Variables create anxiety. We want to remove as many variables as possible from a situation. However, you can embrace the variables in variable refrigerant flow (VRF) systems because they are a major contributor to their efficiency. But you will need to address some variables about your building in order to determine if VRF technology is a good retrofit option.
A VRF system’s variable flow of refrigerant via variable frequency drive (VFD) motors allows the system to adjust to a variety of loads. The refrigerant serves as both the heating and cooling medium that can be delivered to multiple fan-coil units from a single outdoor condensing unit.
A VRF system with heat recovery allows indoor units to cool or heat as necessary. For example, instead of rejecting heat from a server room to the outside, a VRF system can move the heat to perimeter spaces that require it.
For retrofit applications, VRF can offer a solution for inefficient fans and leaky ductwork. Buildings with inadequate cooling may be a particularly good fit. Older facilities and historical buildings, which often cannot easily accommodate new HVAC equipment, may also be targets for VRF solutions.
If you’re considering a major retrofit of an existing HVAC system, take these three steps to do your due diligence for a possible VRF solution, says Anne Wagner, energy efficiency engineer at the Pacific Northwest National Laboratory (PNNL).
1) Screen by building characteristics – Some older buildings may provide better paybacks for VRF than new ones, according to Variable Refrigerant Flow Systems, a technology assessment prepared under GSA’s Green Proving Ground program (www.gsa.gov/greenprovingground). Wagner, who is a co-author of the publication, points out that buildings with high energy usage, particularly those with VAV systems with electric reheat or other electric resistance heat, may be good candidates. Facilities with limited space to add ductwork for additional cooling capacity may also be good candidates. The comparatively small size of VRF refrigerant lines makes them easier to install than ductwork even when buildings have adequate space.
The GSA report notes a number of factors to consider when evaluating a cost-effective match with VRF technology. The zonal capability of VRF fan units are well-suited to facilities with separated spaces, including schools, lodging, multifamily, healthcare, some shopping centers, and office buildings with numerous enclosed spaces and conference rooms. Conversely, big box retail, warehouses, and other facilities with large open spaces are not typically suitable.
Climate is also a factor, with extreme weather being a plus. Colder regions may offer better opportunities for heat recovery and may increase savings if converting from any type of electric resistance heating. Conversely, buildings in mild climates that often operate in an economizer mode will benefit less. According to the report, a sweet spot for VRF in terms of building size is 10,000–100,000 square feet.
2) Do an energy analysis – Have your building surveyed to understand how, where, and when it uses energy – and wastes it. Energy engineers recommend incorporating a minimum of one year’s worth of detailed consumption data in order to model the building through the seasons. The analysis will provide a platform for determining proper size as well as estimating the efficiency and cost of new systems.
3) Assess lifecycle costs – The GSA assessment notes that VRF systems may have higher maintenance, repair, and replacement costs that offset some of the energy cost savings. As a result, when analyzing the economics of VRF, detailed lifecycle costs should be estimated.
The GSA report includes a sample lifecycle analysis that compares a VRF system and a VAV electric reheat alternative for a hypothetical building (see below). The analysis includes costs for a dedicated outside air system (DOAS) because VRF systems themselves do not integrate outside air capability. While some buildings may have adequate natural ventilation, most will require outside air, but existing ductwork may be adequate to deliver volumes that meet code requirements. In many climates, a DOAS needs to preheat and precool outside air to a temperature close to room conditions and its controls must coordinate with a VRF system’s controls.
What Are the Energy Savings from VRF?
That’s the simple question everyone wants to ask – but there is no simple answer. Every building is unique.
The GSA report estimates that VRF systems can achieve 30% and higher HVAC energy cost savings over older inefficient systems and minimally code compliant conventional systems. For retrofit applications, costs and energy savings vary a great deal, making it impossible to estimate a typical payback.
In the case of the federal government’s portfolio, GSA recommends targeting existing buildings whose energy usage is above the average (60.7 kBTU/square foot with conventional systems) and whose initial incremental cost for VRF is less than $4/square foot compared to CAV and VAV systems.
Take a look at some examples of VRF retrofit application examples from the GSA and the private-sector below:
VRF APPLICATIONS IN THE PRIVATE SECTOR
Among the challenges posed by the rehabilitation of a long-unoccupied office building in Towson, MD, were restrictive floor-to-floor heights and the owner’s desire for LEED certification and Class A office space.