Fan Coil Ventilation
The only reason to use central fan equipment for an in-room terminal
unit system is to provide the correct amount of ventilation or
makeup air to the various spaces being served by terminal units.
Ventilation air is generally the most difficult factor to control
and represents a major load component. The designer must select
the method that meets local codes, performance requirements, cost
constraints, and health requirements.
A central, outside air pretreatment system, which maintains neutral
air at about 70°F, best controls ventilation air with the
greatest freedom from problems related to the building’s
stack effect and infiltration. Ventilation air may then be introduced
to the room through the fan-coil unit, or directly into the room
as shown in Figure 4. Any type of fan-coil unit in any location
may be used if the ventilation system has separate air outlets.
Ventilation air contributes significantly to the room latent cooling
load, so a dehumidifying coil should be installed in the central
ventilation system to reduce room humidities during periods of
high outside moisture content. An additional advantage of the
ventilation unit is that, if it is sized for the internal latent
load, the terminal cooling coil remains dry, extending the life
of the unit. However a piped condensate drain is recommended.
This neutral temperature removes the outside air load from the
terminal unit, so it can switch from heating to cooling and vice
versa without additional internal or external heat loads.
In buildings where fan-coil units only serve exterior zones and
a separate all-air system serves interior zones, it is possible
to provide exterior zone ventilation air through the interior
zone system. This arrangement can provide desirable room humidity
control, as well as temperature control of the ventilation air.
In addition, ventilation air held in the neutral zone of 70°F
at 50% rh can be introduced into any fan-coil unit without affecting
the comfort conditions maintained by the terminal units.