Technology spotlight: Heating and cooling with VRF systems
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This column
highlights innovative equipment, systems and applications that can help utilities save energy and improve service. |
There's a new HVAC technology in town that's turning some heads. It's easy to retrofit to existing buildings—even historical buildings without existing ductwork. It can move heat from warmer parts of the building to cooler parts; it provides very good part-load performance; it can limit conditioning to only those rooms that are occupied; and it may cut energy costs. This new technology is most commonly known as variable refrigerant flow (VRF). It's an outgrowth of the "multi-split" systems used in residential applications.
VRF equipment manufacturers include Mitsubishi, Daikin, Toshiba, Fujitsu, Hitachi, LG and Samsung. VRF systems are most popular in Asia and are established in Europe, but are gaining a foothold in the United States. These systems combine one or more centralized, variable-speed, air-cooled compressors and condensers connected to dozens of terminal (fan coil) units throughout a building, each of which can act as an economizer or an evaporator, as needed, to provide heating and cooling. Electric resistance coils provide additional heating and defrost cycles during the coldest weather.
Comparing VRF to HVAC
Comparing VRF systems to conventional HVAC systems is difficult because of the variations in conventional systems and buildings. This challenge is compounded by the lack of standardized protocol for efficiency testing of VRF systems, lack of clear agreement among existing case studies and applicability of VRF systems in various regions in the country.
The current industry standard HVAC system for medium/large commercial buildings is the rooftop direct-expansion, variable air-volume (DX-VAV) air conditioner with an air-side economizer, series terminal boxes with electric reheat in perimeter zones and natural gas for building warm-up. These units are controlled to minimize fan horsepower and optimize supply-air temperatures, and use an economizer to minimize compressor power. Although they require reheat energy in cooler regions, for most of the year this reheat energy may come from air above the ceiling that has been heated by a recessed lighting system. Chillers may be used for larger buildings and split systems for smaller buildings, but rooftop VAV is a common alternative for VRF and therefore a good choice for a comparison.
The following table summarizes many advantages and disadvantages of these two systems in commercial applications.
VRF Systems |
DX-VAV Systems |
Ductless terminals eliminate heat and air gains and losses from ductwork, and eliminate most duct friction that increases fan energy use. |
Long ductwork runs have relatively high static and dynamic pressure losses. |
May require separate ventilation ductwork with fans, controls, etc. Ventilation may be provided through an energy recovery ventilator that transfers much of the heat and moisture from exhaust to incoming air. |
Ventilation air supplied with conditioned air. |
Can recover heat from core zones to offset ventilation and envelope heat losses by adding third refrigerant distribution pipe. Amount of useful heat recovered depends on building envelope design, climate and building operations. |
Series VAV fan boxes recover heat from ceiling plenums to meet reheat requirements (less so on the top floor of a building); water-loop heat pump systems can also recover heat from building core. |
Fans/motors efficiency about 30-40 percent. At least one per room rather than one per zone, and greater pressure drop, due to coil and filters. |
Primary fan/motor efficiency nearly 75 percent at full load, and series box fan/motors 30-40 percent efficient. |
Requires more installed capacity because designer must choose unit with capacity meeting or exceeding each zone's maximum. |
Building load diversity allows cooling system to be downsized as much as 20 percent depending on building. |
Variable speed compressors (less common in DX-VAV rooftop units) enhance part-load efficiency. |
Multiple cooling stages provided by various means (none as effective as variable speed compressors) enhance part-load efficiency. |
No "free" cooling, due to lack of economizer. Without the economizer, energy codes require higher minimum equipment efficiency (10-20 percent higher in the International Energy Conservation Code), increasing the cost of cooling equipment. |
Outside air economizer (which requires only fan energy) can provide substantial portion of cooling load in more moderate climates. |
Heat lost and gained from multiple long runs of refrigerant piping. |
Uses minimal refrigerant piping. |
Can respond to staged occupancies, only conditioning rooms as they become occupied. |
More limited in ability to condition only occupied spaces. |
Morning warm-up (and back-up electric resistance heat on coldest days) increases electrical demand charges and shortens energy-saving set-back periods. Also, electric heating generally costs more than gas heating. |
Morning warm-up uses a central natural-gas furnace that can bring building up to operating temperature quicker with no additional demand charges. Series box strip heaters increase heating costs on coldest days. |
Non-Energy Issues:
Easy retrofit to existing buildings, even older buildings that had steam heating and no cooling, so no ductwork. |
Larger ductwork requires shafts, multiple fire/smoke dampers and adequate ceiling space, reducing usable/rental space. |
Can phase installation to serve portions of the building as funding or program requirements allow. |
Can break system into smaller sub-systems for phasing, but many benefits of VAV systems are lost. |
Some ability to filter air and control humidity. Less cross-contamination among zones. |
Ability to provide various levels of filtration and humidity control in a central location, minimizing maintenance and operation costs for this feature. |
Provides only the required minimum quantity of outside air. |
During economizer operation, improves indoor air quality with up to 100 percent outside air, benefiting occupant health and pre-cooling the space on cool summer nights. |
Is VRF right for your building?
Those interested in VRF systems should review the energy and non-energy benefits above, and obtain cost estimates for a VFR and one or more conventional systems. For a particular project, the impact on rentable area or the need for humidity control could be a key factor in system selection.
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