A fan coil unit (FCU) is a simple and versatile terminal device used in heating, ventilation, and air conditioning systems. It typically consists of an enclosure containing a fan, a heat exchanger coil, an air filter, and basic controls. The coil is connected to a building’s central hot and/or chilled water system, while the fan draws room air across the coil and then discharges the conditioned air back into the space.In operation, water of a suitable temperature is circulated through the coil. For cooling, chilled water absorbs heat from the room air passing over the coil; for heating, hot water transfers heat to the air. The fan ensures adequate air movement, providing uniform temperature distribution in the conditioned zone. FCUs can be configured in two‑pipe or four‑pipe systems. A two‑pipe system uses a single supply and return for either hot or chilled water, depending on the season, whereas a four‑pipe system has separate supplies and returns for hot and chilled water, allowing simultaneous heating and cooling in different zones.Fan coil units are commonly installed in hotels, offices, apartments, hospitals, and other commercial or residential buildings. They are often located in ceiling voids, under windows, or within floor-mounted cabinets. Ceiling-mounted units may be concealed and ducted to multiple diffusers, or exposed with an integral discharge grille. Their compact size and modular nature make them suitable for retrofit projects and for buildings with diverse thermal loads.Control of an FCU is usually achieved with a room thermostat and fan speed selector. The thermostat regulates the water valve or electric heating element, while fan speed control allows occupants to adjust airflow and comfort level. Advanced systems may integrate with building management systems, offering centralized scheduling, temperature setpoint adjustment, and energy optimization.One of the advantages of fan coil units is the decentralized control they provide. Each unit can serve a single room or small area, allowing individual temperature preferences and reducing energy consumption in unoccupied zones. The water-based distribution system can also be more efficient and space-saving than extensive air duct systems.However, FCUs require regular maintenance to perform effectively. Filters must be cleaned or replaced, coils need periodic cleaning to maintain heat transfer efficiency, and condensate trays and drains must be inspected to prevent leaks and microbial growth. Proper design, installation, and commissioning are essential to ensure quiet operation, adequate ventilation (often supplemented by a separate fresh air system), and acceptable indoor air quality.