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.

04/16/2018


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.


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