What is the definition of a mini-split AC system? A mini-split AC system is an efficient, compact and affordable way to cool or heat a single room, or multiple rooms in a home or building. It is a ductless system that is powered by an outdoor compressor and an indoor air handling unit. The air handling unit is connected to the wall or ceiling, and the compressor is installed outside the building, typically on a concrete pad or wall. The compressor pumps refrigerant through a line set, which is connected to the air handling unit. Mini-split AC systems are great for older homes, since they are more efficient and don’t require ductwork like traditional ducted systems. Additionally, mini-split AC systems can be controlled independently, allowing you to individually adjust the temperature of each room in the home. With their energy efficiency and low noise levels, mini-split AC systems are quickly becoming a popular choice for homeowners and businesses alike.