VRF Basics

Heat pump technology

has continued over the years and homeowners now have a variety of systems to choose from depending on their needs. Variable refrigerant flow (VRF) is a heating and cooling solution that does not require ductwork so it is an ideal solution for homes and living spaces where installing ductwork is not possible. Installation of a VRF system can be accomplished with much less alteration to existing buildings as well.

VRF system

A VRF system typically uses a single outdoor condensing unit that connects with one or more indoor air handlers that distribute the air. The indoor air handlers are usually mounted on the wall or ceiling and the unit will have its own internal thermostat. Each air handler can be controlled as its own unique zone to provide just the right amount of heating or cooling for that area. With certain systems, it is even possible for one air handler to be heating while another is cooling.

Of course, the compressor needs energy and most compressors used today have electrical plugs built-in. The plugs use a material called Fusite which uses glass to enable electrical power transmission through the compressor shell while protecting the electrical conductors from high pressure. Once the electricity gets inside the shell the refrigerant, motor and compressor are all inside.

Heat pumps

Heat pumps already provide significant energy savings but a VRF system takes that even further. There is minimal energy loss when compared to the 30-40% loss seen with traditional forced air systems that use ductwork. VRF heat pumps rarely need a backup heat source, a heat strip, to maintain heat in the winter.

While some of the energy savings come from small factors like not having cool air flowing through ducts in a hot attic, most of the reduction in energy needs comes from the fact that the compressor in a VRF system runs at a lower capacity than a regular heat pump. In a traditional system, the compressor is either on or off, either at a hundred percent or zero, which is not the most energy-efficient way to cool or heat a space.

Over the years, some manufacturers have tried to improve this efficiency by using multiple phase compressors but this didn’t totally solve the problem. VRF technology uses variable speed and capacity modulated inverter duty compressors that respond to the load required. These compressors can run anywhere from 10% capacity to full capacity as needed. Paired with outdoor fans with variable frequency drive motors this system is much more energy-efficient.

During a cooling cycle, the traditional heat pump system has the compressor and condenser coil outside which feeds liquid refrigerant into the indoor evaporator coil from a metering device. The liquid draws heat in from the air blowing past and eventually “evaporates” into a gas. At a predetermined point along the coil, the gas reaches a superheated state before being suctioned back to the compressor.

Everything in the entire system has been designed to work together. The compressor is going to draw in a certain amount of refrigerant and compress it to the correct pressure. Condensing into a liquid state that refrigerant goes through the metering device creating another pressure drop. The correct amount of refrigerant has been calculated out by the engineers so that there is enough refrigerant to absorb the heat and reach the superheating phase. Everything should work out perfectly.

A VRF system consists of a similar outdoor unit but is coupled with multiple indoor units that transfer the heat from the room directly to evaporator coils located within the conditioned space. A minimum of two lines (a liquid line and a gas suction line) are run to each indoor air handler. All these new lines can’t just be joined into the main liquid and suction lines without affecting that balance mentioned above. Simply splicing all the lines together with T joints will make significant changes to the pressure in the line.

Branch-Circuit Controller

A branch-circuit controller, or multiple branch selectors, are added to a VRF system. The controller monitors which air handlers require refrigerant and will increase or decrease the speed of the compressor as the system needs change. Thus the amount of refrigerant “varies” but the pressure is correct.

The branch controller is a sealed system, nothing should be able to get into that system. The controller cannot take any dirt, moisture, or debris. Manufacturers will want you to run nitrogen through the lines as you are brazing to keep carbon from building up inside.

There is quite a bit of variety with VRF systems. Some use two lines, some use three lines. Some are air-cooled, some are water-cooled. The design of the air handlers vary. A careful review of all installation instructions and training will be important as VRF systems become more popular in the US.