A two-pipe / four-pipe fan coil unit is a terminal air-conditioning device widely used in commercial buildings, hotels, offices, hospitals and residential developments. It combines a heat exchanger (coil), a fan, a filter and a control system into one compact unit, providing zone-level temperature control with relatively simple installation and maintenance.The key feature is the water-based coil system. In a two-pipe configuration, there is one supply pipe and one return pipe serving the unit. Chilled water or hot water is circulated alternately through the same coil, depending on the seasonal requirement. During summer, chilled water is supplied to remove heat from the room air; in winter, hot water is supplied to provide heating. Because the same pair of pipes is used for both heating and cooling, the system is economical to install and suitable for buildings where simultaneous heating and cooling are not generally required in different zones.A four-pipe fan coil unit uses two separate water circuits: one supply and one return for chilled water, and one supply and one return for hot water. The unit is equipped with either a single coil with separate sections or, more commonly, two separate coils within the same casing. Control valves regulate the flow of chilled or hot water independently. This arrangement allows individual spaces to be cooled or heated at the same time, even within the same floor, depending on local demand. It is particularly useful in buildings with diverse exposures, such as south- and north-facing façades experiencing different solar gains.In both two-pipe and four-pipe versions, the fan draws room air through a return grille and filter, blows it across the coil, and then discharges conditioned air back into the space. The fan may be multi-speed or equipped with an EC motor for variable air volume control. Thermostats or room controllers modulate fan speed and valve positions to maintain the desired temperature while balancing comfort and energy efficiency. Optional fresh air connections or integration with a separate ventilation system can enhance indoor air quality.Two-pipe fan coil units offer lower initial cost, reduced piping complexity and easier installation, making them attractive for many projects. However, changeover between heating and cooling modes usually affects the whole system or large zones, limiting flexibility in shoulder seasons. Four-pipe fan coil units require more piping, valves and controls, leading to higher investment, but they provide superior comfort and operational flexibility, supporting true year-round, simultaneous heating and cooling in different areas of a building. By selecting either a two-pipe or four-pipe configuration according to the building’s use, climate and comfort requirements, designers can achieve a balanced solution in terms of cost, performance and energy consumption.