Variable refrigerant flow (VRF)
Published: February 6, 2014
- BENEFITS & BARRIERS
- CASE STUDIES
Variable refrigerant flow (VRF) systems are typically all-electric systems that use air or water source heat pumps to provide space heating and/or cooling to a building's spaces. VRF systems can condition multiple zones in a building, each of which may have different heating and cooling needs. Using sophisticated control technologies, VRF systems have the ability to modulate the amount of refrigerant sent to each zone independently and in tune with diverse and changing space conditioning loads, thereby increasing energy savings. VRF systems use refrigerant to move heat throughout a building (as opposed to water or air), which allows them to use energy more efficiently.
VRF technology has been available in global markets (Europe, Asia) for some time. Until recently, there has been little interest in VRF technology in North America. Multiple forces, including reductions in the environmental impacts of refrigerant, and technological advancements and market adoption of the key components of VRF systems (eg., variable speed fans and compressors, heat pump technologies and advanced controls) have helped increase the awareness of VRF systems in the United States.
VRF systems are essentially large-scale versions of the ductless mini-split air conditioning systems that have made inroads into several niche residential and commercial markets in the Upper Midwest. Using refrigerant, VRF systems move heat between an outdoor unit (typically an air source or ground source heat pump) and one of many indoor units used to heat or cool an individual zone in a building. Sophisticated controls allow multiple indoor units to be placed on the same main refrigerant loop so that the number of lines needed to move refrigerant throughout the building can be minimized. Variable speed compressors and fans are used to minimize energy use when the system is operating under part-load conditions.
The main heating and cooling plant of a VRF system is usually an air source or water source heat pump. Ground source or "geothermal" heat pump systems have proven effective in some Midwestern applications. The purpose of the outdoor unit is to exchange heat with the outdoor environment, either by expelling heat (when cooling) or absorbing heat (while heating). Heat is exchanged with the outdoor environment using a heat exchanger filled with R-410a refrigerant, which is then pumped throughout the building to one of many indoor units. Variable speed compressors are used so that lower compressor speeds can be used during part-load circumstances.
Refrigerant from an outdoor unit is pumped to one of many indoor evaporator units, each of which is responsible for heating and cooling an individual zone in a building. Indoor evaporator units control the amount of heat being dumped to (or collected from) a space using linear or electronic expansion valves (EEVs). Indoor evaporator units exchange heat between the refrigerant and ambient air by blowing air over the unit's evaporator coil. During the heat exchange, the refrigerant either condenses (when in heating mode) or evaporates (when in cooling mode). Refrigerant returns to an intermediary heat recovery unit or directly to the outdoor unit, where it is subsequently heated or cooled again.
VRFs use a R-410a refrigerant, which is more energy dense than water or air. This means that a smaller volume of vapor or liquid refrigerant is required to move the same amount of heat. This can result in pumping motor savings and a more efficient transfer of heat to/from zones. R-410a is a non-ozone depleting compound; however, R-410a does have a high global warming potential, so it must be collected and disposed of properly.
Heat recovery unit
A heat recovery unit (HRU) allows for the simultaneous heating and cooling of several individual zones that are co-located on the same main refrigerant loop. For example, an HRU might extract heat from the refrigerant returning from a cooled zone to heat refrigerant leaving for a different zone that is currently in heating mode. By reusing the heat that can be extracted locally, HRUs can reduce the size of the overall heating and cooling loads that must be satisfied by the outdoor unit.
Fan coil units can be outfitted with electrically efficient variable speed motors, such as electronically commutated motors (ECMs), to modulate air flow rates with changing heating and cooling loads. Fans that operate at lower speeds when heating and cooling loads are low use far less energy and often have the added benefit of being quieter.
Vary with complexity of project and depend on the chosen heat pump technology. Price premiums over VAV w/electric reheat are approximately 20%. Other estimates peg air-to-air VRF systems at $18/sqft and ground source heat pump VRF systems at $30/sqft
Potential energy savings
Savings rates are variable and depend on the project and baseline technology. Generally ranges of 10-30 percent are provided in the literature.
Buildings with diverse heating and cooling loads Commercial office buildings can have spaces with different heating or cooling needs. For example, a building with a data center or server room as well as office space may need to supply cooling to the data/server room while heating the offices. VRF systems can simultaneously heat and cool different building zones.
Limited forced-air based retrofit options Options for adding new heating and cooling capabilities to historic buildings can be limited. For example, adding additional duct work might be cost prohibitive, or conflict with the building owner's need to preserve as much of the historic character of the building as possible. VRF might be an option in these circumstances since installing refrigerant line is often less intrusive than adding ductwork to extend a forced-air system or adding additional plumbing for a hydronic system.
Buildings with space constraints Individual VRF units are generally compact, requiring less space than other heating and cooling equipment. Additionally, refrigerant lines are small in diameter, so they require less space during and after installation.
Efficient movement of heat throughout the building: VRF systems can be more efficient than traditional heat pump systems. Heat is transferred between the outdoor and indoor units using refrigerant, which can hold more heat per unit volume than either air or water. Typically, only one central or main refrigerant loop is required, with the flow of refrigerant to indoor units being controlled independently with individual expansion valves and sensors. This means that less pumping energy is required to move a relatively smaller volume of refrigerant throughout the building.
Fewer duct losses: VRF systems don't usually require their own air ducting since they move refrigerant instead of conditioned air. Ducting in buildings with VRF systems is usually limited to what is required for ventilation purposes only. The amount of air required for ventilation is usually less than the amount of air that must be moved in forced air heating and cooling systems, so ducts in buildings with VRF systems are typically smaller with lower static pressures and fan requirements.
Simultaneous heating and cooling: VRF systems have the added benefit of being able to heat and cool different zones simultaneously. For example, a VRF system in a commercial office building might supply heat to one of several conference rooms while cooling another. Or, as is becoming more common in today's office spaces, a VRF system might supply cooled air to a data center or server room while simultaneously heating office space. To accomplish these tasks, VRF systems are outfitted with heat recovery units (HRUs). HRUs collect heat returning from a cooled zone to help heat refrigerant being sent to a heated zone (and vice versa).
Improved comfort: Since terminal units are controlled independently using individual sensors and expansion valves, the temperature in each zone can be held within a narrow temperature band.
Quicker and easier to install: Refrigerant lines are small in diameter, so they require less space during and after installation. Since they are relatively small in diameter, refrigerant lines may only require that small-sized holes be bored to pass lines through interior walls. Both indoor and outdoor units are typically smaller and lighter than their alternatives, making them easier to install.
Relatively low noise: Indoor and outdoor units are typically quieter than their alternatives, especially when running in part-load conditions.
Challenges and market barriers
First cost: The cost of VRFs can be relatively high compared to conventional alternatives. However, for some renovation projects that need additional heating and cooling capacity but are constrained for space, VRF systems may be less expensive than conventional systems.
Statewide energy savings
We took a high-level look at the potential energy savings in Wisconsin from VRF systems. The estimate is meant to provide a sense of scale showing the impact this technology might have on Wisconsin energy customers.
To estimate statewide impacts, we assumed that this would be a retrofit opportunity to install VRF systems for where applicable in commercial applications. We applied a technical savings rate of 20% to HVAC energy in commercial buildings. We assumed an applicability rate of 20%.
All data used for these estimates are from the Wisconsin Energy Statistics (2012) and Department of Energy's Commercial Building Energy Consumption Survey (2010).
There are not currently financial incentives available for the single zone VAV technology specifically. However, Focus on Energy offers custom incentives for the Commercial building custom projects and this tecynology may be applicable under that program. https://focusonenergy.com/business/custom-projects
Seventhwave's Madison office
Seventhwave (formerly Energy Center of Wisconsin) occupies half of the third floor of the newly constructed 749 University Row building located in Madison, WI. Tenant spaces are conditioned using a ground source VRF heat pump system with an energy efficiency ratio (EER) of 22. The ground source heat pump, which relies on approximately six miles of underground piping, extracts heat from a 250-ft deep borefield that surrounds the building. The decision to choose a geothermal-based heat pump system was partially made to avoid requirements for a back-up heating system. In cold climates like Wisconsin's, some air-to-air based VRF heat pump systems require a back-up heat source, which can increase costs and add complexity to a VRF project. In Seventhwave's building, refrigerant is pumped from the basement's "boiler" room to one of three condensing units located in a small mechanical room on the building's third floor. Refrigerant is then piped to fan coil units located throughout Seventhwave's space. Through a partnership with one of its member utilities, Madison Gas and Electric, the Energy Center added sub-metering capabilities for its space to independently track the HVAC, lighting and plug loads. An energy usage dashboard driven by the building's automation system (BAS) can be viewed online.
Lewis County Public Utility District Office Building, Chehalis, Washington
The Lewis County Public Utility District (PUD) building in Chehalis, Washington shows how a building's HVAC system can be retrofitted in situ with VRF technology while remaining open to the public. The PUD's VRF system provided a noticeable improvement in comfort over the building's existing air-to-air heat pump systems, which forced many of its occupants to rely on electric heaters for personal comfort. The VRF heat pump system was chosen over several design alternatives, including VAV and water sourced heat pumps; although the VRF system wasn't the cheapest, it offered the best solution for meeting the occupants' expectations for comfort while not disrupting day-to-day operations during the remodel. The 76-ton VRF heat pump system maintains 54 individual temperature zones. An HRV was installed to pre-heat and pre-cool the air from the dedicated-outside-air system (DOAS) before it is routed through existing duct work to the building's individual zones. Demand control ventilation (DCV) technology is used to modulate the amount of ventilation air with changes in occupancy or the use of natural ventilation.
Little Deschutes Lodge, La Pine, Oregon
This recently constructed 26-unit senior living center is located in a region of Oregon that has seasonal temperatures that are similar to those of Wisconsin. The lodge's heating and cooling is provided by a ground source VRF heat pump system. The ground source heat pump is connected to a piping field covering approximately 50,000 square feet. Four 8-ton heat pumps and ducted indoor units are used to condition the lodge's apartments. Common areas are conditioned using ductless indoor units and either an 8-ton outdoor unit or a smaller 5.5-ton heat pump. The lodge uses solar heat and two ERVs to offset at least part of its annual heating load. The lodge's site EUI is half that of average EUI values for similar multi-family residential projects when evaluated using the Energy Star Portfolio Manager. The lodge exemplifies how a VRF system can be combined with ground source heat pumps in a multi-family new construction project in order to provide significant site energy savings and to meet the individualized comfort requirements of many occupants.
Madison Children's Museum
The Madison Children's Museum chose to repurpose an existing building in downtown Madison when it needed more space. As part of the building renovation, the museum included several sustainable design features, including a variable refrigerant volume HVAC system. The Daikin VRV uses air-source heat pump technology to heat the building for as little as 20% of the energy needed with a traditional gas-fired heating system. Heat recovery technology captures the internal heat of the building and uses it to minimize the amount of new energy needed.
Lewis County PUD Variable Refrigerant Flow
summary Brief profile of a VRF retrofit of an existing building's variable air volume HVAC system. The document discusses why VRF can be an attractive retrofit option for a building whose occupants exhibit varied requirements for space conditioning and who wish to minimize breaks in services during HVAC system replacement.
citation Project Brief: Lewis County PUD Variable Refrigerant Flow, Bonneville Power Administration, June 2012. Accessed May 23, 2014
Ground-Source Variable Refrigerant Flow Heat Pumps: A Solution for Affordable Housing, Assisted Living, Hotels and Dorms
summary Case study published by the Washington State University Extension Energy Program that profiles the recently constructed Little Deschutes Lodge, an affordable housing project for seniors located in La Pine, Oregon. Although located in the Pacific Northwest, the average seasonal temperatures in La Pine are not too different than those of Wisconsin. The case study describes the Lodge's HVAC system, which exemplifies how VRF technologies can be integrated with ground source heat pumps in a new construction project.
citation Kerr, M., Ground-Source Variable Refrigerant Flow Heat Pumps: A Solution for Affordable Housing, Assisted Living, Hotels and Dorms, Washington State University Extension Energy Program, Feb 2011. Accessed May 23, 2014.
Green Guide to the Museum
summary Publication from the Madison Children's Museum cataloging and describing the sustainable features of their renovated museum space.
citation Green Guide to the Museum, Madison Children's Museum. Accessed May 23, 2014.
Emerging Technologies Showcase Webinar, Variable Refrigerant Flow (VRF) Question and Answer Session
summary This publication from the Washington State University's Extension Energy Program's website, provides answers to questions submitted following a technology showcase webinar about VRF systems. It provides useful background on the technological details of VRF systems. Although the webinar was presented by organizations focusing on the Pacific Northwest, the topics covered have broad applicability and are relevant to VRF systems that might be installed in the Midwest.
citation Emerging Technologies Showcase Webinar, Variable Refrigerant Flow (VRF) Question and Answer Session, Washington State University, Extension Energy Program, Energy Efficiency Emerging Technologies, November 2012. Accessed May 23, 2014.
Variable Refrigerant Flow Systems (2012).
summary This report was prepared for the General Services Administration (GSA) by the Pacific Northwest National Laboratory (PNNL). The report critically examines the applicability of VRF technology to the GSA's numerous assets, a large portion of which are older buildings, some with historical significance. Most of the applications for VRF in the GSA's existing building stock are retrofit projects, since new construction is a small portion of the GSA's overall portfolio. The study summarizes the conditions that will most likely result in cost-effective VRF retrofit projects.
citation Thornton, B., Wagner, A. (2012). Variable refrigerant flow systems. Technical report prepared for General Services Administration by the Pacific Northwest National Laboratory. Accessed May 23, 2014.
Measure Summary Report: Variable Refrigerant Flow (2011).
summary This report was prepared for the Bonneville Power Authority (BPA) to summarize the available energy efficiency data and information on VRF systems, as applicable to the Northwest. The report assesses technical energy savings potential for VRF systems in the Northwest. It offers savings and cost information from a case study, a review of savings estimates supplied in the existing literature and a range of cost estimates. The technical potential assessment provided in the report uses a 20 percent savings estimate over baseline technologies, although it acknowledged that actual savings rates would likely be highly variable and would depend heavily on whether or not a system has a heat recovery capability.
citation EES Consulting, (2011). Measure Summary Report: Variable Refrigerant Flow. Prepared for Bonneville Power Administration by EES Consulting. Accessed May 23, 2014.
Variable Refrigerant Flow: An Emerging Air Conditioner and Heat Pump Technology
summary A concise summary of the potential benefits of VRF systems and market barriers to their adoption in the United States.
citation Amarnath, A. and Blatt, M. (2008) An Emerging Air Conditioner and Heat Pump Technology. 2008 ACEEE Summer Study on Energy Efficiency in Buildings. Accessed May 23, 2014.
Variable Refrigerant Flow Systems.
summary A summary of VRF technology and its advantages and disadvantages relative to other commercial heating and cooling solutions.
citation Goetzler, W. (2007). Variable refrigerant flow systems. ASHRAE Journal, 49(4), 24-31. Accessed May 23, 2014.