VRF Systems Explained: From Cooling-Only to Simultaneous Heat and Cool

Part 2 of 3 in the Building HVAC Plants Series. 

Introduction

Vary the flow, vary the capacity, they said. And they were right.

Variable Refrigerant Flow systems have established a large part of the marketplace for commercial HVAC in North America since the early 2020s. Their installation ease for retrofit applications has made them largely replace classic boiler/chiller plants, and they are currently in contention with large hydronic solutions utilizing heat pump chillers.

In the series on Large Commercial HVAC Plants, this article is the second of three: formerly Boilers and Chillers, and next covering Air-Source Heat Pump Chillers. While VRF systems often have their refrigerant circuit directly enter the conditioned space, they can also be utilized through heat exchangers in a more traditional “plant” style setup.

Vary the Flow, Vary the Capacity

The operating principle of VRF systems is to react to changes in load by varying the amount of refrigerant sent through different parts of the system. This is largely controlled by inverter-driven compressors varying their speed, coinciding with the level of opening of the Electronic Expansion Valve (EEV) at a given zone.¹ VRF systems utilize intricate control systems to operate effectively.

Special tees and fittings are installed in specifically planned locations in the piping system to allow pipe sizing that matches the system layout. This is part and parcel of the layout of the VRF system’s main two components: the Outdoor Unit(s) (Condensing Unit) and Indoor Unit(s) (Heads).

Daikin’s RefNet joint and installation locations within a building layout are shown below. These fittings distribute refrigerant to multiple indoor units, and their sizing and placement directly affect flow distribution and pressure drop across the system.²

Why VRF?

Instead of application as an indirect system (refrigerant tempering secondary refrigerant), VRF systems are often applied as direct systems with their refrigerant heads right in the conditioned space. In this arrangement, increased thermal efficiency is realized from the direct expansion cooling effect or “direct condensation” heating effect occurring within the space’s head.³

Installation of VRF systems can be relatively simple with smaller main system components and reduced piping sizes. Minimal modification to existing buildings will be required with VRF systems’ “split” arrangement. Both of these considerations stand in contrast to traditional chiller/boiler plants, or large air or water source heat pump systems and their piping.

VRF systems can obtain high energy efficiency by use of their load-matching abilities due to the inverter-driven compressors working with EEVs. They can realize a higher plant COP (Coefficient of Performance) when integrated into a plant retrofit, replacing older, less efficient equipment.⁴ For a deeper look at how these retrofits compare to traditional system upgrades, check out the full retrofit guide.

When compared to other sources of heating and cooling, VRF systems have a small relative physical footprint in consideration of their capacity, and also offer low levels of noise during operation.

Setbacks of VRF Systems

VRF systems can be extremely complex, and their installation must be planned specifically considering their component and piping layout, with unique considerations detailed below in “VRF Types and System Layout.”

In low ambient conditions where a VRF heat pump is running in heating mode, oil return problems can occur, which lead to insufficient compressor lubrication that can cause damage or component failure.⁵ For more on how oil return affects compressor life, see the article on compressor oil return and oil failure controls.

With their commonly applied direct expansion/direct system application, refrigerant detectors are required to be installed in the conditioned space and require continued calibration.

It is important to consider the refrigerant phase-out status at the time of writing. With many refrigerants being phased out that have been utilized in VRF applications, A2L refrigerants look to step in.⁶ When this is done in VRF applications, this will generate a condition where a partially flammable refrigerant is directly in the space, or in contact with the system’s secondary refrigerant through a heat exchanger.

VRF Types and System Layout

There are three main “flavours” of VRF systems:

Cooling Only

Like ductless splits, VRF systems can also come in an arrangement to offer cooling only where suitable. For many areas in North America this is less common, and we will focus on the more popular and complex arrangements.

Heat Pump

Heat pump splits and chillers have increased in popularity, and VRF systems have followed. Further to a VRF system’s benefits and arrangement described earlier, use of a reversing valve within the VRF system gives it the ability to provide heating or cooling to the conditioned space.

Opposed to having the VRF head directly in the conditioned space, an arrangement is shown below with the VRF piped into a DOAS (Dedicated Outdoor Air System). Arrangements like this are quite common so that a VRF’s heating or cooling coincides with the operation of a building’s fresh air system.

It is noted that installations like this can serve many ERVs, HRVs, or MUAs (Energy Recovery Ventilator, Heat Recovery Ventilator, Make Up Air) to provide heating and cooling to a full building.

A VRF heat pump in this arrangement can only provide heating or cooling at one time.

Simultaneous Heat and Cool

VRF systems can serve a building’s heating and cooling load at the same time when they are installed to suit this arrangement. This can be achieved by a “3-pipe” piping arrangement for direct system application.

By sending piping to the indoor heads from the outdoor unit for both heating and cooling, and having a return line to the outdoor unit, this system has the ability to react to load changes in spaces, and also handle opposing calls for heating or cooling at the same time.

VRF systems can also provide heating and cooling as an indirect system layout, like that of a traditional boiler/chiller arrangement. This can be done on the basis of the VRF system’s heads being utilized more as heating and cooling modules within a plant room to temper water as required.⁷ This can allow for simultaneous heating and cooling of the plant’s secondary refrigerant (water), and allow water to be served to the building’s conditioned space terminal units such as that of a traditional plant system.

A more custom manufacturer-specific arrangement of this concept is utilizing Switch Over Boxes. Switch Over Boxes come in unique arrangements depending on the manufacturer, and can provide different versions of the function outlined below. In this example, the Switch Over Box is allowing the primary refrigerant to transfer its heat energy to the secondary refrigerant, for circulation to the conditioned space for simultaneous heating and cooling.

Note that supplied heating or cooling water temperatures may be lower than those of a traditional chiller/boiler plant, but this is very dependent on specific system arrangement.

Summary

Variable Refrigerant Flow systems have gained much space in the North American market for commercial building heating and cooling. They have unique and specialized equipment and piping arrangements that allow them to provide efficient system performance and react to conditioned space load.

There are considerations that are unique to VRF systems relating to the complexity of their system controls and piping. VRF systems are uniquely suited to retrofit applications due to their smaller footprint and ability to be installed in a split configuration.

Time will tell of the continued success of VRF systems installation for commercial building systems, as contention with heat pump chillers providing more traditional hydronic heating and cooling plays out.

VRF systems are complex but can be worked on and understood like any other heating or cooling system in HVAC/R, so long as care and attention are paid to how these systems are installed, controlled, and maintained. For techs who want to explore how building automation ties into these systems, the BMS series is worth a read.

This article was contributed by Julian Finbow, a commercial HVAC technician and engineer based in Canada with extensive field experience in VRF system installation and service. This is Part 2 of 3 in the Building HVAC Plants series on HVAC Know It All.