by Pamela Androff, PE, LEED AP
Variable refrigerant flow (VRF) zoning and geothermal systems are two of the most energy-efficient options for heating and cooling buildings. Now, the benefits of both systems can be combined. This article provides an overview of geothermal (or water-source) with VRF zoning technology, and its advantages compared to traditional geothermal systems, air-source VRF zoning systems, and conventional HVAC systems. Additionally, the article discusses considerations for specifying water-source VRF zoning systems and provides case studies of successful applications.
VRF zoning provides precise comfort control to buildings with multiple floors and areas by moving refrigerant through piping to the zone to be cooled or heated. Some VRF zoning systems offer highly responsive simultaneous cooling and heating, which maximizes use of heat energy that otherwise would be expelled outdoors. Regardless of the time of day, sun or shade, season, or special requirements, VRF zoning systems can deliver comfort tailored to each zone or space.
Geothermal technology overview
Outdoor temperatures fluctuate with the changing seasons, but underground temperatures do not change as dramatically because of the earth’s insulating properties. Geothermal systems typically consist of heat exchange equipment located indoors, and a buried system of pipes—called ‘loops’—to capitalize on constant underground temperatures to provide energy. Water in the heat exchanger circulates through loops below the surface, absorbing or expelling heat to the below-ground heat sink depending on the time of year. This function ultimately reduces the load on the compressor during the cooling and heating cycles, and results in significant energy savings.
Water-source VRF zoning systems combine a geothermal system’s benefits, with the sophistication of VRF zoning. Together, the technologies take advantage of the inverter-driven compressor coupled with a closed geothermal loop instead of air as a heat exchange medium.
Water-source VRF zoning systems have numerous benefits, including many that users have come to expect from air-source VRF zoning systems.
Flexible installation
Installation is possible in tight spaces because two-pipe designs require less space than ducted systems. Some VRF zoning systems require three- or four-pipe designs, which call for more refrigerant line runs and more brazed connections. Two-pipe designs minimize the total distance of refrigerant line and system connections, which can help reduce installation labor and eventual maintenance costs.
Relatively small water-source VRF zoning condenser units are mounted indoors and can be installed in compact utility closets with minimal access on either side of the unit. Refrigerant, water, and electrical connections are housed on the front of the unit for convenient access.
Application variety
Outdoor water-source VRF zoning units can be connected to an array of indoor unit styles to accommodate the space’s specific needs. All styles are quiet, easy to maintain, and provide optimal comfort. The configuration of the below-ground loop systems can be customized to accommodate the building’s surroundings. For example, loops can be buried under a building or a parking lot by drilling either vertical bore holes or horizontal trenches.
Low maintenance
Water-source VRF zoning systems offer easy maintenance. One VRF unit on the water loop can be serviced without taking the whole system offline. Most indoor units have washable filters that are easy to clean or replace. Condensers can be housed in floor-level utility closets for convenient access and minimal disruption to occupied spaces.
Payback period
Installing a water-source VRF zoning system may have a higher upfront cost. Over time, the system efficiency and low-maintenance expenses can offset the system’s initial cost in energy savings. The average payback period is 10 years.
Advanced controls
VRF zoning systems offer several controller types. A central controller can monitor, schedule, and control up to 50 indoor units. Multiple central controllers can be networked together with integrated centralized control software and systems can be tied to a building management system (BMS) using LonWorks and BACnet (data communication protocol for building automation and control networks).
Clean energy source
Water-source VRF zoning systems use the clean and sustainable energy stored in the earth to heat and cool buildings. This is a more environmentally responsible and efficient source of energy compared to non-renewable fossil fuels.
System efficiency
VRF zoning technology includes the use of an inverter-driven compressor in the outdoor unit that varies the motor rotation speed, allowing it to precisely meet each zone’s load requirement while reducing power consumption. Also, as mentioned, geothermal VRF zoning systems take advantage of stable ground temperatures for even greater efficiencies.
The systems always have a condenser unit in combination with an indoor air-handler unit. The condenser can be located indoors or outside, depending on the system.
Tax credits
Water-source VRF zoning systems may qualify for up to a 10 percent federal commercial tax credit for the total installed cost of the geothermal system.
Water-source VRF zoning system advantages
A significant advantage of water-source VRF zoning over traditional geothermal systems is the ability of the compressor units to connect to up to 50 indoor units. Traditional geothermal systems require multiple compressors in each zone and the units must be installed in the ceiling space. With water-source VRF systems, fewer condenser units result in easier installation, less equipment, and reduced maintenance. Water-source VRF zoning system condensers are serviceable at floor level, rather than the ceiling, for improved access and minimal disruption to occupied spaces.
Another benefit of water-source VRF zoning technology over standalone geothermal systems is the potential for heat recovery. Some systems allow for simultaneous cooling and heating. In these applications, a VRF zoning system’s compressor operating power will be reduced during heat recovery periods compared to an equivalently-zoned traditional geothermal system. Water-source VRF zoning technology often exceeds the efficiency of traditional geothermal systems due to the ability to recover energy more efficiently on the refrigeration side first, compared to multiple distributed traditional geothermal systems.
Water-source VRF zoning systems contain many of the same benefits associated with their air-source counterparts, along with additional benefits. The most significant advantage over air-source systems is the heating de-rates.1 In colder climates, the expected capacity of the air-source VRF zoning equipment can be reduced due to the temperature. Sometimes the equipment must be upsized to handle the peak heating demand, but with water-source VRF zoning systems, this is not an issue. Coupling the VRF zoning units with a geothermal loop provides the benefit of higher efficiencies from milder loop temperatures. The power required by the outdoor units may be reduced by as much as 35 percent when compared to air-cooled systems.
Additionally, there are many advantages of these systems compared to unitary or boiler/chiller HVAC systems. The combination of geothermal and VRF zoning technology results in significant energy saving potential due to the milder loop temperatures requiring less work from the condensers. The water-source VRF zoning outdoor units are small, eliminating the need for large equipment rooms or above-ground space. Regarding indoor units, the refrigerant, water, and electrical connections are on the front of the unit so the condensers can be installed in relatively small utility closets with minimal access on either side of the units. VRF zoning systems also offer the benefit of inverter-driven compressors, which vary the refrigerant flow to each unit for precise and efficient control.
Specifying water-source VRF zoning systems
Water-source VRF zoning systems are ideal for climates experiencing significant temperature variations throughout the year, where having the heat exchange medium located in a temperature-stable underground environment will have the most impact. Due to the combined energy efficiency of ground-source and VRF zoning technology, water-source VRF zoning systems can result in one of the most energy-efficient heating and cooling systems available.
As with all VRF zoning systems, the indoor units require minimal ductwork (if any at all), making them ideal for retrofitting older buildings and replacing old boiler/chiller systems. These systems are ideal for applications such as schools, offices, medical centers, or any building in which individual zone control is essential to occupant comfort.
There are some factors to consider when installing water-source systems. Geothermal systems require a series of wells or loops. Constructing a well field, and having the land to do so, may be cost-prohibitive in some cases, and impossible in large metropolitan cities where space is at a premium. Additionally, anytime digging into the earth is involved, care must be taken to not disturb any existing underground infrastructure. Lastly, installing a geothermal well field may be expensive up front; however, over time, these systems tend to pay back in the form of energy savings.
Muscatine County Courthouse
Iowa’s Muscatine County Courthouse is a 1907 Beaux-Arts style building listed on the National Register of Historic Places. Courthouse officials needed a replacement for a failing 30-year-old cooling and heating system that was costly in both upkeep and energy usage. They wanted a low-maintenance, high-efficiency replacement system.
Aesthetics and ease of installation were also an issue. The indoor units were loud and moldy. The large condenser unit sitting prominently on the historic building’s roof was an eyesore and needed to be replaced with a smaller unit that could be housed elsewhere. The building’s 609-mm (24-in.) thick limestone walls made the possibility of retrofitting ductwork difficult. Court sessions were conducted throughout the installation, ruling out any system requiring extensive demolition and construction.
The Muscatine County Board of Supervisors had already invested in drilling a geothermal test in anticipation of replacing the old HVAC system. The board selected a water-source VRF zoning system that would interface with the existing geothermal field. The specifying engineer and the board felt the geothermal system was especially suited for Iowa’s climate, where it is not uncommon for winter temperatures to dip below –23 C (–10 F).
A ductless, two-pipe design would save on labor and equipment cost because it required far less fittings on the refrigerant lines than a three- or four-pipe system. The ductless design also meant the installers did not have to drill through the thick limestone walls and disrupt court sessions. The compressor units could be easily transported down the stairs into the courthouse’s basement where they would be housed, rather than on the roof.
The installation proceeded smoothly and court sessions were not disrupted by the installation.
“Judges were holding court during the transition, and we could not afford to have a lot of banging, pounding, and installation of new equipment,” said Sherry Seright, county budget director.
The system also provided the desired energy-efficiency levels. Courthouse officials observed in the first summer of operation the geothermal field could handle the campus system heat transfer requirement with just 20 percent of the design flow.
Due to the installation’s success, the board decided the water-source VRF zoning systems would be the new standard for any future HVAC retrofits for Muscatine County. They currently have plans to design geothermal VRF zoning systems for two additional county-owned buildings.
Elliot and Stoddard Halls, Miami University
Miami University of Ohio, in Oxford, is the 10th oldest public university in the United States. Its two oldest buildings—Elliott and Stoddard Halls—needed a new cooling and heating system to replace the ineffective and outdated existing coal-fired steam heat system.
Selecting a new HVAC system presented two distinct challenges. The first was the need to preserve the building’s architectural integrity. Both were listed on the National Register of Historic Places and neither had existing ductwork, making a central air solution impossible. The second challenge was meeting the school’s energy-efficiency goals. The university developed a utility master plan that prioritized energy efficiency and mandated moving away from inefficient technologies like coal burning.
“Renovation of any historic building is a complex undertaking requiring a balance between the original architecture and modern building systems,” said Alec R. Carnes, PE, CEM, LEED AP, and senior principal of mechanical engineering, Heapy Engineering, designers of the system.
The design team specified a geothermal VRF zoning system for the job. “Geothermal is advantageous for our climate in Ohio, where heating and cooling loads are closely matched over the year,” said Doug Hammerle, PE, Miami’s director of energy systems. “This helps balance the well field temperature and maximize the efficiency of the system.”
Seventeen 182-m (600-ft) deep geothermal wells were placed under the sidewalks surrounding the halls. As modern footings were unknown 150 years ago, the hand-dug basements had no space for heat pumps. An easy-to-access mechanical room was built into the attic of each hall for the three heat pumps and centralized controller.
In 2011, Elliott and Stoddard switched from coal-fired steam heat to geothermal heating and cooling. Metered as one, the two halls showed an annual 61 percent decrease in energy consumption compared to 2010. The oldest buildings on campus became the most energy-efficient.
The VRF zoning systems’ ductless design also preserved the original aesthetic of the buildings. The team selected floor-mounted indoor units that could be concealed in cabinets, lending the historic look of a radiator case.
“None of this would have been possible without the two-pipe system design,” Hammerle said. “The interior would have been severely cut up with a hydronic four-pipe system.”
Strawberry Mansion, Philadelphia
Built in 1789 as a summer home, Philadelphia’s Strawberry Mansion is now a museum, but the historic building needed renovations.
One of the most pressing needs of the rehabilitation process was a new cooling and heating system to replace the existing system that dated back to the 1930s. The museum had never been centrally air-conditioned and it needed reliable climate control for the museum’s collection of antiques. With 929 m2 (10,000 sf) on four levels, 23 rooms, and masonry-bearing walls, it was a challenge to find a central cooling and heating system that would be unobtrusive and have little impact on the home. Additionally, the museum wanted to adhere to the Greenworks Philadelphia Plan—a six-year plan launched in 2009 with the goal of making Philadelphia the greenest city in America. There was a strong desire to find an inventive, green building solution to this challenge.
The mansion already had an existing 12-well geothermal field and the project team first considered a four-pipe water-based system. They soon found it was impractical to install. The 200-year-old stone walls could not accommodate many of the 609 X 203-mm (24 X 8-in.) deep chases required for the four-pipe design. In many areas, there was no space for ductwork or for pipe routes perpendicular to structural elements.
A water-source VRF zoning system was able to meet the unique needs of Strawberry Mansion. The two-pipe system and compressor units could easily fit in the old chase walls and ceiling where there was no space for ducting. The flexibility of the refrigerant lines, as opposed to ductwork and diversity of indoor unit styles, were well-suited for the design restrictions posed by the historic structure.
The design team placed the two water-source heat pumps in the basement. They are specifically engineered for closed water loop systems that would efficiently interact with Strawberry Mansion’s existing 12-well geothermal field. Designed to fit in small spaces, these units take up little room and are flexible enough to cool or heat up to 50 individual zones maximizing energy, equipment, and installation efficiency.
The system met the museum’s energy efficiency expectations and also saved the owners $50,000 upfront, compared to the original four-pipe proposal.
“Our collection has suffered from decades of no air-conditioning or humidity control,” said Beth Kowalchick, president of the all-volunteer 1926 Committee who presided over the restoration. “After a few months, I could see a noticeable difference in the appearance of our textiles, prints, antique furniture, and fine art.”
Conclusion
Technology combining variable refrigerant flow zoning and geothermal systems—two of the most energy-efficient options for heating and cooling available—can benefit projects compared to traditional geothermal systems, air-source VRF zoning systems, and conventional HVAC systems.
Notes
1 De-rates are a reduction in capacity. Lower de-rates is an advantage, meaning the system operates more efficiently. For example, if a system operates at 100 percent heating capacity at ?18 C (0 F) degrees and 85 percent heating capacity at ?23 C (?10 F), it has a heating de-rate of 15. Water-source VRF zoning systems tend to have lower heating de-rates than air-source systems—an advantage. (back to top)
Pamela Androff, PE, LEED AP, is product manager–commercial and product planning for Mitsubishi Electric’s U.S. Cooling and Heating Division. In 2013, she became the youngest person to serve as president of American Society of Heating, Refrigerating, and Air-conditioning Engineers’ (ASHRAE’s) Atlanta chapter. In 2014, Androff was named one of Consulting-Specifying Engineer’s “40 Under 40,” which recognizes young engineers shaping the future of the industry. Androff earned a bachelor’s degree in mechanical engineering from the University of Central Florida (Orlando), where she served as president of the American Society of Mechanical Engineers (ASME) chapter. She can be contacted by e-mail at pandroff@hvac.mea.com.
