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iWorx® VPU2
505-010-2, Effective: April 29, 2014
15
© 2014 Taco Electronic Solutions, Inc.
APPLICATION DESCRIPTION
The VPU2 is a Variable Air Volume (VAV) package unit controller for controlling supply air temperature and supply air
duct static pressure. Figure
illustrate
typical
VPU2
applications.
The
VPU2
operates
in
conjunction
with
up to 56 VAVI zone controllers. Control is achieved by modulating the economizer position and sequencing the heating
and cooling stages based on the current supply air temperature requirements. In addition, the VPU2 controls the sup-
ply fan speed to maintain a supply duct static pressure setpoint.
Figure 5: VPU2 Application with Staged Heating & Cooling and Modulated Economizer
The VPU2 controls the starting and stopping of the supply air fan. During the occupied and warm-up periods, the fan
runs continuously. The fan cycles on during the unoccupied periods when a zone is in extended occupancy or when
there is a call for emergency heating. The fan speed control operates to maintain a configurable system static pressure
setpoint. Fan speed is calculated by a Propor Integral (P+I) control loop based on the measured static pressure
and setpoint. As the pressure increases above the pressure setpoint, the fan speed output is throttled back. As the
pressure decreases below the pressure setpoint, the output is increased.
The enthalpies of the outside and inside air are calculated periodically. A comparison is performed to determine if “free
cooling” is available. If “free cooling” is available, the economizer is enabled. Free cooling can also be enabled based
on a dry-bulb comparison of the outdoor air temperature and indoor temperature. The system can use either a two-
position or modulated economizer. If a two-position economizer is employed, it is energized when there is a call for
cooling. It is used as the first stage of cooling to take advantage of the energy savings. The two-position economizer
output is off when the modulating economizer feature is disabled.