the total refrigerant charge, refrigerant pipe sizing and diagrams, equipment selection options and wiring diagrams. If changes are required, the selection software can be easily updated and system parameters recalculated. Note, this calculation can be completed manually using manufacturer installation manuals and refrigerant pipe-sizing charts but is time consuming.
A special note should be made that Standard 15 exempts small systems with 6.6 pounds of refrigerant or less than the RCL requirements if the system is installed per the manufacturer’s installation instructions and the equipment listing. This exemption applies to all refrigerants regardless of the refrigerant safety classifications.
Step 3 — Verify compliance: The purpose of this step is to verify the initial variable refrigerant flow system layout complies with Standards 15 and 34 by:
- Determine the occupancy classification for the rooms — The occupancy classifications used by Standard 15 are not the same as the occupancy classifications used by the International Building Code. The classifications are similar, but the definitions established by Standard 15 must be used when determining the refrigerant concentration limit for the project. The occupancy classifications established by Standard 15 are: institutional, public assembly, residential, commercial, large mercantile, industrial and mixed. Institutional occupancies RCLs are restricted to 50% of the values shown in Standard 34 Tables 4-1 and 4-2. This makes the refrigerant 410A RCL for institutional occupancies 13 pounds/1,000 cubic feet of occupied area. Industrial occupancies have several special conditions that may apply, see Standard 15 Section 7.2.2.
- Determine the minimum allowed floor area — Calculate the minimum allowed floor area (square feet) based on the total system refrigerant charge and ceiling height using the formula below.
Minimum allowed floor area (square feet) = total system refrigerant charge (pounds)/[(RCL (pounds/1,000 cubic feet) x ceiling height (feet)] x 1,000
- Verify all the room’s volumes are adequate — Verify that none of the rooms that have a part of the refrigeration system installed are smaller than the minimum allowed floor area. Standard 15 Section 7.3 states volume calculations shall be based on the volume of the space that refrigerant disperses to in case of a leak. This includes all spaces that have any refrigerant containing part installed in addition to rooms where indoor fan coil units are installed. This step is completed by comparing the room areas served by the VRF system against the minimum allowed floor area determined above. Note, the project may contain several minimum allowed floor areas for consideration dependent on several factors such as: varying ceiling heights, multiple occupancies in the building or multiple VRF systems with different refrigerant charges.
- Verify refrigerant piping installation requirements — ASHRAE Standard 15 stipulates specific installation requirements for refrigerant piping systems as follows:
- Refrigerant piping cannot be installed less than 7.25 feet above the floor.
- Piping cannot be installed in a shaft containing a moving object (e.g., an elevator or dumbwaiter).
- Piping cannot be installed in enclosed stairways, landing or means of egress.
- Piping must be properly supported and isolated.
Step 4 — Apply corrective actions to achieve compliance: If the smallest room is below the minimum allowed floor area the following steps may be applied.
- If the room is too small, it may be possible to increase the room volume by connecting it to other rooms, which then includes the volume of both rooms in the calculations. Common connection methods are to use HVAC construction methods such as louvers, ducts, transfer grilles or similar methods.
If the plenum space above the ceiling is used as a supply or return plenum, then that area may be included in the room volume calculations per Standard 15 Section 188.8.131.52. Locating the indoor unit fan coil in the plenum space above the suspended ceiling and ducting it to one or more rooms and using the plenum as the return air path will comply with the standard and increase the room volume.
For systems with ducted supply and return, the ceiling could be raised to a height that provides the required room volume or removed altogether. See Figure 1 for minimum allowable floor areas for refrigerant 410A calculated for common ceiling heights.
Standard 15 Section 7.3.1 nonconnecting spaces states rooms can be connected by permanent openings, but does not define the minimum size or location of a permanent opening; thereby, engineering judgment is necessary when incorporating a permanent opening. These openings should be located low, as the refrigerant is heavier than air and settles at the floor. Typically, designs have connected spaces by undercut doors and door transfer grilles. Some VRF manufacturers state in their technical manuals that a permanent opening is defined as equal in area to 0.15% or more of the total floor area of the smaller enclosed occupied space in which refrigerant containing parts are located. The 0.15% definition is based on the Japanese guideline mentioned previously, but has not been formally adopted by ASHRAE.
- Another solution would be to review the piping layout to see if it can be altered or optimized to reduce piping lengths and refrigerant charge. This is a trial and error iterative process best accomplished using manufacturer system selection software and assistance from a manufacturer’s representative.
- Depending on the system layout, size, building characteristics and other mitigating factors the variable refrigerant flow system may be divided into multiple smaller and separate decentralized systems. Subdividing a VRF system is often an iterative process completed by using the manufacturer’s system design software to develop and compare multiple subdivision options. This typically decreases the refrigerant charge in a single system to at least half of the original system layout. Additional benefits to decentralizing the VRF system can be reduced installation costs due to smaller refrigerant pipe sizes and smaller condensing units.
- If none of the above corrective actions to increase the room volume can be used, a last resort option may be to use the Standard 15 small system exemption. This solution entails removing the room from the VRF system including removing all refrigerant containing components from the room. The room can be served by a dedicated mini-split system that complies with the Standard 15 small-system exemption.
Refrigerant monitoring and alarming systems
Standard 15 only requires a refrigerant monitoring and alarm systems in refrigerant machinery rooms in case of a leak as stated in Section 184.108.40.206. However, local codes or client standards may require a monitoring and alarm system in occupied spaces. Refrigerant monitor system manufacturers have anticipated a requirement for leak detection in occupied spaces and offer monitor systems that fit within standard electrical gang boxes. These detectors are typically mounted 12 to 18 inches above the floor and within the ceiling plenum space.
Most systems are compatible with BACnet and ModBus building management systems. The refrigerant monitoring system should initiate a variable refrigerant system shutdown, local notification through an audible alarm and notify the building management system. Alarming a specific monitoring zone area can help direct maintenance staff and reduce the time it takes to locate the leak.
VRF systems are relatively new to the U.S. market and the construction experience and quality control knowledge are developing. Quite often, VRF system operational issues can be traced back to improper installation. Installation details and requirements can vary from manufacturer to manufacturer. Therefore, it is important for the contractor to follow the unit arrangements, refrigerant piping diagrams, control wiring diagrams and installation manuals provided by the manufacturer. If field changes are required, the contractor should check with the engineer of record and manufacturer to verify if the changes will have an effect on the VRF system.
One of the best ways to have a compliant and reliable variable refrigerant flow systems is through proper training. Many manufacturers offer training courses focused on the design and construction of the system. Design side training focuses on equipment sizing and design tools. The construction side training emphases installation details, startup, operations, troubleshooting and system maintenance. Engineers and owners should require that the installing contractor attend an instructor-led hands on construction training program provided by the VRF manufacturer within the contract documents. Often, manufacturers will offer an extended warranty if the contractor attended their training.