Know when, where to specify VRF systems
Variable refrigerant flow (VRF) offers an alternate HVAC solution. Its key attributes include zonal control, energy efficiency, and indoor air quality (IAQ). To help facilitate their successful design and operation, engineers should take advantage of the available resources and training offered by manufacturers, utilities, and others.
- Discuss variable refrigerant flow (VRF) technology.
- Assess VRF design and implementation.
- Make use of manufacturers to better understand VRF installation, controls, etc.
Variable refrigerant flow (VRF) systems were invented in Japan in the early 1980s and have become common in Asia, Europe, and Australia. Over the last 5 to 10 years, VRF also has started to penetrate the U.S. market. This technology provides a great alternative to traditional HVAC systems.
VRF systems are common on existing building renovations in classroom and office buildings where existing conditions make this technology more constructible. Often, the owner has an interest in upgrading the existing HVAC system and is looking for improved energy efficiency and increased controllability. However, this technology is in use in new construction projects, as well, and should be considered.
At a high level, the system uses multiple evaporators connected to a single condensing unit. The compressor in the condensing unit is inverter-driven and operates as a variable speed system, as opposed to only on-off functionality. This allows only the required flow of refrigerant to be delivered to the system and results in lower energy use at part-load conditions. These systems are thermodynamically similar to a traditional direct expansion (DX) refrigerant system; however, the key differentiator is that multiple evaporators are connected to the system.
The system can be set up as a heat pump, cooling-only, or heat-recovery configuration. Zoning of the system is paramount to capturing the possible energy savings. In the heat pump mode, all indoor units connected to the system are either operating in heating or cooling at any given time, with each unit having control over its zone temperature.
VRF is most beneficial when spaces can be zoned such that heat can be rejected from spaces in cooling and diverted into those requiring heating. This is the basis for the heat-recovery system. All indoor units are provided with individual control and can individually operate in heating or cooling mode at a given time.
VRF systems use refrigerant as the driver to move heat throughout the system. Although refrigerants are very effective and efficient at doing so, they also present other concerns. Most of the VRF systems in the market today operate using R-410A refrigerant. ASHRAE Standard 15 (packaged with Standard 34): Safety Standard for Refrigeration Systems and Designation and Classification of Refrigerants sets the maximum limit of R-410A refrigerant in a single circuit as 25 lbs/1,000 ft3 before the space must be defined as a refrigeration machinery room. As more evaporators are connected to a single condenser, and additional piping runs are necessary to distribute to the system, the quantity of refrigerant in this circuit increases.
A rule of thumb for refrigerant charge in a VRF system is 4 to 6 lbs of refrigerant per ton of cooling. Actual numbers will be dependent on the capacity of equipment, number of pieces of equipment, and piping lengths. As an example, a project with a 16-ton VRF system would require roughly 64 to 96 lbs of R-410A.
A typical 10x10-ft office space with a 10-ft ceiling would easily exceed the refrigerant limits under the assumption above. To adhere to the ASHRAE 15 requirements, the system may need to be broken down into smaller refrigerant circuits, thus losing some of the benefits of diverting loads. Space size, system capacity, and piping length all impact the allowable refrigerant in a given circuit.
The ventilation-air strategy of the system must be given careful consideration to ensure the best indoor air quality (IAQ). There are three main methods of introducing outside air into the system:
- Direct to the unit
- Via a dedicated outside air system (DOAS) or energy-recovery ventilator (ERV)
- Directly to the space.
Direct-to-the-unit configurations are best suited for mild climates and spaces with low ventilation-air requirements, as the evaporators are limited in latent capacity (typically 70% to 80% sensible heat ratio) and low entering conditions (typically above 50° F). DOAS and ERV will pretreat the ventilation air and allow for the ventilation loads to be mostly decoupled from the space loads. This allows the VRF to be sized for only the space loads, which reduces the VRF system capacity.
The last method of supplying directly to the space requires a DOAS or similar equipment and completely decouples the ventilation load from the VRF.
VRF manufacturers offer several options for controls including integration to a centralized energy management system (EMS). At the simplest level, factory-packaged simple controllers offer programming modes of operation, setpoints, and fan speed. The controller also offers feedback on failure codes for troubleshooting. This is essentially a self-contained system with no other interaction necessary.
However, this option is limited in the interface and information available. VRF manufacturers offer central controllers that can group multiple systems into an interface with even further controllability, such as scheduling. Lastly, a gateway controller can be provided to interface with a central EMS, which allows for more advanced monitoring and control.
It is worth noting that several VRF manufacturers offer controls for energy tracking, which allow owners/operators to track energy to individual units. This is most often used for metering and charging tenants for the HVAC system associated with their space.
Designers and contractors have become more familiar with VRF technology; however, many manufacturers offer training focused on both the design and construction of the system. These manufacturers will often offer extended warranties for projects where the designer and/or contractor attended their training.
The training on the design side will often focus on proper sizing of equipment and available tools for design, while the construction side will offer education on proper installation practices, startup, operations, etc. Owners should ensure that the installing individual onsite actually participated in the training and not just the contracting company. It's best to identify the available resources offered by the manufacturer and use them appropriately, as it can improve the delivery of the final product to the owner.
James Del Monaco is group manager/sustainability director at P2S Engineering.