Fan coil units (FCUs) are widely used terminal devices in heating, ventilation, and air‑conditioning systems for residential, commercial, and industrial buildings. They are designed to control the temperature and, to some extent, the air quality within individual zones or rooms. An FCU typically consists of a heat exchanger coil, a fan, a filter, a drain pan, and a housing. It can be connected to a central chiller or boiler system that supplies chilled or hot water to the coil.The basic operating principle of a fan coil unit is straightforward. Water of a certain temperature flows through the coil, and the fan draws room air through the filter and across the coil surface. As the air passes over the coil, heat is transferred between the air and the water. In cooling mode, heat from the air is absorbed by the chilled water, lowering the air temperature and often causing moisture to condense on the coil and drain away. In heating mode, hot water in the coil releases heat into the passing air. The conditioned air is then supplied back into the space, raising or lowering the room temperature as required.Fan coil units can be configured in several ways to suit different installation conditions. Common types include ceiling‑concealed units, exposed ceiling units, floor‑mounted units, and wall‑mounted units. Ceiling‑concealed models are often installed above suspended ceilings and connected to short duct runs or grilles, providing a discreet appearance. Exposed ceiling units are visible in the space and are often used in commercial or industrial interiors where mechanical equipment is intentionally left in view. Floor‑mounted and wall‑mounted units are typically used in spaces with limited ceiling voids or where easy access and simple installation are priorities.Control strategies for FCUs vary from simple manual switches to sophisticated automatic systems. Basic units may use a three‑speed fan switch and a local thermostat to select heating, cooling, or fan‑only operation. More advanced systems can integrate with building management systems, using electronic valves, variable‑speed fans, and digital controllers. These features allow precise temperature control, scheduling, and energy optimization based on occupancy patterns and outdoor conditions.One of the main advantages of fan coil units is zoning flexibility. Each unit serves a specific area and can be controlled independently, allowing occupants to adjust conditions according to their needs. This is particularly useful in hotels, offices, apartments, and mixed‑use buildings, where different spaces may require different temperatures at the same time. FCUs also reduce the size and complexity of central air‑handling equipment, since a large portion of the air movement is handled locally.However, proper design and maintenance are essential. Filters must be cleaned or replaced regularly to maintain air quality and fan efficiency. Condensate drain pans and piping must be kept clear to prevent water leakage. Coils and fans require periodic inspection to ensure reliable operation and to sustain energy performance over the life of the system. When correctly selected, installed, and maintained, fan coil units provide a versatile, efficient, and adaptable solution for indoor environmental control in a wide range of building applications.