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142 VAV-PRC011M-EN
Application Considerations
Control Types
VAV terminal units are available with many different options. These options fall into three main
categories of controls: direct digital (DDC), pneumatic, and analog electronic. All of these control
types can be used to perform the same basic unit control functions, yet differences exist in accuracy
of performance, versatility, installed cost, operating cost, and maintenance cost.
Direct Digital Control (DDC) Systems
Benefits
PerformanceDDC controls offer PI control capability. A PI control scheme is the most accurate
and repeatable control scheme available in the VAV terminal unit industry.
Versatility—DDC controls accepts software commands to determine how its outputs will be
controlled. When a control sequence must be modified, making changes to the software
instructions is easier and quicker than changing hardware.
Figure 12. Ventilation reset
OA
SA RA
VAV Controllers
Required outdoor airflow
Current primary airflow
Current OA fraction
Tracer SC
Find highest OA fraction
Calculate current system
intake airflow (ASHRAE 62.1)
RTU controller
Reset intake airflow
Direct digital control (DDC) systems became
available as advances in computer technology made
small microprocessors available and affordable.
Much of the hardware in DDC systems is similar to
analog electronic systems. The primary difference is
that DDC controllers allow system integration, remote
monitoring, and adjustment. The microprocessor is
programmed using software that gives the controller
a higher level of capability than either the pneumatic
or analog electronic options.
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