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138 VAV-PRC011M-EN

Application Considerations

VAV System

There are two primary types of VAV systems—single-duct and dual-duct.

Single-Duct Systems

Single-duct systems include one supply fan and a single supply duct, which is attached to each

zone. The supply fan delivers cooled air to the VAV zones in variable volumes, depending upon the

cooling requirements. The supply fan is usually designed to modulate airflow delivered to the VAV

zones.

Many VAV zones require heating as well as cooling. The supply air-handling unit provides either

no heat (cooling only), morning warm-up heat or occupied (changeover) heat. In addition, heat may

be provided at any individual VAV zone (within the zone or within the VAV terminal) by reheating

cool air provided by the central air handler.

No Heat

Central Cooling Only—In some systems, the central air handler provides only cooling and

ventilation during zone occupied periods. The supply air is maintained at a constant temperature

and the supply airflow is modulated to match the VAV airflow rate with the zone cooling

requirements.

Central Heat

Central Heat for Morning Warm-up—Many buildings cool down during the night. To be at a

comfortable temperature in the morning when the building is again occupied, heat must be added

to the spaces. Heat provided by the central air handler for morning warm-up is supplied at constant

air volume to the zones, prior to the time of occupancy. During the morning warm-up period, the

VAV terminal units must open to allow heated air to flow into the zones. In most instances very little

additional heat is needed once the building is occupied.

Central Occupied Heating-Changeover—Some buildings use the same air handler to provide

both occupied cooling and occupied heating. This is commonly referred to as a changeover system.

The system changes between heating and cooling depending on the need of the zones on the

system. In a changeover system, the operation of the VAV terminal units must also change over,

opening to provide heat in the heating mode and opening to provide cooling in the cooling mode.

supply

fan

cooling

coil

variable-

speed drive

thermostat

VAV

box

Variable-Air-Volume (VAV) System

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