The definition of a VRF unit in HVAC is a variable refrigerant flow system. It is a type of air conditioning system that uses a variable speed compressor to control the flow of refrigerant. This allows the system to adjust its cooling or heating capacity to meet the needs of the space. It also provides individual temperature control for each zone in a multi-zone application. The VRF system can provide simultaneous cooling and heating and can operate in both cooling and heating modes. The system works by circulating refrigerant through an indoor unit, outdoor unit and piping system that connect the two. The outdoor unit houses the compressor, condenser, and expansion device. The indoor unit contains the evaporator, fan coil, and cooling coil. The piping system works to transfer the refrigerant from the outdoor unit to the indoor unit. The VRF system is designed to provide maximum energy efficiency and comfort while minimizing the amount of energy used. By minimizing energy usage, the system can help reduce energy costs.