Representing the fastest-growing segment of the U.S. HVAC market, variable refrigerant flow (VRF) is a flexible, cost-effective, and highly efficient HVAC option for buildings with multiple floors and areas.
VRF provides precise personal-comfort control by moving refrigerant through piping to zones in need of cooling or heating. Some VRF systems cool and heat simultaneously, providing personalized comfort to each zone or space regardless of time of day, sun or shade, special requirements, or season. Operation regardless of season is a key point here, as recent technological advances have made VRF systems applicable in even the coldest and harshest climates.
This article discusses these advances and provides an overview of VRF technology.
Four trends have helped to shape the current landscape for VRF, highlighting what users demand and, thus, specifying engineers must consider:
- Demand for personalized comfort. From one space to the next in multifamily buildings, hospitals, high-rise office buildings, and many other applications, occupants often prefer vastly different temperatures.
- Demand for greater energy efficiency. HVAC accounts for as much as 40 percent of a building’s energy costs. As sustainable design increases in popularity, as utility rates increase, and as energy codes become more stringent, a HVAC system’s efficiency and responsibility become more important.
- Demand for space-saving, flexible design. Engineers are called on to provide HVAC solutions that not only are effective, but minimize the space occupied by outdoor units, ductwork, and indoor units.
- Demand for better control. As they become increasingly reliant on cell phones and tablets, end users want to operate their HVAC systems via mobile device or, in the case of larger commercial operations, an advanced controls system. End users are demanding access at any time from any place.
VRF addresses the demand for personalized comfort control. Consider a hospital in which an elderly patient in constant need of warming neighbors an active patient in need of cooling or an apartment building with tenants in need of cooling in a sun-drenched living room and warming in bedrooms. With zoning capabilities and simultaneous cooling and heating, VRF makes this possible.
Fixed-speed compressors typically found in conventional HVAC systems operate at two speeds: full power and off. In the United States, a zone exhibits part-load conditions more than 90 percent of the time. Conventional systems cannot handle part-load requirements, which leads to energy fluctuations and poor set-point satisfaction. With inverter-driven compressors, VRF systems offer full-range variable capacity to deliver only the amount of cooling or heating a zone needs. Working in tandem with integrated controls and sensors that measure each zone’s load, inverter-driven compressors seamlessly adjust speed to maintain the desired capacity level. This function, along with a low-profile ducted or ductless design, typically increases energy efficiency by about 30 percent, compared with conventional ducted systems, partly because of the energy lost by forcing air through ductwork.
As a result, VRF can help facilities meet ENERGY STAR, Green Globes, and LEED (Leadership in Energy & Environmental Design) requirements and achieve the highest integrated energy-efficiency ratios.
VRF is highly adaptable, which makes it well-suited for projects requiring space-saving, flexible design. Modular outdoor units are easy to install and can be tailored to almost any building design, including ones requiring their placement on the roof. In the case of many VRF units, both air and water-source models with a wide variety of capacities are available. Indoor units are compact and come in multiple styles, designed to blend into their environments. Ductless options can include wall-mounted, floor-standing (exposed and concealed), ceiling-recessed cassette, and ceiling-suspended models. Ducted options include vertical-concealed and ceiling-concealed models. The ductless options in particular free valuable interior space and enable increased ceiling height.
Advanced control systems that integrate VRF address the industry’s demand for better control. Capabilities include occupancy sensing, dual-temperature set point and setback, mode scheduling, the ability to integrate third-party equipment, and storage of maintenance data. Meanwhile, advanced control systems can integrate an unlimited number of additional automated systems from any manufacturer.
The Need for Heating Technology
Traditionally, VRF faced challenges in cold climates. At outside temperatures below 17°F, VRF systems would lose capacity or shut off, and auxiliary heating would kick in. This was far from ideal—gas often is much costlier than electricity, and most end users do not want to deal with the operation and maintenance of two separate systems if they instead can use one. After years of research and development, hyper-heating technology was developed.
Developed for areas where outdoor temperatures regularly are below 0°F, hyper-heating technology provides full-rated heating capacity at 5°F and substantial (up to 80 percent) heating capacity at −13°F. This is accomplished in one of two ways.
The first, which is most common with residential equipment, involves the use of a highly efficient, robust compressor. The compressor is capable of producing high capacity at lower temperatures and gives connected units the ability to produce ample heat. This method generally is typical of units with capacities below 24,000 Btuh.
The second involves the use of the same highly efficient compressor and the introduction of two changes to the refrigerant circuit:
- A tube-in-tube heat exchanger that pre-cools the refrigerant in condensing-unit coils. Lowering an outdoor unit’s coil temperature achieves higher capacities in extreme cold.
- The introduction of subcooled refrigerant directly into the compressor. This keeps the compressor cool while operating at high frequencies and adds volume back to the refrigerant system.
Both of these technologies generally are utilized in units with capacities exceeding 24,000 Btuh.
Features and Benefits
Efficiency. Hyper-heating technology provides not only high heating capabilities, but high efficiency. On the residential side, the main highlight is an impressive seasonal energy-efficiency ratio. In many situations, the additions to a system made to achieve hyper-heating boost performance and efficiency.
Cost. While hyper-heating technology represents an investment for an owner, it provides long-term savings opportunities. Units with hyper-heating technology often feature an inverter compressor. In many cases, these systems are capable of saving up to 40 percent on utility bills, compared with less-efficient conditioning methods.
Design. VRF equipment with hyper-heating technology is simple to design and specify. Using manufacturer-designed tools, engineers can build systems, produce AutoCAD or similar-format drawings, and even import custom-built Revit models.
Ultimately, the result of hyper-heating technology is that VRF can be the sole source of heat in all but the most extreme environments, and even in those environments, it is capable of meeting the majority of conditioning requirements. No longer is fire the best way to heat a space.
VRF with hyper-heating technology has seen a multitude of successful applications across the country. These applications often have been projects where comfort is of the utmost importance—where it must be guaranteed (e.g., multifamily condominium building, nursing home, extended residence). Examples range from a high-rise mixed-use building in downtown Detroit to a home in Chicago seeking Passive House certification. In all cases, hyper-heating technology has kept occupant comfort high and utility bills low.
As senior manager, commercial marketing, for Mitsubishi Electric US Inc. Cooling & Heating Division, Kevin Miskewicz leads marketing strategies, programs, and initiatives for the division’s commercial product line. Previously, he served as product manager for condensing units and controls for Heatcraft Worldwide Refrigeration. He is a certified LEED Green Associate.
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