73
VCCX2 Controller Technical Guide
Economizer Operation (Standard)
NOTE: For Waterside Economizer Operation, see page
80.
Economizer operation is enabled when the Outdoor Air
(OA) drybulb, wetbulb, or dewpoint temperature falls below
the Economizer Enable Setpoint by 1°F and if the Outdoor
Temperature is at least 5°F below the Return Air Temperature (if
that value is available). Economizer operation is disabled when
the OA temperature rises 1°F above the Economizer Enable
Setpoint.
The Economizer acts as the 1st stage of cooling and controls
to the Active Supply Air Cooling Setpoint. An Economizer
Minimum Position can be programmed into the controller.
During Economizer Operation, the economizer will modulate
between this minimum position and 100%. If the economizer
reaches 100% for 2 minutes and the Supply Air Temperature is
still above setpoint, mechanical cooling is then allowed to stage
up while the economizer is held at the full open position. Any
time cooling stages are currently running, and the economizer
becomes enabled, it will immediately open to 100%.
During Heat and Vent Modes, the Economizer will remain at
its minimum position. The only exception to this can occur
during “VAV Operation with Outdoor Air Temperature Control
(VAV Tempering)”. See that section for more details. During
Unoccupied Mode, the Economizer can be used for night setback
free cooling; otherwise it will remain closed.
IAQ (CO2) Override of the Economizer simply resets the
Economizer Minimum Position higher. See IAQ (CO2) Control
Operation section for more details.
If utilizing the Title 24 Economizer option, an Economizer
feedback signal (0-10 VDC) can be wired into the VCC-X EM1
Expansion Module for status monitoring. Several Title 24 alarm
conditions can also be annunciated and are listed in the Alarms
section of this manual.
Economizer Override Via BACnet
®
As stated above, the economizer must reach and remain at 100%
before compressors will be allowed to stage on to meet the
Cooling Supply Air Setpoint. Because of this 100% requirement,
if a value of less than 100% is written to BACnet point AV:70
(Outdoor Air Damper Override) while compressors are operating,
the controller eectively recognizes this as the economizer being
active at less than 100% – so the compressor(s) will disable. To
avoid this condition if the economizer is disabled but compressors
are enabled, AV: 49 (Minimum Economizer Position) can be used
instead. This will change the minimum position, while allowing
compressors to remain on. However, if using AV:49 and the
economizer operation is enabled, the damper can drive further
open than the minimum position for true economizer operation.
Comparative Enthalpy Economizer
Operation
A Comparative Enthalpy Economizer option is also available.
The E-BUS Outdoor Combination Temperature/Humidity
Sensor and the E-BUS Return Air Combination Temperature/
Humidity Sensor must be used for this operation to be available.
If the Outdoor Enthalpy is below the Comparative Economizer
Enable Setpoint by the Comparative Economizer Enable
Deadband amount, and the Outdoor Enthalpy is less than the
Return Air Enthalpy by the Comparative Economizer Enable
Deadband amount, then Economizer Operation will be enabled
to act as the rst stage of Cooling. If the Outdoor Enthalpy
rises above the Comparative Economizer Enable Setpoint by
the Comparative Economizer Enable Deadband amount or if
the Outdoor Enthalpy rises above the Return Air Enthalpy by
the Comparative Economizer Enable Deadband amount, then
Economizer Operation will be disabled.
SEQUENCE OF OPERATIONS
Economizer Operation
To Next Page
Loading...
Need help?
General
