Dual duct terminals units have two air valves in a common
VAV box enclosure: one controls cool primary air and the other controls warm
air. This system provides variable air volume as well as variable
temperature. With the dual duct system, adjustable air mixing point is
provided to minimize air movement when the unit changes over between cooling
and heating, and vice versa. Terminals are connected to temperature sensors
located in the zone.
Dual duct systems can be very energy efficient when there is little call
to mix cool and heated air, and separate supply fans are utilized for
heating and cooling. A major shortcoming of single-zone systems is that the
heating and cooling capacity supplied to each comfort zone cannot be
adjusted to match changing load conditions within the zone. As a result,
although the central thermostat can be satisfied, individual zone comfort is
often compromised.
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