Four Steps to Designing a VRF System | Consulting - specification engineer (2023)

Variable refrigerant flow HVAC systems are direct expansion heat pump systems similar to other DX systems and share many of the same system components. There are two different terms used to refer to this type of technology:

• Variable refrigerant volume, a protected term.
• VRF.

VRF is the more common term for these types of systems. The VRF was introduced in theCommercial HVAC Building MarketIn Japan in the early 1980's. Since then, the VRF market has expanded its global presence in Europe in the early 1900's and the United States in the early 2000's.

The US VRF market is relatively small compared to the Japanese market. In Japan, variable refrigerant flow systems are installed in nearly 50% of medium-sized commercial buildings and 33% of all large commercial buildings.

Many factors are contributing to the growth of VRF systems in the US market, including but not limited to:

• budget constraints.
• Simple application of the VRF system in retrofit projects.
• WE.energy efficiencyCodes of Restrictions on the Commercial Use of Energy in the Market.
• Reasonable return on investment.
• Small mechanical footprint that can increase usable space.

Variable refrigerant systems use a refrigerant as the heat transfer fluid like traditional direct expansion systems without mini-split ducts. Multiple indoor fan coil units are routed to a central outdoor condensing unit. VRF systems can be configured 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 either heat or cool interior zones, but not both at the same time. All indoor units connected to the heat pump system work in the same heating or cooling mode.

VRF systems with heat recovery can heat or cool interior spaces at the same time. Indoor units connected to a heat recovery system can operate independently in heating or cooling mode. Depending on the manufacturer and the type of system chosen, the VRF system is a two-pipe or three-pipe system with multiple refrigerant branch/loop controllers to route the refrigerant to the indoor fan coil units.

Refrigerant safety for VRF systems

A major concern for many consulting engineers and owners is the presence of refrigerant lines in occupied areas. Occupant safety should not be an issue when applicable refrigerant safety standards and guidelines are applied. In the United States, VRF system safety requirements are addressed and incorporated into the design through the useASHRAE Standard 15 (packaged with Standard 34): Safety standard for refrigeration systems and designation and classification of refrigerants.

Standard 15 sets out policies and practices for "design, construction, testing, installation, operation and inspection of mechanical and absorption refrigeration systems, including heat pump systems". Standard 34 creates a "uniform system for assigning refrigerant reference numbers, safety ratings and refrigerant concentration limits to refrigerants" depending on the type of refrigerant and room occupancy classification.

Pattern 15 and Pattern 34, respectivelyNational Voluntary Consensus Standards, which means that its provisions are not binding until they are adopted as a code by a state or local government. As the adoption process often involves changes, it is recommended that you check local regulations for any changes or revisions to Standard 15 and/or Standard 34. If you are designing a project outside of the United States, you may need to use local regulations to refrigerant safety standards. Some examples of international standards conforming to ASHRAE Standard 15 are theInternational Organization for Standards 5149eJapan 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 may pose a health risk to the general public. In addition, your customer (the owner) may have additional requirements that are more stringent than the minimum standard of care set out in the documents above.

ASHRAE standards 15 and 34 specify minimum refrigerant concentration limits and the minimum allowable area in occupied spaces and require monitoring of refrigerant leaks in machinery spaces under certain conditions. How does this standard apply to occupied spaces where the refrigerant is the heat transfer fluid in a variable refrigerant flow system? How do you enforce refrigerant safety standards and what safety measures apply to commercial refrigerant air conditioning systems in residential buildings? The following sections attempt to answer these questions and provide recommendations for compliance. In addition, ASHRAE recently released a new policy to provide information and guidance on VRF systems, Policy 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 that the VRF indoor unit evaporator coils are in direct contact with the air conditioning flow and have a high potential for refrigerant leakage into the occupied space.
• All VRF systems sold in the US market use 410A refrigerant. ASHRAE Standard 34 Table 4-2 lists R-410A as an A1 safety classification group. Group A1 refrigerants are non-toxic and non-flammable and have no ozone depleting potential.
• Refrigerant 410A is heavier than air and displaces oxygen. Therefore, Standard 34 specified the maximum refrigerant concentration of 26 pounds/1000 cubic feet 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 the traditional HVAC design process and should be familiar. Develop a complete preliminary system layout, including all refrigerant piping, indoor fan coil units, branch/loop controllers, and outdoor condensing units that meet HVAC load, control, and capacity requirements. The objectives in this step are to place branch selectors in accessible locations, limit the number of indoor fan coil units and minimize pipe lengths to control costs and reduce refrigerant charge.

Stage 2 - Estimate the refrigerant charge:

In this step, you determine the total amount of 410A refrigerant in the VRF system. The preferred option for completing this step is to request EO selection and refrigerant quantities from a local manufacturer's representative based on the preliminary layout developed in Step 1. In addition to an efficient and effective use of your time, the representative can provide the you all refrigerant charging, sizing and refrigerant piping diagrams, unit selection options and wiring diagrams. If changes are required, the design software can be easily updated and system parameters recalculated. Note that this calculation can be done manually using manufacturers installation manuals and refrigerant line sizing charts, but it is time consuming.

It should be noted that Standard 15 exempts small systems with 6.6 pounds of refrigerant or less than the RCL requirements when the system is installed in accordance with the manufacturer's installation instructions and equipment list. This exemption applies to all refrigerants, regardless of the refrigerant's safety classification.

Step 3 - Check compliance:

The purpose of this step is to verify that the initial layout of the variable refrigerant flow system complies with standards 15 and 34:

• Determine occupancy rating for rooms – The occupancy ratings used by Standard 15 do not match the occupancy ratings used by Standard 15International Building Code. The ratings are similar, but the Standard 15 definitions should be used in determining the design refrigerant concentration limit. The occupancy classifications specified by Standard 15 are: institutional, public meeting, residential, commercial, large commercial, industrial, and mixed. RCLs for institutional occupancy are limited to 50% of the values ​​indicated in Rule 34 Tables 4-1 and 4-2. This makes the 410A RCL institutional grade refrigerant with 13 pounds/1000 cubic feet of floor space. Various special conditions may apply to commercial uses, see Bylaw 15, Section 7.2.2.
• Determine Minimum Allowable Floor Area – Calculate the minimum allowable floor area (square feet) based on the total system refrigerant charge and ceiling height using the formula below.
  Minimum allowable floor area (square feet) = Total System Refrigerant Charge (lbs)/[(RCL (lbs/1000 cubic feet) x Ceiling Height (ft)] x 1000
• Ensure that all room volumes are adequate - Ensure that none of the rooms in which any part of the cooling system is installed are smaller than the minimum allowable floor area. Section 7.3 of Standard 15 states that volume calculations shall be based on the volume of the space into which the refrigerant will disperse in the event of a leak. This includes all rooms where parts containing refrigerant are installed, in addition to rooms where fan coil unit indoors are installed. This step is completed by comparing the areas of the room served by the VRF system with the minimum allowable area determined above. Please note that the design may have multiple minimum allowable areas to consider depending on various factors such as: For example: different ceiling heights, multiple building occupancy or multiple VRF systems with different refrigerant charges.
• Review refrigerant piping installation requirements - ASHRAE Standard 15 specifies specific installation requirements for refrigerant piping systems as follows:
  • Refrigerant lines must not be installed lower than 7.25 feet from the floor.
  • Tubes cannot be installed in a shaft that contains moving objects (for example, an elevator or lift).
  • Ducts must not be installed in stairwells, landings or closed exits.
  • Piping must be adequately supported and insulated.

Step 4 - Apply corrective actions to achieve compliance:

If the smallest room is below the minimum allowable area, the following steps may apply.

• If the room is too small, the volume of the room can be increased by connecting it to other rooms, in which case the volume of both rooms is included in the calculation. Common connection methods are using HVAC construction methods such as shutters, ducts, transmission grids or similar methods.
  If the plenum above the ceiling is used as a supply or return air plenum, this area may be included in room volume calculations in accordance with Standard 15 Section 7.3.3.2. Locating the fan coil unit of the indoor unit in the duct above the false ceiling and leading it to one or more rooms and using the duct as an air return path meets the standard and increases the volume of the room.
  For ducted flow and return systems, the ceiling can be raised to a height that provides the required volume of space, or removed entirely. See Figure 1 for the minimum allowable floor areas for 410A refrigerant calculated for typical ceiling heights.
  Bylaw 15 Section 7.3.1 Unconnected spaces states that spaces may be connected by permanent openings, but does not define the minimum size or location of a permanent opening; therefore, engineering judgment is required when installing a permanent vent. These openings must be deep because coolant is heavier than air and will settle to the bottom. Designs typically have rooms connected through recessed doors and entry railings. Some VRF manufacturers state in their technical manuals that a permanent opening is defined as an area equal to 0.15% or more of the total floor area of ​​the smallest enclosed space occupied containing parts containing refrigerant. The definition of 0.15% is based on the aforementioned Japanese guideline, but has not been officially adopted by ASHRAE.
• Another solution would be to review the piping layout to see if it can be modified or optimized to reduce piping length and refrigerant charge. This is an iterative, trial-and-error process that is best accomplished with vendor-supplied system selection software and assistance from a vendor representative.
• Depending on system layout, size, building characteristics and other mitigating factors, the variable refrigerant flow system can be divided into several smaller, separate decentralized systems. Breakdown of a VRF system is generally an iterative process, completed using the manufacturer's system design software to develop and compare various breakdown options. This typically reduces the refrigerant charge on a single system to at least half of the system's original design. Additional benefits of VRF system decentralization can include reduced installation costs due to smaller refrigerant pipe sizes and smaller condensing units.
• If none of the above corrective actions to increase room volume can be applied, the last resort is to use the Standard Small Systems Exception 15. This solution involves removing the room from the VRF system, including removing all refrigerant-containing components from the living room. The room may be serviced by a dedicated mini-split system that complies with standard 15 small system exemption.

Refrigerant monitoring and alarm systems

Standard 15 only requires a refrigerant monitoring and alarm system in refrigerated machine rooms in case of leakage, as specified in clause 8.11.2.1. However, local regulations or customer standards may require a surveillance and alarm system in occupied spaces. Manufacturers of refrigerant monitoring systems have anticipated the need for leak detection in occupied spaces and are offering monitoring systems that fit into standard electrical junction boxes. These detectors are typically mounted 12 to 18 inches from the floor and within the ceiling void.

Most systems are compatible with BACnet and ModBus building management systems. The refrigerant monitoring system shall initiate variable refrigerant system shutdown, local notification via an audible alarm, and notification to the building management system. Alerting a specific area of ​​the surveillance zone can direct maintenance personnel and reduce the time needed to locate the leak.

VRF-Training

VRF systems are relatively new to the US market and design experience and quality control knowledge is evolving. VRF system operational issues can often be traced back to improper installation. Installation details and requirements may vary from manufacturer to manufacturer. Therefore, it is important that the contractor follow the manufacturer's equipment layouts, refrigerant piping diagrams, control circuit diagrams, and installation manuals. If site changes are required, the contractor should verify with the recording engineer and the manufacturer that the changes will affect the VRF system.

One of the best ways to obtain a compatible and reliable variable refrigerant flow system is with proper training. Many manufacturers offer training focused on system design and construction. Design-side training focuses on equipment sizing and design tools. On-site training focuses on the details of installing, commissioning, operating, troubleshooting and maintaining the system. Engineers and Owners should require the installing contractor to participate in an instructor-led hands-on construction training program provided by the VRF manufacturer in the contract documentation. Manufacturers often offer an extended warranty if the contractor has attended training.

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