Variable Refrigerant Flow
Variable Refrigerant Flow (VRF) systems are HVAC systems that use a vapor compression cycle and move heat by refrigerant using pipes throughout a building, attaching small indoor coils to an outdoor unit. They are often configured to also provide heating and cooling similar to heat pumps and often can include an optional heat recovery feature so it can provide some zones of a building with heating while other zones are cooling at the same time.
VRF systems can vary in size but are typically a single or small group of outdoor units connected to a group of indoor units connected by long refrigerant pipes. The compressor in the outdoor unit can operate over a large range of speeds, often based on changing the frequency of the electricity provided to the motor. The variable speed of the compressor translates into the refrigerant flowing at different rates, which can significantly reduce the amount of energy used when not operating at design conditions, which occurs most of the time.
The most typical configurations are:
- Cooling only, where the system only provides cooling to every indoor unit in every zone
- Cooling only or heating only, where all the indoor units operate either in heating or cooling mode at the same time.
- Two-pipe heat recovery where some groups of zones can operate in heating while other groups of zones operate in cooling depending on the operation of the heat recovery control units, a component that manages refrigerant flow.
- Three-pipe heat recovery where every indoor unit for each zone can independently provide heating or cooling since they have access to high-pressure vapor, low-pressure vapor, and liquid refrigerant pipes.
The heat recovery configurations mean that if the amount of heating and cooling are nearly balanced, the compressor does very little work, which makes VRF an ideal system for buildings in which a large portion of the year has some zones heating and others cooling. The most common configuration for VRF is to use air-source condensers, but VRF systems, like other heat pumps, can also be configured to use water-source, ground-source, or groundwater-source condensers. While the industry does generally consider these systems to be called Variable Refrigerant Flow (VRF), some older documents still refer to them as Variable Refrigerant Volume (VRV) instead.
How It Works
At the heart of any VRF system is the same vapor compression refrigeration cycle that is used by most air conditioning and heat pump systems. The differences are how the refrigerant flow can vary as well as how the refrigerant is provided to multiple indoor units.
The indoor units are often small ductless cassettes that fit into the ceiling but can also be wall or floor-mounted like a fan coil unit. For smaller buildings, they are often installed just as ductless mini-splits with an indoor unit mounted on the wall at or above head height. Ducted indoor units are also common. Most VRF systems only provide cooling and heating and rely on a separate system to provide ventilation air, often as part of a Dedicated Outdoor Air System (DOAS); other indoor units integrate with the room terminal or small air handling units.
The configuration of the overall system typically includes the outdoor units, the indoor units, and often the heat recovery control units. When heat recovery control units are included, part of the system can be cooling while other parts heating. The heat recovery control units route the liquid line either to or from the indoor units, along with switching between either the high-pressure or low-pressure vapor line to the indoor unit. This allows the indoor units to provide either cooling or heating. The heat recovery control units also coordinate and control the flow of refrigerant to the outdoor unit. By routing the refrigerant between different indoor units, heat recovery can effectively be performed by taking the subcooled liquid from an indoor unit in heating and sending that to an indoor unit in cooling.
A system that uses all three pipes for every indoor unit has the most flexibility and can flip between the operation for each indoor unit between heating and cooling, irrespective of the operation of nearby units.
Information Needed for the Model
According to the 2020 ASHRAE Handbook for Systems and Equipment in Chapter 18 - “Variable Refrigerant Flow,” modeling considerations include:
- Matching the sizing ratio of the total indoor terminal unit rated capacity and the total outdoor unit rated capacity (i.e., connected capacity) to manufacturer specifications
- Part-load performance of the heat pump’s variable-speed compressor
- Losses associated with the refrigerant piping lengths
- Heat recovery (simultaneous heating and cooling) in some energy models
In addition to these parameters, the following is often needed to develop a good model:
- Understanding the exact configuration of the VRF is critical related to when indoor units can independently provide heating and cooling.
- Even though many efficiency metrics are available, these often do not directly translate into the nominal efficiency that many models need. Downloading detailed performance data from the manufacturer is the best approach.
- If necessary, adjust the capacity at design conditions (outdoor air and return air temperature, for example) to the capacity from the rating point by examining the performance curves, asking the manufacturer, or using the manufacturer’s selection software.
- Control sequencing for supplemental heat and defrost
Note that models of VRF systems are more complex than models of many other HVAC system types, so if one goal of the energy modeling analysis is to compare the performance of a VRF system to other system types, then extra attention to VRF model inputs is warranted, and model output checks described below are important.
Please also take a look at capacity and performance curves related to heat pumps since they apply to VRF as well. Some manufacturers provide tool-specific modeling guides and some tools include performance curves related to specific products from manufacturers. The Additional Resources section below includes specific links.
Some simulation models rely on a large number of performance curves which may only be available from a limited number of manufacturers and other sources.
Since VRF depends on a large number of pipes carrying refrigerant at temperatures usually higher and lower than typical chilled and hot water piping, insulation is very important. Accounting for these losses is important for any VRF model.
Common Measures
Some common energy-efficiency measures to consider are shown below:
- Use of heat recovery when not otherwise specified
- Higher nominal efficiency for cooling and heating
- Better low-temperature performance
- Use water-source or ground-source condensers instead of air-source condensers, though keep in mind that the heat recovery features of VRF become somewhat redundant relative to the natural heat recovery functionality offered by a water loop
- Combine with the use of a dedicated outdoor air system (DOAS)
- Size for the heating load if predominant type to reduce any supplementary heating
- Added insulation for refrigerant pipes to minimize losses
- Three-pipe heat recovery system instead of two-pipe
Common Control Options
The most common control options include the following:
- Constant or variable airflow
- Cycling or continuous fan operation
- The time that is necessary to switch between different operating modes.
- Control of defrost cycle for outdoor coil, including whether the length of the cycle is fixed or automatic, the temperature above and below when it occurs, the humidity range that it occurs, the performance impact of the use of defrost, and if reverse cycle or separate heater.
- Control of the backup heat, the maximum temperature that it can occur, and the lowest temperature that the heat pump cycle can be used.
- Control of the basin or crankcase heater by specifying the maximum temperature for operation.
Most VRF systems have integrated controls that are proprietary to the specific manufacturer, so limited opportunities may exist for some control options.
Common Applications
According to the 2020 ASHRAE Handbook for Systems and Equipment in chapter 18 - “Variable Refrigerant Flow,” the most common VRF applications are:
- High- or low-rise offices
- Educational facilities (schools, universities)
- Health care facilities, including clinics and long-term-care nursing homes
- Multiple-tenant residential buildings
- Historical buildings
- Retail stores
- Hospitality centers, restaurants, banquet halls, hotels, and motels
- Data center cooling-only applications
- Cultural facilities, including religious centers
This list of possible applications is quite long but can be narrowed by three possible criteria that play to the advantages of VRF and VRF heat recovery:
- Applications with many zones of different space types, perhaps with different thermostat setpoints, and are adjacent to one another.
- Applications with a large portion of the year having some zones heating and others cooling.
- Existing building applications where the small size of the components is important or where installation without ducts can be beneficial.
Conversely, applications that are probably not favorable for VRF are when large spaces act as single large zones, such as big-box retail and warehouses
Model Output Checks
The following output checks should be considered when modeling heat pumps:
- Find heating only time and cooling only time when the operating conditions, primarily the outside temperature, are similar to rated conditions and make sure the output for the overall efficiency and capacity is as expected. This should be repeated for multiple cases if off-rated performance and conditions are known.
- For heat recovery systems, find simultaneous heating and cooling times and confirm the total heating output for the heating zones and the total output of the cooling zones are consistent with the reduced demand on, and output from, the outdoor unit.
- For a non-heat recovery system, confirm conditions in all zones directly after the transition between heating and cooling to make sure they are not being overcooled or overheated.
- At different outdoor temperatures and loads, check if the capacity and efficiency are as expected.
- Check that the backup heat only activates below the temperature that is specified.
- Check that just above the activation temperature for the backup that the heating load is satisfied.
- During the defrost cycle, make sure the performance is as expected.
Related Energy Code Requirements
ASHRAE 90.1-2019 includes requirements related to heat pumps, and you can learn more about them on the Heat Pump page. Besides that, the 90.1-2019 standard includes many references to efficiency requirements for VRF systems and equipment in Tables 6.8.1-8 and 6.8.1-9. The test procedure used for both tables is AHRI 1230 Performance Rating of Variable Refrigerant Flow (VRF) MultiSplit Air-Conditioning and Heat Pump Equipment. The metrics used include SEER and HSPF for smaller residential-sized units and EER, IEER, and COPH for larger units. For non-air condensers, the rating conditions for cooling for entering water temperature vary:
- 86F for water-source condensers
- 59F for groundwater-source
- 77F for ground-source
For heating mode, the rating conditions are:
- 47F dry-bulb/43F wet-bulb and 17F dry-bulb/15F wet-bulb for outdoor air
- 68F for water-source condensers for entering water temperature
- 50F for groundwater-source for entering water temperature
- 32F for ground-source for entering water temperature
AHRI 1230 also includes a metric called SCHE for Simultaneous Cooling and Heating Efficiency that so far is not used in 90.1 but tests the performance when part of the system is cooling, and another part is heating.
For ASHRAE 90.1-2022, a new path was added in Appendix G for non-substantial alterations, which does not use the normal baseline creation path that Appendix G traditionally had. For this path for non-substantial alterations, the HVAC type for the baseline is the same as the proposed with the following exception: “If the proposed design includes variable refrigerant flow heat pumps or single-zone systems with electric resistance heat, then air source heat pumps shall be used in the baseline design.” This exception would usually provide a credit for variable refrigerant flow heat pumps when using 90.1-2022 Appendix G for non-substantial alterations.
Similar or Related Systems
Heat pumps are the most similar system to VRF.
Additional Resources
- 2020 ASHRAE Handbook - Heating, Ventilating, and Air Conditioning Systems and Equipment. Chapter 18 - “Variable Refrigerant Flow”
- Thornton, Brian; Wagner, Anne. Variable Refrigerant Flow Systems. GSA, PNNL. December 2012.
- Wikipedia article on variable refrigerant flow
- ASHRAE Guideline 41-2020 - Design, Installation and Commissioning of Variable Refrigerant Flow (VRF) Systems
Links to external websites are provided as a convenience for further research, but do not imply any endorsement of the content or the operator of the external site, as detailed in BEMcyclopedia's general disclaimers.
References
| Content is available under the Creative Commons Attribution-ShareAlike License; additional terms may apply. By using this site, you agree to the Terms of Use. |