Variable refrigerant flow (VRF) systems are relatively new to the HVAC industry. They have gained popularity over the last 10 years and find their best application where there is a need for multiple zones of temperature control, simultaneous heating and cooling loads served by the same VRF system, and restricted ceiling space for piping and ductwork.
System Configurations
VRF systems utilize refrigerant as the working fluid and can provide cooling only to multiple indoor fan-coil units connected to a single condensing unit, either cooling or heating to multiple indoor fan-coil units connected to a single heat pump unit, or both cooling and heating simultaneously to multiple indoor fan-coil units connected to a single heat recovery unit.
The condensing units, heat pump units, and heat recovery units can be either air- or water-cooled. The indoor fan-coil units are available as recessed ceiling cassettes, concealed fan-coil units for ducted applications, surfacemounted wall and ceiling units, and floor-standing (cabinet) units. In order for a single condensing unit, heat pump unit, or heat recovery unit to serve multiple fan-coil units, it is necessary for the compressor(s) to operate at varying speeds; thus, providing a variable flow of refrigerant to the fan-coil units based on load.
For air-cooled condensing units and heat pump units, variable speed condenser fan motors are utilized to match the cooling or heating capacity of the outdoor unit(s) to the net cooling or heating load of the indoor fan-coil units. In the case of heat recovery units, a branch circuit controller is connected between the indoor fan-coil units and the heat recovery unit which allows some fan-coil units to operate in the cooling mode while other fan-coil units operate in the heating mode.
Application of Heat Recovery Systems
Heat recovery systems are particularly energy efficient when the area served has simultaneous heating and cooling loads, such as in the case of an office space with a large floor area. In this case, simultaneous heating and cooling occur during the wintertime when the interior zones, which require cooling year-round, exchange the heat they reject to the perimeter zones which have a heating load in the wintertime. This energy exchange occurs when energy is absorbed by the refrigerant from the units providing cooling and is redirected by the refrigerant to the units that are in the heating mode.
Capacities
Multiple indoor fan-coil units controlled by a single space temperature sensor may be required for larger spaces to meet the cooling and heating load of the space. Also, where the cooling load of the fan-coil units exceeds the approximately 25-ton maximum capacity of a single outdoor unit, two or three outdoor units can be connected together through the use of a twinning kit.
Outdoor Air Ventilation
For most applications, outdoor air ventilation is required in addition to the heating and cooling provided by the indoor fan-coil units. This can be accomplished either by providing outdoor air ventilation directly to the fan-coil units through a small outdoor air duct connection on each unit or through a dedicated outdoor air system which would deliver conditioned outdoor air either directly to the spaces served or to the return air duct of concealed fan-coil units used for ducted applications.
System Efficiency
Because of the unique operating characteristics of VRF systems, such as variable speed compressors and the simultaneous heating and cooling accomplished by heat recovery units, the Air-Conditioning, Heating, and Refrigeration Institute (AHRI) developed the integrated energy efficiency ratio (IEER) to measure the part-load cooling performance of unitary equipment and VRF systems. VRF energy recovery systems are capable of obtaining an IEER rating as high as 22.1 (based on the AHR! 1230 test method). Actual system efficiency will depend upon the level of energy recovery that can be achieved.
Annual Maintenance Cost
The annual maintenance cost of a VRF system can be much higher than a conventional HVAC system, such as a variable air volume (VAV) system, due to the complexity of the equipment that comprises a VRF system. Not only is the equipment itself complex, with the variable speed compressors and refrigerant metering devices in the branch circuit controllers of the heat recovery type VRF systems, so is the computerized control system.
Each manufacturer has a proprietary control system and associated software and computer interface that requires specialized training for a technician to become proficient in its use under normal operating conditions and as a troubleshooting tool when the equipment malfunctions. This requires a level of specialization that the in-house maintenance staff of most building owners does not possess. Therefore, when a problem occurs with a VRF system, the owner must call a contractor who has a service technician trained in the equipment and computerized control system of the particular VRF equipment manufacturer.
This may result in an extended timeframe between when a problem is experienced by the building owner with the VRF system and when the issue is finally resolved by the HV AC service contractor. Furthermore, the cost of repairing VRF systems can be high due to the cost of the VRF equipment and computerized control components that may need to be replaced.
Also, VRF systems operate at elevated refrigerant pressures, higher than conventional HVAC equipment. As a result, VRF systems are prone to refrigerant leaks. When a refrigerant leak occurs in a VRF system, the leak must first be located, which can be difficult considering the extensive lengths of insulated refrigerant piping associated with VRF systems. Once the leak is located, the entire VRF system, which may consist of upwards of 20 fan-coil units and two to four outdoor units per system, must often be fully evacuated of all refrigerant and moisture before the leak can be repaired and the system can be recharged with refrigerant. This process of evacuation and refrigerant recharge usually takes a minimum of 24 hours.
Therefore, the timeframe between when a refrigerant leak is identified and when the leak is repaired and the system is evacuated and recharged with refrigerant can be several days. In the meantime, the VRF system, if it has lost its entire refrigerant charge, is completely unable to provide heating or cooling to the spaces served. The cost to fully recharge an extensive VRF system that has lost all of its refrigerant charge can also be quite high due to the amount of refrigerant that is required.
W. Larsen Angel (HVAC Design Sourcebook)