What is VRF?
VRF is an acronym for Variable Refrigerant Flow. VRF is an energy-efficient method of providing precise HVAC comfort control to indoor environments. VRF systems vary their rate of output to meet load demand. Traditional systems have a larger plus/minus margin before turning on or off which means occupants suffer a greater temperature gradient and the system uses more energy to bring the temperature back to the set-point. VRF systems are almost always on but are operating in a greatly reduced capacity. This maintains a more comfortable environment and provides tremendous energy savings.
VRF systems utilize variable motor speed and variable refrigerant flow, where the outdoor condensing unit conditions the refrigerant and circulates it within the building to multiple indoor units. These indoor units are known as fan-coils or cassettes.
VRF offers a wide variety of applications – everything from spot-cooling or heating a single room using a split-ductless system to a large commercial building with multiple floors and areas that require individual comfort control delivered by a split-zoning system.
VRF cooling and heating technology was invented in Japan in 1982. Its popularity has been growing steadily throughout Asia, Canada and the United States. VRF is also sometimes known as VRV – Variable Refrigerant Volume (Daikin).
VRF System Types
Cooling only: Heating is not available in this system type. Some examples of where this is used are server rooms, elevator rooms and tight-tolerance machine shops.
Heat Pump: The indoor units can heat or cool but not simultaneously.
Heat Recovery: Cooling and heating are available for independent zones…simultaneously.
How Does VRF Work?
A VRF system uses refrigerant to transfer heat between an outdoor condensing unit and indoor fan coils. It can simultaneously cool some zones while heating other zones or simply provide comfort control to zones that are in use while not operating in zones that are not occupied. Multiple indoor fan-coil units can be connected to each VRF outdoor condensing unit. The system varies the volume and rate of transfer of the refrigerant to meet the demand of the fan-coils and maintain a closer temperature set-point in individual zones.
VRF systems have an air conditioner inverter compressor which supports variable motor speed and variable refrigerant flow unlike traditional systems that are either on or off. Inverter compressor technology is highly responsive and efficient. The indoor units of a VRF system are quieter and more compact than a traditional system. By operating at varying speeds, VRF systems work only at the needed rate which allows for substantial energy savings.
VRF moves refrigerant to the zone to be heated or cooled, allowing the temperature of that area to be more precisely controlled. The refrigerant absorbs heat from a space requiring cooling and transfers it to those areas that need extra heating.
What Are Zones?
Zones are single or multiple rooms grouped together that receive conditioned air from the same outdoor condensing unit – specified by the temperature set-point of each individual zone. They operate independently from other zones within the same building and VRF system.
A building that has air-conditioned zones has been divided into grouped or individual rooms whose similarity is the need for heated or cooled air. Rather than relying on a single unit to cool or heat the entire building to a single temperature set-point leaving many occupants uncomfortable, a multi-zoned building utilizes a network of small, energy efficient mini-split units and thermostats to send the right type and amount of conditioned air to each zone providing temperate air to the occupants of each zone. The creation of zones allows building owners to have room-by-room comfort and control, independent of solar gain and interior shading.
The number of zones a building needs depends on its size and layout. In some cases, the entire floor of a building can be comfortably heated or cooled by a single zone. In others, multiple zones are utilized to provide a stable temperature to the central area of the building, heat the shady side and cool the sunny side. The greatest benefit of zoning is the level of customization that can be achieved. Conditioned air can be provided to occupied spaces and the system can be turned off in unused or unoccupied spaces within a building. This customization reduces overall energy consumption and costs. Buildings that benefit from zoned systems include:
- Buildings with two or more stories
- Rooms that get limited usage or are blocked off in the winter or summer because of comfort issues
- Rooms with large windows, no windows or high ceilings
- Occupants that often find themselves battling over the thermostat
- Older, historic buildings where traditional ductwork isn’t feasible
Simultaneous Heating And Cooling
Heat recovery VRF allows indoor units to heat or cool as needed while the compressor load benefits from the internal heat recovery. This results in energy savings of up to 55% over comparable unitary equipment and also allows for greater comfort control of the building by the occupants.
VRF systems that feature simultaneous heating and cooling use a technology called heat recovery. In a heat recovery system, some zones can be in heating mode while other zones are simultaneously in cooling mode. These systems enhance the capabilities of zoning technology by using a BC (Branch Circuit) Controller. The BC Controller can remove heat from a zone that doesn’t need it and apply it to another zone that does. For example, if an indoor unit in one room is calling for cooling and an indoor unit in another room is calling for heating, the BC Controller can take the heat absorbed by the refrigerant in the room that requires cooling and transfer it to the room that requires heat. The BC Controller performs all the work, bypassing the compressor …saving the electricity that would be required to operate the compressor.
Two-Pipe or Three-Pipe?
VRF systems are configured as either two or three-pipe. In a standard two-pipe system, all of the zones must be in either heating or cooling mode. Some 2-pipe systems can simultaneously heat and cool. These systems incorporate a BC Controller. With the BC controller, the heat extracted from the zones requiring cooling is put to use in the zones that require heating. In this case, the heating unit is functioning as a condenser providing sub-cooled liquid back into the line that is being used for cooling. This is also called a heat recovery system. And, while it costs more, it allows for better zoned temperature control and overall better efficiencies.
In a 3-pipe, heat recovery system, some zones can be in heating mode while other zones are simultaneously in cooling mode without the need of a BC controller. The 3-pipe system transfers heat extracted from zones needing cooling to zones needing heat allowing for comfort for all occupants from the same system.
Ducted HVAC systems can be zoned but they cannot achieve the same level of customization and control precision that is possible with a ductless system. When every zone in the building is managed by its own ductless unit, every room and every occupant of the building can achieve total comfort, no matter what time of day or year.
Traditional ducted systems seldom had the joints of the ductwork sealed. When the furnace or air conditioner of a traditional system is operating, a certain portion of the conditioned air leaks out the joints, cooling the joist and wall cavities. Those leaks waste energy and money. Also, the AC or heating system must work harder to get enough conditioned air into the rest of the building. Ductless mini-split systems automatically eliminate that air and energy loss.
Duct leakage can be fixed by sealing the joints from the inside with an aerosol coating. However, that won’t fix the energy loss. When a ducted system is operating, it is cooling the metal ductwork and all of the air around the ducts. This is energy inefficient and costly. Ductless mini-split systems save energy by eliminating duct cooling loss. Ductless systems provide more flexibility to the designer of the building and designer of the HVAC system. The lack of ductwork and smaller equipment unit sizes result in more usable space within the building.
Intelligent indoor units have sensors that measure room air temperature at the return air inlet, or if the design requires it, air temperature can be measured at a remote thermostat. The ability to choose the measurement location allows for better air temperature management and provides the opportunity of maintaining the set point goal of within 1degree Fahrenheit (half of a degree Celsius).
Some split and VRF system indoor units feature a sensor accessory which compares air and floor temperatures and then adjusts vane direction as needed to optimize the comfort within the space. Some indoor units feature linear expansion valves to ensure the precise amount of refrigerant is delivered to the zone.
Cooling & Heating VRF and split systems with multiple indoor (air-handling) evaporator units connected to an outdoor compressor unit can deliver just the right amount of refrigerant to precisely meet the load requirement of each zone. Indoor units can be managed to operate only in those occupied areas that need conditioning and indoor units in vacant areas can be turned off. With the ability to turn off units in unoccupied areas, system capacity is preserved and there are tremendous savings in energy usage and operating costs.
Designing a VRF System
The HVAC system designer will size the system based on the peak heating and cooling loads of the building; the combined capacity of the indoor units can match, exceed, or be lower than the capacity of the connected outdoor unit. The total combined capacity of the indoor units should not exceed the capacity of the outdoor unit. It is, however, very common to see a split-system with a total capacity of indoor units exceed the capacity of the outdoor unit by 20%. Inverter-driven VRF systems can be designed and sized on a zone-by-zone basis to adjust for changing seasons and solar gain of the building.
There are several factors to be taken into consideration when determining the locations of the indoor and outdoor units. The direction of air-flow from the indoor units should not fall directly on the occupants and there should be no obstacles that might interfere with the airflow and prevent it from covering the space. The outdoor unit should be located in open space…far away enough from the building to not be affected by the discharge from the building. Sensors should be installed away from external heat sources such as printers, coffee makers and computer monitors. All units in the system should be installed where they are easily accessible for maintenance.
One of the main considerations when installing a split system is the length of refrigerant piping connecting the external unit to the internal units. To minimize the lengths of refrigerant piping, the system designer will locate external and internal units as close to each other as possible.
- Wide variety of applications – everything from spot cooling or heating a single room with a split-ductless system to a large commercial building with multiple floors and areas (zones) that require individual comfort control delivered by a split-zoning system
- VRF moves refrigerant to the zone to be heated or cooled, allowing the temperature of that zone to be more precisely controlled and providing greater occupant comfort.
- VRF systems can simultaneously cool some zones while heating other zones or only provide comfort control to zones that are in use
- Ductless systems require less physical interior space than ducted systems.
- Ductless systems provide more flexibility for building design and result in more usable occupant space.
- Ductless systems allow the flexibility of combining multiple rooms into a single zone or having a single room as an individual zone.
Select Mechanical can help with your VRF system design and installation. To arrange a consultation, please call our offices at 925-447-1500 or send us a request for an estimate; firstname.lastname@example.org