VRF

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What HVAC contractors should (and shouldn’t) know about variable refrigerant flow technology.

Photos of VRF systems courtesy of Mitsubishi Electric Trane HVAC US

Variant refrigerant flow technology is designed to give building occupants more comfort and more energy-efficient control via zoning. Supported by integrated controls and sensors, VRF accomplishes cooling and heating through the transfer of conditioned refrigerant between each zone’s indoor unit(s) and an outdoor unit.

As the name variable refrigerant flow indicates, VRF systems are able to modulate the flow of refrigerant so that the system only uses the precise amount of energy needed to meet each zone’s conditioning requirements.

The powerful global movement toward a more sustainable, but technologically advanced built environment is a key driver for VRF system adoption. As architects design high-performance buildings to satisfy the requirements associated with LEED, Green Globes, Passive House, Zero-Net Energy (ZNE), deep-energy retrofits and ambitious sustainability goals set by governments and private entities, the need for VRF technology as an energy-efficient HVAC system has become more widely recognized.

Even if you’re an HVAC contractor who deals mostly with forced air, ducted systems, VRF systems have become a fixture in the HVAC industry, and contractors would do well to know how and why they work for some spaces over others.

A line of text iExcerpted from a White Paper by Mitsubishi Electric Trane HVAC US, the following points describe the requirements, advantages and opportunities associated with specifying and applying variable refrigerant flow (VRF) systems and addresses common misconceptions HVAC contractors may encounter in the field.

Sizing Guidelines Don’t Apply to VRF Systems

The ability of VRF systems to successfully manage part load conditions and vary capacity to meet conditioning requirements should not be taken as license to forgo load calculations and ACCA sizing guidelines. A VRF system is still an HVAC system, and optimal performance requires precise load calculations that account for all variables including ventilation, geospatial orientation and how occupants will use each zone.

Installing VRF Systems Is Just Like Installing All Traditional Systems

VRF systems are electronics-based allowing continuous communication between components. While wires in conventional systems transmit electricity, VRF systems use wires designed to carry information as well as voltage. For example, while older systems used thermostats that were little more than a series of switches, a VRF system uses a controller that transmits system performance data. This is a key difference. Installing the familiar wiring of a traditional system will distort and disrupt necessary communication within the VRF system.

HVAC contractors must concern themselves with maintaining the integrity of signals rather than just the flow of power.

When installing a conventional system, HVAC contractors consider the maximum length of refrigerant piping and follow manufacturer guidelines concerning how much refrigerant to add based on environmental factors including temperature readings, outdoor conditions and room temperatures.

With a VRF system, pipe sizing is important, but the length of pipe is the key factor used to determine the specific amount of refrigerant needed when the system is charged. HVAC contractors will want to follow VRF manufacturer guidelines and refrigerant density considerations informed by ASHRAE Standards 15 and 34.

To communicate and function properly, the indoor units, outdoor units and controllers must be addressed according to VRF manufacturer specifications. Addresses are assigned to each unit of equipment via a set of dials or rotary switches. In addition to providing necessary training on how to address systems, VRF manufacturers may provide software that automatically selects address numbers to help HVAC contractors avoid errors.

VRF Indoor Units Are Always Wall-Mounted

In fact, depending upon the application and aesthetic considerations, a zone may be served by a wall-mounted unit, a low-wall unit, a ceiling-recessed cassette or a ceiling suspended unit, among other styles. Ductless systems were once primarily considered for renovations, but the diverse options available today make ductless systems appropriate for new construction and a wide range of applications.

VRF Applications Are Always Ductless

HVAC contractors may encounter the misconception that VRF applications are always ductless. This is false. HVAC contractors can install multi-position air handlers and fully-concealed, horizontal-ducted units that connect to VRF outdoor units via refrigerant lines. Whether ductless or ducted, VRF systems provide the advantages of variable capacity and simplified zoning. Also, for some projects, a hybrid application is the best choice.

HVAC Contractors Can Save Clients Money With Heat Pump VRF systems

While both types of VRF systems are equipped with sensors, controls and an inverter-driven compressor to meet the load of a zone in a particular moment, comfort issues will arise if the HVAC contractor and project team specify heat-pump VRF systems for buildings where some zones may require heating at the same time others require cooling. Multi-family buildings, office buildings and hotels are examples of applications that require VRF systems with heat recovery. Selecting the correct VRF system and educating clients about the difference between heat-recovery and heat-pump systems are key to each project’s success. Yes, heat-pump VRF systems have a lower upfront cost, but selecting the appropriate system is less expensive than a callback that might ultimately result in replacing the system.

Triple Evacuation Is Optional for VRF systems

Since a refrigeration system should not contain moisture inside its pipes before startup, HVAC contractors should not neglect to perform triple evacuation to a minimum of 500 microns when installing a VRF system. Many of the maintenance requirements for a VRF system are the same in principle as for traditional HVAC systems. With the addition of diagnostic software provided by VRF manufacturers, HVAC contractors will use familiar servicing equipment to clean coils, clean drains and ensure quality airflow. VRF indoor units are equipped with easily accessible, washable filters that can last for up to 10 years.

Want more on VRF technology? Visit SNIPSmag.com.