Define a fan coil unit (FCU) and how DDC controls it.

• A. A unit with a cooling/heating coil and a fan; DDC controls the cooling coil only.
• B. A fan coil unit (FCU) is an HVAC unit with a cooling/heating coil and a fan; DDC controls supply air temperature via thermostat signals and fan speed.
• C. A simple fan module with no temperature control.
• D. A heater-only unit with fixed airflow.

Answer: (B)

A fan coil unit combines a heating/cooling coil and a fan to condition air for a single zone, delivering conditioned supply air directly to that space. In a Direct Digital Control setup, it maintains the zone temperature by adjusting both how much air is conditioned and how fast it moves. The DDC uses the thermostat setpoint and sensor readings to modulate the coil’s valve (controlling cooling or heating) and to vary the fan speed (or fan drive) to set the supply air temperature and airflow. This creates a feedback loop: the sensors tell the controller the current temperature, the thermostat setpoint tells it the target, and the DDC responds by fine-tuning the coil output and fan speed to reach and hold the desired condition. The FCU typically taps into a central cooling or heating loop (like chilled or hot water) and can be more energy-efficient by varying both temperature and airflow instead of just turning a unit on or off.