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.
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.
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.