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Sequence of operation
Sequence of operation
The VVT Bypass II maintains supply duct static pressure under all system operating conditions and can
operate as part of a linked VVT system or as a stand-alone controller.
Duct static pressure control
The VVT Bypass II modulates a bypass damper or VFD to maintain the static pressure in the supply duct of the
pressure-dependent VVT system. The VVT Bypass II has a damper actuator that provides 45 in./lbs (5 Nm) of
torque.
You can use the controller's 0–10 Vdc output to:
•
Drive a larger actuator if the VVT Bypass II's actuator's torque is insufficient. The actuator must be
installed so that 0 volts causes the bypass damper to fully open. See NOTE below and High-torque
actuator.
•
Drive multiple damper actuators from a single bypass controller. See NOTE below and Linked actuators.
•
Control a VFD
NOTE You cannot use the VVT Bypass II's actuator if external actuator(s) are wired to the analog output.
If linkage is active, normal duct static pressure control is overridden if the linked air source is in either of the
following modes:
•
Fire shutdown or evacuation mode – The bypass damper will be fully open. If controlling a VFD, the VFD
output will be at 0%.
•
Pressurization mode – The bypass damper will be fully closed. If controlling a VFD, the VFD output will be
at 100%.
LAT monitoring and limiting
To protect the air source from excessive supply air temperature conditions, the VVT Bypass II monitors the
supply air temperature. If the temperature reaches either the Minimum Cooling SAT [45°F (7.2°C) default] or
the Maximum Heating SAT [120° F (48.9°C) default], the Bypass controls to the LAT Duct Static Pressure
Setpoint which increases airflow through the air source.
The amount of increase in airflow is related to the square root of the pressure increase. For example, a 25%
increase in airflow at 1 in. (.249 kPa) of static pressure would require a 56% increase in duct static pressure
[1 in.H
O (.249 kPa) * 1.252 = 1.56 in. H
2
in.H
O (.199 kPa) provides a maximum 25% increase in airflow over the default Duct Static Pressure Setpoint
2
of .5 in.H
O (.125 kPa). [.5 in.H
2
the Maximum Heating SAT or Minimum Cooling SAT, the controller detects and indicates the excessive LAT
condition and displays the Controlling Pressure Setpoint and the associated LAT Airflow Increase (%).
VVT Bypass II Controller
Installation and Start-up Guide
O (.388 kPa)]. The default LAT Duct Static Pressure Setpoint of 0.8
2
O (.125 kPa)* 1.252 = .75 [~.8] in.H
2
26
O (.199 kPa)]. When the LAT exceeds
2
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