Variables create anxiety. We want to remove as many variables as possible from a situation. However, you can embrace the variables in variable refrigerant flow (VRF) systems because they are a major contributor to their efficiency. But you will need to address some variables about your building in order to determine if VRF technology is a good retrofit option.
A VRF system’s variable flow of refrigerant via variable frequency drive (VFD) motors allows the system to adjust to a variety of loads. The refrigerant serves as both the heating and cooling medium that can be delivered to multiple fan-coil units from a single outdoor condensing unit.
A VRF system with heat recovery allows indoor units to cool or heat as necessary. For example, instead of rejecting heat from a server room to the outside, a VRF system can move the heat to perimeter spaces that require it.
For retrofit applications, VRF can offer a solution for inefficient fans and leaky ductwork. Buildings with inadequate cooling may be a particularly good fit. Older facilities and historical buildings, which often cannot easily accommodate new HVAC equipment, may also be targets for VRF solutions.
If you’re considering a major retrofit of an existing HVAC system, take these three steps to do your due diligence for a possible VRF solution, says Anne Wagner, energy efficiency engineer at the Pacific Northwest National Laboratory (PNNL).
1) Screen by building characteristics – Some older buildings may provide better paybacks for VRF than new ones, according to Variable Refrigerant Flow Systems, a technology assessment prepared under GSA’s Green Proving Ground program (www.gsa.gov/greenprovingground). Wagner, who is a co-author of the publication, points out that buildings with high energy usage, particularly those with VAV systems with electric reheat or other electric resistance heat, may be good candidates. Facilities with limited space to add ductwork for additional cooling capacity may also be good candidates. The comparatively small size of VRF refrigerant lines makes them easier to install than ductwork even when buildings have adequate space.
The GSA report notes a number of factors to consider when evaluating a cost-effective match with VRF technology. The zonal capability of VRF fan units are well-suited to facilities with separated spaces, including schools, lodging, multifamily, healthcare, some shopping centers, and office buildings with numerous enclosed spaces and conference rooms. Conversely, big box retail, warehouses, and other facilities with large open spaces are not typically suitable.
Climate is also a factor, with extreme weather being a plus. Colder regions may offer better opportunities for heat recovery and may increase savings if converting from any type of electric resistance heating. Conversely, buildings in mild climates that often operate in an economizer mode will benefit less. According to the report, a sweet spot for VRF in terms of building size is 10,000–100,000 square feet.
2) Do an energy analysis – Have your building surveyed to understand how, where, and when it uses energy – and wastes it. Energy engineers recommend incorporating a minimum of one year’s worth of detailed consumption data in order to model the building through the seasons. The analysis will provide a platform for determining proper size as well as estimating the efficiency and cost of new systems.
3) Assess lifecycle costs – The GSA assessment notes that VRF systems may have higher maintenance, repair, and replacement costs that offset some of the energy cost savings. As a result, when analyzing the economics of VRF, detailed lifecycle costs should be estimated.
The GSA report includes a sample lifecycle analysis that compares a VRF system and a VAV electric reheat alternative for a hypothetical building (see below). The analysis includes costs for a dedicated outside air system (DOAS) because VRF systems themselves do not integrate outside air capability. While some buildings may have adequate natural ventilation, most will require outside air, but existing ductwork may be adequate to deliver volumes that meet code requirements. In many climates, a DOAS needs to preheat and precool outside air to a temperature close to room conditions and its controls must coordinate with a VRF system’s controls.
What Are the Energy Savings from VRF?
That’s the simple question everyone wants to ask – but there is no simple answer. Every building is unique.
The GSA report estimates that VRF systems can achieve 30% and higher HVAC energy cost savings over older inefficient systems and minimally code compliant conventional systems. For retrofit applications, costs and energy savings vary a great deal, making it impossible to estimate a typical payback.
In the case of the federal government’s portfolio, GSA recommends targeting existing buildings whose energy usage is above the average (60.7 kBTU/square foot with conventional systems) and whose initial incremental cost for VRF is less than $4/square foot compared to CAV and VAV systems.
Take a look at some examples of VRF retrofit application examples from the GSA and the private-sector below:
VRF APPLICATIONS IN THE PRIVATE SECTOR
Among the challenges posed by the rehabilitation of a long-unoccupied office building in Towson, MD, were restrictive floor-to-floor heights and the owner’s desire for LEED certification and Class A office space.
The 15-story, 170,000-square-foot Towson City Center was completed in 1967. Alleged ventilation problems and a reputation as a sick building led to the facility’s closing in 2002. It remained unoccupied for more than 10 years until new owners undertook a giant renovation project, which included mechanical and electrical systems and curtainwall.
For Steve Wagner, director of engineering for design/build mechanical firm MEC2, the gutted tower had limited space for new ductwork. The floor-to-floor height on 12 of the building’s floors is only 10 feet, 6 inches. To earn LEED certification, energy efficiency was also a top priority for the new HVAC.
The choice of a Mitsubishi Electric VRF system met those requirements. “We knew that if we went the VRF route, the sheet metal could be kept to a minimum,” says Wagner. The low, 9 7/8-inch profile of the indoor fan coil units also saved space.
On the building’s penthouse mechanical level there was more space to be saved. With no need for the cooling tower and boiler of the previous central system, this equipment was removed. The small footprint of the VRF system’s 15 outdoor units (one for each floor) freed up 12,000 square feet of the penthouse for storage or other uses. A dedicated outside air system with energy and desiccant recovery wheels was installed on the steel grillage of the former cooling tower. The rentable square feet on each floor was increased due to the reduction in vertical shaft space needed for ventilation.
Uncommon Flexibility and Efficiency
Listed on the National Register of Historic Places, the former Imperial Hotel in Atlanta was built in 1910. Gutted and converted into subsidized residential housing units, the building is now known as the Commons at Imperial Hotel. The renovated building has more than 90 housing units, which made it a good fit for a VRF solution.
The building needed individual HVAC controls for each apartment but the rooftop area was too small to accommodate so many split-system condensing units. However, the VRF system from LG Electronics required only one outdoor unit for each of the eight floors and the basement, resulting in nine rooftop units.
According to Robert Barfield, vice president of construction services for the developer, Columbia Residential, the VRF system also increased the area of the housing units by eliminating many vertical ventilation shafts. Indoor fan coil units for the VRF system were installed above the ceilings in the apartments.
The VRF system’s heat recovery capability was also important for efficiency. Barfield explains that the north and south facades of the building had large expanses of historic metal bay windows but the east and west facades had much less window area. As a result, indoor temperatures on different sides of the building were often unbalanced. The VRF system allows heat from warm zones to be transferred to cooler zones instead of rejected to the outside. Residents also have a high degree of control over heat and cooling within their units.
The building’s renovation was designed to achieve LEED Gold certification.
GSA Applies VRF to Diverse Buildings
GSA has implemented VRF technology in 10 projects, according to Kevin Kampschroer, director of GSA’s Office of Federal High-Performance Green Buildings. BUILDINGS content director Chris Olson spoke with Kampschroer about three retrofit projects: the Wayne Aspinall Federal Building and U.S. Courthouse in Grand Junction, CO; the Bishop Henry Whipple Federal Building in Fort Snelling, MN; and the Byron Rogers Federal Building in Denver.
What has been GSA’s experience with VRF retrofits?
I’ll mention three projects because each is interesting in its own way.
The smallest is the Aspinall building, which is the first building on the National Register of Historic Places to achieve net zero. It uses a combination of VRF, ground source heat pumps, rooftop solar, efficient windows, and extraordinary attention to increasing the envelope insulation from the inside as the historic exterior can’t be tampered with.
This is a particularly good facility for the application of variable refrigerant flow because of its location in what is essentially a high desert. For half of the year there is a large temperature differential between the east and west facades. As the sun heats up the east side of the building, it requires cooling while the west side calls for heat. Variable refrigerant flow allows you to move heat from one side of the building to the other, making everyone more comfortable.
The building’s heat pumps make a great combination with VRF. Geothermal wells and VRF are like a marriage made in heaven.
Has GSA used this VRF/geothermal combination on other facilities?
Yes, the Whipple building in Fort Snelling, MN. It’s a much bigger building – 618,000 square feet – but it also needed replacement of the entire HVAC system. It is one of the largest buildings in our inventory with geothermal wells and VRF.
For Whipple, the east-west orientation was not such a big factor. Much of the savings from the VRF involves the ventilation. The energy it takes to push air around a building typically makes ventilation the largest single energy cost in maintaining building temperature. Pumping refrigerant in the VRF system is more efficient than using fans to move air. You can vary the pump speed and the pump pressure, which saves on energy distribution.
The renovated facility is 72% more energy-efficient, in part due to the VRF system. The efficiency makes it a beautiful building!
Does your third example also use geothermal wells?
The third example is not ground source. It’s the Byron Rogers Federal Building in Denver, which was built in 1964. At 620,000 square feet, it’s about the same size as Whipple but it’s tall. Because of its downtown location, ground source heat pumps were not an option.
From an energy perspective, the Denver building’s orientation could not be worse – long facades on the east and west and short facades on the north and south. Through the day we need to move heat from the east side of the building to the west or back in the other direction. By switching over to VRF, we save space because we’re not moving as much air. Instead, we’re moving chilled water through beams for cooling.
The condensing units are installed within the existing boiler room. They serve VRF fan coils on each floor, which are the equivalent of a radiator that heats and cools. And the separation of heating and cooling from the ventilating system enables us to concentrate on high-quality, super-filtered outside air.
The retrofit saved a lot of area on every floor by eliminating ductwork running up and down the building. On the top mechanical level, it was the equivalent of adding a new floor. If you imagine an 8 ½ by 11 sheet of paper as the cross section of a duct, you need a pipe about the diameter of a nickel to move the same amount of heat. That gives you an idea of the space that you’re saving.