A Comprehensive Guide to Variable Refrigerant Flow (VRF) Systems
Functions: Heating and cooling
VRF systems use refrigerant as the heat transport medium instead of water, as their name implies. A pump is controlled by a variable speed drive to adjust the refrigerant flow depending on the current load. These systems are easy to install once contractors become familiar with them, while providing flexible operation and remarkable energy efficiency. Another advantage of VRF systems is their modular design, which makes them a great option for projects that will be built in stages.
The most basic version of a VRF system uses an outdoor condensing unit that can be used for heating or cooling. There are two refrigerant lines, one for supply and one for return, and they are shared by several internal fan coils. Although this basic VRF configuration does not allow for simultaneous heating and cooling for different areas of the building, there are two ways the system can be modified to achieve this.
Two-tube system
When VRF systems use only one supply line and one return line, it is possible to deploy a branch controller for simultaneous heating and cooling of different areas of the building. The branch controller is installed between the condensing unit and the piping network that supplies refrigerant to the indoor fan-coil units.
- The areas that require cooling are supplied with refrigerant in liquid form (subcooled), and the areas that require heating are supplied with refrigerant in gas form (superheated).
- Basically, the heat extracted from the cooled areas is delivered to the heated areas, and the condenser provides only the difference in output necessary to balance the heating and cooling.
- Two-pipe VRF systems with branch controller are recommended when capacity is expanded in the future. Because the controller provides a hub to which all indoor units connect, there is no need to modify existing refrigerant lines during an expansion.
Simultaneous heating and cooling is only possible in a two-pipe system if a branch controller is implemented. Alternatively, a three-pipe system can be used for buildings with simultaneous heating and cooling needs.
Three-tube system
This VRF system configuration utilizes three lines connected to the condensing unit: one for heating, one for cooling, and a common return line. The basic operating principle of a three-tube VRF system is as follows:
- Instead of using a branch controller to supply liquid or gaseous refrigerant, this function is built into the outdoor condensing unit.
- The three lines (liquid, gas and return) are connected to all internal fan coils, and each unit is equipped with a branch selector that switches power depending on the specified operating mode – heating or cooling.
- The feedback is common for all fan coils, regardless of the operating mode.
A three-pipe system generally provides higher heat recovery efficiency than a two-pipe system with a branch controller, but the system offers reduced flexibility for future expansions – existing refrigerant lines must be modified to add more fan -coils.
