April 10, 2009
  New and Cool: Variable Refrigerant Flow Systems
Superior control and efficiency are bringing VRF systems to America

by Sara Fernández Cendón

Summary: Variable refrigerant flow (VRF) systems have been around for almost three decades, but they’re new to the U.S. HVAC market. As American engineers become familiar with the technology, and especially as they learn of its energy efficiency advantages, more in the industry might be willing to give the systems a try.

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1: CITY MULTI VRFZ systems are installed on the roof of the Burlingham Hall residence hall at Pacific University in Pacific Grove, Ore. Image courtesy of Mitsubishi.
2: Daikin compressing units installed on the roof of a building. Each of these can be connected to multiple evaporators inside the building. Image courtesy of Daikin.
3: A central monitor that controls the entire VRF system. Image courtesy of Daikin.
4: A heat recovery system diagram.

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Cooling the old-school way
If you’ve been shopping for HVAC systems lately, you might have encountered a new contender among the usual choices. Introduced in the U.S. about five years ago, VRF systems were invented in Japan more than 20 years ago. They’re widely used not only in Asia, but also in Europe and South America.

VRF systems manufacturers highlight qualities such as energy efficiency, design flexibility for architects and engineers, quiet operation, and the ability the system grants individual users to control temperature in their own areas. Another appealing feature offered by most manufacturers is a centralized monitoring application that gives users control over the entire system from a single location or via the Web. The technology that makes it all possible is sophisticated, but VRF systems (also known as VRV, or variable refrigerant volume systems) are not very complicated.

A quick review of air-conditioning principles might be useful in describing VRF technology—the most basic principle, of course, being that air conditioning removes heat from the space to be cooled by pushing refrigerant through a cycle. The cycle comprises four elements common to all HVAC systems, which is based on the fluid dynamics that when a refrigerant expands, it becomes cooler; when it is compressed, it becomes warmer; and changing phases from fluid to gas or back again adds to the cooling/warming effect. So the system is composed of a compressor, a condensing unit, a metering device (or expansion valve), and an evaporator or heat sink.

In a direct expansion (DX) system, the simplest among air conditioning systems, the “hot” part of the cycle starts at the compressor, which compresses refrigerant vapor and turns it into a high-temperature gas. The refrigerant then goes through a condensing unit, a series of coils in which the gas loses heat and becomes liquid. The “cold” part of the cycle begins as the liquid refrigerant passes through the metering device, which causes a drop in pressure. The refrigerant then goes through the evaporator (another series of coils), and in the process of evaporating it absorbs heat from the surrounding area, producing a cooling effect that is dissipated through fans. After completing the cycle, the refrigerant goes back to the compressor in its initial low-pressure, gaseous state.

Slight variations in the refrigerant cycle have led to different applications designed for different uses. Window units, for example, pack all the elements of the cycle into one small device—the hot side being on the outside, the cool part facing the space to be cooled. Split-system units split the hot side of the cycle (placed outside the building) from the cold side (inside). In these types of systems, cool air is often transferred from the evaporator to many different rooms by an air-handling unit, which distributes the conditioned air through a series of ducts.

Industry standards set limits on the length of piping running between the condenser and the evaporator in DX systems. When the needs of a particular project exceed such limits, chilled water systems are often used as an alternative. In chilled water systems water is cooled by a regular refrigeration system and then circulated through ducts to air handlers throughout the building. Because there is no limit to the permitted length of water pipes, these systems are often used to cool large buildings or entire campuses. Chilling is often cycled at night to take advantage of off-peak energy rates.

The variable beauty of VRF technology
Configurations vary among the types of air-conditioning systems available, but one key ratio remains the same: always one condensing unit to one evaporator. For DX systems, this means that once a condensing unit is connected to an evaporator inside the building, providing cool air to several spaces requires either ductwork or additional condensing units and evaporators.

Not so with VRF systems, in which one condensing unit can be connected to multiple evaporators, each individually controllable by its user. Similar to the more conventional ductless multi-split systems, which can also connect one outdoor section to several evaporators, VRF systems are different in one important respect—although multi-split systems, like DX systems, turn on and off depending on whether the room to be cooled is too warm or not warm enough, VRF systems constantly modulate the amount of refrigerant being sent to each evaporator. By operating at varying speeds, VRF units work only at the needed rate, which is how they consume less energy than on/off systems, even if they run more frequently.

Although systems vary among manufacturers, VRF technology is usually available as heat pump or heat recovery units. Heat pumps provide either heating or cooling. Heat recovery systems allow for simultaneous heating and cooling—which means, for example, that one condensing unit might be connected to six indoor units, three of which could be used to cool some areas, and three of which could be used to heat other areas, all at the same time.

The modular nature of VRF offers a dizzying array of options. And, to help engineers interested in exploring the use of this technology, the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) has assembled a group to study VRF. ASHRAE included a description of the VRF system in its 2008 handbook on HVC systems and equipment and is now working on a separate chapter on VRF to be published in 2012.

Several manufacturers of VRF systems are part of the ASHRAE committee working on documentation for the technology, but only a handful are already marketing their systems in the U.S., with Mitsubishi Electric HVAC Advanced Products Division in Suwanee, Ga., and Daikin Industries (based in Osaka, Japan, with U.S. headquarters in Dallas) currently being the major players.

Both Mitsubishi and Daikin are taking steps to educate U.S. engineers, architects, and contractors on the technology. According to Meredith Emmerich, director of application support with Mitsubishi, about 10,000 people went through the company’s training on ductless and VRF systems last year alone. The company offers support and training through 1,100 locations across the U.S.

Daikin’s Dallas location, too, includes a training facility where VRF equipment is installed and exposed, so engineers, architects, and contractors may come in and see the outdoor and indoor units, the piping, the installation, and controls on all the models.

Breaking it down
VRF systems offer an energy-efficient solution that provides considerable flexibility. But, as with any other HVAC system, their cost-effectiveness and usefulness needs to be evaluated on a building-by-building basis. VRF systems are a good option for buildings with varying loads and different zones: structures such as hotels, schools, and office buildings where individual users want to have control over the temperature in their areas. VRF systems tend to have greater piping length allowances than DX systems and use copper piping with small diameters, which makes them suitable for buildings with low-ceiling spaces or for adaptive reuse and other projects aimed at preserving historic value with minimal destruction during installation.

Less likely candidates to benefit from VRF technology are large open volumes, such as gyms, theaters, or sanctuaries. These building types often fail to maximize the potential of the system, which is ideal for areas with different zones.

Lee Shadbolt, AIA, principal with Commonwealth Architects, based in Richmond, Va., says his firm is considering a VRF system for the renovation of Hotel John Marshall, a historic landmark built in the 1920s. Energy efficiency was one of the main factors considered, but there were other reasons to look at VRF.

“First, it’s a great application for multi-familiy residential use,” he says. “Second, it was extremely efficient and gave us a lot of points toward LEED certification. And third, it allowed us to work with the high-rise nature of the building.”

Shadbolt says other options (such as split systems or a central chiller boiler plant) have been considered for the project. But VRF, which is about 20 percent more expensive than other alternatives considered, is also significantly more efficient, according to his team’s assessment.

Regarding cost, Jami Billman, sales engineer with Daikin, says that VRF systems can be designed both in expensive and more affordable ways. For example, a system with one evaporator in every single room may be more costly initially, but the installation might require less ductwork. Or, in a different arrangement, several spaces might share a nearby evaporator. The smaller footprint of VRF equipment can also reduce costs. According to Billman, in most cases the system eliminates the need to have mechanical rooms, so useable space is given back to the client.

Joe Bush, application specialist for City Multi, Mitsubishi’s line of VRF systems, explains that Mitsubishi is the only manufacturer to use two refrigerant lines, instead of three, for heat recovery systems. He says this patented technology translates into considerable installation cost savings as well.

Ramez Afify, PE, LEED-AP, director of engineering at New York-based Clifford Dias Consulting Engineers, is a member of the ASHRAE group devoted to the study of VRF. In general, he estimates the initial cost of a VRF system to be 20 to 40 percent higher than a traditional split/heat pump HVAC system, but, he says, operating costs might be at least 10 percent less. According to Afify, the difference in price between a VRF and a conventional system might be recovered in fewer than five years.

Beyond the initial cost of VRF systems, disadvantages often cited revolve around refrigerant lines and ventilation. Afify explains that if VRF systems are large, as many chilled water systems are, a significant amount of refrigerant gas, instead of water, ends up running through the building.

“Of course refrigerant is not dangerous within certain volume limits, but if the system grows huge, it becomes a concern,” he says, adding that ASHRAE Standard 15, “Safety Code for Mechanical Refrigeration,” discusses the topic in detail.

Concerning ventilation, Afify explains that providing outside air can turn into a hurdle, because VRF units may require a separate ventilation system, especially in hot and humid climates or when dealing with high occupancy areas. Major manufacturers do generally offer outside air processing solutions that can be tied into the same control systems used for VRF units.

More than any of the above setbacks, however, Afify believes that what has kept U.S. engineers away from VRF systems has been a lack of familiarity and clear documentation. As more U.S. engineers become familiar with the technology, many in the industry expect to see VRF systems grow in popularity.


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