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.