The fan coil unit, commonly abbreviated as FCU, is one of the terminal devices of an air conditioning system, composed of a small fan, motor, and coil (air heat exchanger). When chilled water or hot water flows through the coil, it exchanges heat with the air outside the coil, cooling, dehumidifying, or heating the air to regulate indoor air parameters. It is a commonly used terminal device for cooling and heating.

The fan coil should be selected for surface or concealed installation based on the specific room conditions and decorative requirements, and the installation location and type should be determined. Vertical units are generally placed under the exterior window sill; horizontal units are suspended from the upper part of the room; wall-mounted units are hung above the wall; column-type units can be placed on the ground or within partition walls against the wall; cassette units are embedded in the ceiling.

The outdoor unit should be placed directly indoors without decoration, but it should be chosen with a color that complements the room's interior. The concealed unit should be equipped with supply and return air grilles that match the architectural decoration, and a filter should be installed in the return air grille. Additionally, a removable or accessible maintenance opening should be left during the architectural decoration for easy removal and maintenance of the unit's fans and motors, as well as cleaning the air heat exchanger.

Horizontal installation units are often mounted on ceilings, with two types of air delivery: side delivery from the ceiling box or downward air delivery from the ceiling. If side delivery is used, low static pressure fan coils can be chosen, with the unit's outlet directly connected to a double louvre air outlet. For downward air delivery from the ceiling, high static pressure fan coils should be selected, allowing the air inlet of the unit to be connected to a section of ductwork, which in turn connects to several diffusers for downward air delivery. There are two methods for return air in horizontal concealed installations: either installing louvres or other types of return air inlets and air filters on the ceiling, connecting them to the unit's return air box via ductwork, or not using ductwork, allowing indoor air to enter the ceiling and be drawn in by the unit mounted on the ceiling.