Variable refrigerant flow (VRF) systems are air conditioning systems based around the vapor compression mechanical refrigeration cycle used to produce heating and cooling, similar to a standard direct expansion (DX) heat pump.What sets VRF apart? In a standard DX heat pump, there is a single refrigeration coil, connected to the condensing unit by refrigeration piping. The coil is inside of a sheet metal box, with a fan to blow air across the coil and produce conditioned air. That box is connected to the spaces requiring air conditioning by ductwork, and the fan pressurizes the duct system to move air into the occupied space. For VRF, rather than having a big box with ductwork running out to each space – each space has its own smaller sheet metal box with a smaller fan and smaller refrigeration coil. Instead of large ducts, we instead bring small refrigeration pipes from the outdoor unit to each indoor box.
Given that the VRF refrigeration system is spread across a larger footprint than the simple DX heat pump version, the VRF system is more custom engineered by the manufacturer. As a result, the system can be flexible and highly customizable. Modulating compressors are used, allowing for higher efficiency, in addition to limiting the fan energy required. VRF also takes up minimal space, allowing for easy retrofitting.
Air source VRF is popular around the globe with widespread use for 20+ years in Asia and Europe. Within North America, it’s steadily capturing a larger share of the residential and commercial market. A growing trend is the use of VRF in Passive House projects, or other projects seeking to reduce their site usage of fossil fuel. The latter is an especially hot topic these days, as many towns, cities, and even states are pushing for full electrification. The goal of building electrification being to transition all energy uses over to electricity only as the power grid moves to become cleaner and less reliant on fossil fuel. VRF can facilitate this goal by using an air source heat pump rather than a gas fired furnace or boiler. VRF is also a great fit when a project is spatially challenged, as small refrigerant piping can be snaked through a building much easier than larger sheet metal ducts.
There are many different styles, manufacturers, and options for VRF. Each project is different and will need careful consideration to determine the best design. Cooling Only and Heat Pump Operation.
Cooling only VRF is available and works the same as a heat pump (just without the reversing valve in the outdoor unit). Typically, these are used in the residential and light commercial marketplace. Think of the “mini-split” type units seen in homes, cafes, etc. Typically, these units are air source only. Some typical indoor units are shown in the following pictures as well.
Figure 1: Cooling only VRF System (Fujitsu website)
Figure 2: Common VRF Indoor Unit Types (ASHRAE Systems and Equipment 2020, Ch. 18)
Provides heating and cooling, but both options are not available simultaneously. Again, this will look very similar to the cooling only options. Air source and water source heat rejection options are available, but air source is most common.
This is the category where VRF really shines and provides the most versatility for the built environment. It’s also the most complicated to design. Heat recovery VRF provides heating and cooling, with both options available simultaneously. Air source is the most common option, but water source is also available.
Figure 4: 2 Pipe VRF Heat Recovery (ASHRAE Systems and Equipment 2020, Ch. 18)
Figure 5: 2 Pipe VRF Heat Recovery (ASHRAE Systems and Equipment 2020, Ch. 18)
The typical workflow an HVAC design is to estimate size of key central components (chiller, boiler, cooling tower, etc) based on preliminary block loads within a space and develop the space by space design as the project progresses and those spaces are further refined. VRF requires the opposite approach. To properly size the outdoor units, all indoor units must be defined first. So, a space by space load calculation must be done, desired zoning determined, indoor units selected, and then finally outdoor unit design can be determined. This is due to refrigerant charge limitations, oil return, branch selector box capacities, and ASHRAE standards 15 and 34 requirements. Most outdoor units have a capacity requirement of approx. 70%-150% of connected indoor units.
Figure 7: Enclosed Air Source VRF Outdoor Unit
Specific options must be selected for proper operation in particularly cold regions. This is especially important as we travel from the Hallam-ICS Southern New England office up to the South Burlington, VT office. (Southern New England counts here, too, not just you folks up in Vermont!). This will influence heat recovery vs heat pump, in addition to specific accessories required.
Low ambient heating packages can be provided to allow for heating operation below 0°F (see Daikin VRV IV or Mitsubishi Hyper Heat option). The Designer needs to be aware that defrost cycle will reduce capacity – outdoor unit design to be reviewed to make sure that the de-rated capacity of the unit at design weather conditions can meet peak loads. (Heating capacity reduced by about 25% at -13°F compared to nameplate).
Figure 8: VRF Piping Limitations (Daikin VRV Reference Guide)
VRF can be utilized across many different building types and occupancies. It’s share of the market is only going to increase as the AEC industry pushes towards electrification. It’s among the most energy efficient options for providing heat via site electricity only.
Here at Hallam-ICS, we love to design for the unique, “oddball” situations. VRF is a great tool we can utilize to fit the space heating and cooling needs for those unique designs. We’ve explored installing outdoor VRF units inside of a warm pump room and using them for as a simple means of recovery of low-grade heat – transferring heat that would otherwise be exhausted out the building. VRF can be utilized in tandem with a DOAS unit in labs to provide space heating and cooling, minimizing the outside air that must be conditioned. If the project space is critical and the associated heating and cooling must be on generator power – what do you do if the central boiler/chiller plant isn’t? VRF is a great option in that situation to avoid a potentially significant electrical upgrade project.
About the author
Jeff joined Hallam-ICS in April 2020. He joined as a Sr. Mechanical Engineer, and has a strong technical background in building mechanical systems and MEP/FP project management.
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About Hallam-ICS
Hallam-ICS is an engineering and automation company that designs MEP systems for facilities and plants, engineers control and automation solutions, and ensures safety and regulatory compliance through arc flash studies, commissioning, and validation. Our offices are located in Massachusetts, Connecticut, New York, Vermont and North Carolina and our projects take us world-wide.