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Variable Refrigerant Flow (VRF) Systems

June 24, 2020

What is Variable Refrigerant Flow (VRF)?

VRF systems are quickly becoming the specified HVAC system of choice for residential, commercial, and industrial applications in the United States. VRF systems feature multiple zone flexible installations and the lack of ductwork is a distinct advantage where space is limited making it an excellent choice for existing building HVAC system retrofits. Inverter technology allows for a variable speed compressor and the variable capacity of VRF systems is an excellent choice to meet both peak and part load demand as opposed to constant speed single capacity systems.

Types of Air-Cooled VRF Systems

Heat Pump VRF – 2 Pipe System

Heat pump VRF systems, also known as 2-pipe VRF, allow heating or cooling in all indoor units but not simultaneous heating and cooling.  Heat pump VRF systems are like split system heat pumps but the major difference is an inverter driven compressor. The inverter driven compressor allows for a variable capacity system that more closely matches the true space load condition resulting in fewer starts and stops, reduced energy consumption, improved temperature and humidity control, and longer equipment life. Heat pump VRF systems typically include indoor ductless terminal devices that can be mounted on a wall, recessed in a ceiling, or even mounted on the floor. When the indoor terminal units are in the cooling mode, they act as evaporators; when they are in the heating mode, they act as condensers.

Heat Recovery VRF – 3 Pipe System

Heat Recovery VRF Systems, also known as 3-pipe VRF, allow heating and cooling in all indoor terminal devices simultaneously. Each outdoor air-cooled condenser is connected via 3 pipes to an indoor heat recovery unit; a high pressure gas refrigerant line (for heating), a high pressure liquid refrigerant line (for cooling), and a low pressure gas suction line (for return to the outdoor unit). Each indoor heat recovery unit works together with the indoor terminal units and respective thermostats in each zone to determine if they require heating or cooling. An indoor terminal unit in heating mode is supplied with high-pressure gas refrigerant from the heat recovery unit. The heating mode indoor terminal unit acts like a condenser and the refrigerant exits as a high pressure liquid and proceeds back to the heat recovery unit. The heat recovery unit combines high pressure liquid exiting heating zones with high pressure liquid from the outdoor condensing unit and directs it to any indoor terminal units that are in cooling mode.  The cooling mode indoor terminal unit acts like an evaporator and the refrigerant exits as low pressure gas, returns to the heat recovery unit, and then proceeds to the outdoor condensing unit to begin the cycle again.

Heat Recovery VRF – 3 Pipe System

Common Methods of VRF Outdoor Ventilation Air

Dedicated Outdoor Air System (DOAS)

A dedicated outdoor air system (DOAS) is an air handling unit that filters and conditions 100% outdoor air which is distributed throughout a building as ventilation air. One of the advantages of using a DOAS is that it removes the ventilation air heating and cooling load from the VRF system. The DOAS is selected to handle the ventilation air heating and cooling load and not the full heating and cooling load of each space, which reduces the required size of the DOAS unit.

Exhaust Recovery Ventilator (ERV)

An exhaust recovery ventilator (ERV) uses building exhaust air to pre-condition inbound building ventilation air thereby recovering some of the energy that otherwise would have been lost through the building exhaust fan. ERVs can be selected to transfer some of the ventilation air heating and cooling load to the indoor terminal units of the VRF system or selected to handle the heating and cooling of the ventilation air exclusively.

Potential Issues with VRF Installation

Refrigerant R-410A

Most of today’s VRF systems use R-410A refrigerant, achieving a very high energy efficiency ratio (EER) of 15 to 20 and integrated energy efficiency ratio (IEER) of 17 to 25. They are 20% to 30% more efficient than conventional HVAC systems due to partial load operation, speed modulation, zoning capabilities, and heat-recovery technology. The classification of R-410A in ASHRAE Standard 34-2019 is Safety Group A1 (meaning non-toxic and non-flammable), it has no ozone depletion potential and it meets the stringent mandates of both the Montreal Protocol and the U.S. Environmental Protection Agency.

However, due to its ability to displace oxygen, ASHRAE Standard 34-2019 has established the maximum refrigerant concentration limit (RCL) of R-410A to 26 lbs./1000 ft3 of room volume for occupied spaces. Additionally, the RCL of R-410A is reduced by 50% to 13 lbs./1000 ft3 of room volume for all areas of institutional occupancies in accordance with ASHRAE Standard 15-2019.


For best results, VRF system refrigeration piping should be constructed of copper tube, ASTM B 75, UNS C12200, H55 Temper (Light Drawn) for straight lengths, and ASTM B 280, UNS C12200, O60 Temper (Soft Annealed) for coiled. The three basic principles for refrigerant piping installation include dry, clean & tight. 

Great care must be taken during installation to prevent moisture from entering the refrigerant piping, no dust or contaminants must be allowed to enter, and of course, it must be installed tight with no refrigerant leaks.

  • Refrigerant piping ends should always be covered when stored or during installation, and piping should never be stored on a floor but rather on racks or shelving at the construction site. 
  • Refrigerant piping should be installed with a slight upward gradient toward the outdoor air-cooled condensing unit to prevent the build-up of refrigerant oil in low lying pockets, and piping supports should be installed as to not crush or otherwise damage the piping insulation.
  • Pipe supports on horizontal piping runs should be a minimum of 5’ on center for piping with an outer diameter (OD) < 3/8” and up to 6.5’ on center for piping with an OD > 1/2”. 
  • Piping supports adjacent to flared fittings should be no more than 1’ away from the fitting to reduce the stress on the solder joint during VRF system operation.

Outdoor Air-Cooled Condenser

Proper installation of outdoor air-cooled condensing equipment is critical to ensure that the VRF system performs to its specified capacity. Always refer to the manufacture’s installation literature for installation specifics but as a best engineering practice always allow adequate spacing between each condensing unit to ensure proper cooling airflow and space for equipment maintenance. Condensing units must always be mounted high enough above the ground or roof deck to allow for possible snow accumulation during the winter months. An 18” clear height is typical but as mentioned previously, always refer to the manufacture’s installation literature to be certain that your equipment is properly installed.

How Can VERTEX Help?

VERTEX’s team of mechanical engineers have expertise with all aspects of VRF system design and installation. We are available to analyze your VRF system and help you identify potential installation, operational, or maintenance challenges which could lead to VRF component failures.

To learn more about VERTEX’s Forensic Engineering services or to speak with an Engineering Expert, call 888.298.5162 or submit an inquiry.

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