VRF: Overcoming challenges to achieve high efficiency

Variable refrigerant flow systems can result in effective, high-efficiency HVAC designs, but care must be taken to achieve proper ventilation, humidity control, and compliance with ASHRAE Standard 15.
By Cory Duggin, PE, LEED AP BD+C, BEMP; TLC Engineering for Architecture April 16, 2018

This article has been peer-reviewed.

Learning Objectives:

  • Understand the components of variable refrigerant flow (VRF) systems.

  • Know the options to provide sufficient conditioned ventilation air with VRF designs.

  • Appreciate the importance of ASHRAE Standard 15 compliance and how it relates to occupant safety.

Engineers are always looking for a better, more efficient way to accomplish tasks. Variable refrigerant flow (VRF), aka variable refrigerant volume (VRV), systems are innovative systems that can provide flexibility, control, and a high level of energy efficiency.

VRV systems were invented in 1982 by Daikin. The company trademarked the name VRV, resulting in all other manufacturers calling their product VRF. Each manufacturer has a slightly different approach to delivering the same basic technology. Most manufacturers use three refrigerant pipes from the condensing unit (CU), but some only use two.

Figure 1: The piping networks for both heat recovery and heat pump VRF systems is shown at the Titus Landing Medical Office Building in Titusville, Fla. All graphics courtesy: TLC Engineering for ArchitectureVRF systems come in three main types: heat recovery (HR), heat pump (HP), and cooling only. HP VRF systems do not allow for simultaneous heating and cooling on a single refrigerant network. HR VRF allows for simultaneous heating and cooling on a single refrigerant network by recovering heat from one space to be rerouted and used in another zone. This is similar to how hot-gas reheat uses the hot gas from an evaporator to provide free reheat during a dehumidification cycle in direct expansion (DX) equipment. If the zones on one of the refrigerant networks do not need the ability to have simultaneous heating and cooling, it may make sense to save some budget dollars by selecting an HP VRF system.

Zoning for a VRF system generally refers to which thermal zones (fan coil units, or FCUs) are on each of the independent refrigerant networks. It is common for a building to have multiple refrigerant networks with dedicated CUs.

VRF systems, like most HVAC systems, have advantages and drawbacks. It is important to consider how to provide proper ventilation, complying with ASHRAE Standard 15: Safety Standards for Refrigeration Systems while maximizing the system type’s energy efficiency potential.

What is VRF?

VRF systems use single or multiple CUs acting as one that are connected to several FCUs on a common refrigerant network to provide heating and cooling. In contrast, split DX units have one condenser per evaporator, resulting in more outdoor equipment and no ability for heat recovery.

VRF systems are comprised of CUs, branch-selector (BS) boxes, refrigerant piping, and indoor units (also known as FCUs). The CU functions the same as in a typical split system, but in VRF systems they employ high-efficiency inverter compressors and variable-speed condenser fans that can modulate to serve the varying common refrigerant network load.

Figure 2: A VRF indoor unit is hung from a structure, showing refrigerant piping connections. BS boxes are only used in HR VRF systems and have several different names depending on the manufacturer, such as branch-circuit controller, heat-recovery module, and mode-control unit. They can be recognized as the refrigerant header box between the CU and the FCUs. BS boxes are “where the magic happens” in HR VRF systems. They control how much and which (hot gas or subcooled liquid) refrigerant goes to each FCU.

The FCUs function like any other DX FCU. They have small electronically commutated motor fans that are typically three-speed. The advantage with VRF FCUs is their ability to modulate both airflow (if not set to constant volume) and leaving-air temperature (by varying the amount of refrigerant) to match the space load. The CUs in VRF systems allow for reduced sizing due to the diversity of the non-coincident FCU peak loads. This makes sizing a VRF CU more comparable to sizing a chiller than a split DX unit, because the CU should be sized to the block load of the FCU connected to it and not to the sum of the peaks.

Design challenges

The main challenges with VRF design are minimum indoor unit size, limited ventilation capacity, and system refrigerant charge.

Minimum indoor unit sizes: One of the advantages of VRF designs is they lend themselves to greater zone control. The problem is, the smallest VRF FCU may be larger than needed for a given space. The minimum FCU for most manufacturers is around 0.5 tons. This doesn’t cause an operational issue because the FCU can vary the refrigerant and airflow to match the load still, but it does inflate the first cost by causing the owner to buy more FCU capacity than required.

Figure 3: This branch-selector box has three pipes coming from the condensing unit and two pipes coming out to each of the connected fan coil units. One particular VRF manufacturer has a zoning kit that is essentially a plenum box with dampers on it, to allow multizone airflow control from a concealed ducted FCU. This can reduce the first cost while still allowing some control for each of the rooms/zones on the FCU without having to buy indoor unit capacity that isn’t needed.

Ventilation: Achieving ventilation that complies with ASHRAE Standard 62.1: Ventilation for Acceptable Indoor Air Quality while also maintaining humidity and filtration can be challenging with VRF. Ducted FCUs can simply have the outdoor air (OA) injected into the return duct, but ductless units have to use a knockout that will allow a limited volume of air to be ducted to the unit. The amount of latent capacity in VRF FCUs is limited by the amount of coil surface area and is not designed to handle raw OA. The manufacturer can provide a range of entering mixed-air temperatures the FCU can handle.

Concealed ducted units can have untreated OA ducted to their return duct, but the latent load limitation is the same. The best way to provide ventilation with a VRF system is to precondition it with a dedicated outdoor air system (DOAS) and supply it directly to the space. This takes the indoor unit limitations out of the equation and allows them to function independent of the ventilation system.

Many think a DOAS has to be a large piece of rooftop equipment, but most VRF manufacturers now offer special OA FCU with a higher latent capacity that can be connected to a refrigerant network. If designed thoughtfully, having a DOAS or multiple small DOAS connected to the VRF refrigerant networks can provide a lot of useful heat recovery.

If space is available, one central DOAS delivering air directly to spaces or FCUs can be a great option. Having all the OA conditioned in one place creates an opportunity to maximize air-side energy recovery potential. An enthalpy wheel can precool/heat the raw OA, which reduces the peak cooling and heating capacity of the DOAS required and reduces the energy required in operation, as well.

Refrigerant charge: The consequence of having several FCUs on a single, shared refrigerant network is it increases the total refrigerant charge of the system. Complying with ASHRAE Standard 15 is a crucial component of any VRF design, but many designers overlook it. ASHRAE Standard 15 sets the refrigerant concentration limit based on if the entire system were to leak its refrigerant charge into a single, occupied space.

The easiest way to check for compliance is to figure out the smallest an occupied space can be based on the refrigerant charge of the system that serves it. Any occupied space that has refrigerant piping running through it must be considered, even if it isn’t conditioned by the VRF system.

Figure 4: A VRF condensing unit is mounted on the ground with the refrigerant mains shown. Section 7.3 of ASHRAE Standard 15 specifies how to calculate the volume when checking compliance with the refrigerant concentration limit. For example, the volume above a suspended ceiling cannot be included unless it is a return-air plenum. The volume of the supply and return ducts that serve the space can be included. If ductless cassette FCUs are being used, the only duct volume to count would be from the DOAS system.

The last factor to consider with ASHRAE Standard 15 is the occupancy type. Section 7.2.1 states that, for institutional occupancies (which include hospitals, nursing homes, asylums, and spaces with locked cells), the refrigerant concentration limit (RCL) is half of the limit prescribed in Tables 4-1 and 4-2 of ASHRAE Standard 34: Designation and Safety Classification of Refrigerants. VRF systems use R-410a, which has an RCL of 26 lb/Mcf.

To comply with ASHRAE Standard 15, refrigerant networks may need to be split into multiple networks to reduce the total refrigerant charge. Care should also be taken to not route refrigerant piping through small occupied rooms, especially if they aren’t already being served by the VRF system. VRF systems are highly efficient, with many selling points, but it is important to comply with ASHRAE Standard 15 to maintain the safety of occupants in buildings using high-refrigerant-charge systems.

Energy efficiency

There are three features of VRF systems that significantly increase their energy efficiency:

  • Heat recovery.

  • Inverter compressors.

  • Decoupled ventilation load.

Both heat recovery and decoupled ventilation load are options for a VRF design, but inverter compressors are inherent to the ability to serve multiple zones with a single CU and vary the amount of refrigerant each receives. Heat recovery reduces energy consumption both by getting free heating or cooling up to the balance of the predominant load’s rejected heat and by reducing the amount of work required by the CU fan and compressors.

The goal of zoning a VRF system with heat recovery is to maximize the number of hours heat recovery can be used. The simplest example of how this can be achieved is to include core spaces that need cooling year-round along with perimeter spaces that often need heating. This would allow whichever is the non-predominant load to be served without any additional compressor work. Assuming the cooling load is predominant, the CU would be in cooling mode and the BS box would reroute as much hot gas as needed to meet the heating load to the perimeter FCU before sending it back to the CU.

Another example would be to divide a building into separate east- and west-facing zones on the same CU, which would result in a smaller CU because the east and west zones wouldn’t peak at the same time.

Not all VRF designs employ decoupled ventilation; however, it is the most efficient design strategy that also effectively controls indoor air quality. Decoupling ventilation refers to conditioning the required OA using a dedicated piece of equipment, so the zone equipment only has to meet the heating and cooling loads of the space. The concept applies to all zone-based heating and cooling systems, including active chilled beams, passive radiant heating and cooling, and water-source heat pumps. Some designs don’t include a DOAS to save cost and end up running it through the VRF FCU, or in rare cases, introducing raw outdoor air to the space.

As discussed previously, the VRF FCUs have very limited latent capacity for dehumidification, so raw OA added to the return duct or knockout typically needs to be 10% or less of the supply volume. At a minimum, the OA should at least be routed through an energy-recovery ventilator (ERV) to reduce the OA load to where the VRF FCU can handle it. An ERV that can transfer both sensible and latent heat is the most effective for reducing the OA load. These can be wheels or static cores.

Using a DOAS also allows the exact amount of required OA to be supplied to each zone, which reduces the overall system OA load from what would be required in a multizone VAV system and reduces the ventilation energy usage. The best-case scenario for designing a VRF system is to have a DOAS with an ERV supplying OA directly to the space at a neutral temperature (65° to 70°F), which will minimize the energy associated with ventilation and allow the VRF FCU to cycle on and off based solely on the space load, minimizing the fan energy. If the DOAS is ducted to the VRF FCU, the FCU fan has to run continuously in order for the ventilation to be provided.

Inverter compressors are the heart of VRF technology. They allow for high full-load coefficients of performance and even higher part-load efficiencies, as illustrated in the integrated energy efficiency ratio. Without their ability to modulate to meet the varying loads of all the FCUs on the refrigerant loop, VRF wouldn’t be possible. Because of their efficiency, inverter compressors are now being used in other HVAC equipment, such as packaged rooftop units.

Another option in the high-performance engineer’s toolbox is the VRF HVAC system. It is a flexible system type that leads to less energy consumption than central VAV heating and cooling systems. For a VRF design to be truly successful, the designer must overcome the inherent design challenges and maximize the energy-saving capabilities of the system type.

Cory Duggin is a principal and energy modeling wizard at TLC Engineering for Architecture. He is a member of the Consulting-Specifying Engineer editorial advisory board.