Variable Refrigerant Flow (VRF) Systems
VARIABLE REFRIGERANT FLOW SYSTEMS
A Variable Refrigerant Flow (VRF) heating and air conditioning system is a specific type of HVAC system which utilizes one outdoor unit that connects to multiple indoor units. These indoor units can be single systems; serving one space, or a ducted system; serving multiple spaces. The indoor units are controlled individually by a room thermostat in a variable flow mode, while the outdoor unit provides the refrigerant. This provides for the heating and/or cooling of the individual interior space. The outdoor units can be either air cooled or water cooled and the indoor units are connected to the outdoor unit via refrigerant piping.
VRF systems have been in existence for approximately 30 years. The VRF system is a variation on the individual mini ductless split systems which were commonly used to condition small data rooms or individual rooms where a heat producing load was located, or in a space where a larger system was not required.
These systems have become very popular across the globe, particularly in places such as Europe and Japan. These areas tend to have high electricity costs. By utilizing the high efficiency of the VRF systems the cost of operation can be reduced.
In the Pacific NW, the manufacturing of VRF systems was previously dominated by three (3) main companies. However, as the popularity of this type of system has increased, the number of manufacturers entering the field has tripled, growing from three main dominating companies to somewhere from 8 to 10 that we know of. The individual room control as well as the type and size of a building now utilizing VRF technology has heightened the expansion of these systems. They have now made their way into the construction industry because of their high efficiency.
What is a Variable Refrigerant Flow System?
A VRF system is an air conditioning/heating system where multiple indoor units can be connected, via refrigerant piping, to a single outdoor unit. There are two types of systems: Heat Pump and Heat Pump with Heat Recovery. The heat pump system operates similarly to a standard heat pump where the outdoor unit is either in the heating or cooling mode and all indoor units are operating in the same (heating or cooling) mode. The heat recovery heat pump system allows the individual space connected indoor units to be in either the heating or cooling mode whilst not requiring the outdoor unit selecting the mode. This, is accomplished by piping refrigerant from the outdoor unit to a distribution box (circuit controller, branch selection box, etc.) and then routed using a second set of refrigerant pipes to each individual indoor unit. This allows diversification in the overall cooling and/or heating load and greatly increases energy efficiency.
Indoor units range in size from less than 1 ton up to approximately 8 tons. These units can be a fan coil type; either ducted or non-ducted, an exposed ceiling mounted unit, a concealed ceiling mounted unit or a wall mounted unit. Outdoor units range in size from approximately 5 tons for a single unit, up to 30 tons for multiple units connected together to form one single larger tonnage unit. Example: (2) 5 ton units can be connected to form a single 10 ton unit or an 8 ton unit can be connected to a 10 ton unit to yield a total tonnage of 18 tons and an 8 ton unit can be combined with a 10 ton unit and another 10 ton unit to yield a total tonnage of 28 tons. While being configured from multiple units and requiring separate power connections, the resulting “combined” unit is considered a single unit.
While a conventional HVAC system has outside ventilation air as part of the packaged system, VRF systems, although generally having outside air capability, usually require a dedicated outdoor air system (DOAS) to operate in conjunction with the VRF to provide ventilation air to the occupied spaces where the VRF provides the heating and cooling and the DOAS provides for the ventilation. The DOAS only provides for the ventilation required by code thus the air side capacity of the system is not as great as a conventional all-air system which is providing ventilation air as well as enough air to be heated and cooled to satisfy the temperature control of the space. Most VRF manufacturers do offer DOAS equipment which integrates into the control system for the VRF.
Temperature control and system control of a VRF system is provided by the manufacturer of the system. Manufacturers have their own proprietary control systems. A component of one manufacturer cannot be used in another manufacturer’s system.
Design and Installation
The design of the VRF system should be done by an individual who is knowledgeable in VRF systems and who has been trained by the manufacturer in the design of their particular product. The recommendations of the manufacturer should be followed completely in the design process. It is important to “zone” the systems properly and to always keep the length of refrigerant piping as short as possible. The use of isolation valves, both in “zones” and on individual pieces of equipment is recommended for both an ease of installation and testing basis as well as for ease of maintenance after construction.
Since there is considerably more refrigerant piping being installed in a VRF system than a more conventional system, the quality of the installation becomes paramount. Piping needs to be cleaned and capped. Air inside the piping needs to be purged and nitrogen used during the brazing of the pipe.
Pressure testing of the piping should be accomplished in accordance with the manufacturer’s recommendations.
The installation should be provided by a contractor who has completed the installation training classes provided by the manufacturer and the designer should include in the installation specifications that the manufacturer or the manufacturer’s representative should inspect the installation.
Pro and Cons of VRF Systems
- Heat Recovery systems can be very energy efficient.
- VRF systems (per current code) can be exempt from the economizer code requirements.
- Minimal ceiling space or minimal space inside the building is required.
- Very effective in remodel projects.
- This option can be costly.
- VRF systems are single manufacturer driven. If a system is provided by manufacturer A, components from manufacturer B cannot be utilized in the repair/modification of the system.
- Refrigerant capacity. The total capacity of refrigerant in the overall system is greater than the amount allowed by code for indoor air quality.
Authors: Don Pfaff, P.E. and Kristin Trone
Mr. Pfaff, during his 40 years of experience, has been a designer, a Professional Engineer, a Senior Mechanical Engineer and Project Manager. At R&W Mr. Pfaff acts as a mentor to other designers and engineers as he is very knowledgeable in the design and installation of various mechanical systems including HVAC, Plumbing, Controls, Piping and Fire Protection.
Ms. Trone has over 28 years in the engineering field with 20 years as a HVAC designer. Ms. Trone is very familiar with the energy code issues of various states and is also very proficient at calculating building HVAC cooling and heating load and has designed tenant improvement systems for commercial office buildings that include single and double duct VAV, reheat VAV, constant volume, hydronic heat pumps, air cooled split systems and computer room cooling systems. She has designed projects and managed construction for both new and remodeled buildings.