Fan Coil Unit

Overview

A fan coil unit is the terminal device of a central hydronic HVAC system. The building's chiller or boiler does the heavy lifting and sends chilled or hot water through pipes; the fan coil unit, mounted in each room or zone, simply blows local air across a water coil to deliver cooling or heating exactly where it is needed. Because the energy is generated centrally and only water is piped to the unit, fan coils are compact and quiet, which is why hotels, offices, and apartments use them by the hundreds.

The unit is built around a sheet-metal Chassis & Casing that carries the coil and blower behind a Washable Air Filter. Return air enters through a grille, passes the filter, crosses the Water Coil, and leaves conditioned through the Discharge Louver. A Control Module thermostat watches room temperature and drives both the fan and the water Modulating Control Valve; a Condensate Pan & Drain pan catches the water that drips off the coil when it runs cold.

How it works

Conditioning is straightforward heat exchange. The Water Coil is a serpentine of finned copper tube. In cooling mode chilled water flows through it and room air blown across the fins gives up heat and moisture; in heating mode hot water warms the same air. There is no refrigerant and no compressor in the unit itself, so the only moving part of consequence is the fan.

That fan is the Squirrel-Cage Blower: a forward-curved squirrel-cage wheel on a multi-speed motor. It pulls return air through the filter and pushes it across the coil at one of several speeds the occupant selects. Air volume sets how much capacity the coil delivers, so fan speed is the coarse comfort control.

The fine control is the water side. The Modulating Control Valve throttles flow through the coil under command from the Control Module board: its motorized actuator opens and closes a brass valve to hold the room setpoint, modulating capacity without changing fan speed. When the coil runs below the room dew point, moisture condenses on the fins and drains into the Condensate Pan & Drain pan, through a trap, and out a hose, which is why correct drain slope matters in installation. A manual air vent on the coil lets trapped air bleed out so the coil fills completely with water and transfers heat across its whole face.