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16 TRG-TRC014-EN
notes
period two
Components of a VAV System
VAV reheat terminal units provide supply-air tempering or space heating by
reheating the cool primary air. This is accomplished by adding an electric, or
hot-water, heating coil to the discharge of a cooling-only unit.
The VAV reheat terminal unit is typically used for those zones that require
seasonal cooling and heating, such as exterior zones of a building.
In the cooling mode, the unit is controlled in the same manner as the cooling-
only unit. The supply airflow is reduced as the cooling load in the space
decreases. When the space cooling load drops below the minimum primary
airflow setting for the unit, overcooling the space, the heating coil warms
(tempers) the primary air to balance the low space cooling load. When space
heating is required, the heating coil further warms the primary air to satisfy the
space heating load.
VAV Reheat
terminal
terminal
heating coil
heating coil
primary
primary
air
air
supply
supply
air
air
Figure 25
VAV Reheat
100%
0%
heating coil active
heating coil active
primary air
primary air
space load
design
cooling load
design
heating load
% airflow to space
minimum
cooling
airflow
minimum
heating
airflow
maximum
primary
airflow
heating
heating tempering
tempering
Figure 26