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Variable-refrigerant flow (VRF) systems have been used for the better part of three decades in Europe and Asia. While clearly not as common in North America, the design has been catching on — mostly for its ability to respond to fluctuations in space load conditions. Because of this, it excels at saving money during part-load system use. VRF is appealing for reasons beyond energy savings. After all, the systems can simultaneously heat and cool separate spaces in the same building. VRF systems also can vary compressor speed to meet load condition and have quieter operation than a direct exchange system. But that's not to say that it’s perfect for every building or every climate. Facility managers have to weigh benefits and limitations.
Affify is careful to note that a VRF system isn't an off-the-shelf solution. It generally Ramez Affify, principal at E4P consulting engineering the assistance of a design engineer, who needs to review the load profile for the building so that each outdoor section is sized based on the peak load of all the indoor sections at any given time; then the outdoor unit can be specified.
Designing a VRF by selecting the outdoor unit first, Affify says, is a sure way to end up with an oversized system.
A VRF isn't suitable for all applications. Limitations include:
There is a limitation on the indoor coil maximum and minimum dry- and wet-bulb temperatures, which makes the units unsuitable for 100 percent outside air applications, especially in hot and humid climates.
The cooling capacity available to an indoor section is reduced at lower outdoor temperatures. This limits the use of the system in cold climates to serve rooms that require year-round cooling, such as server rooms.
But in many cases, VRF systems work well. Affify references a recent VRF installation in the desert southwest where - shortly after installation - the area experienced a heat wave where ambient outdoor temperatures reached 120F, well exceeding the manufacturers recommended range.
"To our great pleasure, the system functioned and cooled the building during [those] hot times," Affify says.