The 101 on Variable Refrigerant Flow Systems
Popular in Asia and Europe since the 1980s, Variable Refrigerant Flow (VRF) was introduced to the U.S. market for commercial applications in 2003. According to a report published by Prescient and Strategic Intelligence, the fastest growth in the global VRF market between 2018 and 2023 will occur in North America.
VRF is an HVAC technology designed to provide energy-efficient comfort control according to the conditioning needs of a building’s zones. Supported by integrated controls and sensors, VRF systems accomplish cooling and heating through the transfer of conditioned refrigerant between each zone’s indoor unit(s) and an outdoor unit. VRF systems are able to modulate the flow of refrigerant, so the system only uses the precise amount of energy needed to meet each zone’s conditioning requirements.
The Inverter Advantage
As the mechanism that controls the capacity within the system, the compressor has been referred to as the heart of HVAC systems. The compressor serves the same mechanical function in both traditional HVAC systems and VRF systems, but has distinct electrical characteristics in each.
In traditional HVAC systems, the compressor runs at the same speed regardless of the load, while in VRF systems it is equipped with an inverter that enables it to vary its speed and capacity to match the cooling or heating load of a space at a particular point in time. A heart metaphor is apt for the inverter-driven compressor of VRF systems. Just as a human heart beats faster during exercise and slower during sleep, the inverter-driven compressor enables VRF systems to automatically adjust conditioning in response to user commands and environmental conditions.
HVAC systems are sized to provide heating on the coldest day of the year and cooling on the hottest day. This means that the fixed-speed compressors in more traditional HVAC systems always run at a speed best suited for extreme temperatures, or the full projected load of a building. Variable speed enables VRF systems to save energy and money by modulating capacity and electricity use to match part-load conditions, which occur most of the time.
Sustainability Drives Demand for VRF
The global movement toward a more sustainable, but technologically-advanced built environment is a key driver for VRF system adoption. As architects design high performance buildings to satisfy requirements associated with LEED®, Green Globes®, Passive House, ZeroNet Energy (ZNE), deep-energy retrofits and ambitious sustainability goals set by governments and private entities, the need for VRF technology as an energy-efficient HVAC system has become more widely recognized.
The 2018 report published by the U.N. Intergovernmental Panel on Climate Change indicates that a complete modernization of the built environment with net zero carbon dioxide emissions is needed to keep planetary climate change from exceeding 1.5° Celsius above preindustrial levels.
Through zoning and advanced controls, VRF can address the expectations formed by smart technologies that provide individuals with near-instant access to information, commerce and entertainment.
To achieve a decarbonized built environment, Strategic Electrification is essential. Strategic Electrification is the modernization of technologies, transportation systems and the built environment to be powered by electricity drawn from renewable energy sources that do not generate CO2 emissions. Along with greener electrical grids and innovative batteries to store excess power, highly-efficient, electric-powered mechanical systems such as VRF are critical to Strategic Electrification. VRF systems’ efficient use of electricity makes them ideal for applications that use solar, wind or hydroelectric power. With minimal electrical waste, VRF systems’ precision promotes a more sustainable, decarbonized built environment that retains modernity and comfort.
True First Costs
With conventional systems, the first cost may appear lower compared to VRF systems on a mechanical bid, but, typically, the bid only accounts for equipment and installation.
By delivering conditioned refrigerant through small-diameter piping to spaces requiring heating or cooling, VRF systems reduce expenses related to duct runs and required plenum space. Perhaps instead of a 30-inch plenum for a Variable Air Volume (VAV) system, the building will only require an 18-inch plenum for refrigerant piping and ventilation air conditioned by a dedicated outdoor air system.
VRF systems are also lighter and more compact. This efficiency of design reduces expenses related to structural support requirements. Additionally, VRF systems can be installed in less time than many conventional systems, which can reduce labor costs.
The modular design of VRF systems may also open opportunities for deferred costs as tenant leases are secured.
Zoning and Integration
To condition interior spaces, HVAC contractors must install ductwork that runs from the main unit and splits off into different rooms. Conventional HVAC systems require ductwork, pipes and a controls system to connect components from different manufacturers. Zoning is possible but requires dampers and space-intensive ductwork.
In contrast, a VRF system consists of an outdoor unit and a network of indoor units connected via refrigerant lines and governed by a network of controls and sensors. The outdoor unit contains the condenser coil, heat exchanger, and fan(s). The indoor units contain fan(s), a filter, a heat exchanger and environmental sensors. Each outdoor unit may be connected to multiple indoor units, and individual outdoor units may be combined to increase tonnage.
Rather than running extensive ductwork, the HVAC contractor installs refrigerant pipes and wires from the outdoor unit to each zone’s indoor unit. Giving each zone its own indoor unit simplifies zoning and saves space with the largest refrigerant pipe having a width of an inch and three eighths.
Heating, cooling and controls are fully integrated and in continuous communication in VRF systems. This eliminates the complexity of conventional systems, which may include components from multiple manufacturers.
Through zoning and advanced controls, VRF can address the Through expectations formed by smart technologies that provide individuals with near-instant access to information, commerce and entertainment. Additionally, some indoor units incorporate sophisticated algorithms that allow VRF systems to change conditioning and airflow based upon the number and location of occupants.
While occupants want personalized local comfort control, building owners and facility managers are demanding better-centralized control through sophisticated controllers and building management systems (BMS). VRF offers sophisticated controls and can integrate with a BMS through standard protocols like BACnet® for efficient management, reporting and tighter control of usage and utility costs.
VRF systems are able to modulate the flow of refrigerant so the system only uses the precise amount of energy needed to meet each zone’s conditioning requirements.
In addition to personalized, energy-efficient, thermal comfort, VRF systems provide a more comfortable auditory environment by minimizing operational noise. VRF indoor units run at whisper quiet sound levels between 19 and 34 decibels while VRF outdoor units operate at levels as low as 55 decibels, no louder than a typical conversation. The low level of operational noise coupled with VRF units’ relatively compact size gives architects, engineers, HVAC contractors and building owners a range of options that maximizes usable space.
Indoor Air Quality and VRF
VRF systems provide zone filtration and allow for the integration of ventilation air. Each zone has an indoor unit equipped with its own filter to reduce contaminants such as allergens, viruses and bacteria. To ensure compliance with the ventilation requirements of ASHRAE Standard 62.1 and the International Mechanical Code, HVAC contractors can apply an Energy Recovery Ventilator (ERV) or a Dedicated Outdoor Air System (DOAS) with VRF. Specialized mechanical systems engineered for use with VRF systems are energy-efficient with all the benefits of variable capacity and can deliver ventilation rates of 30 percent or more above ASHRAE 62.1-2007 requirements.
Heat Pump, Heat Recovery
VRF heat-pump systems are either in cooling mode or heating mode, delivering refrigerant gas for heating or liquid refrigerant for cooling to indoor units. These VRF systems are appropriate for large, single zone applications or in warm climates where cooling is required all year.
When applied with heat recovery, VRF systems can provide simultaneous cooling and heating, with sophisticated controls and a heat recovery module directing refrigerant gas or liquid refrigerant depending on what mode indoor units are in. These systems can transfer refrigerant to indoor units directly from the outdoor unit, but can also transfer refrigerant between indoor units that are in opposite modes. With this capability, heat that would otherwise be rejected at the outdoor unit’s condenser can be repurposed as the system leverages load diversity to increase total applied capacity by up to 150 percent over the rated capacity of the outdoor unit.
Both types of VRF systems are equipped with sensors, controls and inverter-driven compressors, but comfort issues will arise if heat-pump VRF systems are applied in buildings where some zones may require heating at the same time others require cooling. Selecting the correct VRF system is key to each project’s success.
As the VRF market continues to expand and VRF technologies continue to evolve, it is increasingly important for facility managers to understand the benefits of VRF. When used to their potential, VRF systems can provide exponential cost-savings, can support a more sustainable and technologically-advanced built environment and can lead to a host of other benefits for building occupants, including better indoor air quality and improved comfort.
By Cain White, Senior Manager of Commercial Product Planning, Mitsubishi Electric Trane HVAC US