Fan Coil Capacity Control
Fan-coil unit capacity is usually controlled by coil water flow,
fan speed, or a combination of these. Water flow can be thermostatically
controlled by either return air or wall thermostats.
Fan speed control may be automatic or manual. Automatic control
is usually on-off with manual speed selection. Units are available
with variable-speed motors for modulated speed control. Room thermostats
are preferred where automatic fan speed control is used. Return
air thermostats do not give a reliable index of room temperature
when the fan is off. Residential fan-coil units have manual three-speed
fan control with the water temperature, both heating and cooling,
scheduled by outside temperature. On-off speed control is poor
because (1) alternating shifts in fan noise level are more obvious
than the sound of a constantly running fan, and (2) air circulation
patterns within the room are noticeably affected.