Ductless split-system air-conditioners are used for houses with non-ducted distribution systems (where extending or installing ductwork is not feasible), and for additions such as bonus rooms, enclosed patios, etc.

Ductless split-system air-conditioners combine the zoning flexibility of a conventional room unit (a single air-conditioner installed through a wall or a window frame) with the whole house cooling potential of central systems. Like central systems, they have two main components: a compressor/condenser, as well as an air- handling unit, which contains an evaporator and a fan. Some units operate as heat pumps and provide both summer cooling and winter heating.

The compressor and condenser are housed as one unit and located outdoors. The quiet fan/evaporator unit is indoors, located in the area to be cooled or heated. A conduit, which houses the power cable, refrigerant tubing, suction tubing, and condensate drain, links the outdoor and indoor units.

Some advantages of a ductless system include:

- quiet operation

- versatility in zoning and design

- eliminate the loss of cool air as it passes through the ductwork

- provides zone cooling which saves energy since only the rooms that are occupied need to be cooled or heated. A thermostat independently controls each zone. And, because they do not rely on ductwork (which is often leaky and can account for a third of the energy usage for heating and cooling), but instead on insulated refrigeration lines, mini-split heat pumps can boost energy efficiency.

- if purchasing an air conditioner for the whole house is not affordable, buying the system one zone at a time is an ideal option. A single outdoor unit controls from one to four zones, depending on the size of the unit.

- variable speed high efficiency fans are also available

Some of disadvantages of a ductless system include:

- higher initial cost

- Not everyone likes the appearance of the air-handling unit. While less obtrusive than a window unit, they seldom have the built-in look of a central system. There must also be drainage for condensate outside the building. If the drainage is not well placed, the condensate can stain concrete or building materials.