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).
System types
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.
Source: Back to basics: VRF systems | Consulting – Specifying Engineer (csemag.com)