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66 VAV-PRC011M-EN
Dual-Duct VAV Terminal Units
Dual-duct units have two air valves. One heating valve and one cooling air valve modulate
simultaneously to provide occupant comfort. These systems were popular prior to the energy crisis
of the early 1970s. Popularity is increasing with system concepts which use one valve for
maintaining and monitoring 100% ventilation air.
The features of the dual-duct VAV terminal units are described by the product categories shown in
bold. Within each category the options available are listed
Selection Procedure
This section describes the catalog selection of dual-duct VAV terminal units with specific examples.
A computer selection program is also available to aid in selection of VAV terminal units.
Selection of dual-duct VAV terminal units can involve two elements:
Air valve selection
Acoustics
Air Valve Selection
The wide-open static pressure and airflows are found in the performance data section of the
catalog. To select the air valves, locate the required design cooling and heating airflows for your
terminal unit type and find their vertical intersection, with the smallest air valve size that has a
pressure drop equal to or lower than the maximum wide-open static pressure requirement.
Example:
VDDF Terminal Unit Design cooling airflow: 1000 cfm
Maximum wide-open Air pressure drop: 0.25 in. wg
Minimum cooling airflow: 500 cfm
Design heating airflow: 1000 cfm
Maximum wide-open Air pressure drop: 0.25 in. wg
Minimum heating airflow: 400 cfm
From the performance data charts, select a valve size 10 for cooling, which has a wide-open static
pressure drop of 0.09 in. wg. Select a size 10 for heating, which has a wide-open static pressure drop
of 0.09 in. wg.
Check the minimum and maximum cfm desired with the minimum and maximum cfm allowed in
the table in the general data section. The maximum setting of 1000 cfm is within the acceptable
range. The desired minimum setting of 500 cfm is acceptable for the unit desired.
Figure 1. Dual duct terminal unit
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