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Sequence of operation
The VVT Bypass Controller maintains supply duct static pressure under all system operating conditions.
The controller can operate as part of a linked VVT system or as a stand-alone controller.
Duct static pressure control
The VVT Bypass Controller modulates a bypass damper or VFD to maintain the static pressure in the supply
duct of the pressure-dependent VVT system. The controller has a built-in damper actuator that provides 35
in./lbs (4 Nm) of torque.
You can use the controller's 0–10 Vdc output to:
•
Drive a larger actuator if the built-in 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 (page
13).
•
Drive multiple damper actuators from a single bypass controller. See NOTE below and Linked actuators
(page 14).
•
Control a VFD
NOTE You cannot use the built-in 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 Controller
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 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
Carrier Proprietary and Confidential
21
Sequence of operation
O (.199 kPa)]. When the LAT exceeds
2
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