Variable Refrigerant Flow
A Variable Refrigerant Flow (VRF) system is a decentralized HVAC system with terminal units at or near the occupied space serviced by centralized condensing units.
The VRF system is used in many building types including hotels, multi-family residential, offices, schools and institutional. The VRF terminal units come in a wide range of configurations including wall, ceiling exposed, horizontal ceiling concealed, cassette etc. A terminal unit consists of an enclosure, fan and motor, refrigerant coil with drain pan, electronic expansion valve, controls and a filter.
Variable refrigerant Flow (VRF) units are like fancoils but use refrigerant instead of water for heating and cooling. Multiple terminal units can be connected to centralized refrigeration units. Heat-cool VRF behaves like a 2 pipe fancoil system where all the terminal units are in either heating or cooling mode. Heat recovery VRF behaves like a 4 pipe fancoil system where individual terminal units can be in either heating or cooling allowing individual housing unit control. Heat recovery VRF also can move energy from one part of the building to another (like watersource heatpumps). The condensing units can be air or water cooled.
The terminal units have drain pans so consideration for condensate piping must be given. Providing enough pitch for good drainage can be a challenge. In those cases, small condensate pumps with overflow safety switches can be used.
Temperature control comes from the VRF controls which are specific to the manufacturer. A wall sensor can control the fan and whether the terminal unit is in heating or cooling mode. The controls are integrated into the main condensing units controlling the overall refrigeration system including oil management. Most manufacturers have a gateway such as s BACnet to integrate their controls into the full building management system.
VRF uses HFC-410A refrigerant instead of water to move energy around the building. While there is no ozone depletion potential for HFC-410A, it is a greenhouse gas. HFC-410A is an A1 refrigerant (low toxicity and no flammability) however it is piped throughout the building. Special consideration is taken in the design of VRF system to ensure occupant safety. VRF is an all electric system so it has a zero site carbon load and is a good choice for all electric buildings.
Since the VRF system is decentralized, a Dedicated Outdoor Air (DOAS) system is required to deliver outdoor air to maintain acceptable indoor air quality. It is good practice and often is a code requirement that the ventilation unit have some form of air to air energy recovery.
