Variable Refrigerant Systems Make for Zoned HVAC Control

Several manufacturers offer variable refrigerant volume or variable refrigerant flow (VRF) HVAC systems. And while there are differences among the various systems, all of them react to changes in heating and cooling requirements by varying the flow of refrigerant. This is accomplished by using inverter technology to vary the speed of a compressor, or by making use of multiple compressors of var...

01/01/2007


Several manufacturers offer variable refrigerant volume or variable refrigerant flow (VRF) HVAC systems. And while there are differences among the various systems, all of them react to changes in heating and cooling requirements by varying the flow of refrigerant. This is accomplished by using inverter technology to vary the speed of a compressor, or by making use of multiple compressors of varying capacity.

This type of system is not new. In fact, the technology developed in the late ’50s, but mainly have found acceptance in Europe and Asia. However, manufacturers have been promoting these systems in North America of late, because of the design flexiblility that they offer—especially for retrofit projects—and the energy efficiency of the variable speed compressor, which avoids on/off cycling.

The typical VRF system is ductless—but could be ducted—with as many as 40 or more indoor units connected to one outdoor condensing unit.

Obviously, this type of system offers zoning that is well suited to restaurants, hotels, theaters, hospital, universities and industrial facilities—in fact, anywhere it pays to deliver a personalized comfort zone to buildingoccupants.

Basically, there are two types of VRF systems: Two-pipe systems often are installed in large, open spaces such as for retail and office facilities that require heating or cooling during the same operational periods. Three-pipe systems work best where there is a need for some spaces to be cooled while others are heated.

One vendor, Mitsubishi Electric, which manufactures a two-pipe system, claims that installation is relatively simple. The outdoor unit, a branch-circuit controller and each of the indoor units is connected via a two-pipe refrigerant system. The outdoor unit coordinates with the controller to deliver a variation of refrigerant flow to each of the indoor units. A direct digital control (DCC) system controls a network link between the controller and outdoor compressor unit to provide control of the entire system. However, there are separate remote controllers at the individual units to provide the individual occupant control that the VRF HVAC system boasts.

In addition to the advantage of zoned, individual control, another benefit that the manufacturers of these systems point to is their low noise levels. For example, Mitsubishi claims that its indoor units operate as low as 24 dB(A), while the outdoor units demonstrate noise levels as low as 56 dB(A), making these systems suitable for facilities such as health-care, schools and libraries.

Finally, manufacturers emphasize the design flexibilities that VRF ductless systems offer. In historic preservation work—or any other type of retrofit where a less invasive approach is needed—ductless systems avoid the need for tearing out walls and ceilings to install new ductwork. Also, systems can be reconfigured easily for changes in space. Mitsubishi says that its systems can be configured and reconfigured into 20 different zones in a building, with 2,000 zones managed from a single network computer.

The United States may still be a “ducted” society, but that’s beginning to change.



Advantages of Variable Flow

• Flexible HVAC zoning options that are suited to large spaces.

• Building occupants have individual control of air in their personal spaces.

• Variable-speed compressors avoid on/off cycling and offer greater energy efficiency.

• HVAC noise levels are greatly reduced for facilities such as schools, libraries and hospitals.



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