Back to basics: VRF systems
Know the basics of variable refrigerant flow (VRF) systems to determine if they are the right choice for your next HVAC project.
- Summarize the different types of variable refrigerant flow (VRF) systems available.
- Explain the pros and cons of using VRF systems in a commercial building application.
- Identify the codes and standards that dictate the design and use of VRF systems.
Variable refrigerant flow (VRF) systems are gaining in popularity and are used as an enhanced version of multi-split systems, featuring simultaneous heating and cooling as well as heat-recovery capabilities.
Modern VRF systems provide some major advantages, such as zoning, individual temperature control, minimized ductwork, excluding the need for secondary fluids (chilled-water or hot-water distribution), and associated costs. This all-electric technology consists of a single outdoor condensing unit, multiple indoor units serving various zones, refrigerant piping with branch selectors, and associated controls.
VRF systems use R-410A refrigerant as the heat-transfer fluid and the working fluid, achieving a very high energy efficiency ratio (EER) of 15 to 20 and integrated energy efficiency ratio (IEER) of 17 to 25. They are 20% to 30% more efficient than conventional HVAC systems due to partial load operation, speed modulation, zoning capabilities, and heat-recovery technology.
In recent years, gas heat pump technology has been increasingly used in certain applications where natural gas utilities offer incentives. As a result, VRF systems can contribute a great number of points toward U.S. Green Building Council LEED certification.
VRF system operation
VRF systems are nontraditional HVAC systems, in comparison with conventional ducted systems circulating the air or chilled-water throughout the building. The term VRF indicates the ability of the system to vary and control the refrigerant flow through multiple evaporator coils to provide individual temperature control in various mechanical comfort zones.
Using direct expansion (DX) as part of the basic refrigeration cycle, VRF systems transfer the heat from the room directly to evaporator coils located within the conditioned space. The heat-transfer media, in this case, is the refrigerant, which delivers heating and cooling to various zones with less energy as compared with air or water.
VRF systems act as multi-split systems, connecting multiple indoor units with one centralized outdoor condensing unit assembly, providing simultaneous heating and cooling and heat recovery in various zones as follows:
- The VRF heat pump system provides heating and cooling for all indoor units at a specific time (see Figure 1)
- The VRF system provides nonsimultaneous cooling and heating at any time
- Heat-recovery systems provide simultaneous cooling and heating as well as heat recovery, transferring the energy from cooling zones to heating zones of the building.
All of the above features are accomplished by VRF-enhanced technology using:
- Variable-speed and capacity-modulated inverter duty compressors
- Outdoor fans with variable frequency drives motors
- Indoor units with electronically commutated motors (ECM).
There are two different types of VRF systems:
Air-cooled, where multiple compressors are connected to a refrigerant-piping loop. Special attention should be paid to equipment selection in locations with high ambient conditions—outside air temperatures above 95°F. For example, in Las Vegas, with ambient temperatures at 115°F and above, the equipment de-rating can be as high as 30%.
Water-cooled, where multiple compressors are connected to a water-source loop, allowing the heat recovery between compressor units.
Various manufacturers have developed refrigerant-piping loop systems for different applications, such as:
Two-pipe systems, which are normally used in VRF heat pump applications to provide cooling or heating only during the same operating mode (see Figure 2). Branch-circuit controllers are used with two-pipe systems to perform the following functions:
- Separate refrigerant into gas and liquid
- Ensure that zones in heating mode receive superheated gas
- Ensure that zones in cooling mode receive subcooled liquid
- Facilitate removal of heat from one zone and apply it to a different zone.
Three-pipe systems, which are configured with a heating pipe, a cooling pipe, and a return pipe (see Figure 3). Branch selectors are used with three-pipe systems to perform the same functions as two-pipe systems with the exception of separators.
- Branch selectors do not require separators because they are connected to a three-pipe system: refrigerant liquid line, refrigerant suction gas line, and high-pressure/low-pressure (HP/LP) mixture line.
- Branch selectors perform a similar function as branch-circuit controllers, directing the superheated gas to heating zones and subcooled liquid to cooling zones. The HP/LP mixing pipe is routed back to the outdoor condensing unit.
The VRF system is best suited for applications with simultaneous needs for cooling and heating during the same mode of operation. Branch selectors are used as control devices directing the liquid refrigerant or gas refrigerant to particular zones requiring cooling or heating.
In heat-recovery systems, the branch-circuit controller can take the heat recovered from the cooling zone and use it to warm up the room in heating mode. This way, the compressor cooling or heating requirements are reduced, which saves energy.
Outside air ventilation
Dedicated outside air units with energy recovery are used to provide ducted ventilation air directly to the space or indoor unit.
ASHRAE Standard 62.1: Ventilation for Acceptable Indoor Air Quality is used to calculate the required outside airflow to each space.
VRF systems require much less ceiling space than conventional systems because only the refrigerant piping and ducted outside ventilation air are accommodated.
VRF system applications
Heat pump systems are used in restaurants, lobby areas, clubhouses, or religious facilities where there is a defined cooling or heating mode of operation. All indoor units will operate in either cooling or heating mode (nonsimultaneous).
Heat pump systems with heat recovery are used in historical buildings, schools, office buildings, assisted living facilities, hotels, banks, and other commercial buildings where simultaneous cooling and heating is a design requirement.
The advantages of VRF systems include:
- Increased energy efficiency and energy savings, for an average of 20% to 30% energy savings relative to variable air volume systems with reheat and constant air volume systems with gas heat
- Very good part-load performance due to inverter-duty variable-speed compressors modulating the capacity from 10% to 100%
- Good zoning control, providing simultaneous cooling and heating with heat recovery
- Reduced ductwork and duct losses are confined to the ventilation air system (approximately 20% of the conventional HVAC system).
The disadvantages of VRF systems include:
- The need for a dedicated ventilation system to deliver the outside air to various zones
- Long refrigerant lines and a large number of branch connections could result in refrigerant leakage
- The need for condensate drain lines for each VRF indoor unit
- Use of supplemental heat may be required for a quick warm-up
- Compliance with maximum allowable refrigerant quantities within a given volume.
Codes and standards
VRF systems must comply with ASHRAE Standard 15 (packaged with Standard 34): Safety Standard for Refrigeration Systems and Designation and Classification of Refrigerants. This addresses refrigerant capacities and possible leakage, especially if the system serves small rooms, which could cause oxygen depletion.
VRF systems use refrigerant R-410A. The safety classification of R-410A in ASHRAE Standard 34 is group A1: nontoxic and nonflammable refrigerant with zero ozone-depletion potential.
Due to the ability to displace oxygen, ASHRAE Standard 34-2013 Addendum L has established the maximum refrigerant concentration limit (RCL) of 26 lbs/1,000 ft3 of room volume for occupied spaces.
According to Standard 15, a VRF system is classified as a direct system/high-probability system where a refrigerant leak can potentially enter into the occupied space.
ASHRAE Standard 15 requirements should be applied to each VRF system design in the following steps :
- Determine the occupancy classification for the rooms
- Calculate room volume
- Determine the amount of refrigerant in the system including the outdoor unit, indoor units, and associated piping
- Verify that the room is not too small using the following formula:
Minimum allowed floor area (sq ft) = Total system refrigerant charge (lbs) x 1,000 RCL (lbs/1,000 ft3) x ceiling height (ft)
Integrated energy efficiency ratio (IEER)
Per Air-Conditioning, Heating, and Refrigeration Institute’s (AHRI) standard, AHRI 1230: Performance Rating of Variable Refrigerant Flow Multi-Split Air-Conditioning and Heat Pump Equipment, IEER has been established as a measure of the cooling produced for the amount of energy required to produce it in Btu per Watt per hour.
IEER is calculated as the sum of four part-load conditions: IEER = (0.02 x A) + (0.617 x B) + (0.238 x C) + (0.125 x D). Where:
A = EER at 100% net capacity at AHRI standard conditions (95°F)
B = EER at 75% net capacity at reduced ambient (81.5°F)
C = EER at 50% net capacity at reduced ambient (68°F)
D = EER at 25% net capacity at reduced ambient (65°F)
Note: Full-load EER (100% capacity) represents only 2% of the overall IEER rating. As overall capacity is reduced, the system EER is increased significantly (see Figure 5).
VRF systems have a significant impact on energy consumption in comparison with ASHRAE 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings-2010 base building, providing a highly efficient HVAC system and achieving a great number of points in Energy and Atmosphere Credit 1: Optimize Energy Performance.
The main advantages of VRF systems in energy efficiency are due to:
- Part-load operation and energy-performance optimization
- Zoning capabilities
- Heat-recovery potential
- Use of inverter-duty compressors
- Reduced kilowatt-per-ton energy input, which results in reduced overall energy cost budget.
A regular rooftop unit with EER 13 has 0.923-kW/ton input
A VRF system with an IEER of 17.4 has 0.689-kW/ton input.
This will result in the VRF system having more favorable energy performance and achieving more than 20% (or more) energy reduction as compared with the ASHRAE base building.
Alex Jankovic is a mechanical project engineer with JBA Consulting Engineers.